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  1. #1

    Capacity needed for 2500 sq ft house

    We are in the final throes of decision about the installation of a new gas furnace and a heat pump in my 1937 vintage house with minimal if any in-wall insulation. The house, in Boston, is 2500 sq ft and the ceilings on the first floor average about 9 ft with 8 ft ceilings on the 2nd floor. Two vendors are promoting different systems, as follows:

    A. Higher priced system:

    Carrier 58MVC120-20 Infinity Series 3-Stage 120K btuh Gas-fired furnace
    Carrier Infiniti 25HNA960 heat pump
    Carrier SYSTXCCUID01 Infinity Series Controller
    Carrier CNPVP6024ACA 5 ton Vertical DX Coil
    Nortec steam humidifier

    B. Lower cost system:

    Carrier 58MVC100-1-F-20 furnace
    Carrier 25HPA542H003 heat pump
    Carrier SYTXCCU1DOL-B Infinity controller
    Carrier CNPVP4221ACA coil
    Nortec steam humidifier

    The vendor of A said that my house needs the 5 ton heat pump because it is drafty and has some high ceilings. The vendor for the smaller system said that the house would be too cold using the 5 ton unit and that if stage 1 kicked in it would be insufficient. It is tough knowing how to proceed. Any suggestions?

  2. #2
    Join Date
    Dec 2002
    Location
    Toledo, Ohio, United States
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    12,896
    Twilli like C
    No Heat No Cool You need Action Fast

  3. #3
    Join Date
    Sep 2002
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    I don't know
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    Both of them are wrong.

    The only way to size correctly is with a load calculation. (Some may do it based on square footage, shading, and experience with similar houses, which is better than just guessing)

    Having an over-sized unit will not make your house too cold unless you set the temperature too low; however, a 5 ton unit may not run enough to remove the humidity and prevent heat stratification.

    Prior to even sizing equipment, every practical way to improve to building envelope should be explored. Many government offer grants for energy efficiency upgrades, and adding insulation doesn't always have to entail ripping out walls. Insulation can be easily added to the attic; it can also be blown into empty walls. If the house is drafty, seal the drafts prior to replacing equipment. If you plan on replacing windows, postpone the installation and do the windows first. If the equipment is sized based on the existing efficiency level, it will be oversized if you decide to make changes down the road.

    A professional energy audit and blower door test can help you identify areas which need improvement, and determine which upgrade options have to best rate of return.

    The bottom line is that there's no point of putting high efficiency equipment into an inefficient house.

    120 000 BTUs for a 2500 sq ft house is way out of line; the same goes for five tons of cooling.

    Where I live a house of your size would get a 2.5 or 3 ton condenser paired with a 60-75k BTU furnace. (However, I live in a colder climate with a mild cooling season.) The other thing is that the equipment can only be as good as the distribution system; leaky/undersized ductwork can reduce capacity and efficiency dramatically.

  4. #4
    Join Date
    May 2007
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    DC Metro Area (MD)
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    3,350
    Quote Originally Posted by twilli3967 View Post
    Twilli like C
    Twilli means he likes neither of the contractors! The best way to size the system would be through a load calculation--did any contractor do one? Oversizing does crush efficiency. 5 tons is a lot of cooling.

    2500/5 = 5 tons. Perfect! (Just kidding, silly 500 sq. ft./ton rule of thumb.)

  5. #5
    Join Date
    Dec 2002
    Location
    Toledo, Ohio, United States
    Posts
    12,896
    No Twilli meant Coleman
    No Heat No Cool You need Action Fast

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    Call number 3 and 4.

    2 stage units aren't suppose to be big enough to do the whole place in first stage when it 95 outside.

    Screen contractors over the phone, only schedule contractors that will do a load calc.
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    How many times must one fix something before it is fixed?

  7. #7

    Unhappy I need a suggestion fo similar situation, please :

    Hi everybody,
    I'm on a research for exchange of my 25-years OLD! furnace and heat pump.
    Just for puttini in situation : furnace is SMART from James Smart Mfg Ltd, Montreal, CAnada, Model SHG-200-5, Input 200,000, Output 160,000 BTUH !!
    As you can see, lot if this juice is going right away trough the chimney ... :-(
    Heat pump : GE Elite Super Efficience Wathertron Heat Pump, BWX736A100A0, Mfg 06/83.
    I use them since 1998 when did purchage of the house, and God thanks! they did a good job. Now, pump failed with 2 broken copper tubes and I think I shoul put all that on the scrap and purchase new modern system.

