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View Full Version : Advice on New Furnace Size??? too big?



Sunsetblvd79
10-12-2010, 02:35 PM
hello, i was just wondering if any one had any input for me.
my uncle owns a lot of houses, he buys/sells/rents...he sold one before he put in a brand new heater he had, so he sold the new heater to be for 500.00. its a Heil 90k BTU.
the problem is this:
my house is 1500 sq feet, plus a finished basement, which i dont care how cold/hot the basement is. we currently have a 70k BTU furnace.
today the heating guy came for an estimate to install the furnace with an estimate of 849.00. he said when he saw the heater that it was too big for the house, which after reading online, i see that it is correct information...but im wondering if it will still be okay and i should go ahead with the install?
i dont want huge eletric bills, or for it to catch fire or break all the time from cycling too much. i was told by another heating person last year, that the old furnace wasnt big enough, but looking online and after today, i think he might of been wrong?
any advice/suggestions/comments would be greatly appricated, so i know if to cancel the appointment and look at a correct size furnace. (sorry so long winded)

Thatguy
10-12-2010, 03:23 PM
It won't run as efficiently as a properly sized unit.
My oversized 83% efficient furnace was only on 1/4th of the time in a typical winter, based on my gas bills and the input BTU/hr. We used maybe 300 therms in January and 10% of that was for the water heater.
Getting furnace efficiency data so you can do financial tradeoffs can be pretty difficult. It also depends on how much you pay per therm for gas or electricity. 1 therm = 100,000 BTU = 29.3 kwh.

Wally Hays
10-12-2010, 04:33 PM
The furnace must be sized to the heat loss and to the duct work.

Dana
10-13-2010, 08:43 AM
hello, i was just wondering if any one had any input for me.
my uncle owns a lot of houses, he buys/sells/rents...he sold one before he put in a brand new heater he had, so he sold the new heater to be for 500.00. its a Heil 90k BTU.
the problem is this:
my house is 1500 sq feet, plus a finished basement, which i dont care how cold/hot the basement is. we currently have a 70k BTU furnace.
today the heating guy came for an estimate to install the furnace with an estimate of 849.00. he said when he saw the heater that it was too big for the house, which after reading online, i see that it is correct information...but im wondering if it will still be okay and i should go ahead with the install?
i dont want huge eletric bills, or for it to catch fire or break all the time from cycling too much. i was told by another heating person last year, that the old furnace wasnt big enough, but looking online and after today, i think he might of been wrong?
any advice/suggestions/comments would be greatly appricated, so i know if to cancel the appointment and look at a correct size furnace. (sorry so long winded)

What's your zip code, and how much fuel did you use from September through June? It's pretty easy to work backwards from the furnace nameplate numbers and heating degree-day weather data to find out how close your furnace is to being "right sized" for the design-day heat load. Hot air furnaces can be up to 2x oversized without losing too much on efficiency, but it'll cost more in maintenance due to cycling issues. Given the size of your house, it's possible/likely that at 70K 78% AFUE unit may be 1.5-2x oversized for a modestly insulated but fairly tight home in northern IL, but a 90K 90% AFUE unit is likely way over 2x. (It would be about 3x oversized for my not superinsulated not super tight antique central MA home of comparable but slightly bigger size.) If the Heil is a 2-stage &/or condensing version, it might be reasonable (even if it had to be hard-wired for low-stage only) but I suspect your HVAC guy wouldn't have made the comment about the oversizing were that the case. Got a model number?

As Wally correctly points out, duct sizing may be an issue if the air hander on the newer-bigger system is appreciably bigger than the unit currently installed, and could add considerably to the loading of the blower.

Is there a reason you're swapping out the old one, other than your uncle snookering you into this "great deal"?

gator37
10-13-2010, 06:50 PM
My experience has been in reference to gas furnaces is that when you select an off the shelf cooling coil you have limited selections on the heating furnaces and the heating furnace you would like to select based on BTUH will not produce enough static pressure to deliver the CFM required for the cooling coil, therefore the heating ends up oversized. This is probably not as much of a problem in your area as it would be in the southern states. If you don't have a cooling coil to worry about you should not have a problem getting the correct size furnace. The question is what is the heating requirement by calculation?

