Author: Bryan Orr

One of the most common parts to fail on a single phase HVAC system is a run capacitor, so much so that we sometimes refer to junior techs as “capacitor changers”. While capacitors may be easy to diagnose and replace, here are some things many techs may not know.

Capacitors Don’t “Boost” the Voltage 

A capacitor is a device that stores a differential charge on opposing metal plates. While capacitors can be used in circuits that boost voltage they don’t actually increase voltage themselves. We often see higher voltage across a capacitor than the line voltage, but this is due to the Back EMF (Counter electromotive force) generated by the motor itself, not the capacitor.

Current Doesn’t Flow Through The Capacitor, Just in and Out of It 

Techs notice that the one side of power is connected to the C terminal or the side opposite the run winding. Many techs imagine that this power “feeds” into the terminal, get’s boosted or shifted and then enters the compressor or motor through the other side. While that may make sense it isn’t actually how a capacitor works at all.

A typical HVAC run capacitor is just two long sheets of really thin metal, insulated with an insulation barrier of very thin plastic and immersed in an oil to help dissipate heat. Just like the primary and secondary of a transformer the two sheets of metal never actually touch, but electrons do gather and discharge with every cycle of the alternating current.  For example, the electrons that gather on the “C” side of the capacitor never go “through” the plastic insulation barrier over to the “HERM” or “FAN” side. The tow forces simply attract and release in and out of the capacitor on the same side they entered.

The Higher the Capacitance, the Higher the Current on the Start Winding 

On a properly wired PSC (Permanent Split Capacitor) motor, the only way the start winding can have any current move through is if the capacitor stores and discharges. The higher the MFD of the capacitor, the greater the stored energy and the greater the start winding amperage. If the capacitor is completely failed with 0 capacitance it is the same as having an open start winding. Next time you find a failed run capacitor (with no start capacitor) read the amperage on the start winding with a clamp to see what I mean.

This is why oversizing a capacitor can quickly cause damage to a compressor. By increasing the current on the start winding the compressor start winding will be much more prone to early failure.

The Voltage Rating is What it Can Handle, Not What it Will Produce

Many techs think they must replace a 370v capacitor with a 370v capacitor. The voltage rating displays the “not to exceed” rating, which means you can replace a 370v with a 440v but you cannot replace a 440v with a 370v. This misconception is so common that many capacitor manufactures began stamping 440v capacitors with 370/440 just to eliminate confusion.

You Can Test a Capacitor While the Unit is Running

You simply measure the current (amps) of the motor start winding coming off of the capacitor and multiply it time 2652 and then divide that number by the voltage you measure across the capacitor. For a full write up on the process, you can look here

 

 

 


The term “short” has become a meaningless phrase in common culture to mean “anything that is wrong with an electrical device”.

A short circuit is a particular fault that can mean one of two things in technical lingo.

Any two circuits that are connecting in an undesigned manner. This would be the case if a control wire had two conductors connected together due to abrasion. Like a Y and G circuit “shorted” in a thermostat wire between the furnace and the thermostat. This would result in the condenser running whenever the blower is energized.

A short can also be described as a no load path between two points of differing charges. This would be a traditional “short to ground” low voltage hot to common connection or a connection between legs of power without first going through a load of appropriate resistance.

Both of these conditions will result in something occurring that should not be occurring. Either something being energized when it shouldn’t be or fuses and breakers tripping or blowing or damaged components.

This is different than an Open circuit which is no path at all. So if a load has power applied and NOTHING is happening it is an open. If power is applied and breakers or fuses trip or blow or something comes on at the wrong time or order, that is a short

— Bryan

Back in the “good old days” controls were all analog and mechanical which simply means that they acted in a directly connected and variable manner based on a change in force. Both pneumatic (air pressure) or hydraulic (fluid pressure) systems are examples of mechanical, analog controls. When pressure increased or decreased on a particular device it signaled a change in in action on another device like a pump / valve etc…

In the HVAC/R industry, we still see these types of controls with a TXV being a common example. The TXV is controlled by pressures in the suction line, bulb and spring to set the outlet superheat. These forces are all mechanical without electrical inputs or specific “data points”. The feedback from these forces are in constant tension to output the proper amount of refrigerant to properly feed the evaporator coil.

