Month: April 2017

We had a really great conversation on the HVAC School Facebook Group today about some belt tension best practices and it turns out that even a lot  of really smart and experienced techs are not aware of all the factors related to belt tensioning.

Myth #1 is that amperage is used to set belt tension. Now don’t get me wrong, checking amperage before and after changing belt tension is an excellent practice to ensure you are not binding the bearings from over tension, it does not tell you whether or not the belt is at optimum tension.

I think Browning summarizes it best in this statement from their Browning tool box technician app

Ideal tension is the lowest tension at which the belt will not slip under peak load conditions

Getting a belt too tight shortens the life of the belt and bearings and can cause high amperage. Leaving a belt too loose will shorten the belt life and result in loss of airflow and noise.

Many techs confuse the sheave adjustment, designed to alter the pulley ratio and the airflow with the belt tension adjustment. These are not the same thing and serve separate purposes.

The adjustable sheave allows the pulley faces to adjust closer or further from one another, resulting in a belt that rides closer to the hub when looser (halves further apart) or closer to the edge when tighter  (halves further separated) THIS ADJUSTMENT IS FOR FAN SPEED ONLY NOT TENSIONING

With a properly tensioned belt the belt should not slip significantly when starting, it should not be noisy and it should not bounce around. If you tighten the belt check the amps before and after and the motor should not overamp.

The correct tension method is to get the belt close to the correct tension by feel with a deflection of 1/64 of an inch for every 1″ of distance between the two pulley centers. You can then use an app or a chart like THIS ONE to find the proper force to generate this deflection.

You would then use a belt deflection tool like the one one shown above to test the deflection force required and adjust accordingly. The video below demonstrates this.

 

I like what Jeremy smith stated in the group “Belt tension has less impact on motor amperage than pitch diameter of the sheave and how that affects total airflow.Use the Emerson tool and the app (or paper chart if you’re all stone age) Record tension and other data (sheave diameter, center to center length, rpm and proper tension) on blower housing.”

Check those belts.

— Bryan



No matter what trade you work in you will need to use a concrete anchor at one time or another. Here are some mistakes I have seen (and made) that you are going to want to avoid.

Know your concrete

Is it concrete block hollow cell? Poured cell? Concrete slab? What is the PSI? Not all anchors are created equal for every type of concete. Make sure you know exactly what you are fastening into and choose the right anchor.

Tapcons are light duty

Your typical threaded “tapcon” is for light duty tasks. While a tapcon may be fine to hold down a condenser (that already want to stay down), it would probably (definitely) be the wrong option for mounting a heavy motor assembly to the wall. For big jobs go with a tougher anchor. Hint: if it has threads and you “screw” it in, it probably isn’t the toughest option.

Anchors embedded too shallow

Most concrete anchors have a minimum embedment depth. You can usually embed deeper but you you need to at least hit the minimum, read the directions.

Anchors too close

All anchors have a minimum distance they can be from one another. If you get them too close the whole kitten kaboodle can pull right out on you (I’ve done this).

Don’t overdrill

Wedge anchors (Red Heads) are a common heavy duty anchor. When drilling the hole wrap electrical tape on your bit at the depth you want to go so you don’t overdrill the hole or worse…. blow out the other side.

Clean the hole

With a wedge anchor the dust in the hole can act like a lubricant, making it easier for it to pull out. When done drilling use a round bottle brush or vacuum to clean the holes out. This is especially true when using an epoxy “chemical” bolt.

Warbling the bit

With concrete anchors, use the right size bit and run it straight it. No warbling the bit around… unless your desired result is having the anchor pull out and destroy a monastery, then warble away.

Torque it down

Sorry, you do really need a torque wrench, especially if you are working with large wedge anchors. Torque that sucker down to factory specs and you won’t need to worry. Keep in mind, you may wanna retorque it after it’s been put under load a few times, especially in safety critical applications.

Don’t overload it

Before you get started make sure you know the  Ft/lbs of force the anchor will be under and play on the safe side. Like GI Joe says… now you know, and knowing is just scratching the surface.

— Bryan

P.S.- you can watch a video on wedge anchors HERE

The most common and often most frustrating questions, that trainers and senior techs get goes something like this. “What should my ______ be?” or “My _____ is at ______ does that sound right?

Usually, when the conversation is over both the senior and junior techs walk away feeling frustrated because the junior tech just wanted a quick answer and the more experienced tech wants them to take all of the proper readings and actually understand the relationships between the different measurements.

