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  1. #1

    How to determine proper size for replacement gas furnace???

    Hi everyone & thanks for your thoughts.

    We've had 3 different contractors come in and look at our ~20 year old gas furnace and provide quotes. The question I can't seem to get a straight answer on is what size furnace to get.

    Our current 80% efficiency gas furnace is rated at 100k which means it puts out 80k BTU. Ok, I understand that. The new system is rated at 95% efficiency, so do I get a 80k or 100k two-stage unit? The smaller 80k unit (as recommended by 2 contractors) will give us nearly 76k output which is slightly less than we have now, but the larger furnace will give us 95k which is well over - and from what I understand, oversizing these units will lead to problems down the road. The contractor recommending the larger one says the smaller one will not put out enough heat.

    We live in the Mid-Atlantic so winters are reasonable.

    System looking at on the 80k side:
    Trane XL15i A/C - 4TTX5036A1000C
    Trane XV95 furnace - TUH2C100A9V4VA
    Aspen Coil - ACE36D44210L015

  2. #2
    Join Date
    Jan 2009
    Location
    Keokuk, IA
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    5,520
    IF you're in the mid atlantic and only need a 3ton ac, you won't need more than 80k BTU and likely only need a 60k BTU furnace. But it will depend on insulation, how much shading you have and overhangs over windows on how proportionaly you cooling load is vs. heating load.

    Are you sure your AC is sized right?

    Look at it this way, if your current 100k BTU furnace is adequate, then a 80k 95% furnace will be as well. Is your furnace running almsot constantly when it's dipped below 10F? It should be if sized correctly. IF not, then you may be a little oversized right now anyway.


    A good contractor, for furnace sizing will either 1) DO a proper load calulcation or 2) get a copy of your gas bill which usually has the average daily temeprature for that month, then compare that to you design temperature, factor in the average temeprature you keep it in your home and can use that info to work backwards into a size. I personally think that's one of hte best methods. But you want to run hte calculation for 2 or 3 months to create a trend, then extrapolate that for the coldest average mothly temperature anticipated. Why? You'll see a increasing trend due to stack effect. The colder it is, the more infiltration your have. So using extrapolation, will factor that in.

  3. #3
    Bah ... 3 contractors onsite and none did a proper load calculation / asked for additional details re: sizing. Each contractor "rated well" by Angie's List fwiw. It does dip below 10F here (30 miles West of DC) but we have been comfortable here the last 5-6 years.

    Two zone heating btw, 3 story house, heatpump for upstairs, this unit for the bottom 2 floors. The first contractor with a quote also did our A/C for the upstairs (also a Trane XL15i) about 2 years ago and came in $1k under the other two contractors (but I was leery about the outlier).

  4. #4
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,841
    Quote Originally Posted by RLymburner View Post
    Bah ... 3 contractors onsite and none did a proper load calculation / asked for additional details re: sizing. Each contractor "rated well" by Angie's List fwiw. It does dip below 10F here (30 miles West of DC) but we have been comfortable here the last 5-6 years.

    Two zone heating btw, 3 story house, heatpump for upstairs, this unit for the bottom 2 floors. The first contractor with a quote also did our A/C for the upstairs (also a Trane XL15i) about 2 years ago and came in $1k under the other two contractors (but I was leery about the outlier).
    Forget those companies. Return to the telephone and start asking the question up front, "How do you size a replacement gas furnace?". When you find one that does it by Manual 'J' load calculation, invite them over for a quote. But under no circumstances do I recommend dealing with a company that is cutting corners from the very start, gambling with YOUR money that he GUESSES correctly on furnace sizing. The science to do it correctly is readily available. Those who choose to not do a proper Manual "J" are just too lazy to learn the process to deliver proper results for the customer. As far as Angie's List is concerned, the people who are reporting on those 'large' companies probably don't know what they're missing. So how can they make a valid evaluation? They may think that on/off/on/off/on/off all day is normal and that they're comfortable. Then someday they may find someone who does the job properly and gives them on/on/on/on/off/off/on/on/on/on operation and find out what real comfort is.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  5. #5
    Join Date
    Dec 2002
    Location
    Houston,Tx.
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    16,121
    Quote Originally Posted by RLymburner View Post
    Bah ... 3 contractors onsite and none did a proper load calculation.
    All them there manuals are important, but make sure you also get a good installation. I personally would rather have a exact replacement, good quality install, than a shoddy manual installer. Might even ask for a portfolio or referrals.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

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  6. #6
    Quote Originally Posted by skippedover View Post
    Forget those companies. Return to the telephone and start asking the question up front, "How do you size a replacement gas furnace?". When you find one that does it by Manual 'J' load calculation, invite them over for a quote. But under no circumstances do I recommend dealing with a company that is cutting corners from the very start, gambling with YOUR money that he GUESSES correctly on furnace sizing. The science to do it correctly is readily available. Those who choose to not do a proper Manual "J" are just too lazy to learn the process to deliver proper results for the customer. As far as Angie's List is concerned, the people who are reporting on those 'large' companies probably don't know what they're missing. So how can they make a valid evaluation? They may think that on/off/on/off/on/off all day is normal and that they're comfortable. Then someday they may find someone who does the job properly and gives them on/on/on/on/off/off/on/on/on/on operation and find out what real comfort is.
    Nuts - wish I had found this forum. Good advice, looks like I'm back to square one here and need to pick up the phone.