Digital vs. Analog

As controls have changed from mechanical to electrical we now have systems that are controlled by analog electrical signals and digital electrical systems. Analog is simply a varying electrical signal (either voltage or amperage) that signifies changes in a system or device. A digital signal means data encoded into “digits” which can be communicated using many different computer languages, rules or protocols (these all mean essentially the same thing). In digital controls the “signal” can include a combination of voltage, amperage and On/Off changes to communicate between devices.

So what about 4-20ma?

When industry started to change over from mechanical to electrical they created a protocol (set of rules) that controls could use that would still function in a similar way to the old pneumatic controls. They decided that the range would be  4ma(milliamps) as the bottom reference of any sensor and 20ma would be the top reference. If you were setting up a sensor to indicate the fluid level in a tank you would set the bottom output to 4ma (meaning empty) and the top output to 20ma (meaning full).

In the case of a pressure transducer, you set the top range to the max rating of the sensor to 20ma and the bottom pressure reading to 4ma… you get the point.

ma controls are great because of their simplicity and ruggedness. You supply power to a “sensor” (Actually a sensor / transmitter to be exact) and based on the measurement the sensor reports to the transmitter that produces a milliamp signal. This signal is connected to an input on the control that measures the amperage and converts that to a reading.

Because amperage is the same at all points in the circuit the 4-20ma circuit is not impacted by voltage drop or interference like a voltage sensing circuit. Because the “bottom” of the scale is 4ma the control can also sense the difference between an “out of range” condition below 4ma and an open circuit.

The downside of a 4-20ma control is that each device requires it’s own conductor. In digital controls, many devices can be controlled by a single conductor set or “trunk” making it much easier to route, configure and manage complex controls.

Testing 4-20ma Circuits 

There are two different ways to measure milliamps. One is to use a special milliamp clamp called a “process clamp meter” that allows reading the amperage without disconnecting wires. These meters are expensive and it is unlikely that a typical HVAC/R tech will have one.

The more common way it to use alligator clips on a quality meter set to the milliamp scale and connect in series with the circuit. this means you will need to disconnect a wire or terminal and put your meter in the path. This is the same way we test microamps on a gas furnace flame sensing rod only using the milliamp scale.

— Bryan

 

We have discussed DTD (Design Temperature Difference) quite a bit for air conditioning applications, but what about refrigeration? Let’s start by defining our terms again

Suction Saturation Temperature

Saturation temperature is the temperature the refrigerant will be at a given pressure if it is in the process of changing state. This change of state would be from liquid to vapor (boiling) in the case of the low side (evaporator / suction line). When we look at saturation temperatures instead of pressures we can use similar rules and we will see similar saturation temperatures across all refrigerants when the application is the same. Experienced HVAC and refrigeration techs pay far closer attention to the saturation temperatures than they do pressures.

Evaporator TD and DTD

Evaporator TD (temperature difference) is the measured difference between the suction saturation temperature (evaporator boiling temperature) and the box temperature. DTD (design temperature difference) is the designed or expected TD.

Delta T

Many A/C techs will confuse TD with Delta T. Delta T is the difference between the evaporator AIR temperature entering the coil to the air temperature leaving the coil. The Delta T will vary based on the humidity in the box where TD will not.

Target Box Temperature 

The temperature the refrigeration box should maintain when the system is operating properly

Superheat

The increase in temperature between the suction saturation temperature and the suction line temperature leaving the evaporator. Superheat is the temperature (sensible heat) gained between the point that all of the liquid boiled off in the evaporator coil and the suction line at the outlet of the coil. in refrigeration, like HVAC 10°F(5.5°K) of superheat  is average with a range from 3°F to 12°F(1.65°K – 6.6°K) depending on the equipment type (10°F(5.5°K) for med temp, 5°F(2.75°K) for low temp, 3°F(1.65°K) for ice machines ).

Hot Pull Down

Refrigeration equipment is unlike HVAC equipment in that the evaporator will spend most of its life running in a very stable environment with minimal fluctuation in the box temperature.

On occasion a refrigeration system will see a huge change in load in cases where it was off and needs to “pull down” the temperature, or when doors are left open or when a large quantity of warm product is placed in the box. When a piece of refrigeration equipment is in hot pull down it cannot be expected to abide by the typical DTD or superheat rules and must be allowed to get near the design box temperature before fine adjustments are made to the charge, TXV superheat settings or to the EPR (Evaporator Pressure Regulator) if there is one.

