HI Guys!
Question to ask regarding the ratings of heating and air conditioning. I have a turn of the century Four-Square in Chicago and an having a new zone installed on the second floor for HVAC. My contractor has recommended the following:
Comfortmaker 90% Efficiency furnace (50,000 btu input) and
Comfortmaker 2 ton air conditioning unit
The total square foot in this zone is 1200 square feet.
Does this sound correct? Any advice would be greatly appreciated!
Thanks. Rick
Replies
37.5 btuh/ft^2 for an old structure in Chicagoland is a little on the light side for heating, unless the buidling has been updated with better insulation and windows. 2 tons for 1200sf should be a good match.
The only way to know is to do a heat loss/heat gain calculation. It's a bit of work (maybe 2hrs) but it will take far more into account than rules of thumb. Our house is insulated quite well, and a -15*F design day requires about 15BTU/sq ft. to keep the place at 70 degress.
Due to the insulation, the summertime max heat gain is about 12BTU/sq ft. (mostly due to windows), so our tonnage requirements are on the order of 1ton/1,000 sq. ft. However, if you don't do a calculation, you won't know what factors are relevant and which ones aren't.
All I can say is: The era of cheap energy is likely coming to an end. Insulate while you can, then reap the benefits of lower upkeep costs and smaller systems to keep the place comfortable.
Thanks for your reply. How do I perform a heat loss/gain calculation? Can you point me in the right direction?
I would consider a Manual-J compliant Heat Gain/Heat Loss calculator like Hvac-Calc. Many contractors use it and if you only intend to need it for 2 months, the $50 2-month license is a pretty good investment. I even bought the full HO version ($300, IIRC) because I needed the program for more than 2 months. You can also read a homeowner review of HVAC-Calc on my web-site.
However, any quality HVAC installer will do a Manual-J-compliant calculation as a matter of course. It's a great way to see what installers cross their t's and dot their i's vs. the ones that do not.
Constantin,
I am curious about your location and some details on the construction of your obviously well insulated home. 15btuh/sf at 85 deg DT is a very effective construction. I run about 20 btuh/ft and 1200 sf/ton (with heavy shade). I do have a great deal of glass.
Tim
Hi Timbo, our house is in the Boston area (long winters at 5650 DD heating, 777 DD cooling). The old exterior walls have 3.5" of Corbond in them, the new walls have 5.5" of Icynene. R13 between floors, R10 under and outside the basement walls. The roof is sealed with 6" of Corbond in one section, 10" of Icynene in the other. HRVs are a must. The numbers I gave you are theoretical, as we have not yet had a real heating/cooling season yet. For cooling purposes, I did not account for shading by trees. What did strike me though is that even though the house RFH is set at 60 degrees during construction, 40 degree sunny weather on the outside still results in a home that is 65degrees+ warm on the inside.
Sounds like you should get close to hitting those theoretical numbers. I have 2-2x4 stud wall, each with r-13 fiberglass batts and offset plates and studs. Ceilings have r-25 between the trusses and r-25 "rolls" at 90 degrees to the trusses. Cathedral ceilings have 2" EPS 1" below the roof decking, and r-19 batts below that.
Performance is not quite up to calcuated values, but I'm not finished sealing and triming. I expect to heat 5000 ft with a single 125 MBH, 94% furnace. That might be stretching it a little. I have a 4 ton AC that runs about 16 hours a day on a 95 degree DD, other than that, about 500 hours total during the summer.