What is Fenestration and Why Do I Care?

Fenestration is a fancy architectural term that means “openings in the outside of the building”. You will see this word a lot when you read ACCA manual J or when you are doing a manual J load calculation.

Fenestration loads include heat losses and gain through windows, doors, skylights etc… and can make up a significant portion of the overall load as well as being the biggest variable load throughout the day as the sun moves across the horizon.

There are two big things to watch for when entering fenestration loads

  1. Look for tags on doors and windows that say NFRC (National Fenestration Rating Council) certified. If they have this mark it means that the entire door or window was rated including the glass, frame etc… over the entire rough opening rather than the glass only. If it is not NFRC certified then you are better off using ACCA manual J tables.
  2. Use the full rough opening of the doors or windows you are entering into manual J rather than just the part you can see. Many doors and windows will be pretty standard or at least consistent across the building so once you get one rough opening and U-factor you will often be able to use it over again.

There you have it… Try to use the word “fenestration” next time you play scrabble with your grandma for extra awesome points.

— Bryan

Pressure / Enthalpy Diagram Example

This article is written by my buddy and Canadian Supertech Tim Tanguay. Thanks Tim!

This P/E chart shows R410a at 100°F Saturated Condensing Temp, 10°F SC
40°F Saturated Suction Temp, 20°F SH at the compressor.
The green highlighted thumb shape is the saturation zone. Everything that occurs in the saturation zone is a latent (change of state) process.
Everything that occurs to the right (superheating) and left (subcooling) is a sensible process.
Go to the movies in your mind, imagine that you are one pound of 410A. We commence our journey at the rightmost point on the upper orange highlighted line.
At this point, you have just left the compressor. You are a superheated vapor, with a temperature of 145°F. You enter the condenser and start rejecting heat to the atmosphere. After rejecting 45°F of sensible heat (desuperheating), you hit the saturated condensing zone (100°F) and you turn into a drop of liquid. As you march your way along the condenser (follow the line left), you reject latent energy but stay the same temperature. As your latent energy decreases you become more liquid until finally, you are a solid column of liquid and you exit the saturation zone to the left of the thumb. You then give up another 10°F of sensible heat to the air and become a 90°F subcooled liquid.
You approach the sight glass as a 90°F subcooled liquid under approximately 350 PSIG of pressure. As you pass the sight glass, you fart a few bubbles just to mess with the refrigeration mechanic observing the process. You squeeze your way through the tiny orifice in the metering device and emerge into the evaporator, solidly back into the saturation zone. You find yourself as a 40°F saturated liquid at 125 PSIG (approx 78% liquid, 22% vapor, indicated along the constant quality lines).
 Now you make your way along the bottom line towards the right side of the thumb, you absorb heat energy from the warm return air rushing over the copper and aluminum evaporator fins. The heat you absorb boils you dry. You are naught but a vapor, and as such, the energy from the return air increases your sensible heat by 10°F. You emerge from the evaporator as a 50°F superheated vapor. As your journey progresses
towards the suction inlet of the compressor, you pick up another 10°F of sensible heat.
You enter the suction port of the compressor as a 60°F, superheated vapor. The compressor puts you through a strenuous workout, squeezes you into a smaller volume and in the process increases your temperature by about 85°F.
You emerge as a superheated 145°F vapor. The process begins anew.
A few things to look at. The numbers on the top represent enthalpy energy, as BTU's per pound.  In this particular example, the sensible portions of the condenser account for approx 20% (eyeball estimate) of the total heat rejected in the condenser. The other 80% of the process is latent.
On the right-hand side of the PE diagram, you have specific volume, represented as curved dotted lines. As SST decreases, specific volume increases and vapor density decreases. This fact alone is why refrigeration compressors need to be physically larger. As specific volume increases, the volumetric efficiency of compressors decrease.   Lower SST's (suction saturation temp) require larger compressor displacement because they need to move more gas to obtain the required mass flow. In AC and refrigeration, the mass flow of refrigerant through the system ultimately determines your system capacity.
At 40F, the latent heat of vaporization of 410A is approx 75 BTU/LB. Compare that to water, which has a latent heat of vaporization of approx 970 BTU'S per pound at 212°F/14.69 PSIA and you begin to realize why dehydration of a system takes so darn long.  It takes a LOT of energy to boil water off.
In the evaporator, about 10% of the process is sensible.  This is why a unit that is short on refrigerant isn't able to cool properly. The refrigerant boils off leaving a large portion of the coil to collect sensible heat (higher superheat). The amount of heat that sensible processes remove from the air stream is relatively tiny, thus we lose capacity.
So too with things like water. The sensible heats involved with changing temperature are minuscule when compared to the amount of heat required to change state. Universally, latent changes require orders of magnitude more energy than sensible changes.
— Tim

Diagnosing ECM & X13 Motors

First, let's give proper credit. Most of the best practices and tools for the diagnosis of ECM and X13 motors come from Regal / Genteq and their site thedealertoolbox.com and their app the dealer toolbelt. Your best bet is to follow the practices shown there and use their TECInspect diagnostic tool shown below.

