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I received an email from a podcast listener with some furnace related questions. Based on the nature of the questions I figured it would be better to ask an experienced furnace tech. Benoît Mongeau agreed to help by answering the questions. 


My name is Matt and I am a newer tech (fully licensed this September, have been doing the work for 2ish years) who lives in Northern Ontario, Canada. I really enjoy the HVACR school podcast. I don’t do any A/C stuff but I still enjoy listening and wrapping my brain around it. I have always struggled with the theory behind getting cold from hot. The bulk of my work is residential gas heating, mainly high-efficiency furnaces, and gas fireplaces. My questions for you are, (these are just ideas for your podcast though help is never turned down)

On a millivolt system (runs off of a thermopile)
– How to easily test for gas valve failure, what are the expected resistances across the solenoid in the gas valve?
– What expected readings should we consistently get from a properly working system (voltage of thermopile alone, with gas valve open, with thermostat closed etc)

On high efficiency
– What is the relationship between the pressures in the collector box of the secondary exchanger and the pressure switch?
– How does a clogged condensate trap lead to the pressure switch not closing?

Another Question
– Is it possible to check readings from the circuit board when the wires are in a harness? For example, I troubleshot a gas valve failure. It was either the board or the valve. The wires coming to the gas valve from the board are in a harness. How do I know which to check and what am I checking for. (Given that everything else was working I leaned toward a faulty gas valve and was right, just so you know!)

Thanks for your time and for doing the podcast.

All the best,
–Matt


For the collector box/pressure switch:
During normal operation, the collector box is under a vacuum (negative pressure) when the inducer is running. That vacuum is what the pressure switch checks for. If the vacuum is sufficient the contacts will close and signal the board everything is good. If your condensate trap is blocked, the collector box will still be under a vacuum. That doesn’t change.

However, the pressure switch port (where the tube is attached on the collector box) should be at the bottom of the box, usually near the drain port. The backed up condensate will simply end up blocking that port and the switch will no longer be able to ”feel” the vacuum, the contacts won’t make and you will get an error (pressure switch not closing or stuck open).

What may also happen, but not always, is that the port will block during a cycle and the vacuum will remain stuck in the pressure tube. As your inducer comes off and normal pressure returns, the air can’t go in the pressure tubing because it’s blocked with condensate, and you’re basically trapping that vacuum inside. So the contacts will stay closed, until the next call for heat. When that call starts, the contacts will already be closed before the inducer starts, and that will also give you an error (pressure switch stuck closed).

Now if your exhaust is blocked, this will create back pressure and your collector box won’t be under the appropriate vacuum, and once again won’t close.

For millivolt systems:
Unfortunately, I can’t say what typical resistance values would be for a mV gas valve because I don’t know. I would say however that in three and a half years I haven’t had to replace a fireplace gas valve. They rarely go bad. In most cases the pilot tube/orifice is dirty, the thermopile is too weak, or, if it works with a wall switch, very very common: the switch is bad. Standard wall switches are meant for AC voltage.

Running millivolt DC thru them will work, but as soon as you have a bit of resistance in the switch contacts, that voltage will not get through. If it runs on a thermostat, usually it works better but you can still get the same problem.

For typical readings, I’d say between 450-650mV from the thermopile alone, open circuit. With the valve open (so, closed circuit) around 200-300mV. But this is very general, it may vary a lot between systems.

If your thermopile alone doesn’t produce enough mV’s, check your pilot flame. Make sure it hits the thermopile well. You might be able to adjust it (on some valves) to make it bigger. As I mentioned, the orifice or tubing may be blocked. That is relatively common especially if the pilot was kept off for a long time.
If your thermopile gives enough voltage but the valve won’t open, check your switch/tstat and even the wire itself for any significant resistance or short.

Isolate section by section and ohm it out. If everything is good and sufficient mV’s come back to the valve and it still won’t open, then yes, that valve might be bad. But I’d probably even replace a switch/tstat before I condemn the valve regardless, just to be sure, just because changing those valves in most cases is a total pain in the butt.