Design Temperature Difference (DTD)

In air conditioning applications a 35°F DTD is a good guideline for systems that run 400 CFM(679.6 m3/h) of air per ton of cooling (12,000 btu/hr). In refrigeration the DTD is much lower than in air conditioning.

There are several reasons for this but one big reason is the desire to maintain relatively high relative humidity levels in refrigeration to keep from drying out and damaging product. Keep in mind that NOTHING is a substitute from manufacturer’s data but here are some good DTD guidelines for traditional / older refrigeration equipment. Keep in min dthat the trend is toward lower evaporator TD on newer equipment.

Walk-ins  10°F(5.5°K) DTD +/- 3°F(1.65°K)
Reach-ins  20°F(11°K) DTD +/- 5°F(2.75°K)
A/C 35°F(13.75°K)) DTD +/- 5°F(2.75°K)

You then subtract the DTD from your box temperature / return temperature to calculate your target suction saturation. You can then use this target saturation / DTD and compare it to your actual measured saturation and DT once the box is within 5°F – 10°F(2.75°K – 5.5°K) of it’s target temperature to help you set your charge, TXV and EPR as well as diagnose potential airflow issues when compared with suction superheat and subcooling / clear site glass.

For Example –

If you have a medium temp walk-in cooler with a 35°F(1.66°C) box temperature you would expect to see a suction saturation of  25°F +/- 3°F

When doing a quick inspection of a piece of refrigeration equipment without gauges you can use this data to do the following calculation –

35°F – 10°F DT + 10°F superheat = 35°F suction line temperature +/- 3°F 

In this particular case logic tells us that the suction line could be no WARMER than 35°F(1.66°C) because that is the temperature of the air the refrigerant is transferring its heat to. However by the time you factor in the the accuracy of your box thermometer and line thermometer and the assumed saturation temperature you would still expect a 35°F(1.66°C) suction line temperature +/- 3°F(1.65°K)

For a -10°(-23.33°C) box, low temp reach-in you would calculate it this way

-10°F- 20°F DT + 5°F superheat = -25°F suction line temperature +/- 5°F 

Clearly, this is NOT the way to commision a new piece of equipment or to benchmark a system you haven’t worked on before, but it can give you a quick glimpse at the operation of a piece of refrigeration equipment without attaching gauges, especially on critically charged or sealed systems.

The best practice is to know the equipment you are working on, read up on it and properly log benchmark data the first time you work on a piece of equipment or during commissioning.

It should also be noted as Jeremy Smith pointed out, in recent years TD’s have been decreasing as manufacturers seek higher efficiency through higher suction and lower compression ratios.

This means that TD’s as low as 5 can be designed into some units but keep in mind… the suction line can still be no warmer than the box so as DTD drops so does superheat and the critical nature of expansion valve operation.

— Bryan

 

 

I get questions all the time about performing “load calculations” and “rules of thumb” as well as how to do it properly. This article isn’t about load calculation but the only good answer is to find a quality ACCA approved Manual J software and get used to using it.

You may have heard from others in the field that Manual J tends to “undersize” the equipment. If you are an engineer or designer you may be shocked at how “oversized” most equipment is when compared to Manual J. Like most things, the truth can be found somewhere in between and here are the reasons for this.

System selection is just as important as Manual J because if you don’t match the proper equipment to the BTU load or if you fail to consider the factors you can easily end up with a system that is undersized or that does not deal with the humidity load of the space.

Here are some common factors that contractors & designers fail to properly factor into their Manual J

Duct leakage rate 

We can guess, but the only way to really KNOW the rate of leakage is to do a duct leakage test like the one shown below by Corbett Lunsford.

Building Envelope Leakage Rate

The rate of leakage in and out of a structure is one of the most overlooked aspects of a load calculation. Once again, we can guess based on the age and construction type but the true leakage rate can vary wildly. The only true way to test leakage rate is by measuring it with a blower door.

Insulation 

Even insulation is often a guess in areas where you cannot access walls or portions of the attic. It takes a combination of experience and thermal imaging or other R-value measuring tools to truly calculate heat loss / gain through insulation.

Shade

This one is very challenging to calculate. You may have two homes with nearly identical construction, orientation, and layout, one with significant shade from trees and another without. This shade can represent a significant decrease in radiant heat transfer to the walls, windows and roof and will vary seasonally based on the season and various times of day. Shade is something you can factor in using common sense. While I wouldn’t suggest “under sizing” just because of shade, you can be sure that a well-shaded structure will have lower radiant gains which will have an impact in all seasons.

