Off the hip furnace sizing - Contractor Talk - Professional Construction and Remodeling Forum

IHI · 08-18-2005, 07:47 PM

I realize I'm supposed to have last winters heating bills for the math equation to nail this right on the head, but the furnace I have installed when we bought this place was bad-limped us through the winter, and with rising fuel prices I'm swapping it out in the next few weeks.

Swap is being done by local HVAC buddy, but I just want to get a little more feedback to be sure furnace will be adequte-I'm not second guessing him, just reassuring myself...you know how that goes.

located in Iowa so we get all kinds of weather from over 100* to -30 to -40 at times. This house is 1200sqft single level ranch with 3 bedrooms, unfinished thus far basement. I installed new roof, windows, siding, and house is 2x6 fully insulated walls. What size would one recommend? Was also told that finishing the basement in the near future has little to do with furnace size...or does it and how much? Furnace will be high effeiecncy natural gas unit to replace 25yr old natural gas currently installed.

Any help is appreciated.

Josh

georgia dawg · 08-18-2005, 09:21 PM

HVAC guys will kill me, but a rough rule of thumb is 300 sq. ft. per half-ton. If you're between two sizes, I'd size down for less cycling. Variable speed and high efficiency helps, too. If you intend to finish your basement and run ducts from this unit, then include it in your calculations for this unit. Better yet, ask the HVAC guy to run the numbers for an exact fit. More than a half-ton in either direction of the rough estimate and I'd get a second opinion or a good explanation to be sure.

IHI · 08-18-2005, 10:21 PM

Quote:

Originally Posted by georgia dawg

HVAC guys will kill me...

I completely understand, we're all under scrutiny somewhat when voicing opions here or other home improvement forums, but obviously in all our "real worlds" questions such as this get answered and hip shot all the time.

Thanks for your time and information.

HVAC Doc · 08-18-2005, 10:40 PM

And 20 years ago in Kansas we sized 400 sq. ft to the ton. At least though do this one thing and keep in mind that with a high efficiency furnace (90%+) that if you had a 100,000 BTU furnace that was 20 yrs. or older, you most likely got no more than 78,000 output of heat. So if you put in a 90% efficient furnace and put a 100,000 BTU furnace, you would be putting out 90,000 btus output which means- grossly oversized and potential for very early heat exchanger failure due to short cycling not to mention the overall lack of complete comfort. But hey, "shoot from the hip" or do it right the first time, it's up to you.

IHI · 08-19-2005, 06:50 AM

I was told a 75,000 BTU would be sufficent, just seems kinda small since that's what an outside company installed in my old house (fix up and dump house) but that was only 621sq ft at 82% efficency rating.

sjohio · 09-27-2005, 04:50 PM

Just my two cents worth. Heat load caculation is highly recommended. Three ton for 1200 square feet seems kind of high.

Bestheating · 01-03-2006, 08:15 PM

This is kind of an old post, but I am going to answer this anyway. Your estimated max heat load without seeing the house would be 48,000 btu. In Michigan where I am, I would sell you a 60,000 btu fruance and a 2 ton air conditioner, with a 2 1/2 ton a-coil for better dehumidification. My customers love that setup.