Here is the general process to follow when checking an ECM and X13 motor that isn't running. Most of it is very common practices you would follow with any motor.

  1. Check for proper line voltage and 24v calls to the proper terminals in the equipment
  2. Check for proper control signal entering the motor from the 24v field wiring or boards. The fan speed selections and programming will vary by manufacturer but the intent is to see if there is a proper control input signal. This can be a bit a challenging and is the primary purpose of the TECInspect tool.
  3. Disconnect power and remove the blower housing
  4. Check for abnormal sounds and side to side bearing play. Because these motors have permanent magnets on the rotor they won't spin freely like a normal motor and you will get an “indexing” feel on the shaft as you turn it.
  5. Look for signs of overheating, burned spots etc…
  6. Remove the module from the motor and disconnect the plug that connects the motor to the module. 
  7. Measure winding to winding on the plug feeding the motor (called phase to phase below). resistances should be less than 20 ohms and nearly the same between all phases/windings.
  8. Measure from each winding to ground on the casing and you should see no less than 100k ohms to ground.
  9. If the motor checks out OK and the module is receiving inputs but the motor still isn't running then it is the module that needs to be replaced

The diagnosis of ECM and X13 motors is actually pretty easy if you do a bit of reading and take a practical “process of elimination” approach.

— Bryan

The Friction Rate Chart (and What it Means)

A lot of proper duct design comes down to a understanding of available static pressure and friction rate. We've covered this topic several times on this site and on the podcast but I wanted to focus on this ACCA chart specifically (shown above).


The horizontal axis is available static pressure or ASP and it's indicated along the bottom from 0.05 and 0.40

There is only one way to know your ASP and that is to calculate how much ADDITIONAL static pressure your blower can work against and still provide the correct airflow once the everything in the airstream is accounted for OTHER than the ductwork.

This can include coils, filters, dampers, grilles and heat strips with the static pressure drop calculated in inches of water column at the design airflow.

This means before you can choose an ASP you need to

  1. look at the manufacturer blower curve or chart
  2. Choose a fan speed to achieve design airflow
  3. Add up all the other resistances external to the furnace / fan coil
  4. Subtract those calculated resistances (frictions) from the rated system operating static pressure (from the fan chart

This ASP # you come up with can often be adjusted later by choosing a different speed tap on traditional PSC systems or by sacrificing efficiency in modern ECM / X13 blowers but to start with we will generally use the 0.50 TESP target as our goal to start with at rated airflow.


The vertical axis along the left side displays the total equivalent length of duct in the critical duct path. This is the highest friction path all the way from the return through the supply and includes the straight duct runs as well as the equivalent lengths from duct fittings and transitions.

The TEL will always be longer and generally significantly longer than the actual lineal length from the longest return to the longest supply.

The Chart

Once the TEL and ASP are calculated you simply use the chart to intersect the two to calculate the design friction rate for the duct design.

For example if we calculated a 300′ equivalent length with an available static pressure of 0.2″wc (which is a very typical situation) that could be interpolated to a design friction rate of 0.07 for the design.

From a practical standpoint you need to stay within the wedge to have a duct system that will be realistically sized. If you end up above the chart wedge at 0.06 then you need to either find a way to reduce your equivalent length by reducing fittings, moving the furnace to a more central location, using a less restrictive filter etc…. or you need to find some more ASP headroom by choosing a higher blower speed or a more powerful blower that can work against more static pressure.

This next part is MY opinion but it's backed up by common sense and ACCA Manual D A15-5 (look it up).

If you end up under the wedge you could POTENTIALLY end up with too much airflow but with modern X13 / ECM motors that's pretty unlikely because they will ramp down to maintain constant torque / airflow.

Because of this (In most cases nowadays) being under the wedge isn't a real problem and 0.18 could still be used as the design friction rate.

Now keep in mind…. this chart is from ACCA Manual D and only designed for residential. Commercial is a different process.

— Bryan

Electronic Leak Detection DOES WORK

I hear many techs complain about the finicky and ineffective nature of electronic leak detection. So much so that some claim that is is a waste of time altogether. we recently located a leak inside the fins of a ductless evaporator coil, pinpointed to an exact spot using an electronic leak detector. For demonstration purposes, we took that coil and performed a definitive test to locate it in the video below.