  7. #7
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Heating and cooling equipment really is a much bigger purchase than buying a car. You're stuck with it for about 10-20 years, you can't just trade it in, it affects your comfort and energy bills all year long 24/7. You can't simply trade it in on a different model if you don't like it and on a residential system it loses nearly 100% of it's value the moment it's placed in service. 50% of the cost roughly, are installation expenses.

    Do you get 3 quotes for a car and simply pick the cheapest one? If car mfg's shipped their cars with the engine, transmission seperate of the rest of the car, wouldn't you want a dealer with a very compitent service department with well trained mechanics to do the assembly? What if a sales "engineer" was the guy that selected which combination of entgines and transmissions you should put in that partucular model and then had to select the correct dirveshafts, brakes, wheels size, and final drive ratios. Still want to use the low bid?

  8. #8
    Sage advice, thank you everyone. I think I'm asking the right questions, but the guys I went to first are the ones that want the most of my $$$! A couple of quick questions before I start hitting the phone lines:

    (1) I have an Aprilaire 700 humidifier on the furnace (manual control) - can it be refitted on the new furnace to save a few bucks? If not, do I get one that can be controlled by thermostat? Right now, the existing contractor quotes include the XL802 controller, but I could get the Honeywell IAQ (or similar) to maintain humidity.

    (2) Two of the contractors have offered to throw in Aprilaire 2210/2410 whole house media air cleaners free of charge (suspect!). In online googling, it appears these are better than the standard 1" drop-ins I have now. No problem with this? We do have 2 indoor cats but no real allergy problems in the past. I hesitate to ask, but should I consider an electronic air cleaner while I'm having the furnace work performed?

  9. #9
    Join Date
    Jan 2004
    Location
    PA
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    68,758
    Your 700 can be controlled by a thermostat with a built in humidistat.
    Contractor locator map

    How-to-apply-for-Professional

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

  10. #10
    Join Date
    Jun 2006
    Location
    Richmond, VA
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    2,989
    Quote Originally Posted by RLymburner View Post
    Sage advice, thank you everyone. I think I'm asking the right questions, but the guys I went to first are the ones that want the most of my $$$! A couple of quick questions before I start hitting the phone lines:

    (1) I have an Aprilaire 700 humidifier on the furnace (manual control) - can it be refitted on the new furnace to save a few bucks? If not, do I get one that can be controlled by thermostat? Right now, the existing contractor quotes include the XL802 controller, but I could get the Honeywell IAQ (or similar) to maintain humidity.

    (2) Two of the contractors have offered to throw in Aprilaire 2210/2410 whole house media air cleaners free of charge (suspect!). In online googling, it appears these are better than the standard 1" drop-ins I have now. No problem with this? We do have 2 indoor cats but no real allergy problems in the past. I hesitate to ask, but should I consider an electronic air cleaner while I'm having the furnace work performed?
    Using the analogy above....when you buy a new car do you want to use any pieces from the old car?

    I personally like the april aire's and have them in my own house. They work very well keeping the equipment clean. I do not care for electronic because whenever I service them, most are not working and aren't picking up anything.

  11. #11
    Good enough for me, thank you everyone. Hitting the phones.

    Awesome advice, great service provided!




  12. #12
    Join Date
    May 2008
    Location
    Missouri
    Posts
    3,614
    The large pleated 5" filters are the way to go especially if you're getting them at a discount. Don't allow a contractor to "oversize" your new unit. USUALLY the old equipment was significantly oversized, so the newer, higher efficiency units are smaller input, but many times closer to what is needed. One of these guys (or gals) SURELY knows how to do a manual J figure on your actual needs. That way you'll know and not guess at what your home actually will need. The fully "electronic" air filters have a tendancy to quit working after a few years (usually after warranty runs out). We no longer recommend them to homeowners, just use the large pleated 5" models. As moto mentioned, if your new higher efficiency furnace and AC are sized RIGHT, they will run nearly continuously at extreme cold or hot temps. That is the way you max out your savings. Nobody buys a 45 MPG car and shuts it off and start it again at every stop light, and expect any kind of fuel savings. When it's 100 outside, or zero, the units should be running nearly full time (if sized properly for your temperature zone).

  13. #13
    Join Date
    Mar 2007
    Location
    tx
    Posts
    1,088
    no comparison to a job done right. Find someone who wants to work with you and set down and do a properly completed manual J load calculation based on the construction materials your house is built out of. Someone who will spend time actually doing there best to inspect your homes insulation, windows, doors, leakage, to the best of their ability. Once a load calculation is done, then a discussion on different types of equipment that will satisfy your homes needs can be held. You can discuss with this contractor any special things you might want from your ou may just want something that will satisfy the load, and then you may want something that will do the job but be able to cycle into second or even third stage for maximum energy savings. Whatever you want, just make sure it was sized right. Not in all cases, but in most, someone who will spend the time to properly size equipment and do load calculations will also take the time to install the system correctly also.
    Bad information is worse than no information at all.

    There are three kinds of people in this world. Those who can count and those who can't!

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