For the gas valve/board dilemma:
If your wires are all in a harness with a big fat connector on the board, there’s a good chance you won’t be able to pull it off and diagnose on the board pins, because by removing the connector you remove most or all of the safety circuits.

If you want to look at the gas valve, you need to hook your meter on the wires at the valve itself. If it’s just a standard 24v valve with 2 or 3 terminals (Common + hot or common + low and high solenoids) just pull the wires off (or connector) at the valve and you have to check for 24V on the wire across common and hot. Even with the valve disconnected if your board is OK it will still send 24V in that wire at the proper time in the sequence of operation (i.e. wait until the ignition sequence completes!!). If you don’t have 24 volts, the board is bad. If you have 24V, the gas valve is bad.

If it’s one of those Honeywell SmartValves, then that’s another story entirely. A good portion of the controls are actually inside that gas valve and it will have multiple wires going to it. They are a bit more difficult to diagnose. My best advice is to follow your electrical diagram. If there’s no way for you to disconnect wires at either end (which should never happen as far as I know…) you could always cut the wire and check your voltage in the wire itself. But try to avoid doing that.

— Ben

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It should be stated and restated that codes and code enforcement vary from location to location within the US. The IMC (International Mechanical Code) is one of the most widely utilized and referenced and the 2015 version of the IMC section 307 is what I will be referring to in this article.

Condensate Disposal 

The code as it relates to condensate disposal in the IMC is pretty vague. It says that it must be disposed of into an “approved location” and that it shouldn’t dump on walkways, streets or alleys as to “cause a nuisance”.

This leaves us a lot of wiggle room for interpretation and a lot of authority to the AHJ (authority having jurisdiction) and design professionals to establish what is and what isn’t an “approved location”. Here are a few good guidelines –

  • Don’t dump condensate in places that could cause people to slip
  • Don’t dump condensate around foundations, basements or other areas that could cause ponding, erosion and/or leakage
  • Don’t dump condensate on a roof
  • When discharging into a shared drain or sewer system ensure that it isn’t piped in such a way that waste fumes could enter the system or occupied space

Drain Sizing

IMC 307.2.2 tells us that an A/C condensate drain inside diameter should not be smaller than 3/4″ and should not be smaller than the drain pan outlet diameter. 3/4″ is sufficient for up to 20 tons according to the IMC unless the drain outlet size is larger than 3/4″.

Drain Pitch 

The IMC dictates a 1% minimum pitch of the drain which is equal to 1/8″ fall for every 12″ (foot) of horizontal run. In practice, it is safer to use 1/4″ of fall per foot to ensure proper drainage and provide some wiggle room for error.

Support

Drains can be made out of many materials but PVC is by far the most common. When a drain line is PVC the IMC dictates that it should be supported every 4′ when horizontal (while maintaining proper pitch) and every 10′ vertically.

Cleanout

IMC 307.2.5 states that the condensate assembly must be installed in such a way that the drain line can be “cleared of blockages and maintained” without cutting the drain.

Traps & Vents 

The IMC states that condensate drains should be trapped according to manufactures specs HOWEVER, wording was added in IMC 307.2.4.1 that states that ductless systems must either have a check valve or a trap in the condensate line. While most manufacturers don’t specify this on this gravity ductless drains, it is something to look out for. James Bowman wrote a great article on some code compliance issues in the code that you can read HERE

Venting after the trap (like shown on the EZ Trap above) is a really good idea in most applications because it helps prevent airlock that can occur due to double traps and shared drains as well as prevent siphoning. This vent is AFTER the trap and must remain open to be effective. The vent opening should always rise above the trip level of the condensate overflow switch when it is in the primary drain line or pan or above the secondary / aux overflow port on the primary drain pan. This helps ensure that if a backup occurs that the water properly trips the switch instead of overflowing out of the vent. While venting is a common best practice it isn’t part of the IMC code.

