Results 1 to 8 of 8
  1. #1
    Join Date
    Jul 2009
    Posts
    5

    Oversized furnace a BAD thing?

    Hi there folks,

    My contractor wanted to put in a Carrier base model 45k btu furnace. I asked him to upgrade to a variable speed and he told me that in order to get variable speed in a Carrier, I would need to have a 70k btu furnace because that is all Carrier makes. My contractor seemed to think going up in size wasn't a big deal, but it seems to me that it is. I'm concerned about cfm, amount of fuel used, and long-term system wear.

    Are a 70kbtu furnace and 2ton heat pump compatible?

    Am I better off with the single speed properly sized unit, or the over-sized variable speed?

    Some important info:
    I live in N. Florida and planning on adding a heatpump to this system in the future and using the gas as backup. My house is ~800 sq. ft. The future heat pump will be a 2ton unit.

    Thanks a lot everybody for giving me your opinion.

  2. #2
    Join Date
    Dec 2003
    Posts
    709
    That furnace with variable speed I think is also a 2 stage meaning in low fire it is close to a 45k btu anyway. This would be fine especially with a heat pump because it will be really cold when the furnace comes on anyway.

    I would definitely choose the variable speed and leave the fan running all the time. With the dual fuel, I like to use the Honeywell IAQ stat, and I set the heat pump lockout really low like 15* and this stat has a timer that will kick in the gas furnace if the heat pump runs continous for 60 min or the temperature drops by 2* or more.
    I think this will run the heat pump as much as possible and only use the gas in the coldest temps.

  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,245
    That's my problem. On high it sounds like a jet taking off in the house. I have a manual toggle that keeps high off except in the most bitter weather. So really I have an expensive single stage furnace but that was the only way to get variable speed.

  4. #4
    Join Date
    Jul 2009
    Posts
    5
    many thanks!

    that's what I'm wondering about. In my case, I will definitely be running this thing on the first stage. I can't imagine ever needing it to run it full blast. If I do have it setup like this, will the heat pump be able to utilize the full range of the variable speed, especially for dehumidification purposes.

    I'm afraid I get this blower that is unbearably loud and forceful at all but the slowest speed, forcing me to run it slow all the time and hence negating the benefit of a variable speed blower. I guess what I'm wondering is if the variable speed blower in this BIG unit can get SLOW enough to properly and quietly run a 2stage 2ton cooling/dehumidifying system.

    just semantics: my assumption is that a small unit's variable speed goes from 1-6, and a large unit goes from 4-10. If the lowest speed on the bigger unit is 4, then I'm using a lot more electricity, right? Does this assumption hold any water?

  5. #5
    Join Date
    Dec 2003
    Posts
    709
    No, you will get the full effects of the variable speed with this furnace. It can range somewhere from around 400 cfm up to around 1600 cfm, I would definitely reccomend the honeywell IAQ as well if you are doing dehum, it will run your blower at a very low fan speed and your a/c at a high speed ringing out the air in the home. I would also consider adding a good air cleaner and uv light and running the fan all the time to get even more value from the variable speed.

    The important part is that your installer puts in the right controls and sets it all up right, then you will be happy with this set up.

    Also, you are getting the small blower, and its minimum cfm is actually lower then the standard blower in the 45k furnace.

  6. #6
    Join Date
    Jul 2009
    Posts
    5
    Hivacer,

    you da man! thanks for your knowledge. I now feel much better with what is going to be installed. I'll tell my contractor about that stat and ask him about their recommended air cleaner.
    thanks again,
    daniel

  7. #7
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    The only concern I would have .. is as mentioned... is ductwork size. A 70k BTU furnace will need close to 1000CFM. It could be noisy if you have ductwork only sized for 800CFM for the 2 ton A/C. But, it wouldn't go to high very often, or very long in Florida. But it is something to consider.

    Just a thought. Might not really be an issue.... just dependant on the ductwork. As Loonie said, you could just disable 2nd stage. You might also be able ot spend a little money and increase the plenum sizes and maybe add 1 vent downstream somewhere.

  8. #8
    Join Date
    Dec 2007
    Location
    Cedar Grove, Wi-Sheboygan
    Posts
    1,582
    If your getting a Carrier equipment they do have a 60KBTU furnace 96% EFI. Why hasn't he offer that to you instead of the 70K BTU ?

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event