In this series of articles we will explore the, “What should my _______ be?” questions one at time and hopefully learn some things along the way.


So what should the superheat be?

First, what is superheat anyway? It is simply the temperature increase on the refrigerant once it has become fully vapor. In other words, it is the temperature of a vapor above it’s boiling (saturation) temperature at a given pressure.

The air around us is all superheated! Head for the Hills!

How can you tell that the air around us is all superheated? Because the air all around us is made of vapor. If the air around us were a mixture of liquid air and vapor air, first off you would be dead and secondly, the air would be at SATURATION. So the air around us is well above its boiling temperature (-355° F) at atmospheric pressure which means it is fully vapor and SUPERHEATED. In fact, on a 75-degree day, the air around you is running a superheat of 430°

But why do we care?

We measure superheat (generally) on the suction line exiting the evaporator coil and it helps us understand a few things.

#1 – It helps us ensure we are not flooding the compressor

First, if we have any reading above 0° of superheat we can be certain (depending on the accuracy and resolution of your measuring tools) that the suction line is full of fully vapor refrigerant and not a mix of vapor and liquid. This is important because it ensures that we are not running liquid refrigerant into the compressor crankcase. This is called FLOODING and results in compressor lubrication issues over time.

Image courtesy of Parker / Sporlan

#2 – It gives us an indication as to how well the evaporator coil is being fed

When the suction superheat is lower it tells us that saturated (boiling) liquid/vapor mixture is feeding FURTHER through the coil. In other words, lower superheat means saturated refrigerant is feeding a higher % of the coil. When the superheat is higher we know that the saturated refrigerant is not feeding as far through the coil. In other words higher superheat means a lower % of the coil is being fed with saturated (boiling) refrigerant.

The higher the % of the coil being fed the higher the capacity of the system and the higher the efficiency of the coil.

This is why on a fixed orifice system we often “set the charge” using superheat once all other parameters are properly set. Adding refrigerant (on a fixed orifice / piston / cap tube) will feed the coil with more refrigerant resulting in a lower superheat. Removing refrigerant will increase the superheat by feeding less of the coil with saturated (mixed liquid and vapor) refrigerant.

This method of “setting the charge” by superheat does not work on TXV / TEV / EEV systems because the valve itself controls the superheat. This does not negate the benefit of checking superheat, it just isn’t used to “set the charge”.

#3 – We can ensure our compressor stays cool by measuring superheat

Most air conditioning compressors are refrigerant cooled. This means that when the suction gas (vapor) travels down the line and enters the compressor crankcase it also cools the motor and internal components of the compressor. In order for the compressor to stay cool, the refrigerant must be of sufficient volume (mass flow) and low temperature. Measuring superheat along with suction pressure gives us the confidence that the compressor will be properly cooled. This is one reason why a properly sized metering device, evaporator coil, and load to system match must be established to result in an appropriate superheat at the compressor.

#4 – Superheat helps us diagnose the operation of an active metering device (TXV / TEV/ EEV)

Most “active” metering devices are designed to output a set superheat (or tight range) at the outlet of the evaporator coil if the valve is provided with a full liquid line of a high enough pressure liquid (often at least 100 PSIG higher than the valve outlet / evaporator pressure). Once we establish that the valve is being fed with a full line of liquid at the appropriate pressure we check the superheat at the outlet of the evaporator to ensure that the valve itself is functioning properly and /or adjusted properly. If the superheat is too low on a TEV system we would say the valve is too far open. If it is too high the valve is too far closed.

#5 – Superheat is an indication of load on the evaporator 

On both TEV / EEV systems and fixed orifice systems (piston / cap tube) you will notice that when the air (or fluid) going over the evaporator coil has less heat, or when there is less air flow (or fluid flow) over the evaporator coil the suction pressure will drop. However, on a TEV / EEV system as the heat load on the coil drops the valve will respond and shut further, keeping the superheat fairly constant. On a fixed orifice system as the load drops so will the superheat. It can drop so much on a fixed orifice system that when the system is run outside of design conditions the superheat can easily be zero resulting in compressor flooding.

When the load on the evaporator coil goes up a TEV / EEV will respond by opening further in an attempt to keep the superheat constant. A fixed metering device cannot adjust, so as the heat load on the coil goes up, so does the superheat.

When charging a fixed orifice A/C system you can use the chart below to figure out the proper superheat to set once all other parameters have been accounted for

Using this chart requires that you measure indoor (return) wet bulb temperature so that the heat associated with the moisture in the air is also being accounted for as well. This is one of MANY target superheat calculators out there, you can use apps, sliderules etc… Here is ANOTHER ONE

Remember, this chart ONLY applies to fixed orifice systems.

