Conflicting heat load calculations -- who is correct?

[dbb1]

A coworker of mine is looking to replace his old furnace, and since I'm going through the same process I've been trying to educate him on all the stuff I've learned from this site. In particular, I got him to convince two of his four bidders to perform heat load calculations. He got two very different results.

After talking to the contractors to try to pin it down, it looks like they treated his basement very differently. His basement is unfinished (and will stay that way), and has no supplies or returns. The furnace is in the basement, and the ducts run along the ceiling, then go up into the first floor. There is a door at the top of the basement stairs, which is generally closed but often open, and not particularly tight.

Contractor A treated the basement as part of the conditioned space. He computed heat loss through the walls, floor, and windows of the basement, and factored those into the total heat load to choose a furnace size.

Contractor B treated the basement as an unheated space. He computed heat loss between the first floor and the basement (the joists are not insulated) but didn't count the basement walls, floors, windows, etc.

The difference came out to roughly 12000 BTU, which sounds nontrivial to me. In this particular case, the higher result pushes him over the line for a 80k BTU furnace and into the 100k range.

Which method is correct? Is there a definitive answer?

[browntigerus]

Contractor A guessed that he would want to finish basement one day, and another took it for what it is now.

Who is right?

[dbb1]

Contractor A guessed that he would want to finish basement one day, and another took it for what it is now.

Who is right?

Fair enough, but....

For the sake of argument, let's say he really doesn't think he's going to finish the basement, so he goes for the smaller-capacity furnace. Remember that this furnace is sized more-or-less exactly right for his house, assuming the basement is not a heated space.

Will the furnace have trouble heating his house? Does he need to do something to seal the first floor off from the basement, such as by creating a better seal on the basement door?

In other words, even if he isn't going to finish the basement, is it really *correct* to treat the basement as an unheated space, given the significant amount of air exchange between the basement and first floor?

[dcmcm5]

I would think from a comfort and efficiency standpoint making sure the unheated basement is reasonably sealed off from the rest of the house makes sense. As for sizing, I believe Manual S limits excess heating capacity to 40% for a furnace. Given that, the 100K furnace would not be considered outside the design conditions.

[dbb1]

I would think from a comfort and efficiency standpoint making sure the unheated basement is reasonably sealed off from the rest of the house makes sense. As for sizing, I believe Manual S limits excess heating capacity to 40% for a furnace. Given that, the 100K furnace would not be considered outside the design conditions.

Understood. Given what you said, plus the possibility that he might change his mind about finishing the basement *someday* (even if he says not), the 100k makes more sense.

Thanks to both of you for your advice, I appreciate it.

[dan sw fl]

A coworker of mine is looking to replace his old furnace, and since I'm going through the same process ... In particular, I got him to convince two of his four bidders to perform heat load calculations.
He got two very different results.

...

The difference came out to roughly 12000 BTU, which sounds nontrivial to me. In this particular case, the higher result pushes him over the line for a 80k BTU furnace and into the 100k range.

Which method is correct? Is there a definitive answer?

12,000 out of 64,000 is not a Great (+/-10%) difference.
The Definitive Answer is the Average.

58,000 from one and 70,000 from the other
= a DEFINITIVE 64,000 BTUh ! !! !!!

I think that's WHY
they call it a $64,000 question !!

[Elephanthead]

$64,000 install! Some HVAC salesman is dreaming again.