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  1. #1
    Join Date
    Sep 2003
    Posts
    30

    How to tell if your furnace is oversized

    Maybe too general of a question and since it's usually too late to really do anything about it maybe pointless but I'm curious.
    From reading here there are tell tale signs ie short cycling ect. but w/ so many different types of units (1 stage, 2 stage, modulating) sometimes things aren't easy to spot for non-HVAC people. In some sense this might help people who currently are considering replacement. Not to mention giving them some kind of bargaining power..
    I just replaced mine and none of the 5 estimates even bothered w/ any sort of analysis (I still think this is the norm in the real world) some "hints" might help people from making a costly mistake.
    I actually downgraded my furnace because of all the reading I did here. most estimates wanted to just replace the current BTU rating and in some cases go up a bit. Since my old 1 stage did fine as to heat in my leaky old house (major temp swings of course but never any lack being able to keep up in the coldest of nights) at 90,000BTU's (90 eff duomatic olsen 23 yrs old)
    and there are upgrades planned (which btw is also a wrench in any current analysis) on some windows (tons of single pane/alum storm combos probably every outside wall is 50+ glass, oh joy) but not possible for all that this is something homeowners should also take into consideration on a furnace upgrade. What is fine today may be over sized tomorrow.
    so maybe homeowner eduction based on what there system is currently doing and how to spot if it's not quite right is helpful. not to mention if a "guy" is just selling a bill of goods. Obviously some people just ignore any advice for a bottom line. some things may never change.
    As to my "downgrade" to 80,000btu w/ full mod furnace, sure is more comfortable and w/ these -10 plus nights and single digit days and how the furnace seems to be working it would be helpful to know what to look for... and to figure out if everyone did things right.
    As to cost, no major change one way or another but the electric bill is down a bit, even w/ the mod seemly (seemingly because I can't always tell low fire w/ out sticking my face to the duct) running a blower 24/7 lately so that is a hopeful sign..
    I suppose I shouldn't just pick on the heating aspect but I guess being a "northerner" it is just way more important....
    Pretty sure I could have gone down one more step and still not froze, but oh well....I'll leave that to the next owner.

  2. #2
    Join Date
    Jun 2009
    Location
    Albuquerque NM
    Posts
    2,485
    Quote Originally Posted by jeffkrol View Post
    Maybe too general of a question and since it's usually too late to really do anything about it maybe pointless but I'm curious.
    From reading here there are tell tale signs ie short cycling ect. but w/ so many different types of units (1 stage, 2 stage, modulating) sometimes things aren't easy to spot for non-HVAC people. In some sense this might help people who currently are considering replacement. Not to mention giving them some kind of bargaining power..
    I just replaced mine and none of the 5 estimates even bothered w/ any sort of analysis (I still think this is the norm in the real world) some "hints" might help people from making a costly mistake.
    I actually downgraded my furnace because of all the reading I did here. most estimates wanted to just replace the current BTU rating and in some cases go up a bit. Since my old 1 stage did fine as to heat in my leaky old house (major temp swings of course but never any lack being able to keep up in the coldest of nights) at 90,000BTU's (90 eff duomatic olsen 23 yrs old)
    and there are upgrades planned (which btw is also a wrench in any current analysis) on some windows (tons of single pane/alum storm combos probably every outside wall is 50+ glass, oh joy) but not possible for all that this is something homeowners should also take into consideration on a furnace upgrade. What is fine today may be over sized tomorrow.
    so maybe homeowner eduction based on what there system is currently doing and how to spot if it's not quite right is helpful. not to mention if a "guy" is just selling a bill of goods. Obviously some people just ignore any advice for a bottom line. some things may never change.
    As to my "downgrade" to 80,000btu w/ full mod furnace, sure is more comfortable and w/ these -10 plus nights and single digit days and how the furnace seems to be working it would be helpful to know what to look for... and to figure out if everyone did things right.
    As to cost, no major change one way or another but the electric bill is down a bit, even w/ the mod seemly (seemingly because I can't always tell low fire w/ out sticking my face to the duct) running a blower 24/7 lately so that is a hopeful sign..
    I suppose I shouldn't just pick on the heating aspect but I guess being a "northerner" it is just way more important....
    Pretty sure I could have gone down one more step and still not froze, but oh well....I'll leave that to the next owner.
    You didn't downgrade, you right-sized. Ot at least you got much closer to the ideal size for your house.