Ventilation

For every cubic foot of air  you move out of a building you are also moving one cubic foot of air into the building, either through a designed path, through cracks and gaps, or when a door or window opens. One way or another when you move air out you are also moving it in. It is always better to move that air into the building through a designed path where the air can be controlled, measured and likely treated (ERV, HRV, Dehumidification) instead of through cracks and gaps that can be in any number of undesirable places. In addition to this, there are new standards being enforced surrounding ASHRAE 62.1 & 62.2 that mandate mechanical fresh air be brought into all structures. This fresh air needs to be considered as to heat gains and losses both sensible and latent.

Because of all these factors and industry pressures I have found that design professionals have a tendency to underestimate heat gains and losses (on existing, untested structures) while contractors and field personnel tend to oversize equipment based on “experience” or “rules of thumb” and usually a combination of both. This happens because it is much more common for a contractor to get a complaint of a system “not keeping up” than humidity, or power consumption issues because the thermostat displays the temperature in big bold numbers while humidity and power are a bit more abstract.

Design professionals are under pressure not to oversize equipment by the industry (and rightfully so) but may not be fully aware of all of the “as built” conditions that exist.

But for sake of argument, lets say the heating season losses and cooling season gains have been perfectly calculated

We now bump up against some of the most common areas of misunderstanding by contractors and field staff which is properly matching the equipment to the load. here are some of the biggest mistakes.

  • Failing to cover both sensible and latent (humidity) loads in the cooling season
  • Using nominal tonnage of equipment to estimate capacity (no a 3-ton cannot be counted on to produce 36,000 BTU)
  • Looking at AHRI ratings to find capacity instead of at the manufacturer performance data
  • Oversizing / Undersizing either the heating or cooling side by considering one and not the other

In order to properly select equipment, it is recommended that you use ACCA manual S to ensure that you don’t miss any of the steps. ACCA has a great quick guide on system selection you can read HERE

Here are some tests you can apply to your residential design to help double check that your selection matches the design.

  • The sensible cooling capacity of the system you choose should not be more than 15% greater than the sensible heat gain of the space
  • The latent cooling capacity of the system must be equal to or greater than the latent load of the space
  • In heat pump applications with greater heating loads than cooling loads (common) the cooling system must not be more than 25% than the sensible cooling gain
  • The heat Pump + electric aux heat capacity should not exceed the heat loss by more than 15%
  • In the case of fuel-burning appliances choose the next size greater than the heat loss of the space

All of these need to take into account the manufactures specifications matching the load calculation conditions to the specifications of your equipment at the same conditions. While a furnace may produce the same heat output no matter the conditions, air conditioning and heat pump equipment output will change depending on indoor and outdoor conditions.

The final step is configuring the equipment to the proper air flow levels so that the sensible / latent capacity will match the design. If the system was designed for 400 CFM per ton then ensuring that the equipment is set to output that airflow is critical.

— Bryan

This article was written by Christopher Molnar, a licensed Florida mechanical contractor. While I’m not personally a practitioner of the “check static every time” doctrine, I certainly appreciate Chris and his passion for this topic. Thanks Chris!

Why Check Static Pressures on EVERY call 

You wouldn’t go to the doctor’s office and walk away without having your pulse and blood pressure checked. To do so would be malpractice. Your pulse and blood pressures are a sign of how your blood is flowing through your body and how well your heart is pushing that blood around your body. Measurements outside of the standards are cause for further investigation. 

 Why is it acceptable to walk away from any HVAC service call without checking the air pressures in the ductwork? I would suggest that not taking these pressures on each and every service call is malpractice and you are cheating your employer and customer by not taking these readings.

 

What is static pressure? The definition is the outward pressure of the air (without movement) in the ductwork. The number is either a positive or negative based on which side of the blower/fan you take the pressure on. The air on the supply side of the system is always positive pressure, in other words the pressure is greater than that of the air outside of the duct. Duct pressures are generally measured in Inches of Water Column in the US.

 The air on the return side of the system is always negative, or a pressure that is below the air pressure outside of the ductwork. Think about it, it makes sense. In order to blow air into a space the air in the supply duct must be above the pressure in the room we are moving it into. In order to draw air into the return duct, the air in the return duct must be under that of the room we are pulling from. Air will always move from an area of high pressure to an area of low pressure.

