Hey there Home Energy Pros,

How do you calculate whole home heat loss from the ducts?  To put this in context, I know how to calculate heat loss from the envelope (walls, floor, ceiling taking into consideration insulation levels and air leakage) but I am stumped on how to calculate how much heat the home is losing through ducts located outside the thermal envelope.  A little help, please?

Tags: calculations, ducts, heat, loss

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If they are insulated, assume a 1-2 degree F temperature loss from the heating source until the last vent/terminal/outlet.  Basically it's the energy loss through the duct and insulation.  I hope that answers your question.  If you need me to be more specific, let me know and I'll give you an example.

It may be a little more complicated that that unless you want to talk about rules of thumb.  Is a 7" flex duct suspended 2 feet off of the attic floor? Is it against the bottom of the roof sheathing?  Is it resting on top of the insulation layer so that the bottom (30%? 40%? 50%) of the surface area has higher R-value than just the duct insulation?  Is there a 4° loss to the room over garage because it is 35 feet from the plenum? 

It is hard to get real world numbers

You can use ASHRAE Std 152 that was developed explicitly to do this.  Some of the inputs can be a bit obscure - particularly trying to estimate thermal regain but this paper has some reasonable defaults: On-line PDF version

There is also a simplified version of the 152 calculations in the  California Buildings Energy code  Alternative Compliance manual:http://www.energy.ca.gov/2008publications/CEC-400-2008-002/CEC-400-...

If you pm me I can send you an excel spreadsheet that does the 152 calculations.

Hi Iain, I am very interested in finding a better way to calculate duct losses in gains in the field- could your spreadsheet help me? Thanks, Blake

This is a very needed discussion, IMHO. I personally think that ducts should be viewed as a part of the envelope, which they pretty much are. 

Heat loss would be calculated similarly - you would just need to figure out their surface area, R-value and temperature difference. 

Delta T times U-value times surface area means (oil-fired furnace supply air) 130 - say 50 (outside) = 80 degrees times say a 6" round metal duct = 1.58 ' (circumference) times say a 30 foot run suspended in a crawl space equals 80 X 1.58 X 30 = 3792 BTU/hr if the R-value/U-value is 1. That's for an hour; a ten minute blow would be 632 btu.

If we insulate with an R-11 batt, then that hour's worth of loss drops to 345 btu in the one duct on a mild day. And the 10 minute delivery takes a toll of only 57.5 btu.

But the issue with ducts is often more a matter of duct leakage rather than conductive loss. I recently found a million dollar house with owners complaining of cold room upstairs. I found a 6" by 12" hole in the return in a crawl space which had been cut by  tin snips, and I assumed an HVAC contractor did it because of a depressurized CAZ...in other words, the balance was achieved by matching an unfindable supply leak to the outside with a semi-hidden return leak to the outside so the once-depressurized CAZ didn't kill the young doctor and his family.

Here's my quick and dirty summary of what duct leakage has to do with overall house conditions considering exfiltration, infiltration and indoor air quality.



Wouldn't a supply leak in the CAZ make any pressure problem less bad?  And wouldn't cutting a hole in the return make it more bad?  And have you yet seen an HVAC contractor that has ANY knowledge of CAZ that you haven't taught?

We try to look at the return system in the basement in every house and seal anything that is more than a crack - missing pans (what you have here) are fun and must be replaced to get the house to operate properly and to make sure there are no CAZ pressure problems.  I personally am guilty of opening a couple of small supply leaks in a metal duct to help out a stubborn CAZ problem.

And why just look at the 10 minute delivery - the ducts are up there all the time, the attic is still very hot or very cold, so the delta is just smaller, not zero.  The duct is changing temperature so that it will eat a lot of BTU's when the system does come on, and there is also convective loss in the winter during off cycles - a common complaint in certain styles of house.


Outside ducts are always part of the envelope.  Here is my favorite explanation that gets builders to listen: we pay our bill by the square foot - attic, wall, windows, etc.  If there are ducts in the attic, then that is square footage we have to pay for in heating and cooling.  A 1,000 ft2 attic may have  300 ft2 of ducts in it, meaning we have an extra 300 ft2 of envelope to pay for, and it is at a low R-value.  And that is with a perfectly tight duct system!  

Ed, I assumed that the supply leak was also outside the CAZ and blowing air out, so it was responsible for a depressurization (more bad CAZ) as a result. I also assume that the HVAC contractor could not find it, but had other more important things to do so he whacked a hole in the return side also outside the CAZ in a crawl space walled off from the half basement where the naturally aspirating furnace was, and sohe intended to add pressure to the CAZ and perhaps reduce the backdrafting which the supply leak to the outside (unseen) had caused.