    My 2-floors house has +/- 2,550 sq ft on both floors excluding basement, with one has 690 sq ft area. House onlu volume is approximately 22,700 cu ft, basement is approx. 5,500 cu ft.

    As per I has been told, Area / 750 sq ft/ton gives +/- 3.37 tonnes, or I will need either 3 or 3.5 tons unit. I would think 3.5 is better, knowing that my hous dates 1992 and it is not as well insulated as later houses ... The furnace heating capacity, I've been told I need, is not more than 100,000, even 90,000 BTU. I have no more then 18 - 20 opening for the air in my house.

    Concerning fournace and pump, I'm looking on what is available on our canadian market - Carrier and Lennox.
    I've been told by Guy # 1, that I will get a proposal for two options, both of them consisting of two stage furnaces, wich sounded as miracle to my ears, and a heat pump. Guy # 2 told me, on the other hand, that I do NOT need a two-stage furnace, because heat pump acts as a first stage, so the second stage is simply the one-stage furnace ... Now I'm getting mixed and in trouble ...

    Please, any suggestions or ideas ?

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    Well, by that bogus rule of thumb you need a half ton larger heat pump then you already have that was doing a good job according to you.

    So, instead of guessing. Call contractors that will do a real load calc. Screen them over the phone. If they won't do a real load calc, don't bother having them come out.
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    How many times must one fix something before it is fixed?

  9. #9
    Quote Originally Posted by beenthere View Post
    Well, by that bogus rule of thumb you need a half ton larger heat pump then you already have that was doing a good job according to you.

    So, instead of guessing. Call contractors that will do a real load calc. Screen them over the phone. If they won't do a real load calc, don't bother having them come out.
    Thanks.
    Had already two. Waiting for another one. No one did any special measurment or calculation. What should I be waiting for exactly, let me know, please ...
    Yes, old pump did some job, but not the excellent one, compared with other houses, sorry ...
    What do you think about two-stage furnace combined with pump ? I found some stuff on the net saying it is OK, but # 2 said - NOT ! ;-)

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,808
    Nothing wrong.
    Some contractors think it will go to second stage right away. It won't. It works great and provides better comfort.

    They should be measuring your rooms, windows, doors, asking about your insulation.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  11. #11
    Join Date
    Mar 2008
    Posts
    14
    Quote Originally Posted by amd View Post
    The bottom line is that there's no point of putting high efficiency equipment into an inefficient house.
    .
    Is this right? It seems like an inefficient house is the perfect place to put hight efficiency equipment because that is where it will make the biggest difference in operating cost.

  12. #12
    Quote Originally Posted by Montreal_Canada View Post
    Thanks.
    Had already two. Waiting for another one. No one did any special measurment or calculation. What should I be waiting for exactly, let me know, please ...
    Yes, old pump did some job, but not the excellent one, compared with other houses, sorry ...
    What do you think about two-stage furnace combined with pump ? I found some stuff on the net saying it is OK, but # 2 said - NOT ! ;-)
    A load cal involves a lot of measuring and math. But a short run down is that the contractor will have to measure the square feet of all exterior walls, windows and doors. Est. the R value of wall and ceiling insulation and ACH(how loose/tight the home is). Then do a bunch of math with given factors for your area and ta-daa...now he will know how many btu's your house loses or gains according to avg. low temps. in winter and high temps. in summer. Then they can determine the size unit to suggest.

  13. #13
    Quote Originally Posted by beenthere View Post
    Nothing wrong.
    Some contractors think it will go to second stage right away. It won't. It works great and provides better comfort.

    They should be measuring your rooms, windows, doors, asking about your insulation.
    So, if I do understand well how it works : first heat pump will work up to -10 / -12C (sorry for Celcius), then first stage will get control, right ? So when the second stage will come into the game ?

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