 What causes this static pressure? Ductwork is just a pipe, correct? So why does pressure build up in this pipe, shouldn’t it just “Flow”?

 The blower is designed to work against a certain amount of pressure in order to provide comfort to the customer and achieve peak heating/cooling efficiency. 

The less air that moves through the ducts and over the coil the greater the temperature of the air will drop and more humidity it will give up. However, we also must maintain a proper level of air flow so that we do not overheat a furnace, or freeze a coil. For air conditioning this is about 400CFM (cubic feet per minute) per ton in most regions. 

On the data plate or in the specs of every air handling device or funace you will find the “total external static pressure” that the device can handle. For example, on the nameplate above you will see that the total maximum static pressure is .5 inches of water column. So, what does this mean?

A technician will take two pressure readings using a static pressure tip. One will be in the supply duct above the coil in the case of a fan coil / airhandler or between the blower and coil on a furnace. The other will be in the return duct right before the blower compartment, or sometimes in the return plenum. 

The number in the supply duct will be positive. The number in the return duct will be negative. To find the total external pressures you will drop the sign (+ or -) and use the absolute value. So, a +.25 will become .25 and a -.25 will become .25 as well. Add the two numbers together as .25 + .25 equals .50 inches water column (.50 “w.c.).

 

To properly get these readings you will need a mimumum of one static pressure tip and a manometer, such as the Testo Smartprobe 510i that interfaces with a smart phone.

You want to see a measurement as close to, but not over the “maximum external static pressure” on the data plate. If the number is too low it most likely means that the blower is not operating at the correct speed, or that there is a leak in the ductwork that is preventing static pressure from building up.

 

Low supply static pressure: Look for duct leaks. Look for missing register covers. Look for proper blower speed. Check for proper cooling (if system has a cooling coil that should be wet). Finally, double check supply duct sizing.

 

Low return static pressure: Look for duct leaks. Look for missing filters. Look for proper blower speeds.
High return duct static pressure: Dirty filter? Blocked return grills? Insulation or something else stuck in the return duct. Collapsed return duct (if fiberboard or rusted steel ductwork).

 High supply duct pressure: undersized supply ducts. Closed registers and grills. Closed doors in buildings with central returns. Collapsed ductwork. Blower set at too high of a speed.

 Static pressure that is too low will affect latent capacity (moisture removal) on PSC motors in some cases. 

Static pressure that is too high will cause noise, excess wear on the blower motor, high amperages, cooling and heating inefficiency, and in many cases can pull the condensation off of the air conditioning coil causing leaks and damage. 

 We all learn in trade school to measure voltage, amperage, and the resistance of components. Most technicians understand what causes low superheat, high subcooling, and pressures. We understand what high amperages can do to a system. We know that motors will burn out if the amperages are too high. We know that a clogged condensate line can cause a flood. Taking the additional 5 minutes to take two static pressure measurements when the system is operational will allow you to do a quick air flow check and make sure that there is not something else happening that will impact system health, performance, and customer comfort. Not taking static pressure on each and every Heating or Air Conditioning service call should be malpractice.

— Chris

Photo by Ulises Palacios

Refrigerant circuit restrictions can be common things like a plugged filter drier or a restricted metering device. They can also be more difficult to diagnose and exotic issues like a kinked liquid line, blocked evaporator feeder tube or a compressor connected improperly with a discharge line full of solder (I’ve seen it).

To start with let’s talk about the symptoms.

When an undesigned restriction occurs, refrigerant will “back up” against the restriction resulting in more refrigerant being present before the restriction and less afterwards than designed. Think of it like a refrigerant traffic jam with the refrigerant “road” being congested before the restriction and free and clear afterwards. This restriction will result in a pressure drop across the restriction with higher pressure being on the inlet side and lower pressure on the outlet side of the “traffic jam”.

First we must be aware that a restriction exists in the first place. In the case of the most common liquid line restrictions on HVAC equipment (with no receiver) we will see low suction pressure, high superheat and normal to high subcooling. In cases like this we know it is not simply “low on charge” because of the subcooling reading, and we also know it isn’t just a an evaporator airflow issue because of the high superheat. This leaves us in the realm of restriction.  Like anything else, some common sense, a look at the system history and a visual inspection can find many restrictions without any fancy diagnosis, but sometimes you have to put on your thinking cap, grab a pipe or a cigar, and go to work.