So what should your superheat be in systems with a TEV / EEV? The best answer is… like usual… Whatever the manufacturer says it should be.If you really NEED a general answer you can generally expect

High temp / A/C systems to run 6 – 14 degrees of superheat

Medium Temp  – 5-10 

Low Temp – 4-10

Some ice machines and other specialty refrigeration may be as low as 3 degrees of superheat

When setting superheat on a refrigeration system with any type of metering you often must get the case / space down close to target temperature before you will be able to make fine superheat adjustments due to the huge swing in evaporator load. Once again, refer to manufacturer’s design specs.

— Bryan

P.S. – Trutech has a really great resource on charging best practices

 

 

 

I have spent most of my career being afraid of hard start kits, I heard too many horror stories of start caps exploding and sales technicians telling every customer they need one.

It dawned on me recently that it may be time for me to take a more mature look at start capacitors, potential relays, and hard start kits and find some best practices.

First be aware that not everything commonly called a “hard start” is the same thing. The bottom of the barrel is called a PTCR which is essentially just a resistor that starts off at a low resistance when cool and changes to higher resistance when it gets hot. It creates a direct path from L2 (run side) through the start winding and as soon as it heats up, the higher resistance essentially removes it from the circuit. This is NOT the same technology as a start capacitor in any way and in my experience, they don’t work well and are prone to failure, at least in air conditioning systems.

There are also electronic and timer type “start kits” that utilize a capacitor but remove it from the circuit using a timer.

However, the most traditional and time tested method of start assisting a compressor in HVAC in the good old start capacitor and the potential relay.

Let’s start with how they work.

Photo Courtesy of Rectorseal

When a compressor first starts up, it requires a lot of torque to get from 0% up to 75% of running speed, especially when it has to start under pressure load (unequalized pressures). A start capacitor is designed to create the optimal phase shift for that first 75% of synchronous speed. A run capacitor is sized to create an optimal phase shift for a compressor that is running at full speed and at full design load because the run capacitor never comes out of the circuit.

Photo Courtesy of Rectorseal

While a run capacitor has heat dissipation capability for constant duty a start capacitor MUST be taken out of the circuit VERY quickly to avoid melting down as well as causing compressor damage.

The start capacitor is REMOVED from the circuit by a relay called the potential relay. The potential relay is normally closed and it OPENS when a sufficient PICKUP voltage is present between the 5 and 2 terminals on the relay. This pickup voltage is potential (voltage) that exists in the start winding when a motor gets above about 75% running speed and it is GENERATED in the start winding by the motor itself NOT the capacitor. A capacitor DOES NOT boost the voltage, when you see that increased voltage across the capacitor that is back EMF being generated by the motor, just like in a generator (pretty cool huh?).

Once the compressor shuts off the relay then DROPS OUT which closes the contacts again for the next time.

 

Some hard start manufactures wire the coil on the potential between start and common and some wire it between start and run. You will find that most OEM’s wire between start and common but this does not mean that wiring between start and run is bad… it just needs to be designed correctly for that purpose (Kickstart does it this way for example).

A properly sized start capacitor and potential relay are not BAD for a compressor, they just must be sized and installed correctly and there are some cases where they are more likely to be useful that others.

Cases where they may be very useful useful

  • Long line set applications
  • Hard shut off valves
  • More often on reciprocating compressors than scroll or rotary (but still OK on scroll and rotary when beneficial)
  • on 208V single phase applications

Things to consider

  • Mount the relay properly, there is a proper UP configuration on most potential relays
  • Use hard starts with REAL potential relays not timers, solid state or other relay types (in my experience)
  • Size the relay and capacitor according to manufacturers specs
  • Ensure that you have a good quality, properly sized run capacitor on any system with a hard start

For a complete write up on potential relays, you can read these articles HERE and HERE

Also, we have a podcast out with the technical manager for Rectorseal James Bowman HERE

— Bryan

This is a subject that even many commercial guys don’t have to consider.  For the majority of equipment, even refrigeration equipment, all that is required for proper oil return is to size the suction line properly, trap the suction line as needed, and allow for proper slope towards the compressor.

 

Then we get into larger equipment.   Due to what can be extreme swings in load that result in wide swings in suction line velocity, oil return isn’t always what we’d like to see, even with proper trapping and line slope, so rather than allowing that oil to load up the evaporators and affect heat transfer among other problems an oil logged evaporator can cause, we install systems to prevent the oil from ever leaving the mechanical room.