A leak detector can be tricky to use so here are some of our top tips –

  • Know your detector. Know it's limitations, it's sensitivity and what can cause false positives. For example, some leak detectors will sound off on certain cleaners or even soap bubbles. My detector sounds off when jostled or when the tip is blocked.
  • Keep a reference bottle so you can check your detector every time before you use it.
  • Maintain your detector and replace the sensor as required. Most heated diode detectors require sensor replacement every 100 hrs or so.
  • Keep it out of moisture. Most detectors will be damaged by almost any amount of moisture.
  • Move slowly and steadily. Don't jump around or get impatient.
  • Most refrigerant is heavier than air which means that starting from the top and working down is usually a more effective way to pinpoint.
  • Go back to the same point again and again to confirm a leak. Don't condemn a component based on one “hit”
  • Find the leak WITH BUBBLES whenever remotely possible, even after pinpointing with a detector.

— Bryan

Heat Exchanger Crack Diagnosis

There are two camps I've run into regarding cracked heat exchanger diagnosis. Those who look for it everywhere and those who dismiss it and never look.

I will start by saying that everything I write here is my own opinion and experience. Because this is such a hot button topic don't take my word for any of this, follow manufacturer, industry standards and codes and obviously stop reading unless you are a trained and licensed professional.

Heat exchanger cracks are worth finding but they aren't the most dangerous issue for your customers in most cases

The reason I make this blasphemous statement is twofold.

  1. Many heat exchangers are at a negative pressure in reference to the air moving over the exchanger. This means that the air from the return will move into the exchanger rather than combustion gasses moving into the airstream.
  2. So long as the combustion process is complete there won't be significant CO ( Carbon Monoxide) in the flue,

But let's be clear… If there are any cracks in an exchanger it needs to be replaced, there should NOT be cracks in a heat exchanger. The trouble comes in when we think that looking for cracks in the only or even primary method of finding CO issues with a furnace.

When we rely on our eyes to find every issue we can easily miss problems (including cracks) that our eyes can't see. So here are my suggestions on how to find cracked heat exchangers and other furnace safety issues.

Ambient Carbon Monoxide

One of the first things you should do is measure ambient CO in the conditioned space while the heat is running with either a combustion analysis tool or a personal CO detection device. Anything other than ZERO ambient carbon monoxide is worth investigating. Sure, smoking indoors or cooking can increase ambient CO above zero but when you see it take the time to INVESTIGATE.

Flame Displacement 

One of the oldest ways to check for heat exchanger cracks is to simply observe the flame when the blower starts. Most furnaces will have a blower delay to get the furnace up to temperature before moving air over the exchanger. This procedure is as simple as watching the flame and observing if it the flame moves or changes when the blower starts.

Because the burner and heat exchanger is isolated from the airstream there should be no change in the flame when the blower starts. If there is you need to begin looking for connection or leakage between the burner/heat exchanger and the air.

Combustion Analysis

Testing combustion won't necessarily tell you if you have an exchanger leak but it will tell you if you have a high CO which can help you prevent a dangerous situation for your customers if there is an exchanger crack.

Exchanger Isolation Pressure Test

When you suspect the system may have an exchanger leak you can place a manometer probe in the exchanger and seal off the inlet/outlet of the exchanger as best you can (with the gas shut off obviously). Next, turn on the blower and see if there is a change in the exchanger. Any change is an indication of a heat exchanger leak.

Visual inspection

In some cases, an old-fashioned visual inspection makes the most sense, either by removing the blower or the high limit (or both) and using a mirror or borescope to inspect.  When you do find a cracked exchanger it should be quoted for repair and the furnace turned off.

While finding cracked exchangers is worthwhile I would place it below or on par with other things like –

  • Testing for spillage due to depressurization in the combustion air zone
  • Insufficient Combustion Air
  • Blocked intake and/or exhaust
  • Recirculation of combustion products into intake vents or soffits
  • Gas pressure testing / clocking the meter for overfiring or underfiring 
  • Testing heating airflow
  • Testing for CO in the occupied space
  • Visual inspection of vents, flues, and chimneys for proper installation as well as gaps and cracks
  • Visual inspection for return gaps pulling in air around the furnace

When you do find exchanger cracks on newer units you need to also looks for causes like low airflow, incorrect orifice size or overfiring that could have caused the issue.

Keep your eyes open and don't get fixated on a single issue to keep your customers safe. While you are at it read this great combustion guide from Accutools and Jim Bergmann.

— Bryan


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