In a perfect world we could just connect a gauge anywhere in the system and we could find the pressure drop, in the real world we only have two or maybe three points on connection and they are not sufficient for us to pinpoint a restriction. Luckily we have temperature drop as a proxy for pressure drop, whenever the pressure drops there will also be a temperature drop. The trouble is, by the time the temperature drops enough for us to reliably measure it with a thermometer it is usually pretty bad, making minor restrictions hard to find.  It can also be challenging when the metering device itself is a suspect (and it often is), because the metering device is a DESIGNED RESTRICTION. This means that a pressure drop is it’s very purpose, but is it restricting too much?

So to actually FIND a restriction you are left with a few tools in your arsenal.

Common Sense

Get acquainted with the history of the system. How old is it? What has been done on it recently? Has the refrigerant circuit been open to atmosphere?  If you recently had a burnout compressor then it is very likely that suction and liquid line driers could be restricted. If the system has been running just fine for 7 years it is more likely that that TXV element tube rubbed out and now the TXV is slammed down. If the distributor just a leak repaired on it, it is very possible that they accidentally filled one of the feeder tubes with solder when they made the repair. A little common sense can save a lot of random hypothesis. Any experienced technician will agree with the problem solving principle called Occam’s Razor that states

“With all things being equal, simpler explanations are generally better than more complex ones”

This certainly hold true when looking for restrictions.

Temperature Drop 

Grab your most accurate line temperature clamp and start making measurements across possible restrictions like line filter driers and the liquid line itself. If you find any confirmable temperature drop across a line drier than you can knows it’s restricted, just make sure to double check. Across a typical liquid line you will generally only see a few degrees of temperature drop but it does depend on the ambient temperature, condensing temperature and the line length.

Freeze Test

Sometime the exact point of temperature change can be tough to locate. In these cases when the metering device, distributor, feeder tubes, inlet screen or evaporator are all suspects you can do the freeze test. Disconnect the blower and watch the frost patterns. On a properly functioning system the ice will start right at the outlet of the metering device and extend forward though the feeder tubes and work its way fairly evenly through the coil on the coil piping route. Look for inconsistencies in the pattern and you can often find a restriction.

If for example, you see that the frost is starting BEFORE the metering device instead of after, you can bet the restriction is an inlet screen. This test is finicky and requires a trained eye to track the tubing patterns, otherwise you might think a coil is restricted when it’s just the way it’s piped. Also be aware that the designed pressure drop of metering devices that also contain a distributer and feeder tubes is cumulative across all of those restriction points. This means that in some cases you may get more frost after the distributer than you do between the metering device and the distributer, this is to be expected.

Photo by Ulises Palacios

Thermal Imaging 

The holy grail of finding restrictions is the thermal imaging camera. You are able to see restrictions in real time and pinpoint the exact location where the temperature change begins. Thermal imaging can even be used to find illusive restrictions like discharge line restrictions caused by poor brazing practices, condenser feeding issues, evaporator restriction etc…

Photo by Ulises Palacios

So the process for finding restrictions is –

  1. Prove you have one by looking carefully at your readings
  2. Use some common sense and perform a visual inspection
  3. Take lots of temperature measurements until you find it
  4. Whip out the fancy pants thermal imaging camera and spot that sucker in no time flat and be the hero with throngs of adoring fans

Keep in mind it get’s even trickier to diagnose when you are working on a system with a receiver, because the receiver can usually hold a lot of excess refrigerant, often making a liquid line restriction appear more like a low charge in the readings. Also, minor suction line restrictions like a kinked suction line can be very difficult to find because the temperature drop will usually be unmeasurably low.

This is why taking all the system readings in conjunction with some common sense and knowledge of the systems history are your best allies. And when in doubt… get a thermal imager from TruTech tools .

I told you it wasn’t easy

— Bryan

Here is a great article addressing restrictions in refrigeration systems – Diagnosing A Restricted Liquid Line Can Be Tricky



Relays can be used for many different control applications including controlling fans, blowers, other relays or contactors, valves, dampers, pumps and much more. A 90-340 is a very common, versatile relay that many techs have on their truck so we will use it as the example.


A relay is just a remotely controlled switch that opens and closes using an electromagnet. The electromagnetic portion that provides the opening and/or closing force of the switch is called the coil. Relay coils can come in many different voltages depending on the application, but in residential and light commercial HVAC 24-volt coils are the most common.

