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Community and Q&A

Help with Heat Load Calc

Sofiane | Posted in Mechanicals on


I need the group’s insight on a heat load calc for my home in Gatineau, Quebec, zone 6. 

I will be getting quotes for a new HVAC system, specifically a slim ducted heat pump system and want to have my equipment properly sized.

My concern is that I find my official load calc a bit high at 47 143 btu. (9927 basement + 19 109 1st floor + 18 107 2nd floor)

Using the 15 min heat load calc with out current gas furnace from last winter,  I get a heat load of 33 000 btu.

In what could explain the higher value for the 47 143 btu numer:

Our basement is usually around 18-19 degrees C in the winter and our sunroom needs additionnal electricstrip heating due to a faulty hvac install which only partly covers the needs of that room. The sun room has a sloped roof with what should be r30 in the attic currently. We also usually keep the house close to 20 degrees C (68F).

On the other end, the 47  143 btu value is higher despite accouting for increased insulation in the calculation:
the basement (from r12 in the upper half of the walls to R19 EPS on full height)
the first floor living room + dining room (from r20 fiberglass to r22rockwool + inside r5)
and attic (from r30 to r60)

The initial ACH 50 was 3.12. I unfortunately am still waiting for a new blower test door.

My impression was that I would get something close to 34000 btu in the worst case scenario or an even smaller number. I find the 9927 btu value for the basement a bit surprising as well, given what I have seen in other Q&A questions. 

Does the 47 143 btu heat load seem reasonable to you or is something amiss?

Thank you for your help,


P.S. I have more details on the layout and the windows in the attached file. The title is in French, but I translated all the required info to English. The house facade is facing West.

Edit: added Heat Load Calc documentation

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  1. Jon_R | | #1

    Do carefully review the inputs to load calcs - errors are common. Especially related to infiltration and ventilation.

    But note that you shouldn't expect a Manual J load to match a fuel usage derived design day load. Manual J is adding some margin for weather colder than design day and higher than average winds. It varies, but add 20-25% to a fuel derived value to get a "always adequate" number. Or be prepared to occasionally use some supplemental heat.

  2. Expert Member
    Dana Dorsett | | #2

    From a crude load per square foot rule of thumb perspective 47,143/ 2582 square feet= ~18 BTU/hr per square feet @ -7F/-22C outside, 68F/20C inside is a bit on the high side for newer 2x6/R20 type construction with <U0.30 windows. I would expect it to be under 15BTU/hr per square foot, especially since the basement square footage should have a MUCH lower ratio.

    The 33,000/2582' is a ratio iof 12.5 - 13 BTU/hr per square foot. That is approximately the ratio I would expect a Manual-J to deliver when the inputs are properly aggressive. ASHRAE would recommend installing a furnace with 1.4x the design load, so if the load is 33,000 BTU/hr the right-sized equipment would deliver ~46,000 BTU/hr of output (give or take). So if one sized the equipment to just barely cover the 47,143 BTU/hr estimated load it would still be pretty comfortable during average winter cold, and still keep you warm at temperatures colder than -22C outdoors.

    1. Sofiane | | #4

      Thanks Dana! It's always great to get your input.

      I noticed the plan shows the surface area of the first and second floor on the statistics area. The whole house is 4300 square feet includind basement. I wanted to make sure you saw that as well.

      Regarding right-sizing, I remember reading that 1.1 or max 1.25 for heatpumps with high modulating capacity like the Fujitsu ARU--RGLX/ARU--RLF or Carrier--40MBDQ--. Is this correct?

      How would you set up a ducted mini split system with my heat load? The units would both be placed in the basement.

  3. paul_wiedefeld | | #3

    Does your 33k Btu heat loss from fuel usage calculation include the electric heat strips?

    1. Sofiane | | #5

      No, I don't know how I could have isolated the electric strip consumption from the overall electricity consumption.

      Without the added insulation of the basement/attic, it would make perfect sense for the calculated heat loss to be higher. I don't know whether it holds true once the additionnal insulation is taken into account.

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