Drain Insulation 

The IMC code doesn’t directly state that the drain line must be insulated.  Many will point to the where the ICC energy efficiency code states

N1103.3
Mechanical system piping insulation.[/b] Mechanical system piping capable of carrying fluids above 105?F (40?C) or below 55?F (13?C) shall be insulated to a minimum of R-2. but this really isn’t talking about condensate drains when read in context.

Some municipalities do require that horizontal portions of drain inside the structure be insulated to prevent condensation and this standard makes sense to me. In Florida we always insulate horizontal portions of the drain because if we didn’t we would have consistent issues with growth and water damage due to the high dew points.

Condensate Switches 

IMC 307.2.3 states that all HVAC equipment that produces condensate must have either a secondary drain line or a condensate overflow switch, a secondary drain pan with a secondary drain line or condensate switch or some combination of these installations should be used to prevent overflow if the primary drain line blocks.

This includes rooftop units, ductless units and downflow units but the code does allow for the overflow prevention switch to be placed in the primary drain pan in these cases but NOT the primary drain line according to 307.2.3.1

— Bryan

P.S. – For more info on condensate switch best practices visit https://www.rectorseal.com/installation/

 

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Whenever there is a conversation where “code” is involved it’s important to mention that codes can vary depending on the AHJ or authority having jurisdiction. It’s becoming more common that governments lean heavily on the ICC (International Code Council) and in the case of HVAC/R that is the IMC (International mechanical code) and in the case of fire protection and electrical codes the NFPA (National Fire Protection Association) has become the authority for codes and standards in the US.

So what is the purpose of duct smoke detectors?

NFPA 90A, 2012, A.6.4 makes this pretty clear by stating

“Protection provided by the installation of smoke detectors and other related requirements is intended to prevent the distribution of smoke through the supply air duct system and, preferably, to exhaust a significant quantity of smoke to the outside. Neither function, however, will guarantee either early detection of fire or the detection of smoke concentrations prior to dangerous smoke conditions if the smoke movement is other than through the supply air system.”

In other words, duct smoke detectors are there to keep units from circulating smoke in the space and when possible to send it outside. They aren’t there as a replacement for space smoke detectors.

When do they need to be installed? 

Both NFPA 90 and IMC 606.2.1 state similar things that can be summarized and paraphrased as “If the duct system is designed for more than 2000 CFM the system must have a duct smoke detector installed” and “If the duct system is designed for  more than 15,000 CFM one in the return and supply is required”

NFPA 90 states that the smoke detector should be installed in the SUPPLY after 2000 CFM and IMC 606.2.1 says the RETURN. This means that it up to the AHJ to decide which standard they follow.

NFPA 90A also states “where an approved fire alarm system is installed in a building, the duct smoke detectors shall be connected to the fire alarm system”

Now, these are summaries of more complicated texts with exceptions and lot’s of extras, so if you want to know all details I would suggest you read the code for yourself but in general –

  • A duct smoke detector should shut off a typical blower and fresh air and turn on exhaust
  • Duct detectors aren’t a replacement for room sensors
  • If the duct system is designed to carry more than 2000 CFM (5 tons nominal) of air you need one in the return if IMC is being followed and the supply if NFPA is being followed.
  • If the duct system is designed to carry more than 15000 CFM of air you need one in the return and one in the supply
  • If a central fire monitoring system is in place a duct smoke detector is in use it must be connected to the fire monitoring system

— Bryan

 

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Another quality tech tip from senior refrigeration and HVAC tech Jeremy Smith. Jeremy lives in Ohio so he knows a thing or three about the cold.


As HVAC/R techs, we’re often called on to work in some of the worst weather conditions. With the cold weather, I thought it timely to share some tips and strategies that I use for staying warm in the cold weather, particularly for you guys in the south who think that 40°F Is ‘cold’.

