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  1. #1
    Join Date
    Feb 2014
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    Vancouver, BC, CAN
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    Undersized furnace?

    Went to a customers house today -
    He had a brand new 96% 60,000 BTU - 2 stage furnace installed 1 week ago.
    His old furnace was a 94% 80,000 BTU - 2 stage furnace.
    He had 4 quotes, 1 said for a 60,000 and the other 3 said 80,000.

    He is under the impression that the furnace he had installed is under sized because it runs a lot longer than his previous system. Which makes sense because it is less BTU but I personally think that it is sized correctly, and he is not used to it running longer. The house has 17 supply vents and 2=30"x7" return air (one up and one down).

    He lives in Vancouver, BC, Canada and his home is built in 1981, 2,800 sq. ft.
    He says the system runs all the time.

    I personally believe the system is working correctly and he is not used to it having to run longer.

    Your thoughts?

  2. #2
    Join Date
    Mar 2015
    Location
    Hamersville, Ohio
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    Depending on how Deep you want to get in to it, .....

    Run a Heat Loss/ Gain to determine the Actual Btu requirement.

    Find out the capabilities of this New Furnace, such as the CFM Delivery on heat.

    The R/A size is concerning to me, as well as the number of Supplies.

    As far as "Running all of the Time"? No, ... It shouldn't, the furnace should cycle upon satisfying the T-stat.

  3. #3
    Join Date
    Mar 2017
    Location
    Paducah, KY
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    Sounds like they need a proper load calculation performed.

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  5. #4
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    Jan 2010
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    I have a 2700 Sq.ft. house with an 80,000 btu 96% gas furnace that I bought in a similar climate zone.

    The thing barely runs at design temperature never mind at warmer temperatures. Don't need a manual J to tell me it is oversized.

    You customer may need the proof though.

    Let me guess....he likes a large setback and thinks it runs too long to make it up.

    Sent from my SM-N910W8 using Tapatalk

  6. #5
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    Feb 2014
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    Vancouver, BC, CAN
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    Thread Starter
    I may have to do the manual J on this.

    Would you say, as long as the furnace is able to get to the heat set point then the furnace is not undersized?

    To that question, is it inefficient / efficient / or the same if you had the 60 over the 80 for your cost on the gas bill if the 60 is able to satisfy that set point?

  7. #6
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    Feb 2014
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    Vancouver, BC, CAN
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    Thread Starter
    Quote Originally Posted by BALloyd View Post
    I have a 2700 Sq.ft. house with an 80,000 btu 96% gas furnace that I bought in a similar climate zone.

    The thing barely runs at design temperature never mind at warmer temperatures. Don't need a manual J to tell me it is oversized.

    You customer may need the proof though.

    Let me guess....he likes a large setback and thinks it runs too long to make it up.

    Sent from my SM-N910W8 using Tapatalk
    Well I don’t quite know if he does the large set backs but o know he was saying the furnace was keeping him up all night. He had an old Honeywell stat in before that he didn’t like so now has the new Ecobee 3 with a second sensor he puts in the kitchen ( same floor burn normally a little colder) then he said he got up in the middle of the night brought the wireless sensor to his bedroom at 4am and it was still turning on and off more than his last furnace.

  8. #7
    Join Date
    Jan 2004
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    PA
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    Its not the furnace, its the thermostat.

    Also, did the old thermostat control the staging, or did the furnace time to second stage.
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  9. #8
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    Sep 2008
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    VA
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    Did you explain the proper function of a two stage system? If it's maintaining set point at the lowest expected outdoor temperature then I would say you're sized properly. You don't need a load calculation for that.
    A load calculation is a "calculated" estimate. Real world data trumps load calculations IMO. IMO sizing a staging gas furnace on design temp and not lowest possible temp, is ridiculous. With that said, I'm constantly installing smaller furnaces than what was in the home prior.
    Extreme oversizing is not good either. I just went on a call in a 2 year old home ~2700sqft. It has a zoning system installed on a single stage 100,000btuh 95% furnace. I told her a 60,000 probably would have done the job just fine. The discharge air sensor was located on the bybass duct on the return plenum collar!
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  10. #9
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    Mar 2018
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    Quote Originally Posted by hammersheating View Post
    Well I don’t quite know if he does the large set backs but o know he was saying the furnace was keeping him up all night. He had an old Honeywell stat in before that he didn’t like so now has the new Ecobee 3 with a second sensor he puts in the kitchen ( same floor burn normally a little colder) then he said he got up in the middle of the night brought the wireless sensor to his bedroom at 4am and it was still turning on and off more than his last furnace.
    Couple of quick thoughts... Is the Ecobee controlling both stages or the furnace? Second, why a sensor in the kitchen? You wouldn’t install the thermostat in the kitchen, so you probably shouldn’t put the sensor in the kitchen either. Third, is the bedroom on a 2nd floor or main floor? Notoriously the 2nd floor will be much harder to get to set point than the main floor and will cause it to run much longer to hit that set point.


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  11. #10
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    Sep 2005
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    Atlanta GA area
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    Sizing, IMO, is more a measure of holding set point at lowest design temp (or below)...

    Run time is based on ductwork and circulation.

    IMO it is possible the furnace is running in low more than high, and is running longer to reach temp.
    Result is better circulation of air... which is a positive. Might explain that to the HO... and explain the low stage gas burn, as well as the lowered blower energy usage due to the ECM VS motor.
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  12. #11
    Join Date
    Feb 2014
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    Vancouver, BC, CAN
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    Thread Starter
    Quote Originally Posted by beenthere View Post
    Its not the furnace, its the thermostat.

    Also, did the old thermostat control the staging, or did the furnace time to second stage.
    The old furnace controlled the staging. Only 3 wires to the stat with no C wire and he was able to control the fan from his thermostat. Which leads me to believe the furnace controlled.

  13. #12
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    Aug 2002
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    Southold, NY
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    Quote Originally Posted by hammersheating View Post
    The old furnace controlled the staging. Only 3 wires to the stat with no C wire and he was able to control the fan from his thermostat. Which leads me to believe the furnace controlled.
    How can that be unless it’s Communicating!

  14. #13
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    Feb 2014
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    Thread Starter
    Quote Originally Posted by JeremyEd View Post
    Couple of quick thoughts... Is the Ecobee controlling both stages or the furnace? Second, why a sensor in the kitchen? You wouldn’t install the thermostat in the kitchen, so you probably shouldn’t put the sensor in the kitchen either. Third, is the bedroom on a 2nd floor or main floor? Notoriously the 2nd floor will be much harder to get to set point than the main floor and will cause it to run much longer to hit that set point.


    Sent from my iPhone using Tapatalk
    Furnace is controlling both stages. I checked what it would be to run more stat wires and not possible unless some major drywall work done. I am thinking about picking up a Fast-Stat 5000. I have never used before but looks good from reviews.

    I believe sensor is in Kitchen/ main living room (open space concept) and that it would average between the two sensors. You would normally put it in the room you were more in, correct? This is a rancher style home with a basement - furnace located downstairs and thermostat in hallway upstairs and sensor is upstairs in kitchen/ living room. Bedroom is on the main floor (not the basement). I am not sure if I am reading your response correctly but its normally hotter on the top floor where the t-stat is in my experience - unless there is a long duct run to a far upstairs bedroom like you mentioned.

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