The portion of the relay that opens and closes can be called the switch, contacts or points. These contacts can either be closed meaning there is an electrical path or open meaning there is no electrical path. Often this open or closed circuit will be described as “making” a circuit, meaning the switch is closed or “breaking” a circuit meaning the switch is open.


It is important when connecting a relay to distinguish which two relay points connect the coil. In the case of the 90-340, it is the bottom two terminals of the relay. Even though the coil is unmarked on most 90-340 relays, you can find it easily by locating the terminals with the small strands of wire connected. These two points connect together through the electromagnetic coil. When 24 volts of potential is applied across the coil the switch portion of the relay will switch from open to closed and closed to open depending on the terminal. Keep in mind that in a normal 24v circuit one side of the coil is connected to a 24v switch leg such as the thermostat “G” circuit for blower control, and the OTHER side of the coil is connected back to common.

The other six terminals are switch/contact terminals and the relay has a diagram embossed right on the top for easy reference. The way the circuit is drawn shows the de-energized state of the relay, meaning the state of the switches when no power is applied to the coil. When power is applied to the coil the points that were previously open (broken) now become closed (made) and the ones that were closed become open. When two points are closed when no power is applied to a relay coil we call them “normally closed” when they are open when no power is applied they are called “normally open”.


So based on this embossed diagram on the relay 1 to 3 and 4 to 6 are open (normally open) with no power to the coil and closed when power is applied. 1 to 2 and 4 to 5 are closed (normally closed) with no power and they open when the coil is energized. There is never a path between 2 & 3 or 5 & 6 because between them, at least one of them is always open. There is also no path or circuit between the top three terminals and the bottom three terminals or between the switch and coil portions of the 90-340 relay.

The data tag on a 90-340 shows both the coil voltage as well as the LRA (locked rotor amps) and FLA (full load amps) that the contacts can handle at various voltages for inductive (magnetic) loads like motors. It also lists the amp rating if the relay is controlling a RES (resistive) load like a heater or an incandescent light.


This relay can control a 39.6 LRA and 6.9 FLA Motor or a 15 amp heater at 240 volts based on the data tag.

— Bryan

Diagram courtesy of Emerson

How to wire a condensing fan motor for 3 vs. 4 wires is a common question by new techs. Jesse Grandbois submitted this tech tip to help make it simple. Thanks Jesse!.


This is a quick one on the difference between wiring universal condenser fan motors and why brown+white is the same wire as white. This one seems to confuse even experienced technicians and really is very simple once you see it. Now keep in mind that wire colors ACTUALLY MEAN NOTHING, but on service replacement motors the colors do tend to be consistent. As always, refer to the wiring diagram on the particular motor you are using.

I’ll provide a diagram and explain the wires below.

Here’s the 3 wire method:
– White wire from the condenser fan motor to one side of power on the contactor (T1) and jumped to one side of the fan capacitor. This is AC power and not a dual capacitor, so the terminal side does not matter
– Black wire from the condenser fan motor to other side of power on the contactor (T2)
– Brown wire from the condenser fan motor to the other side of the capacitor opposite the jumper wire.
– Cap off brown+white (unused)

Now for your 4 wire method:
– White wire from the condenser fan motor to one side of power on the contactor (T1)
– Black wire from the condenser fan motor to other side of power on the contactor (T2)
– Brown wire from the condenser fan motor to the capacitor. Again, this is AC power and not a dual capacitor, so the terminal side does not matter
– Brown+white wire to the other side of the capacitor

Now as you can see the only difference is there is no jumper from the contactor to the capacitor. This is because the brown+white and the white is the same wire. They’re joined inside the motor. The brown wire with the white stripe is only there for convenience.

If you want to prove that the white and brown w/ white stripe are the same, take an ohm meter and test between the two. You will find that it either reads Zero or very low ohms proving that they are directly connected within the motor.

flame_sensing_rod

Proving flame is an important part of the gas firing sequence. Without proof of flame you risk dumping unspent gas into the heat exchanger resulting in an explosion.

There are many ways to “prove flame” we are focusing on the flame sensing rod method here.

Here are the facts-

Flame sensing rods, also know as flame rectifier rods or flame rectification rods are common place in modern hot surface and ISI (intermittent spark ignition) gas fired appliances.