Staying warm, particularly in very cold weather, isn’t about having the thickest jacket or the most socks, it’s more about picking the right layers to wear. Let’s be honest, we’re all tough enough to gut out an hour or so in the worst conditions imaginable with a hoodie and a beanie cap. But, when we’re exerting that much mental effort just to endure being cold and miserable, we’re not exerting that effort to work on solving the problem or making the repair, so staying comfortable is both to our benefit and to our employers benefit as we’re more effective when we’re not standing there shivering.

The first and foremost rule in staying warm is to stay DRY. Wet clothing, either from your own sweat or from rain or snow that melts into your gear doesn’t insulate as well. Some fabrics, like wool, retain their insulation better, but if you get wet, you’re going to have a long, cold day. Let’s start with what’s called a “base layer” the base layer isn’t about insulation. It’s about staying dry. Under Armour is a popular brand and their stuff is great, but I go in a different direction and use Merino Wool base layers. They can be cheaper than Under Armour and perform at least as well. These aren’t Grandpa’s itchy wool long johns, they’re very comfortable, warm and they help keep your skin dry which is really the secret to staying warm.

Next, you want insulation. Fleece is good here, so is a nice Under Armour hoodie or similar item. Even a nice, thick sweatshirt is enough to keep your body warm. A vest is also helpful because keeping your body core warm is more important than keeping your arms warm. I’ve also got fleece pants for when it gets really cold but I normally don’t need them for work. Layer one or more on as needed for conditions. On top of all of this is your outer shell. This is the wind and water repellent layer. As HVAC/R mechanics, we need this layer to be abrasion resistant as well. Something like a down ski jacket is probably the warmest thing you can find, but they won’t last very long around those sharp sheet metal edges.

A Carhartt coat and bib overalls are great and are the standard for tradesmen everywhere, but other brands like Walls and Dickies are just as warm and can be had for less money. Personally, I prefer a bib and coat set over a set of coveralls. That way, I can wear just what I need rather than having to either be cold or wear the whole, heavy setup and maybe overheat.

Another nice thing to have is a foam kneeling pad. Not just to cushion your knees from the roof but to keep snow from melting into your insulation and making you colder.

Now let’s talk headgear. Sure, a beanie is great, but when you’ve got to be out there for hours, gotta keep the grey matter warm. My setup starts with a polypropylene fleece balaclava. They’re great because they are so versatile. They’re everything from a neck warmer to a full head, face and neck insulator. Every tech that works in cold weather needs one. When the wind really kicks up or the mercury really drops, though, that just isn’t quite enough for all-day comfort out on that roof. Enter the “Mad Bomber” hat. Big, floppy rabbit fur ears wrap the sides of your face and buckle under your chin. Now, we’re staying toasty. When the weather really gets nasty and the wind is howling, I’ll pull the hood of my coat up then wrap a pair of inexpensive ski goggles over the whole mess. The hood keeps the wind off of the back of your neck and the ski goggles both protect your eyes from the harsh snow glare, cold winds and holds the whole thing together. Best of all, ski goggles don’t fog up like regular sunglasses do.

No matter how cold it gets, I don’t like insulated boots for work. If I’m working inside, my feet get wet and sweaty and uncomfortable then, if I have to go outside I’m already wet and, once you’re wet you will get cold a lot faster. So, I stick with uninsulated boots for everyday wear and I carry 2 extra pairs of socks. A pair of polypropylene liner socks. These are the base layer for your feet. Not keeping you warm but keeping you dry. Over those, a pair of nice, thick wool socks. Lace your boots up over them and you’ll be plenty warm but the there is one more step to warm feet. Spend enough time out of the roof like this and one thing happens.. your boots get all snow packed, then the snow melts and soaks through the boots and you wind up with wet, cold feet. The answer is a pair of overboots. Pull them on over your boots, buckle them up and now your feet stay dry and warm. One quick note on boots. OSHA requires safety toe boots, but that doesn’t mean steel toes. The steel toes on those boots just stay cold, no matter that you do and it radiates what heat you’ve got in your toes and you suffer.

