To Believe, or Not

February 17, 2011

So, the other day a guy called me up and asked for help sizing a heat pump for an addition on a house. I asked him if he had done a load calculation to which he replied “Yes, but I’m not sure about it”. “You’re not sure about it? Why, did it tell you that you looked like a goat?” I asked, looking for clarification. “No, no, I mean that I’m not sure about the size that it came up with. It said that I should put in a 5-ton unit.” Passing right over my prepared defense of the goat resemblance, I asked what was wrong with the 5-ton prescription. “Well you see, the room is 19 feet by 19 feet, and it just seemed like a lot.”

OK, now you’ve got my attention there, Billy. “You have a room that is 361 square feet that is calling for five tons? Are you sure that you didn’t enter ‘19x91’?” “Nope, I checked that.” “Alright then, is it possible that you’re not giving me all of the details here? Are there a lot of windows in this room, is this a Laundromat, an iron smelter, what already? Come on, you know what I’m going to ask!” I was beginning to lose my patience with goat-boy. “Well, oh yeah, I forgot. It’s a three sided addition facing south, windows from floor to ceiling on all sides, with a 12-foot height. Oh, and the roof is all glass.” “Uh, huh; ……details.”

In contrast to the extreme case described above, picture a typical 1,600-square foot, three-bedroom ranch, in Anytown, USA. Load calculations are done on these cookie cutter homes every day resulting in a heating load of 20,000 to 50,000 Btus. Why then, do most of these dwellings have 100,000 Btu furnaces/boilers installed? Do we believe in load calculation technology, or not?

More and more we are required to present load calculations to inspectors, or just to take out a permit. So, doesn’t it make sense to learn how to do it properly and then, possibly, actually, even use the results we come up with to size the system? Believe it or not, it even works better than the “stand at the curb, and count the fingers” method.