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Air infiltration heat loss

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Posted by Jeff on October 13, 2007, 4:02 pm
  Since I have a fair idea of how much heat I'm losing through my walls,
floor and ceiling. I'd like a rough idea of much I'm losing through air
exchanges (air infiltration).

   I started to calculate this, but then realized I had to take in
factors like humidity inside and out, so I thought: Someone must have
done this before and has ballpark figures.

   I'm figuring about .45 BTU * cubic feet * Degrees F * air exchanges
per hour. So far 10,000 CF, 1 air exchange/hour and delta 20 F =
88,000BTU/day. That seems like maybe 2/3 the losses through conduction.
Does that seem right?

   How do I roughly figure my heat loss through air infiltration?


Posted by Ecnerwal on October 13, 2007, 4:11 pm

Roughly figure your heat losses through conduction, factor in the
efficiency of your furnace/heater, and keep track of (or pull from the
weather service) the heating degree days for a period of time
corresponding to a billing or filling interval of your fuel of choice.

How much fuel you used, times the efficiency of the furnace, times the
BTU value of the fuel, minus the losses for conduction roughly equals
the loss by air exchange. Becomes harder to track down if your fuel of
choice is also heating water.

With a reasonably well insulated house, a heat recovery ventilator
begins to make a lot of sense, fuel-use-wise, though Nick seems to
champion no ventilation at all, or at least very little. I'm more fond
of a tight house and controlled ventilation, with heat recovery.

Cats, coffee, chocolate...vices to live by

Posted by Jeff on October 13, 2007, 5:19 pm
 Ecnerwal wrote:

   I can see that is going to be impossible for me to figure, since I'm
using a mix of solar, wood and electric! But I see your point. I think
it may be best just to keep looking for leaks and sealing.

Well, Nick is Nick, he's against any windows too. Nick is a great source
of ideas, but that doesn't mean that you have to use them at face value!

   Since I'm in a 20's house I'm thinking I'll never get it too tight!

  I've found a little bit of tape works wonders around windows and I
think my IR thermometer might help me find leaks once it gets colder.
I'm trying to get a rough idea of how much heat I'll need this winter,
as I'm thinking of shutting off areas that aren't in the heating budget.
There's also a point where going after weak areas is a better value than
adding more insulation to the attic or the floor.


Posted by nicksanspam on October 13, 2007, 8:32 pm

Why 0.45 Btu? Air's specific heat is 0.24 Btu/lb-F, with a 0.075 lb/ft^3
density, so it's 0.018 Btu/F-ft^3 by volume, ie 20x0.018 = 0.36 Btu/ft^3
for a 20 F temp diff, ie 24hx1achx10000x0.36 = 86.4K Btu/day.

I figure 1 cfm adds about 1 Btu/h-F to the conductive house loss. For
instance, 1achx10000ft^3/60m/h = 167 cfm, and 24hx167x20F = 80K Btu/day.

Or if you have a humidifier. You might set it for 60% RH for a cold day
or 2 and measure the outdoor RH and temp and humidifier water consumption
and heat energy needed to keep the house 70 F (easy to do with electric
heaters for a day) and calculate the air leakage, more accurately if you
estimate the moisture contribution from people, 2 gallons per day for
an average family of 4, according to Andersen.

It makes no sense to me, for an average US house with 224 cfm of natural
air leaks. Why deliberately ventilate if the house naturally leaks 10 times
more fresh air than we need?! An HRV makes more sense for a Canadian IDEAS
house that leaks 2.5 cfm, if it's not 100% solar heated.  If it is, adding
15-30 cfm of deliberate ventilation with an exhaust fan that runs when
the indoor RH RISES to 60% (airtight houses with human humidity sources
need winter DEhumidification) makes more sense, with a much lower initial
cost and no fuel cost.

No. I "champion" the ASHRAE 15 cfm per full-time occupant standard :-)

Me too, with the solar house exception above.


Posted by gary on October 16, 2007, 1:18 am
Hi Jeff,
You might find that your utility offers a free blower door test as a
part of an energy audit program.
This should give a pretty accurate idea of how bad your infiltration
is -- and maybe some help on what to seal up.

A simple home heat loss calculator I did:

One air change per hour is fairly loose, but a lot of homes are that

There are some fairly useful articles here on air infiltration and
where most of it comes from (somewhat surprising) -- see the "sources
of Air Infiltration" etc article:


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