# Data on Heat Exchange Effect

| Posted in General Questions on

I read a post on here where Dana Dorsett and one other person mentioned the heat exchange effect.
ie. even through leaks in the building envelope, the outside air coming in is heated by the house before coming in. eg. if its -20 degrees outside, the air coming in is not actually -20 degrees but warmer than that

Is there any research on this I can read?

thanks.

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1. | | #1

I believe it was written about in the 2005 ASHRAE Handbook of Fundamentals as "Infiltration Heat Recovery". As I remember LBL was the author. Lost my CD for the handbook so I could not link it at the time it was brought up on GBA.

1. | | #4

thanks! I will look that up.

2. | | #2

Big O,
The following explanation is from my 2012 article, "Air Leakage Degrades the Thermal Performance of Walls."

Starting after the colon, I'm quoting the article:

When infiltration and exfiltration occur at multiple leak sites in a wall — imagine a colander — the heat flow through the wall is less than when the infiltration occurs at a single hole and the exfiltration occurs at another single hole. This is because a leaky wall acts like a heat exchanger.

Here’s how the phenomenon works in a wall with infiltrating air. First, imagine a perfectly tight wall with a single hole in the center of the wall. All infiltration occurs through this one hole; let’s say the hole admits 20 cfm. This type of infiltration does not benefit from the type of heat exchange under discussion.

If the same 20 cfm of infiltration occurs through one hundred tiny colander-sized holes, the energy penalty attributable to the infiltrating air will be less. That’s because the infiltrating air that passes through all those little holes gains heat as it moves through the wall; in fact, the air gains more heat than it would if it passed through the single hole.

Here’s how the phenomenon works in a wall with exfiltrating air. If all of the exfiltration happens at a single hole, you don’t get any benefit from the heat-exchange phenomenon. But if the infiltration occurs through one hundred tiny colander-sized holes, the warm exfiltrating air will raise the temperature of the wall assembly as it exits, reducing the conductive heat flow through the wall.

“Exfiltration reduces heat flow through the wall,” explained Dave Ober, one of the presenters at the Westford Building Science Symposium on August 1, 2012. “There is a performance benefit. The wall acts like a parallel-flow heat exchanger, so you are getting a little heat recovery.”

Ober continued, “The effect of discrete of holes” — that is, holes that do not resemble a colander — “is easy to calculate because there is no interaction. But uniform air infiltration” — that is, air infiltration through colander-like holes — “recaptures some of the heat loss through the wall by heating some of the air flowing through the wall. Uniform air exfiltration also recaptures a small amount of heat — actually it changes the temperature gradient and reduces the heat flow. You are not recapturing all of the heat; it is a net loss. You are reducing the conductive losses somewhat.”

Because of the heat-recovery effect, Schumacher noted, “Walls with fibrous insulations have reduced heat flows compared to sealed walls with an equal airflow through a discrete hole.”

It’s important to remember the heat carried away by escaping air in the winter incurs a greater thermal penalty than any benefit accrued by the heat-exchange that occurs as air flows in or out of a wall. “There are a lot better ways to achieve heat recovery,” said Schumacher. “It’s better to seal the walls.”

Here’s how the BSC researchers summarized the heat-recovery phenomenon:

● Infiltrating air may recover some heat flow through the windward wall, thus reducing the energy impact of the air leakage;
● Exfiltrating air can reduce the heat flow through walls on the leeward side of the building;
● Any air leakage through a wall assembly will result in increased heat flow.

1. | | #5

Hi Martin, thanks for that write up. I was looking for data on the percentage of heat recovered. I have found some interesting research on the subject

1.The measured energy impact of air leakage on frame wall systems
Bhattacharyya, S
https://www.osti.gov/servlets/purl/7238564

excerpt from conclusion:
For the range of values of the pressurtzatJon, exponent _ typically measured in houses, values of gHEE measured in
the test cell were smaller but still suggest that estimates of infiltration load based
on air exchange rates are likely to systematically overestimate infiltration loads
by an average of 20 percent or more

2.CFD Simulation of Infiltration Heat Recovery
Buchanan, Christopher R., Max H. Sherman

https://eta-publications.lbl.gov/sites/default/files/lbnl-42098.pdf

it concludes that heat recovery can be around 10-20%

3. | | #3

I am no pro on the subject but to me it doesn’t make a difference how is the outside air introduced through a wall. You might be heating the air that is infiltrating, but you are also cooling off the wall. So what temp the outside air comes in at is irrelevant to the indoor temp because you are still losing heat to heat it up and it will drop the temp exactly as much as if the outside air is introduced into the conditioned envelope at outside temp. There is no free heat - the indoor temp will drop the same amount of you introduce the cold air as is or if you heat it up through the wall but cool off the wall resulting in same temp drop in conditioned space, no?

1. | | #6

This is an interesting discussion. The takeaway is unchecked infiltration is not good for building performance. Heat loss due to air infiltration is not a straight calculation, .018 at sea level, but is complicated by the path through the building envelope. I agree there is no free heat, it is a matter of how we apportion the heat loss in a building.

2. Expert Member
| | #7

From a physics standpoint, you're loosing the same amount of energy either way, since it takes the same number of BTUs to heat a given mass of air from whatever you start from to wherever you end up, regardless of if you do it in a bunch of little steps (like a leak through a fiberglass batt), or all at once (if it leaks through a hole in a sheet of polyiso). The big difference is that on a comfort level, you're much more likely to notice frigid air leaking in through a gap around a window than slightly warmed up air that came through the insulation in the wall and leaked out your electrical outlet or baseboard.

If your goal is energy efficiency, which is probably the case with most of the people on the GBA forums, then you want to air seal regardless. If you are mostly concerned with comfort, then that "heat exchange effect" would mean that leaks in places insulated with fibrous insulation would be last on your list to worry about.

Personally, I'd still stick to the usual recommendation of trying to do a thorough job of airsealing regardless of the type of insulation you choose, which will increase efficiency AND comfort in ALL cases.

Bill

1. | | #8

Bill if you read the links I posted in response to Martin you'll see that there is a heat exchange effect and it can be significant. The outgoing air exchanges upwards of 20% of its heat with the incoming air

1. Expert Member
| | #9

I think you're thinking of that 20% of energy exchanged as being something like what occurs in an HRV, so you don't "lose" all of it outright. I would still think of this as a loss though, that better air sealing would help to reduce, even if the overall actual energy losses are less than what you might get from the BTU/airmass loss itself. Hopefully that makes sense, I could probably have worded things a little better :-)

Bill

1. | | #10