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  1. #1
    Join Date
    Mar 2004
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    171
    If you have a small space (800-1000 square feet) to heat and the manual J calculation calls for 25,000 BtU of heating capacity. If the smallest natural gas furnaces available is 45,000 BTU, and this is a replacement system, can you compensate for this by putting in a 2 stage furnace or a modulating furnace knowing that it will only fire on the lower cycle(s)? I understand heat pumps are an option, but I'm wondering if there is any harm (other than overpaying for capacity you don't need to the variable speed/modulating approach. Thanks.

  2. #2
    Join Date
    Mar 2004
    Location
    SE Michigan
    Posts
    17,948
    If your load calls for 25k, and you install a 45k 80% furnace, that furnace output is only 36k. Seems ok to me. I would rather be a little bit over than under.
    Behind the ostensible government sits enthroned an invisible government owing no allegiance and acknowledging no responsibility to the people.

    Theodore Roosevelt

  3. #3
    Join Date
    Mar 2004
    Posts
    171
    That's close, but let's say the load calls for 20,000 btu and you want to install a 90+ furnace. Now that's double the BTU you need, but would a 2 stage or a mod help compensate for this? What other choice do you have if you don't want to use a heat pump with a smaller BTU capacity? I wonder why manufacturers don't make smaller furnaces.

  4. #4
    Join Date
    Mar 2004
    Location
    SE Michigan
    Posts
    17,948
    There are smaller units for apartment applications that use a small duct system. They are ceiling mounted. There is a company in Texas that makes them. I cant remember the name, maybe someone will post a link. There are always options, you just have to look.

    Someone posted a photo of one recently.
    Behind the ostensible government sits enthroned an invisible government owing no allegiance and acknowledging no responsibility to the people.

    Theodore Roosevelt

  5. #5
    Join Date
    Mar 2004
    Posts
    171
    Let's say the ductwork is alread in place, and you are replacing an existing, old furnace on the fire floor. Is there any harm (other than overpaying for capacity you don't need) to the variable speed/modulating approach, meaning that you are counting on the furnace to operate on the lower firing cycle(s)?

  6. #6
    Join Date
    Jul 2005
    Posts
    717
    since you are on this.... "let's say" kick, let's say you install an oil fired furnace, and then you can install a lower sized nozzle to give out the required amount of heat.

  7. #7
    Join Date
    Mar 2004
    Posts
    171
    It's a gas fired furnace being replaced on the first floor. Is there any harm (other than overpaying for capacity you don't need) to the variable speed/modulating approach, meaning that you are counting on the furnace to operate on the lower firing cycle(s)?

  8. #8
    Join Date
    Dec 2005
    Posts
    117
    Originally posted by deejoe
    since you are on this.... "let's say" kick, let's say you install an oil fired furnace, and then you can install a lower sized nozzle to give out the required amount of heat.

    There are no oil furnaces in the US that can get anywhere near that low in heat output. The smallest nozzle is a 0.4 gph. Run that at 100 psi and you'll get 56,000 BTU. Run that at 140 psi like most new burners and you'll be at 65,800 BTU.
    Now since the oil quality in the US is rather poor (high sulfur content), a nozzle this small will clog up frequently and will cause problems.


    To the original poster, getting a two stage furnace will help matters somewhat. Yes you'll pay more for something that you will never use, but it will work. You have to make sure that the duct system can deliver the proper air flow for the full capacity so that if it does run in high, you won't overheat the heat exchanger - which leads to cracks.


  9. #9
    Join Date
    Mar 2004
    Posts
    171
    I think the ductwork was properly sized. I think another problem in my area (NJ) is that I don't think the majority of contractors think outside the gas furnace box and consider heat pumps or other alternatives. Don't know why that is so. In a rental home I have, I'm pretty sure that I have two completely oversized gas furnaces. I'm trying to avoid that in the next one I do, and if nobody will consider gas furnace alternatives, I'll look at the two stage or modulating (likely too expensive) route to compensate, or as coolwhip pointed out, go with an 80% to lower the actual output.

  10. #10
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,337
    Im in NJ myself. We run into a similiar problem with a chain of tanning salons we take care of. For example, the one needed 80,000 btu of heat and 6 tons of coiling. So we put a 135,000 btu furnace with a 6 ton coil and condenser. We set the furnace up to just run on low fire. Works great. Yes you could go with a heat pump, but with the electric rates in NJ, it will be cheaper to run a 2 stage on low fire.....if low fire meets the load.

  11. #11

    2 stage furnace - way to go !

    bydabeach,

    I have 1200 sq ft. HVAC calc says I need 60,000 btu OUTPUT on the coldest day (-25).

    My 2 stage furnace (derated for high altitude) gives 42240 in high mode. The low stage only gives 27456 (high altitude). High altitude reduces the output capacity by 12 percent for the Denver altitude. So, at sea Level (NJ) it would do 31,200 btu (low mode) like the Trane literature says.

    It is one SMOOTH heat output. I only need the 2nd stage around -5 or -11 (rare). I found out because I had a 3 wire thermostat and the installer forgot to install the jumper (2nd stage was NOT available for me).

    My natural gas consumption was slightly lower than the old 1984 amana (dealer says it was made by Ducane) 75,000 btu (78 afue ???) input furnace (new furnace no pilot light).

    I have the smallest Trane XV80 furnace (60,000 btu input). It was manufactured in Trenton (per the shipping box). Is that 45,000 btu furnace you mentioned a 2 stage?

  12. #12
    Join Date
    Mar 2004
    Posts
    171
    I believe that there are several 2 stage 45,000 btu furnaces. I think Goodman makes one. Why is your gas consumption only slightly better. You went from at 75,000 btu (single stage?) to a 60,000 BTU 2 stage that is running on low fire most of the time. Wouldn't your gas usage be much lower?

  13. #13
    Join Date
    Nov 2005
    Location
    Kansas
    Posts
    4,414
    no, your gas usage is not lower, gas produces the same amount of heat output (BTU) per cubic foot gas.
    In the Midwest, furnaces where often sized too big to accommodate a large blower for the ac. A two stage furnace would help to reduce the capacity, give the furnace longer run time what will increase comfort and longevity.
    Don't forget, even a correctly sized furnace is over sized 90% of the run time.

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