Help - confused by estimates that specify different size equipment

[adisb]

I just got a few estimates for my 792sq ft ranch on a crawl space and some say they would install a 70K BTU furnace and a 2 ton AC, others say they would install a 50K BTU and a 1.5 ton AC. One of them said that they will cut 4 10x6 heat supplies in the ductwork in the crawl space to condition the space for the winter.

The house is built in 1954 but I will have all new windows installed so, heat loss through the windows will be minimal.

Based on my readings on-line, a 45k BTU furnace and 1.5 ton AC should be sufficient??? Why oversize to 70K BTU and have the systems cycle on and off a lot??

Any thoughts would be appreciated. Thank you in advance.

[pstu]

You are right, too big on either AC or heating and the unit will cycle on and off a lot. That will be less efficient with energy and you may not be comfortable.

Your size question is difficult to answer. Although you have given much information, it still is not enough. One choice which I (a homeowner) have taken is to license the software HVAC-CALC from this site for a reasonable $XX, and tell it the dimensions of all your walls, windows, etc. This is called a "Manual J" or "Heat Gain/Loss Calculation" and is the most professional way to deliver an answer to your question.

That said, the Manual J is something of an ideal, which is neglected and sometimes disputed by technicians working in the field. We often see techs oversize, presumably so it is difficult for a homeowner to cry "undersized" when it does not heat or cool well (which may happen for reasons other than sizing). The best common arrangement is an HVAC company will agree to do such a calculation upon being promised the business. If done right it will take them some time. Do not expect them to do the calc for free, the pitfall is a less scrupulous homeowner may often use that number to get a deal with a lower bid, leaving the original company a little bit cheated.

In the case of AC, having too much capacity will make your house prone to high humidity... if you live in an area where that is common. However for both heating and cooling, you may be down against the smallest models available from some brands. In my own experience in Texas, I had to buy an oversized furnace for my needs simply because the Trane company did not make a furnace with small enough heating yet big enough airflow for AC needs. You might in an analogous situation. Just a few thoughts from a homeowner to think over until a real HVAC pro addresses your question.

Best of luck -- Pstu

[adisb]

You are right, too big on either AC or heating and the unit will cycle on and off a lot. That will be less efficient with energy and you may not be comfortable.

Your size question is difficult to answer. Although you have given much information, it still is not enough. One choice which I (a homeowner) have taken is to license the software HVAC-CALC from this site for a reasonable $XX, and tell it the dimensions of all your walls, windows, etc. This is called a "Manual J" or "Heat Gain/Loss Calculation" and is the most professional way to deliver an answer to your question.

That said, the Manual J is something of an ideal, which is neglected and sometimes disputed by technicians working in the field. We often see techs oversize, presumably so it is difficult for a homeowner to cry "undersized" when it does not heat or cool well (which may happen for reasons other than sizing). The best common arrangement is an HVAC company will agree to do such a calculation upon being promised the business. If done right it will take them some time. Do not expect them to do the calc for free, the pitfall is a less scrupulous homeowner may often use that number to get a deal with a lower bid, leaving the original company a little bit cheated.

In the case of AC, having too much capacity will make your house prone to high humidity... if you live in an area where that is common. However for both heating and cooling, you may be down against the smallest models available from some brands. In my own experience in Texas, I had to buy an oversized furnace for my needs simply because the Trane company did not make a furnace with small enough heating yet big enough airflow for AC needs. You might in an analogous situation. Just a few thoughts from a homeowner to think over until a real HVAC pro addresses your question.

Best of luck -- Pstu


thanks for your comments and suggestions !

[BaldLoonie]

I'm not sure I'd want to put heat in the crawl. That pressurizes the crawl which isn't that tight, then it causes extra infiltration in the living areas.