100 k gas furnace vs. 80 k

[bartlebee]

I am having a gas furnace and central A/c installed next week, but I am having second thoughts on the size of the furnace. My house is an L shaped ranch (1970's) moderately insulated, 12 windows all double paned, low e, and 1850 sq. ft. of space. I currently have a 11 year old 80K XV90 (92.5 % eff.) gas furnace and a 21 year old A/c unit. The A?C unit is pretty much limping along and needs replace, but with the energy tax credit and the PA state energy rebate (almost as large, can't say amount, I know), and local utility rebate, it costs almost the same to replace gas furnace and the A/C unit, as it does just to install a new A/C unit. And I get another 10 years parts and labor on the furnace and a/c unit as part of the deal. Plus, I cannot get energy credit with my existing gas furnace as it does not have an AHRI rating when matched to an A/C unit (too old), even though it is variable speed.

Anyway, we would like to add heat to the basement. Our contractor recommend bumping up to the 100 K XV95 Trane furnace matched with a 3 ton A/C unit and an aspen coil. The 80 K trane unit will not get the AHRI rating with a 3 ton Trane XL15i A/C unit (need a larger coil). I was willing to forgo the full energy credit and just take a partial for the heater only, but the contractor feels that the XV95 80K unit will not have enough btu's with the additional airflow needed to heat the basement.

The contractor stated that the unit will probably never use the upper stage (95,000 BTU), but he felt that the lower stage (61,500 BTU) would not short cycle for the heating needs of our house. He also said that the 100K unit will be able to throw more air volume as it has a 3/4 horse motor vs. 1/2 HP for the 80 K. I should explain that our house is 70 foot long and the heater is all the way to one side of the house, so the ducting runs have been an issue.

I am a little nervous about going above an 80K unit, because it adequately heats our house now (with out the basement). The basement is approximately 1680 sq. ft. , but I only intend to heat approximately 1400 sq. ft. ( I will box off 480 sq. ft.).

I always hear go with the lower BTU furnace, but does it matter as much with a variable speed furnace since the lower stage BTU rating is below my load needs. Any suggestions?

[garya505]

Does your contractor have a heat loss calculation to support his recommendation? If not, I suggest you get a different contractor.

[bartlebee]

I had 5 contractor out to the house. None of them did a load calculation that I know of.

3 thought 80 K system was best (without heating basement).

1 suggested 100K system because of energy match for combined sytem (without heating the basement).

The last contractor was going to use 80 k system, until I mentioned heating the basement. Then he suggeted 100k

I will ask for a heat loss calculation.

[garya505]

I had 5 contractor out to the house. None of them did a load calculation that I know of.

3 thought 80 K system was best (without heating basement).

1 suggested 100K system because of energy match for combined sytem (without heating the basement).

The last contractor was going to use 80 k system, until I mentioned heating the basement. Then he suggeted 100k

I will ask for a heat loss calculation.

It doesn't take much to heat a basement, but I suspect you will not have good comfort in both the main floor and the basement unless you zone it or put in 2 separate systems.

[beshvac]

You don't have the right contractor yet......

[BaldLoonie]

Why buy a 2 stage furnace that won't need to go into high? That's paying a lot and losing the great benefit of the furnace.

In our area, we have homes well over 2000 sq ft 2 story on a basement heating with 75-80,000 BTU 95s. Sis has 2200 sq ft (no basement) that does fine with 75K down to about -10 out, then it falls behind.

DON'T OVERSIZE!

[cehs]

I live in a 2 story 1860 sq ft house in Iowa, built 1969 and I have a 2 stage variable speed 70K BTU nat gas furnace. I heat the basement, and I have 14 windows in the house and a french door in the family room.
I did a load calc on my house and found I only need 64K of heat, with the basement.
I would stay with the 80K.....
Have your contractors do a load calc, or keep looking...

[bartlebee]

Thanks very much for your suggestions and encouragement. I will call my person today to discuss. I'll let you know how it goes.

I don't think I am going to zone the basement because of the cost of this option, although zoning the basement would make it more comfortable. I would prefer to add auxilliary heat with a wood or gas stove in the basement. I already have a hearth in the basement, but due to the condition of the chimney, I will have to run insulated or double walled stainless steel flue pipe in the chimney to meet code.

[tigerdunes]

bartlebee

I wouldn't go more than 80K without some hard documentation in writing to support this. zoning controls would be nice but at the very least you need damper control on supply duct to help balance main and basement zones. you will get the tax credit on the furnace so I would think twice about the third party coil. I would install a good 4-5" box filter cabinet and of course a good two stg thermostat.

most homeowners do not realize that heating/cooling loads for basements at or partially below grade have small BTU requirements. Have any of the dealers provided a Man J?

IMO
Good Luck!

[tigerdunes]

bartlebee

one other thought. have any of the dealers recommending the 100K mdl even mentioned the possibility and need of ductwork modifications?

humm...

tred carefully before going up in size.

IMO

[garya505]

Thanks very much for your suggestions and encouragement. I will call my person today to discuss. I'll let you know how it goes.

I don't think I am going to zone the basement because of the cost of this option, although zoning the basement would make it more comfortable. I would prefer to add auxilliary heat with a wood or gas stove in the basement. I already have a hearth in the basement, but due to the condition of the chimney, I will have to run insulated or double walled stainless steel flue pipe in the chimney to meet code.

That's what I would do. Get the main floor set up right, and use a smaller unit for the basement. Basements don't take much to heat.

[bartlebee]

I spoke with my contractor. They were adding 20,000 btu's for the basement. He told me that they did a heat loss analysis, but he didn't provide the calcs (so I doubt it). After some discussion, we decided to go with with the Trane XV95 80 K gas furnace, because 100K is overkill upstairs, and I don't to heat the basement all the time.

The only downside is that the A/c coil will be 17.5 inch instead of 23 inch, the enclosure is smaller on the 80 K unit. I don't think this will be an issue, however.

Thanks to all for your insightful comments. This website rocks!