When boot shopping, buy safety toe boots with a composite toe. The plastic safety toe is still OSHA approved but it more comfortable to wear in the cold. One important thing is to avoid cotton. Cotton socks, cotton underclothing, etc absorbs moisture and doesn’t provide any insulation value.

Unfortunately, I don’t really have any all day stay warm tricks for your hands. A pair of ragg wool gloves is about the best protection I’ve found. Usually, I wind up with my hands in and out of the gloves all day, balancing between keeping my fingers warm and and being able to do the job. Another nice thing to have on hand, so to speak, is a couple packages of those disposable hand warmer packets. Unwrap one or two and throw it into your pocket. Cheap guy trick for those. If you’re only out in the cold for an hour or so, seal that handwarmer up in a zip lock and press the air out. Since they’re air activated, removing the air stops the reaction. Open the bag back up, shake it up and it’ll start warming back up again.

In practice, this is how I’ll employ this layering system. Your situation may be different. I dress for work very much the same every day. Work pants and a T-shirt. As the weather cools, I’ll add a sweatshirt and eventually, a vest. That’s just for walking around. When the call drops that I have to go hang out on the roof, I find a place where I can change. Based on experience with in the cold, I’ll select the right amount of layers to keep myself warm without overheating. Sweating can be as bad as shivering and will ultimately lead to shivering. I keep the heavy gear packed in a big duffle bag in reverse order of putting it on, so I can just pull a piece out, put it on and grab the next piece. Peel gear off in the same order and it packs away, ready for next time. If your gear gets wet, be sure to dry it before venturing back out into the cold.

Hypothermia and frostbite

Since we generally work alone, we really need to learn to self-monitor for these two conditions. Hypothermia is a serious, potentially life-threatening condition where the body core temperature drops below 95F and the body is no longer able to warm itself. Your brain and vital organs will stop working and you will go to sleep one last time. The first symptoms of this are violent and uncontrollable shivering. This is your body working your muscles vigorously in an effort to generate heat to keep itself warm. Further symptoms include slurred speech and disorientation. Don’t ignore these symptoms and if you find yourself in that condition of violent and uncontrollable shivering, get yourself somewhere that you can warm up and stay there until you are warm.

Frostbite is the formation of ice crystals in your body. This occurs when exposed skin, typically fingers, toes, ears and noses literally freezes. It is very painful and can result in the loss of flesh in the affected area. Symptoms include pain, itchiness and discoloration of the affected part which progresses to hardening as the flesh freezes deeper. Much like hypothermia, this is much better treated early than allowing it to progress. Stop and allow the area to warm slowly. DON’T RUB IT. This breaks up any ice that has formed and those ice crystals can cause more tissue damage. Cool water is the best way to warm a frostbitten area. One last caution or two to touch on.

A lot of guys like to drink coffee, tea or hot chocolate to warm up. While I love a good, hot cup of coffee, this isn’t always the best way to warm up. Caffeine is a vasoconstrictor meaning that it causes the small blood vessels in your hands, feet and elsewhere to constrict, limiting blood flow. That can increase your risk of conditions like frostbite because warm blood isn’t flowing to those areas. Also, we typically associate dehydration with summertime and sweating, but in cold conditions, this is also a concern because the air is so dry, we’re losing moisture with every breath plus we may be sweating and the wicking layers we wear are dissipating that moisture as they’re designed to and we don’t notice the sweat like we would in the summer. Stay hydrated.