Flame sensing rods stick out into the flame and connect back to the furnace board. Once the board sends a call to the gas valve to open, it monitors current flow on the flame sensing rod. It does this by generating a potential (voltage) at the flame sensing terminal, this terminal is connected to the sensor with a conductor. When no flame is present there will be potential at the rod and no current, when a flame is present a small microamp DC current will be present as a path is made between the rod and the ions in the flame. This small DC current signals the board that flame exists and all is well with the world. If it does not sense this microamp DC current within a few seconds it will shut off the gas valve and try again.

The board outputs this potential (voltage) on the  flame sensing terminal right at the beginning of the sequence to confirm that the path is “open” with no flame. This ensures against false positives (sensing flame / current when there should be none) andonce it goes from 0 current to the rated micoamp current the board “knows” that flame is present. 

These flame sensing rods are “dumb” devices. They do not generate potential (volts) or current (amps), their predecessor the thermocouple (seen in standing pilot systems) does generate a potential itself which is often the source of the confusion.

A flame sensing rod is a piece of metal with a ceramic insulator that keeps it from grounding out. That is all. However because it is conducting in the Millionths of an amp (microamp) a lot can go wrong with it that a normal electrical component wouldn’t have any issue with. Tolerances are tight so small factors make a big difference.

Flame sensors fail when:

  1. They short out due to a cracked insulator
  2. They Fail open because they are broken
  3. They don’t conduct because they are not properly placed in the flame
  4. They become coated in silica (glass) or carbon

Before I go any further I want to address a common question. Do flame sensors have a special coating that can be rubbed off with improper cleaning?

Well… If we are talking about a thermocouple or a thermopile then yes.. absolutely, but we aren’t discussing standing pilot systems here.

I have seen a lot of flame sensing rods, and I have done a good deal of research and I have found no evidence that most flame sensing rods have a special coating on them that can be rubbed off. Now, if you have real, quantifiable proof  from an manufacturer that says otherwise.. PLEASE provide it to me so I can retract this statement.

Here are the steps to test a flame sensor –

  • Ensure the furnace is properly grounded. You can do this by powering down the heater and taking an ohm reading between neutral and the burner assembly. You should read a few ohms of resistance max, the lower the ohm reading the better grounded it is.
  • Make sure your polarity is correct, incoming hot connected to hot, neutral to neutral.
  • Ensure the rod is positioned so it will be covered in flame
  • Get a meter that reads in the microamp scale with a .10 resolution minimum. Use a good QUALITY meter for this and make sure your leads are in the correct locations.
  • Connect your leads in SERIES. This means you have to disconnect lead from the rod, connect one lead to the rod and the other to the terminal to the board WITH THE CONNECTOR UNHOOKED FROM THE ROD
  • When the flame lights you should read between .5 and 10 microamps depending of the furnace. Readings between 2 and 6 are common.

flamerectification

If you do not have a proper microamp reading you can confirm the following

  • That the flame rod is not open. Ohm from tip to terminal on the rod. If the rod is open it is failed.
  • Check the insulator and make sure it isn’t cracked or grounded
  • Check for proper burner grounding and incoming power polarity (as mentioned)
  • Clean the rod… Now this is a controversial one. I suggest using a very fine steel wool or abrasive pad (magic erasers often work). remove and clean the rod and ensure you wipe it clean of any particles left over from cleaning. Handle very gently. Once complete perform an ohm test from tip to terminal again to ensure you haven’t damaged during cleaning. If you want to be real crazy, use some electrical contact cleaner on it after cleaning to help remove any residue… just nowhere near flame, unless you don’t want eyebrows.

Once you have established all of the above and you are still not getting the required microamps then you are left replacing the board.

Word of warning –

Test your tools regularly. If you are trusting your meter and you aren’t 100% sure your meter is working and set up properly you may end up with a misdiagnosis. Test and calibrate your tools regularly.

Do every possible test before replacing a board. Many techs advocate just replacing a flame senor if they suspect it isn’t conducting well. I am cool with that so long as

  1. You don’t charge the customer for it is there was nothing wrong with it
  2. You company is OK eating the cost of rods that were not needed

Or.. you just install a new one long enough to test. That is all fine and good if you have extra flame rods in your truck. Many techs do not have that luxury.

Finally…

If flame rods are getting dirty / coated often, you will want to find out why. There is something in the environment or the combustion that is causing it.

In Summary flame rods should be

  1. In the flame
  2. Clean
  3. Not open
  4. Not shorted

Now is the part where the furnace techs from all over the world tear me apart.

— Bryan

 

 

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