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Sanity check on napkin heat loss numbers

airfix | Posted in General Questions on

I finally got through doing my heat loss calculations to size my boiler for radiant floor heating in the basement.  I’m looking for a sanity check that I am in the right ballpark.  I feel confident about most of the numbers except the heatloss due to air infiltration and my ERV.

Climate zone 6A, 1600 sq.ft walk out basement.  One wall shared with a garage assumed temp of garage 32F.  Assumed outside design temp 0F.  Assumed below slab soil temperature 40F.  Assumed inside design temp 68F.  3 of the 4 exterior walls rise from 10 foot below grade to just about at grade.  Assumed a 1.5ACH50 blower door test and 82 CFM flow rate through the ERV.

Above grade walls 2.5″ polyiso exterior foam over 2×6 frame with BIBS in cavity.  Whole wall R~32
Below grade walls 1.5″ interior polyiso foam with 2×4 frame with BIBS.  Whole wall R~22

Heat losses:
Q windows = 3345 BTU/hr
Q Doors = 806 BTU/hr
Q Slab = 4092 BTU/hr
Q Above grade walls = 1029 BTU/hr
Q Below grade walls = 1638 BTU/hr
Q garage wall = 784 BTU/hr
Q air leakage = 4378 BTU/hr
Q ERV = 1025 BTU/hr

Total Q = 17,097 BTU/hr

Assume boiler efficiency is 95% and over size boiler by a factor of 1.4:

Q boiler = 25,123 BTU/hr

For the infiltration heat loss I got stuck in the weeds a bit.  I had to convert 1.5ACH50 to ACHnat (natural pressure). I went to the ASHRAE2017 Fundamentals book Chapter 16 page 16.24 and used the basic Sherman and Grimsrud equation (48).  The problem came determining the effective air leakage area.  I used a windspeed of 10mph, stack coefficient of .0299 and wind coefficient of .0086. I calculated an effective leakage area of 41

The ELS was calculated using Martins formula in post 5 here:

Blower door tests and Equivalent Leakage Area

I thought my ERV number seemed quite high in relation to the air leakage number.

What do you think?  Am I in the ball park or do I need to go back to the drawing board?



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  1. Expert Member
    Peter Yost | | #1

    Hi Steve -

    how did you decide how much of your air leakage to assign to the basement space? Usually, air leakage is about 10 - 25% of total heat loss in a building BUT air leakage to sold concrete foundation walls to the exterior is essentially zero.

    There are others on GBA Q&A who do heat load calcs in their sleep and can provide more feedback and my response will elevate this within GBA to "suck" them in...


    1. airfix | | #2


      Good point. I looked at an 1.5ACH50 for the whole house then looked at the volume in the basement and did 1.5 x the basement volume. Then converted that to ACH natural. Essentially I assumed an air loss in the basement proportional to the volume of the basement compared to the volume of the whole house.

      As you state air leakage through the concrete will be small. The air leakage is likely any air leaking past my air control layers (mudded and taped sheet rock and taped polyiso) through poorly sealed wall penetrations and joints. Also the sill plate will be another likely spot but perhaps small because the stack effect delta P at the ceiling of the basement is close to zero as the stack transitions from a negative delta P to a positive delta p.

      My air loss number will be a conservative number. I'm interested in any better approaches the other experts might suggest.


  2. Expert Member
    Dana Dorsett | | #3

    >" Am I in the ball park or do I need to go back to the drawing board?"

    It's definitely far enough inside the ball park to specify a heat source and tubing spacing on the radiant floor. The "right" boiler either modulates to something less than half your ball-park 17KBTU/hr load numbers (such as a 10:1 turn down 80KBTU/hr-in or smaller mod-con), or has a self-buffering thermal mass and a burner that can deliver at least 25K (= any tank type condensing water heater). A 7.5kw electric boiler would do it too, without being too oversized even if your infiltration load calculations prove to be too conservative.

    If going all-electric at reasonable efficiency is something you're interested in. The 17K load is just outside the capacity range of the Sanden CO2 refrigratant heat pump water heater, but might just squeak into the 0F capacity of a Chiltrix CX34 chiller if the water temp requirements can be kept under 90F. There are bigger reversible chillers that would definitely have the capacity.

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