    If it runs almost constantly on the coldest nights, that's a sign your sizing is good.

  3. #3
    Join Date
    Aug 2002
    Location
    Southern NJ
    Posts
    962
    If it runs almost constantly on the coldest nights, that's a sign your sizing is good.[/QUOTE]

    And that you have have insufficient insulation in your house!
    I love to Cook...HVAC is a hobby that pays the bills!

  4. #4
    Join Date
    Jun 2009
    Location
    Albuquerque NM
    Posts
    2,485
    Quote Originally Posted by makinmoney View Post
    If it runs almost constantly on the coldest nights, that's a sign your sizing is good.
    And that you have have insufficient insulation in your house![/QUOTE]

    Not really. You could have a really well-insulated house, and a small properly-sized furnace.

  5. #5
    Join Date
    Aug 2002
    Location
    Southern NJ
    Posts
    962
    How could it be properly sized if it's running all night.......?
    I love to Cook...HVAC is a hobby that pays the bills!

  6. #6
    Join Date
    May 2008
    Location
    Myrtle Creek. Oregon
    Posts
    182
    I think the question was how to tell if a furnace is OVERSIZED.
    take a temp, rise thru the furn. 50* t0 70* rise is a bout the norm. on gas furnaces. if its too high it shorten the life of the heat exchanger. and elect heating elements on elect. furn.
    a stupid question is a question you wont to ask, but don't

  7. #7
    Join Date
    Jun 2009
    Location
    Albuquerque NM
    Posts
    2,485
    Quote Originally Posted by makinmoney View Post
    How could it be properly sized if it's running all night.......?
    That's not what I said. Read:
    "If it runs almost constantly on the coldest nights, that's a sign your sizing is good."
    Without doing a lot of research, that's a pretty good indicator.

    If you want to know how to properly size a furnace, you can find a lot of threads on that subject on this site that will help you. I'm not going to try to summarize it all here, but I'm sure you can find it if you're interested.

  8. #8
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,977

    Arrow Wind velocities & air infiltration levels

    During colder weather, check the wind velocity levels & how that affects furnace runtime.

    Also, check air infiltration on windy days, the level of infiltration will have a huge effect on heating runtime (furnace oil, gas, or electric, or heat pump) & total heating energy costs.

    How long between cycles, etc., provides clues; it pays to correct causes of too short off-time problems.

    The home, ductwork & airflow are all major factors affecting lowering heating & cooling utility bills.

    I'm thankful that we had low wind velocities during last weeks 14-F below zero here in SW WI.

    Temporarily, that kept me out of the oil heating poor house! - Darrell

  9. #9
    Join Date
    Jun 2009
    Location
    Albuquerque NM
    Posts
    2,485
    Quote Originally Posted by udarrell View Post
    During colder weather, check the wind velocity levels & how that affects furnace runtime.

    Also, check air infiltration on windy days, the level of infiltration will have a huge effect on heating runtime (furnace oil, gas, or electric, or heat pump) & total heating energy costs.

    How long between cycles, etc., provides clues; it pays to correct causes of too short off-time problems.

    The home, ductwork & airflow are all major factors affecting lowering heating & cooling utility bills.

    I'm thankful that we had low wind velocities during last weeks 14-F below zero here in SW WI.

    Temporarily, that kept me out of the oil heating poor house! - Darrell
    You anywhere near Dodgeville?

  10. #10
    Join Date
    Sep 2005
    Location
    Atlanta GA area
    Posts
    21,079
    Might want to think in terms of basics here:

    The purpose of the furnace is to keep the interior of the house warmer than outside air temperature, for comfort. This requires adding BTU's of heat to the interior, as heat always travels to cold (in this case leaves the house). In the summer, the heat travels into the house... the AC removes the heat.

