Hello,
We were given a copy of a Manual J from our builder. The numbers seem very low as far as equipment size. We have 1525 sq ft upstairs and 1425 in our walkout basement. They said we needed 23,250 btu required and 28,400 btu recommended total. We also had 42,000 recomended on heat. They installed a 2.5 ton Heat pump. I realize they are sized for cooling, but expecting us to make up the 12,ooo btu needed from electric heat is to costly.
Another concern is 1025 total cfm required for the entire house. Do these numbers sound reasonable or did Mr. builder go with the lowest HVAC estimate.
The proper method is size to the cooling needs and let the backup make up the heating. Around here, we sometimes have twice the heat needs than cooling. Many of our 2.5 ton pumps have 20kw backup to make up 30,000 BTU diff in bitter cold weather. Our all electric heat pump homes have cheaper heat than those with gas heat. If you oversize in cooling, you end up humid & clammy. That would make you miserable all summer long.
Without seeing the house and running my own figures, hard to say but I don't think he'd undersize you. He'd get nothing but grief. I'd trust him and not worry about it unless you can't cool in a heat wave.
Originally posted by help3 What does 30,100 btu@47f mean on the Manual J?
This is the output of the equipment he is quoting at 47 degrees outdoor temp. Manufacturers usually will list the output of the equipment at 47f and 17f to assist in selection of the correct equipment.
At 47 degrees my heat pump will produce only 75 % of the required heat. This is B.S., should they have installed a larger unit? Also, the unit runs to long on AC and we can't get the temp below 78 degrees on 90 degree days.
At 47 degrees you won't need 42,000 btu's, you'll only need that much at od design temp, weather thats 0, 5, or -5, degrees for your area I don't know.
I've always used a balance point sheet to size my heat pump equipment. In PA the heat loss is typically double the heat gain but by sizing the equipment between the heating and cooling loads I sacrifice some comfort in the few summer months to help with heating costs in the many winter months.
SRJ
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