 

I’ll try to lay this out in a step-by-step manner, adding layers of complexity as needed.

 

Since oil will be entrained (mixed / carried) in the discharge gas leaving the compressors, we’ll first want to install an oil separator in the discharge line to capture this oil, then we’ll work on managing its return to the compressor crankcase where it belongs.

 

That’s step one:  separating any oil from the discharge gas leaving the compressor.   There are 3 basic methods used for this (in order of effectiveness)

 

Impingement .  In this method, all of the discharge gas passes through a screen where oil vapor gathers into larger droplets and drips off into a vessel where we can deal with it later.

 

Helical.   In this method, the discharge gas enters the vessel at an angle and swirls around plates within the vessel.   The oil droplets entrained in the discharge gas, being heavier than the vapor itself, are flung outward and hit these plates and drain to the bottom of the vessel as before.

 

Coalescing .  Here, the discharge gas is forced through a filter where the oil droplets are captured and, again, drain to the bottom of a vessel.

 

Now that we’ve captured the oil, half of the job is done.   We’ve prevented it from going out into the system, now we basically have a bucket full of oil under discharge pressure we’ve got to manage.

 

One thing to remember is that oil tends to accumulate the worst of the garbage in the system, so a quality oil filter is necessary.   To prevent problems with clogging fine orifices, needle valves and pressure regulators we’ll encounter in our oil management systems, that filter should be installed as close to the oil separator as possible.

 

Things start to get interesting from here, so I’m going to try to explore the simplest methods first and dig into more and more complicated oil management strategies as we build an understanding.

 

Probably the simplest management strategy is one of the most modern ones.  A direct oil level management system.  An electronic float at each compressor monitors level in each crankcase and, as that compressor pumps out the small amount of oil it normally pumps, the electronics package energizes a solenoid valve to let that oil back into the crankcase.  There will typically be a small orifice within this valve so that feed happens rather slowly but fast enough to prevent the level from dropping low enough to cause a problem.

 

Most equipment that is out there however, isn’t quite so simple, direct and easy to understand.

 

None of the other systems use electronics to manage oil flow, so from here on out, all controls are mechanical.

The next type of system uses mechanical float-type regulators bolted to each compressor to monitor the oil level in the crankcase.  As before, when the level drops, the regulator needs to add oil back into the compressor.   Much like a toilet tank or other float controlled device, the float opens a needle valve to allow oil into the regulator . The actual oil level within the regulator is adjustable within a fairly narrow range.

 

For this control to regulate properly, we need to reduce the oil pressure to a safe level. If we fed these regulators oil at discharge pressure, the high-pressure differential would force the small needle valve inside open and allow the regulator to overfeed and overfill the compressor.   Instead, we install a valve between the separator where the oil is at discharge pressure and the regulators on each compressor to reduce the pressure down to typically about 20# above crankcase pressure.

 

Adding a couple layers of complexity to the system,  we arrive at what is probably the most common type of oil system in use on parallel refrigeration equipment today.

 

Oil drains to the bottom of the separator vessel, as the oil level rises there, it opens a float valve. Oil passes through the float valve into a reservoir tank.   The reservoir tank serves two purposes.   First, it simply holds the oil until it’s needed.  Second, through a special check valve installed between this reservoir and the suction header, the oil pressure is lowered to that same 20# above suction pressure figure.  These check valves are available in different pressure differential settings, but 20# is the most common.

 

From the reservoir to the compressor, the system is the same.   An oil line sends oil from the reservoir out to the mechanical float devices that control the level of oil in each compressor.

One other common feature in oil management systems like this is an equalizing line.   We all understand that 2 containers of any liquid will have the same level in them due to the self-leveling nature of liquids.  This equalizing line, in theory, connects the crankcases of all compressors together to create a self-leveling system.   It doesn’t always work quite as well as hoped for because there can be different pressures in the crankcase of a running and non-running compressor.  We’ll dive a little deeper into that as we move into troubleshooting oil problems.

 

Troubleshooting

 

While these systems seem complicated, and they have a lot of moving parts that can fail, they really boil down to oil level and pressure differential.   We need to maintain a level of oil in the compressors and in the reservoir or separator and maintain enough pressure differential to keep that oil moving.   Lose one or the other and you’re not going to stay running for long.

 

Let’s kind of walk step-by-step through a troubleshooting process until we find or eliminate all problems.   First, I look at all compressor levels and reservoir level.   If I’ve got a lot of oil in the compressors, I want to check equalization lines.  If they’re all consistently low, I’m going to start looking at the oil management system.

