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  • furnace sizing

    I understand the pitfalls surround an improperly sized air conditioner. but, what about a furnace? if it's too small it'll run all the time, and not properly heat the house. Too large? is there a downside to upsizing the furnace?

    seems like it would cycle less, burn more fuel when it runs, but have a shorter runtime.
    No, it's not rocket science, it's plumbing and unlike rocket science it requires a license.

  • #2
    Re: furnace sizing

    Originally posted by MoJourneyman View Post
    I understand the pitfalls surround an improperly sized air conditioner. but, what about a furnace? if it's too small it'll run all the time, and not properly heat the house. Too large? is there a downside to upsizing the furnace?

    seems like it would cycle less, burn more fuel when it runs, but have a shorter runtime.

    An oversized furnace (one whose capacity is too large) costs substantially more to operate than a correctly sized furnace. This is because oversized furnaces typically cycle on and off more often than correctly sized furnaces, and cycling on and off is more expensive than running at steady state.

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    • #3
      Re: furnace sizing

      thank you. that one has been bugging me for a while now and I finally got around to asking.
      No, it's not rocket science, it's plumbing and unlike rocket science it requires a license.

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      • #4
        Re: furnace sizing

        To add to the good advice, the limit on a furnace is a safety, not an operating control, like on a boiler. The oversizing besides temp swings resulting in less comfort causes all controls to cycle excessively resulting in excessive wear and premature failure, heat exchanger included.

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        • #5
          Re: furnace sizing

          I heard the same thing about over sized furnaces resulting in excessive wear and premature failure ESPECIALLY with the heat exchanger (too much expansion and contraction). Im not sure I alltogether believe this....and I cannot see how being slightly oversized, say 10,000-15,000 BTU would cause this anyway. How much oversizing are you talking about? 50,000 BTU???

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          • #6
            Re: furnace sizing

            well, in my case I was thinkinf more for my dad's house. it has a 90k btu furnace, my options are an 80k or a 105k furnace. with a 3 ton AC.

            and of course I don't have load calc software
            No, it's not rocket science, it's plumbing and unlike rocket science it requires a license.

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            • #7
              Re: furnace sizing

              If the efficiency rating is similar on both furnaces, and the original furnace was working fine for you, no way I would go from 90 to 80KBTU. (that's smaller furnace may run 24-7) I'd go to 105KBTU (I dont care what anyone says). I did just that when I had a Carrier 92,000 BTU and replaced with a 100,000 Keeprite. (plus an 8,000 BTUs difference is noise).

              A heat loss calculation is what is needed if you want to be 100% acurate (and they arent always perfect)

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              • #8
                Re: furnace sizing

                old furnace is 80% new furnace will be 90%

                I'm still thinking i'll go with the 105k because I don't want to install one that will never shut off. and it doesn't seem like a large enough jump to make short cycling an issue. but i've been wrong plenty of times before.
                No, it's not rocket science, it's plumbing and unlike rocket science it requires a license.

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                • #9
                  Re: furnace sizing

                  80% vs 90% efficiency is a factor (mine was 90% vs 92%...lets see what the experts say...just hang on they will show up....

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                  • #10
                    Re: furnace sizing

                    If you are going up in effieceny then you go down in size. replacing 100,00btu 80% furnace with a 95% you can drop 15% off of the btu's which will give you the same cycle patern. best bet would go with a multistage furnace then going oversize really doesn't matter much cause 90% of the time it runs on the lower stages unless needed. but it also needs to be on a multistage thermostat. i actually have 5 stages of heat so it never shuts down and can heat 2100 sq ft for $40.00 to $50.00 in the dead of winter.

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                    • #11
                      Re: furnace sizing

                      Originally posted by jcanter View Post
                      If you are going up in effieceny then you go down in size. replacing 100,00btu 80% furnace with a 95% you can drop 15% off of the btu's which will give you the same cycle patern. best bet would go with a multistage furnace then going oversize really doesn't matter much cause 90% of the time it runs on the lower stages unless needed. but it also needs to be on a multistage thermostat. i actually have 5 stages of heat so it never shuts down and can heat 2100 sq ft for $40.00 to $50.00 in the dead of winter.
                      What??? Load is Load no matter the efficiency... Not sure I have ever sized a system by attempting to guess its cycling patern? Please elaborate



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                      • #12
                        Re: furnace sizing

                        he was stating that he didn't want it to run contiusly. by changing efficeincy your getting more btu's in the house. example 80% furnace for every $100.00 spent on fuel $80.00 goes into the house & $20.00 goes up the chimmney, with a 95% every $100.00 spent $5.00 goes up the chimmney and $95.00 goes in the house. so a 80% 100,00 btu and a 95% 85,000 btu in the same house will cycle the same. I am not saying how to size it by cycling. a load on the house is whats needed to size properly. as long as it sized to design temp it's actually more efficient to stay running then on and off. ( same as your car, better gas milage on the freeway than stop and go).

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                        • #13
                          Re: furnace sizing

                          Okie, I can sorta understand what he was trying to say but was equally as concerned.
                          Fact is, more efficient is more useful Btu's and less waste. BUT If the furnace was grossly over sized in the first place, can you assume 80 or 105 will be ok. What if a 45,000 input at 95% is the right choice. You say your stuck with two choices and that tells me your not looking at the right brands or your trying to buy out of the back of a truck
                          GET A LOAD CALCULATION

                          For me to give any other advice than that would be careless and give a false sense of security with your decision.

                          Think of it this way, get it wrong and you pay WAAAAAAAAAAAAAAay more than it would cost to get it sized right the first time.

                          Just saying.....
                          And hey, what do I know!

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                          • #14
                            Re: furnace sizing

                            Originally posted by rjm78 View Post
                            GET A LOAD CALCULATION
                            Silly question, but is there a relatively cheep software for that? or possibly a formula to use on it?

                            I barely do any hvac work, almost all service, just a handful of replacements.

                            I certainly have more options than an 80 or 105, those are just the closest to the existing furnace in the brand that my preferred supplier sells.
                            No, it's not rocket science, it's plumbing and unlike rocket science it requires a license.

                            Comment


                            • #15
                              Re: furnace sizing

                              Originally posted by MoJourneyman View Post
                              Silly question, but is there a relatively cheep software for that? or possibly a formula to use on it?

                              I barely do any hvac work, almost all service, just a handful of replacements.

                              I certainly have more options than an 80 or 105, those are just the closest to the existing furnace in the brand that my preferred supplier sells.

                              I have been using http://www.hvaccomputer.com/startpagea.asp and am very happy with it My area now requires an approved load calc before a changeout or new install...

                              No simple formula unfortunately though we have all beeen told them at one time or another...

                              400 CFM Per Ton of Cooling
                              500 SQFT Per Ton of cooling

                              The rules of thumb may work 75% of the time but that will make you wrong the other 25%. Completly throw standardization out the window (no pun intended) when it comes to loading new construction...

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