Do I need a new furnace?

[josephc68]

We bought an old home two years ago, added an addition and since then have experienced some high gas bills to heat our home. :mad: When we put the addition on our house the HVAC contractor said that the furnace might be border line for the new square footage.

In colder weather (sub-zero) the furnace runs almost constantly and on extremely cold days is unable to actually bring the house up to temperature.:eek: I downloaded the HVAC-Calc software and with some meticulous input came up with a Total Heat Loss (BTUH) of 67,741. I currently have a four-year old Heil 75,000 BTU, 80% AFUE furnace.

The problem is that even if I accept that I have a slightly undersized furnace the two 2nd floor bedrooms furthest from the furnace don't warm up very well. They are easily 8 to 10 degrees colder than the rest of the house.

The home has been retrofitted with a warm air system. A trunk line was run from the basement all the way to the attic. From there flexible 6" duct was run to the various bedrooms through the ceilings. There are no air returns on the second floor with the exception of one near the new addition.

I received a quote for a Carrier 100,000 BTU, 96.6% AFUE variable speed furnace. It would have three automatic damper controls to service three zones. One of the zones would service the 2nd floor, including the problematic bedrooms. I also have access in the insulated attic to be able put in air returns in these bedrooms and tie them into the air return by the addition.

Would this larger furnace with the higher CFM airflow have any impact on these 2nd floor bedrooms in terms of actually heating them? Would putting in the air returns and connecting them to a single wall cavity air return have any affect or am I wasting my time on that one?

Any insight or help would be greatly appreciated. Thanks.

[amd]

Do everything possible to reduce heat loss before replacing the furnace. (insulation, draft proofing, duct damper balancing, etc.) Return air vents will help if the bedroom doors are closed for long periods of time.

Getting a larger unit might require duct changes and won’t do much to reduce your gas bills.

[josephc68]

Way ahead of you. We spent big bucks to foam the attic with high density foam. All the windows were replaced by the prior owners with modern replacement windows (double pane, etc) and storms. After we foamed the attic the therms/degree day usage dropped 25% and the house was much more comfortable. Pretty much draft proofed as much as one can with a 95 year old house.

Doors to the bedrooms are closed 10 hours at night when the kids are sleeping and I have closed all the dampers in an effort to force more air in with no real effect.

[amd]

A 75, 000 BTU 80% furnace can supply up to 60, 000BTU/Hr. If the load calculation is correct, it's undersized.

Closing dampers completely increases energy consumption and can damage the heat exchanger. Don't do it.

Since the furnace is fairly new, it might be advisable to install a supplemental heat source (electric baseboards with dedicated circuit or gas fireplace/wall heater) for cold days instead of replacing the unit and making changes to the ductwork. (higher airflow requirements) Wait for a second opinion.

[beenthere]

A 100,000 96% furnace is going to be over sized. Exspecially if you zone it.

Your duct system may not be sized for that large of a furnace, and could cause the furnace to run on its high limit.

Did you show the contractor your heat calc.

Get more estimates.

[climatecontrol]

That 100,000 BTU quote you previously got will be too large for your house, it will have shoter run times (satisfying the T-STAT too early) and not heat your house evenly.

And based on your heat load calculation you need an 80,000 BTU 90%+ furnace, this will give you the proper BTU output needed to heat your house without being oversized.

But then again, you can throw money into supplemental heat sources like fireplaces, baseboard heaters, and things of that nature, but why not get the proper furnace for your house.

Call your HVAC contractor, and have them come over and do a proper heat load calc, they may find the same number you did, they may not, better to be safe then sorry. All I am saying is make sure those numbers are correct before you do anything drastic.

Good Luck!!!