 

To evaluate the oil management system, start by checking the temperature leaving the oil separator.   The line leaving the separator should be warm to the touch (100F).  I like to put a thermometer that logs min/max temps and observe it for 10 or 15 minutes.  You’ll see the temperature climb and drop as the oil float inside feeds.  No feed?  Time to consider the float inside as a problem.

Next, let’s check the pressure in the reservoir or the outlet pressure of the unit’s pressure reducing valve against suction pressure.   20# above and we’ve got level in the reservoir?  OK. No level in the reservoir?  Lets try to find that oil before we go adding oil.   A system with too much oil can be as problematic as one with too little oil… If we don’t have differential in the reservoir, I’ll isolate the inlet and outlet of the reservoir and bleed some hot gas from discharge into the reservoir with my gauges to see if the differential check is faulty.   Since we’ve already demonstrated that the separator is feeding, we need to see if the differential check valve has failed.

 

Next step for me is to start checking each individual oil level regulator.   I’ll normally uncap the adjustment stem, turn it CCW to the top stop, counting the turns then adjust it down (CW) to a midpoint which is typically 5 turns.  If any are wildly out of adjustment, I single that compressor out for some additional attention.  It is very important to not adjust these controls more than 10 turns from the top stop.   The adjustment range is limited and adjusting beyond that limit will ruin the control, regardless of its condition before you worked on it.

I mentioned crankcase pressure earlier, and this is an issue that can be problematic with oil issues.   As the rings wear in a compressor, we can see some discharge blowby into the crankcase.  Not enough to warrant replacement of the compressor necessarily, but enough to sometimes cause issues with oil.   First, if we put additional pressure into one compressor, that unbalances the oil equalization system by pushing oil in unwanted directions.  Second, by increasing the pressure in the crankcase over the suction pressure, we reduce the net oil feed pressure, slowing the oil feed rate.

 

To check a compressor for pressurized crankcase, install a gauge on both the suction port and the crankcase.   If the pressure within the crankcase is more than about 2# higher than suction, you may have some problems.

 

A few other, kind of random thoughts on oil failure trips and trouble.

 

These are 3 phase motors.   A contactor with severely pitted points or a contact that doesn’t make good contact can cause a temporary single phase, prevent the compressor from running and create a situation where the oil control is energized but no pressure is created by the compressor.  Always check the contactor.

 

Screens and filters.   Since the oil system tends to collect all of the garbage in the system, oil systems tend to have a high concentration of filters and strainer screens.   From the impingement screen and coalescing element in a coalescing separator, screens are a huge problem.   Add to that the screens, an oil filter, the float at the bottom of the separator, the pressure reducing valve, the screens and valves in each individual oil level control and, often an oil pickup screen in the compressor itself and there are many points that can become obstructed by the debris in an oil system.   Regular oil filter maintenance is important for a reliable system.

— Jeremy Smith CM

P.S. – Henry Technologies has compiled everything that you can possibly need to know into a handy manual and, most importantly, a quick-reference chart with some basic diagnostic readings and measurements to take HERE

 

 

When you first start checking your supply air with a thermo-hygrometer you may notice that the relative humidity is REALLY HIGH. Often the RH in a supply duct will be between 85% and 96% relative humidity on a system that is functioning as designed. The reason for this is fairly simple.

In order for dehumidification to occur the air must reach dew point, otherwise known as 100% relative humidity

Jim Bergmann explains it this way. Think of a sponge being like air and when it is fully expanded it is like the air in the return. When the sponge is fully saturated and can accept no more water it is at 100% RH and when it is completely dry it is at 0% RH. Let’s imagine that the sponge is 50% saturated and full size in the return. When that sponge (air) goes over the evaporator coil it is compressed, because colder air can hold less moisture. Once that air is compressed (cooled) enough it will begin to give up moisture. This point at which it starts to give up moisture is called dew point or 100% relative humidity. Once that air leaves the coil it still remains in approximately the same temperature state (compressed sponge) as it was when it went over the coil. This means that unless heat is added or removed from that air, it will remain at 100% relative humidity.

So why is it less that 100% RH in the supply?

There are several reasons why the air in the supply will be slightly below 100% in the supply. First is contact factor or bypass factor which are both terms used to demonstrate the efficiency of a coil at “contacting” the air. The greater the surface area of the coil and the longer the contact time of the air on the coil the more efficiently heat will be transferred from the air to the coil.

