MEP engineer drew some plans, and the HVAC sub is raising questions. 3,800 sf space in a mid-rise condo. Windows onn the east and north only. Occupied apartments above and below. 11 ft ceilings so we’re talking about 42k cu ft of space.
What would be a rule of thumb answer for the size of the unit? Engr spec’d 60k btu.
“When asked if you can do something, tell’em “Why certainly I can”, then get busy and find a way to do it.” T. Roosevelt
Replies
http://www.hvaccomputer.com/hvac/sizer.asp
Rules of thumb can be a bad thing. Do a load calc.
dans ... the engr did a load calc, and that's how he sized the unit. The hvac sub says it's too small and wants either a larger unit or a second small one.
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
Did the engineer specify latent and sensible cooling loads? I suppose you know that you will not get 60k BTU of sensible cooling from a 60k BTU unit....
Any ways, if that what is speced, and the engineer has the load calcs to prove it, then that is what you should get.
You should have the sub talk to the engineer if he got questions.
Reminds me of similar pickle and can be like trying to get red and blue to agree on something, but those are the guys who should be talking.
"Let's get crack-a-lackin" --- Adam Carolla
The rule is: don't use your thumb.
Having no information on construction, climate, siting, occupancy, internal loads, etc., second guessing would be illadvised. However, with no roof load and no south or west glass loads, the selection isn't unreasonable.
The proper way to do it is to use the "Manual J" method, which is basically a load calculation. That is probably what that software that Bill gave the link to does - once you get past the joke part :-). In a load calc you enter in a whole bunch of stuff, like for starters, stuff you mentioned plus a lot more. One of the parameters you enter is the specific geographic location too (zip code maybe?). The Engineer has already done it now tell your HVAC contractor to do it. If he cannot do a load calc, he may not be the right sub the job anyway.
Also, I may be wrong on the following, but the 60k btu is the heating part - right? - what is the cooling part, which might be more important in Texas.
I suspect that HVAC contractors who use rules of thumb methods tend to oversize units - this way they have a sure thing, with an "oh well" applied to the clients utility bills and also oversizing an AC unit can actually make it less effective.