Help! - We have a natural gas Armstrong furnace with a single spee...

HOMEOWNER CRIES FOR HELP...

posted: September 8th, 2008 - 2:04pm

QUESTION:
We have a natural gas Armstrong furnace with a single speed blower rated at 91% efficiency combined with a Trane XE 900 heat pump rated at SEER 9. Both were installed in 1991, making them 17 years old. Neither has any known major problems, although the heat pump did require recharging about 8 years ago because of a leak in a line fitting. The heat pump is set to provide home heating when the temperature is 40 degrees and above. The furnace takes over when the temperature falls below 40 degrees. The heat pump also provides cooling in the summer. We are located in north-central Ohio. Our electricity cost is now 8 cents/kwh, but, based on our typical usage, will go to about 10 cents/kwh in September. I don't have the ccf price on natural gas at hand right now, but is "typical" for our part of Ohio. (No insult meant to the pros...this is what I have heard, but not sure of)...Most things I read suggest that a system of this age is expensive to run compared to today's more efficient models. Our rural electric cooperative suggests that a SEER 16 heat pump unit would save 60% on cooling costs compared to a SEER 10 model and that a new heat pump would also save considerably on heating costs at 40 degrees and above (no specific heating cost savings numbers were estimated).
An hvac contractor told me that even though my furnace was rated at 91% efficiency, the newer furnaces rated at 95% would actually save more than the obvious 4% because of their 2 stage design. The design supposedly saves fuel by heating more slowly, but more often, than having the full 80,000 BTU kick in at one time. (My words, not theirs, similar to saving gas going 0-60mph slowly instead of 'pedal to the metal' acceleration.) The full 80,000 BTU force kicks in only with extreme weather. The variable speed system also takes advantage of digital thermostats that may help save energy. I can replace the SEER 9 heat pump with a SEER 15 without replacing my furnace. At least with Trane, I am told that to get the SEER 19 heat pump I would have to also replace my furnace with the complimentary 2 stage unit. Many pages I have read seem to back this up; the higher SEER ratings depend on installing a complete, paired system. I am interested in the advice of pros. What is your opinion? Is this something I should pursue? I am not ready to run out and spend money this instant, but, with rising electricity and natural gas costs all but a certainty, I am looking down the road. Thanks for any input.

ANSWER: 2-stage furnaces are for comfort more than efficiency. A properly-sized two-stage furnace will spend a good portion of time in low fire. However, if you go with a 2-stage backup gas furnace, make certain that you use a thermostat, such as the Honeywell VisionPRO IAQ, that can stage the backup heat. Otherwise, it will do the staging based on a timer. It is difficult to achieve 15 SEER without a variable-speed blower in the furnace, so I would not go above 14 SEER for the heat pump. Hopefully someone else here knows more about the actual savings from 2-stage furnaces running in low stage. I understand what the company is saying, but I'm not sure how true that is when the long run times in low fire are considered. I have a heat pump in my house Set up for dual fuel operation just as yours is and in my opinion it is the only way to go.