— Jeremy

Shopping list
Sportown Men’s Merino Wool Lightweight Long Sleeve Crew Base Layer Top,XXL 

Minus33 100% Merino Wool Base Layer 706 MidWeight Bottoms Black XL
Balaclava Fleece Hood – Windproof Face Ski Mask – Ultimate Thermal Retention & Moisture Wicking with Performance Soft Fleece Construction, Black, One Size

Mad Bomber Supplex Hat with Grey Fur, Black, Medium

RefrigiWear Insulated Wool Gloves Green Large

Fox River Outdoor Wick Dry Alturas Ultra-Lightweight Liner Socks, Medium, White

Wigwam Men’s 40 Below Heavyweight Boot Socks, Grey Twist, X-Large

Ski Goggles for Youth Age 8-16 ¨C UV400 Protection and Anti-Fog ¨C Double Grey Spherical Lens for Sunny and Cloudy Days (Black)

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When mounting a TXV bulb or checking bulb placement there are a few important considerations (listed in order of importance)

  1. Mount the bulb on the suction line. Flapping in the breeze is no good.
  2. Mount TIGHTLY it with a proper metallic strap (usually copper or brass). Not zip ties, not tape.
  3. Position it on a flat, clean, smooth, portion of the horizontal suction line. Not on a coupling or an elbow.
  4. Mount it before the equalizer tube (closer to the evaporator than the EQ tube)
  5. When possible mount it at 8 or 4 o’clock on the suction line (or according to manufacturers specs) . This becomes more important the larger the suction line.
  6. When possible, insulate the bulb so that it is not influenced by ambient air temperature. It never hurts to insulate the bulb even inside the cabinet though not all manufacturers require it.
  7. If you do need to mount it vertically, make sure the tube points up not down

Poor bulb contact will result in a bulb that is warmer than desired, resulting in overfeeding and lower than desired superheat.

Finally… be gentle with the bulb and tube. They break easily.

You can read a more detailed description HERE

— Bryan

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How does a typical motor know how fast to run?

Typical induction motors are slaves of the electrical cycle rate of the entering power (measured in hertz).

Our power in the US makes one full rotation from positive electrical peak to negative peak 60 times per second or 60hz (50hz in many other countries)

This means that the generators at the power plant would have to run at 3600 RPM if they only had two poles of power 2 poles (60 cycles per second x 60 seconds per minute = 3600 rotations per minute) in reality, power plants generators can run at different speeds depending on the number of magnetic poles within the generator. This phenomenon is replicated in motor design.

The more “poles” you have in a motor the shorter the distance the motor needs to turn per cycle.

In a 2 pole motor it rotates all the way around every cycle, making the no-load speed of 2 pole motor in the US 3600 RPM.

A 4 pole motor only goes half the way around per cycle, this makes the no-load (Syncronous) RPM 1800

6 pole is 1200 no load (no slip)

8 pole is 900 no load (no slip)

So when you see a motor rated at 1075 RPM, it is a 6 pole motor with some allowance for load and slip.

An 825 RPM motor is an 8 pole motor with some allowance for slip.

A multi-tap / multi-speed single phase motor may have three or more “speed taps” on the motor. These taps just add additional winding resistance between run and common to increase the motor slip and slow the motor.

This means  a 1075, 6 pole motor will run at 1075 RPM under rated load at high speed. Medium speed will have greater winding resistance than the high speed and therefore greater slip. Low speed will have a greater winding resistance than medium and have an even greater slip.

Variable speed ECM (Electronically commutated motor) are motors that are powered by a variable frequency. In essence the motor control takes the incoming electrical frequency and converts it to a new frequency (cycle rate) that no longer needs to be 60hz. This control over the actual frequency is what makes ECM motors so much more variable in ten speeds they can run.

So in summary. There are three way you can change a motor speed.