And you are spot on with your comment that if the supply leak had been inside the CAZ, then of course it would have positively pressurized that zone and that if the hole were cut in the return running inside the CAZ, that hole would have depressurized that zone. But the holes were outside the CAZ and blowing (supply leak) and sucking (return leak) air outside the envelope.

And you're also spot on about the heat loss or gain in a considerable hunk of surface area which the manual J does not account for specifically. A typical 1500 sq. ft. house with ducts in a crawl or attic may run 40 feet of trunk and at least 250 feet of 6" round. If uninsulated supply are metal ducts, that's another 500 or so sq. ft of R nothing dumping heat in the winter and nabbing heat in the summer, even if they're air-tight (not likely!) which if typical is 5 - 10% of the flow. 

Does anyone know why Manual J doesn't rate the ducts as even a loss or gain to be reckoned with?? They count doors and infiltration but not duct area, R-value or mechanically induced air leakage! That seems a bit goofy, huh?  Duct leaks are  'two-fers' - we suffer not just the forced loss (exfiltration) but the equivalent amount of outside air pulled in (infiltration) with supply leaks to the outside, and vice versa with return leaks. Plus when we depressurize the CAZ, people die or get really sick and have no idea why.

In a better world, we wouldn't have duct leaks to the outside or even outside ducts to leak. They'd all be inside where they belong.


I Understand your scenario now - wasn't clear on it before.  Manual J, If I remember right, applies an efficiency factor if the ducts are in the attic - it makes you oversize the unit by something like 25% as a penalty - can someone verify?

An interesting question.  You didn’t say why you wanted to know what the heat loss/gain might be.  If it is just to calculate the furnace/AC sizes you can probably do this manually (with the help of excel).  If you are looking to calculate annual energy costs you would need to incorporate the annual weather data, at this point you should get a software application that calculates the whole deal, Maximum HVAC requirements, costs correct duct and register size, correct equipment to ensure correct airflow, static pressure, etc., for both heating and cooling, and the software will also calculate your annual energy costs.  On the other hand if this is too much bother, and it is an incredible amount of bother, you can simply bury the attic ducts in R30 and seal and insulate the crawl space. You have brought the ducting into the shell and then you can consider your project done and forget the calculations altogether.

The other consideration is sealing the ducts.  While it is universally understood that air sealing is a good thing to do, there are consequences, which may be expensive for you to remedy.  You should first make the assumption that the ducting system is incorrectly designed, which it will be, because if it was correctly designed (and installed) the customer wouldn’t have called you in the first place.  Sealing the ducts will change the air flow characteristics, increase the static pressure, reduce air flow over the heat exchanger and AC coil and probably make the system noisier and possibly less efficient.  The other issue is ventilation.  HVAC systems tend to be used most often when windows are closed (open windows are the most common form of natural ventilation).  Leaky supply ducts negatively pressurize the space and leaky return ducts pressurize the space.  In either case they are introducing outside air into the space which in many cases may be improving indoor air quality.  These are the considerations.

Just my thoughts,

Michael Kyes


George: As Iain noted the ASHRAE 152 calculation is pretty good and has stood the test of time. There have been field tests that showed how well it did against monitored data. (I can send you some) More recently we have found that ducts in the attic, even ducts with little leakage can loose as much as 50% of their energy in cooling. See http://www.proctoreng.com/utilities/wholehouse.html This of course is not a surprise to anyone, but close attention to duct length, surface area, and R-value can mitigate most of the issue. See http://www.amazon.com/Measured-Home-Performance-Practices-Californi...  

Lets not forget cycling losses. Equipment that cycles 2x per hour will have a lower heat loss through ducts than a system that cycles 4-6 times per hour. each time the system cycles the first 5-10 minutes are spent stabilizing the duct temperature. The less times the system cycles per day, the lower the overall loss. Properly sized equipment will allow lower cycles per hour while the building keeps the same "Temperature swing" between cycles.

Another thing to consider is duct layout. Reduce the total surface area of the duct system if it's in unconditioned space. Use fewer large ducts instead of many small ones. For long runs it's much better to use a 10" then divide into (3) 6", then to run 3 long 6" ducts straight off the plenum. Think "trunk and branch" vs. "spider".

Of course the BEST solution is to just get the equipment/ductwork ALL in conditioned space. This will often allow reducing equipment size by 1/4!! Think a 0.5 - 1 ton smaller AC, or dropping a "burner size" off a furnace.


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