Thatguy
10-14-2010, 07:22 AM
The question is what is the heating requirement by calculation?
IIRC it should come out to 6 to 10 BTU per sq. ft. per HDD for the average house and up to 25 for a leaky house.
It was 300 therms for January for my 2000 SF 1964 house for the DC area.
http://www.degreedays.net/

For 970 HDD I get 300 x 100,000/(2000 x 970) = 15 for this leaky mammyjammer of a house heated by a reliable but inefficient gas furnace from 1982.
If my actual furnace efficiency is 50% at 1/4th duty cycle then my house comes in at about 8.

Dana
10-15-2010, 03:22 PM
IIRC it should come out to 6 to 10 BTU per sq. ft. per HDD for the average house and up to 25 for a leaky house.
It was 300 therms for January for my 2000 SF 1964 house for the DC area.
http://www.degreedays.net/

For 970 HDD I get 300 x 100,000/(2000 x 970) = 15 for this leaky mammyjammer of a house heated by a reliable but inefficient gas furnace from 1982.
If my actual furnace efficiency is 50% at 1/4th duty cycle then my house comes in at about 8.

The curve isn't THAT steep. At a 25% duty cycle you're still over 0.8x (maybe over 0.85x) the AFUE rating, which was probably 75-78% back in '82.

A professional air-sealing job is probably cost-effective for you though.

Also, you're measuring source-fuel BTUs with your equation, not the heat load. To measure the heat load you'd have to factor in the efficiency of the equipment as-used. If it's a 75% AFUE unit runng a sub-25% duty cycle at design conditions, figure on 0.85 x 75% = 64% efficiency. That makes your 15btu/sf heat load number is actually closer to:

0.64x15= 9.6
It also means that if you went with a reasonalbly-sized condensing furnace you would have only burned 200therms.

Thatguy
10-16-2010, 07:08 AM
I also forgot to add the heat generated by the elec. input to the house, almost all of which is converted to heat, and the sunlight power of 1 kw per sq. meter onto your house's apparent cross-sectional area, assuming some absorption factor.

I got a Manual J through an inter-library loan service once, but the calc's are incredibly tedious. The online service is $49.

Dana
10-18-2010, 09:19 AM
I was also being a bit sloppy- that's not actually your instantanenous heat load either, it's the btus per square foot per heating degree-day, not a btu/hour per square foot number. Also the square footage is really irrelant, unless you are calculating the output for radiant floor or ceiling heat, so let's stick to the whole house heat load.

For 300 therms in 970 HDD with a ~65% efficient system you're looking at 0.65 x 300 x 100,000/970 = ~20100 BTU/HDD

Divide that by 24 hrs/day to get 20100/24= 838 BTU/degree-HOUR

To calculate your load at design condtion as BTU/hr, consider that MD your outside design temps are something like 12F, so at base 65F you're looking at 65-12F=53F degrees. Multiply that by your BTU per degree hour number and you get:

838 x 53=~44.4 KBTU/hr

as the first approximation of your design day heat requirements from your heating system.

A typical home is disspating an average of 1-1.5KW , or ~3-5KBTU/hr in electric power, so add that in if you want to get closer to your true absolute heat load. (Look at your electric bills if you want to get closer.)

The solar factors in most homes are even less than that, unless you have a LOT of south facing glass to consider. The solar gain on to the exterior roof & siding doesn't accrue to your conditioned space linearly- it's outside the thermal boundary of the house. It may lower your heat load slightly for those few hours out of 24, but it's less than a 10% factor.

If you upsize the 44.4K number by about 15%, to ~50KBTU/hr, that's more than sufficient to deal with sustained no-sun periods at design temp. A 60K condensing furnace would deliver 55K or more, and would be an appropriate size to be looking at. A 60K, 80% furnace would also work, but could be marginal during 100-year record cold. Any 100K or larger furnace would be oversized beyond AFUE test assumptions.

If you want to convert that BTU/square foot at design conditions to see if you could deliver it all with radiant-ceiling/floor, figure on 50000/2000=25BTU/square foot, which would be doable as a radiant floor if you had that much clear floor area (no cabinets furniture or rugs).

Note, if your heating system is actually more efficient than 65%, your design condition heat load is actually a bit higher than 50K, which would indicate yours is a fairly leaky house, or that your foundation isn't insulated or something. My antique of a house has ~10% more floor area, and needs about 30K out of the heating system at a design temp of ~0-5F (10% deeper design conditions). Before I started tightening it up several years ago and insulated the basement walls & rim joist with rigid foam it needed about 40-45K. Air sealing was the single largest improvement factor (and there's room to tigthen it up more.) In a ducted air situation, your duct leakage & design could also be a significant system-efficiency problem.