New Furnace Size

Discussion in 'HVAC Heating & Cooling' started by rdsxfn, Oct 22, 2007.

  1. rdsxfn

    rdsxfn New Member

    Oct 22, 2007
    Hi, I'm new to the forum and have been reading and getting a ton of great info.

    I currently have a 100,000 BTU furnace but think it is too big. I did a preliminary heat loss calculation and came up with needing about a 70,000 BTU furnace. I'm looking at getting a new High efficiency, 2 stage, variable speed furnace and I have found someone who can get me a screamin' deal on either a 100,000 or 125,000 BTU furnace. He was telling me that with the variable speed it doesnt matter that much if you get a bigger furnace than you need as it will just run on the lower speed most of the time and kind of act like a smaller furnace and acutally on those really cold days, having the extra BTU's might be helpful.

    Does this sound right to you or is it a bunch or baloney?
  2. Furd

    Furd Engineer

    Jul 3, 2007
    Retired energy systems engineer
    Wet side of Washington State
    Yes, it is a bunch of baloney.

    Doing the heat loss you used a design outdoors temperature that you probably rarely experience. The furnace needs to be sized to keep your home at its design indoor temperature (usually between 70 and 75 degrees) in the most severe weather. Since that design of most severe almost never happens the plain truth is that for most of the time your furnace will be oversize and often considerably oversized.

    The best furnace efficiency is gained when the furnace is running constantly.

    The idea behind the two-stage operation is that most of the time the furnace will be firing on the first stage which is about 70% of its maximum firing rate. Even at this reduced firing rate the furnace is going to cycle on and off to maintain the indoor temperature at the desired set point. It will however be firing for longer periods so the efficiency will be higher than if it gave a shorter cycle.

    With the two-stage option you can maintain a closer temperature differential and this will result in greater comfort.

    The variable speed is a term that only applies to the blower. (There are some modulating forced air furnaces but they are pretty rare and usually only in larger sizes.) Specifically, instead of having only three or four selectable speeds for the blower the variable has a multitude of selectable speeds. These speeds are determined by what the furnace is doing and also during the original installation.

    For example, if you have the "Fan On / Auto" switch selected to "Fan On" the blower will run at a very slow (and quiet) speed to maintain a minimum air circulation through the ductwork and your home. This can give you much more comfort and even temperatures in addition to continuously filtering the air. When the thermostat "calls for heat" the furnace will light on the first stage and the blower will "ramp up" in speed to a higher setting to maintain a specified temperature differential across the heat exchanger and "push" the heated air into the living spaces. If necessary to go to the second stage of heating the blower will ramp up to a higher speed so the speed of the blower is always in close control of the furnace operation.

    All that said, the installation of furnaces has become even more of a crap shoot because the sales people no longer need to closely ascertain the necessary size for the particular home. The tendency is to simply install the two-stage with the same, or even higher, BTU rating as the original furnace. I found this out the hared way. My original furnace had a 60,000 BTU rating and it was oversize. I went with the smallest size two-stage variable speed furnace available form Lennox and it was rated at 70,000 BTU. After signing the contract and after the new furnace installation I looked at the nameplate and discovered that a substitution had been made and I had a furnace rated at 90,000 BTU.

    That's 50% larger than the original furnace!

    I called the salesman and his explanation was that this way my furnace would never go to the second stage except when the outside temperature was below zero. Only problem is that in the 57 years I have lived in the greater Seattle area the temperature has NEVER gone below zero. I had received a furnace with the first stage more than 10% larger than the original furnace and it had the very real probability of operating at 50% more than I needed during the coldest weather.

    I made the installer come back and install the proper (and contracted for) size furnace. I think the reason for the initial furnace being installed was that it simplified the duct connections and the contractor didn't give a darn that the BTU output was way in excess of my needs.

    So, DO NOT let anyone talk you into buying / installing a furnace larger than your heat loss calculation shows that you need.
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    For any furnace, as the weather gets colder, it will run or cycle more often until at some point, it will run continuously. That is the value that the heat load analysis will kick'll need x BTU to keep a particular dwelling at some Y temperature on a Z temp outside temperature. ANything warmer than Z means that it won't run as often. Anything colder than Z means that the house will get colder. If for example, it was sized to maintain 72 degrees when it got to zero outside, if it got to -1, the house would likely cool off to 71 degrees. In other words, they should be somewhat in lockstep. The house will not cool off quite as fast as outside, since it has stored up a bunch of heat.

    If you set your thermostat down at night, and then in the morning it is really cold, it may not be able to raise the temperature very fast, if at all. So, if the furnace is sized for a zero degree day, you may not want to use a setback unless you have some reserve. In a typical situation, the furnace won't run all of the time, and you'll get the typical swings of temperature as the thermostat cycles. If you run the fan continuously (and I'd probably not do this on a single speed fan), it does help to keep the house temperature much more even.
  5. leejosepho

    leejosepho DIY scratch-pad engineer

    Nov 23, 2006
    disabled-retired industrial fabricator
    200 miles south of Little Rock
    Agreed, just like not letting someone sell you a dump truck for carrying a few bags of groceries back to the house.
  6. Bob NH

    Bob NH In the Trades

    Oct 20, 2005
    New Hampshire
    There is an advantage to a larger furnace if it is not way oversize and if it is set up correctly. A larger furnace usually has a larger heat exchanger, and a larger heat exchanger will transfer more heat with smaller temperature difference, and therefore more net efficiency.

    But higher BTUs doesn't always mean a larger furnace because they change the firing rate without changing the heat exchanger.

    You can see that effect in some furnaces where they will offer the same physical size furnace with different firing rates or BTU ratings. The efficiency will be lower at higher firing rates because the higher firing rate sends more and hotter flue gas up the stack instead of transferring the heat to your house.

    It is not appropriate to generalize on such matters. You need to get certified efficiency ratings for the specific furnace, and the variation of efficiency for the various firing rates. Compare that efficiency and the cost of the furnace with other brands and sizes.

    Then you will have the information to make a decision.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    Bob makes a good point. While not exactly the same anagogy (but very close), the HE boiler I have has a modulating burner that can throttle itself down to 20% of maximum. It is rated at max output of 94.5%, but they claim it can reach closer to 98% at the lower output. Since a heating system rarely has to produce maximum outputs, this is a real bonus. While there may be modulating furnaces, (I know there are two-stage ones which isn't quite the same) I'm not familiar with any.
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