  • Change the # of poles (more = slower)
  • Increase slip to make it slower, decrease slip to bring it closer to synchronous speed
  • Alter the frequency (cycle rate)

— Bryan

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We get a lot of questions about both evacuation procedure and TXVs so last week we produced videos on both topics including –

  • Before and after testing of piston vs. TXV
  • Using the Bluvac Measurequick app
  • Use of core remover tools for evacuation
  • flowing nitrogen process
  • creating an external equalizer port and much more

If you haven’t hit subscribe on our YouTube channel yet would you mind taking the time to do it today? it would be greatly appreciated. You can do that HERE 

P.S. – I will be at the Rectorseal booth 2545 in Chicago at the AHR conference on January 23rd at 2PM demonstrating the new Pro-Fit flaring tool. If you are at AHR come by and see me and sign up for a chance to win a free Pro-fit!

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This tech tip is written by one of the best techs I know. Neil Comparetto.

I think that we all can agree that duct leakage is not ideal. Our job is to condition the space. If we can’t control the air, that becomes difficult. On top of that anytime you are losing already paid for conditioned air. But really, how bad could it be?

I’m in Richmond Virginia, so we’ll use that as our example location. According to ACCA Manual J summer design conditions our outdoor design temperature is 92° Fahrenheit, with a moisture content of 106 grains per pound. (grains is a measurement of absolute moisture). Let’s use the indoor conditions 75° F and 50% relative humidity, which converts to 65 grains of moisture.

Our example system will be a 3 ton air conditioner moving 1200 CFM with ducts in a vented attic. For this exercise we won’t get into duct sensible heat gain that even a 100% tight duct system will have to overcome.

This system will have a modest 10% supply duct leakage into the attic (Energy Star estimates that the typical duct system has 20-30% duct leakage). Assume 0% return leakage (which is unlikely). So we already know that 10% of our capacity is gone, never to return again into the attic. On a 3 ton air conditioner that will be roughly 3,600 btuh. We are now delivering 1080 CFM of supply air to the living space, and returning 1200 CFM. Where does the additional 120 CFM of return air come from? You guessed it, outside. The supply duct leakage into the attic, outside of our thermal and pressure boundary, has now brought the living space into a negative pressure. No big deal, it’s only 120 CFM… but have you ever done the math!?

Stick with me, it’s not as bad as it looks. Here are the formulas for the sensible and latent heat required to bring the infiltration air back to indoor conditions (75°/ 50%RH).

Sensible BTUH = 1.08 x CFM x (Outdoor temp – indoor temp) Latent BTUH = 0.68 x CFM x (Outdoor grains – Indoor grains)

Let’s use 92° F as our outdoor air temperature number. In all likelihood, considering that the attic floor/ceiling plane is one of the leakiest parts of the house, and the attic is typically > 120° F, that in real life it will be higher than whatever outdoor temperature is.

Our example will look like this:

1.08 x 120 CFM x (92°-75°) = 2,203 btuh of sensible heat

.68 x 120 CFM x (106 grains – 65 grains) = 3,346 btuh of latent heat

2,203 + 3,346= 5,549 btuh of total heat.

That is an additional 5,549 btuh of total heat. The 3,346 btuh of latent heat is the more difficult number to deal with. Next time you are bored flip through your favorite air conditioner’s product data and see what it can produce, you may be surprised. Don’t forget about the 3,600 btuh that’s up in the attic somewhere. And just think, this is from only 10% supply duct leakage, considerably more is very possible.

As you can imagine in the heating season this problem doesn’t go away. Typically outside air is much drier than indoor air, and duct leakage will dry out the indoor space. If the heating system is a heat pump the capacity loss is corrected by electric strip heat, which is bad. That means when you seal the ducts auxiliary heat is reduced, which is good.

Leaky ducts can contribute to many more issues than just energy loss and comfort. Did you know that a one square inch hole in the duct system is equal to thirty inch hole in the building envelope? The potential to create pressure imbalances in the building is tremendous. Pressure imbalances can cause many issues, like flues backdrafting, excess dust and allergens, uneven temperatures, and moisture issues to name a few.

Something as simple as sealing ducts can solve many issues, hopefully you include it in your scope of work.

— Neil

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