Because no coil is 100% efficient, there will always be some air molecules that leave the coil warmer than others, this causes the airstream to be warmer overall and decreases the RH of the air stream. You will notice when a system has a higher coil air velocity (speed) it will have a higher bypass factor (lower supply humidity). When you run lower coil air velocity the bypass factor will drop and the supply RH will increase.

There is also some heat added by the blower motor and possibly even the cabinet or supply ductwork. This added drybulb heat results in a warmer airstream and thus some additional moisture capacity. Imagine a slight expansion of the sponge due to heat from the duct walls and the blower motor.

Once that supply air exits the duct and mixes with the room air it is allowed to “expand” again and the relative humidity drops below what it was initially. This is why supply air has a high RH in cooling mode.

Here is a video we did on the topic –

— Bryan

 

Both wet bulb temperature and air enthalpy are extremely useful to understand when calculating actual system capacity as well as human comfort. Dry bulb temperature is a reading of the average molecular velocity of dry air, but it does not take into account the actual heat content of the air, or the evaporative cooling effect of the air.

Like we mentioned in the last tip, when air is at 100% relative humidity the dry bulb, wet bulb and dew point temperatures are all the same. This is because at 100% relative humidity the air is completely saturated with moisture and can have no evaporative effect.

When air is less than 100% RH it will provide an evaporative cool effect and wet bulb temperature is a measurement of that effect. In fact, wet bulb temperature is the temperature a damp thermometer bulb will read when exposed to a 900 FPM (Feet per minute) air stream. If you have ever seen someone using a sling psychrometer, that is exactly what is happening (Hopefully you have a wrist that is well calibrated to 900 FPM). The lower the wet bulb in comparison with the dry bulb (This differential is called wet bulb depression) the lower the relative humidity and the greater the the evaporative cooling effect.

Enthalpy is the total heat content of the air and is represented in BTUs per lb of air. By converting lbs of air to cfm we can calculate the amount of heat in an air mass as well as the change in the enthalpy across a coil to calculate the heat moving capacity of a coil, BTU losses / gains over a length of duct and much more.

You will notice that wet bulb and enthalpy are slanted lines, descending from left to right and they are equivalent. This means that a particular wet bulb temperature is also equal to a particular enthalpy (At 14.7 PSIA at least). In the chart above you can see that a 62.8 degree wet bulb mass of air contains approximately 28.4 btus per lb. The tricky part is reading at this extreme level of resolution, because 28.4 vs. 28.6 can make a significant difference when it is multiplied out over a large air mass. This demonstrates why VERY accurate tools and careful calculations are required for enthalpy calculations in HVAC/R.

— Bryan

For a full WB ot Enthalpy calculator go HERE and look for the enthalpy chart


Ever since I started in the trade we would take discharge line temperature in the winter on a heat pump system. The reason for this is that in the Winter the discharge line is easily checked while suction superheat and even subcool can be more difficult to access. The old timers that trained me would say that in a properly functioning system the discharge temp will be “about 100 degrees over outdoor ambient” when a heat pump system is running in heat mode. That rule of thumb is actually still pretty close, but it’s isn’t exact…. and what happens if you are getting a different reading?

First off, of your discharge temperature (as measured with a thermometer at the compressor) is over 225, you have an issue. At that temp, the oil will begin breaking down, so if you check for no other reason, check to make sure you are under 225.

High head and/or low suction cause a higher discharge line temp. If your suction pressure is low but the superheat is low (low evaporator airflow or heat load) it will cause LESS of a discharge temp increase than if the suction is low due to low charge, restriction or evaporator underfeeding.

You can also see an increased discharge line temp if you have a high suction superheat at the condenser due to an uninsulated or improperly insulated suction line.

On the condenser side, anything that causes high head will also cause high discharge line temperature. Overcharge, low condenser air flow due to improper motor or blade or dirty condenser coils. In the case of heat pump units running in heat mode, the most common causes are dirty air filters or other indoor air flow restrictions (because the condenser is now inside during heat mode)

In short… high discharge temp can commonly be caused by

– Low charge (high suction superheat)

– Severe Overcharge

– Low condenser air flow

– Damaged condenser fins

– Restricted metering device

– Other restrictions (Liquid line drier, suction line drier, kinked lines, clogged screens)

Low discharge line temp can be caused by

– Low compression ratio (failing compressor)

– Low condensing temperature (Outdoor temperature)

– Metering device too far open / flooding evaporator coil / Low Suction Superheat

— Bryan

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