    According to ACCA manual J, there are 'design temperatures' for each area of the country... equipment is sized to those design temperatures.

    Just for discussion: If the design temp of an area is, lets say, 10* in the winter, and you want to keep your home at 70*, then however many BUT's of heat necessary to maintain this temp rise is the size of furnace you need. Remember that this is output BTU's, not input BTU's.

    Usually, the next larger size furnace is installed... Example: the need is 77,275 BTU's and a furnace with an output of 80,000 BTU's is installed.

    Hence: When the outdoor temp reaches that 10* outside, and there is a requirement of almost 80K BTU's to keep the house at 70*... the furnace is running almost continuously.

    One more thing to think about: Outdoor temps go down over night, and are the lowest just before dawn... so in that last 30 min before the sun rises is when the unit will run almost constantly. At around 11:00 PM when folks go to sleep... it may well be 18* outside... and the unit will have excess capacity... thus cycling.

    Hope this helps.
    GA-HVAC-Tech

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  11. #11
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,977
    Quote Originally Posted by garya505 View Post
    You anywhere near Dodgeville?
    I am around 10 miles SW of Lancaster, WI.
    Estimating as the crow flies, Dodgeville, WI is around 62 miles, from me.

    I live on a farm in a rural area between Bloomington, Beetown & Lancaster.

    Do you have relatives in Dodgeville? - Darrell

  12. #12
    Join Date
    Jun 2009
    Location
    Albuquerque NM
    Posts
    2,485
    Quote Originally Posted by udarrell View Post
    I am around 10 miles SW of Lancaster, WI.
    Estimating as the crow flies, Dodgeville, WI is around 62 miles, from me.

    I live on a farm in a rural area between Bloomington, Beetown & Lancaster.

    Do you have relatives in Dodgeville? - Darrell
    My grandfather was from Highland township in Iowa County. Looks like that is maybe 40 miles from you.

  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,935
    Quote Originally Posted by jeffkrol View Post
    Maybe too general of a question and since it's usually too late to really do anything about it maybe pointless but I'm curious.
    From reading here there are tell tale signs ie short cycling ect. but w/ so many different types of units (1 stage, 2 stage, modulating) sometimes things aren't easy to spot for non-HVAC people. In some sense this might help people who currently are considering replacement. Not to mention giving them some kind of bargaining power..
    I just replaced mine and none of the 5 estimates even bothered w/ any sort of analysis (I still think this is the norm in the real world) some "hints" might help people from making a costly mistake.
    I actually downgraded my furnace because of all the reading I did here. most estimates wanted to just replace the current BTU rating and in some cases go up a bit. Since my old 1 stage did fine as to heat in my leaky old house (major temp swings of course but never any lack being able to keep up in the coldest of nights) at 90,000BTU's (90 eff duomatic olsen 23 yrs old)
    and there are upgrades planned (which btw is also a wrench in any current analysis) on some windows (tons of single pane/alum storm combos probably every outside wall is 50+ glass, oh joy) but not possible for all that this is something homeowners should also take into consideration on a furnace upgrade. What is fine today may be over sized tomorrow.
    so maybe homeowner eduction based on what there system is currently doing and how to spot if it's not quite right is helpful. not to mention if a "guy" is just selling a bill of goods. Obviously some people just ignore any advice for a bottom line. some things may never change.
    As to my "downgrade" to 80,000btu w/ full mod furnace, sure is more comfortable and w/ these -10 plus nights and single digit days and how the furnace seems to be working it would be helpful to know what to look for... and to figure out if everyone did things right.
    As to cost, no major change one way or another but the electric bill is down a bit, even w/ the mod seemly (seemingly because I can't always tell low fire w/ out sticking my face to the duct) running a blower 24/7 lately so that is a hopeful sign..
    I suppose I shouldn't just pick on the heating aspect but I guess being a "northerner" it is just way more important....
    Pretty sure I could have gone down one more step and still not froze, but oh well....I'll leave that to the next owner.
    Did it run at full rate all night when it was -10 or colder outside to maintain your normal indoor set temp? If not, it's larger then you need. But the next size down may have been too small.
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