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  1. #1
    Join Date
    Feb 2012
    Posts
    12

    Sizing new furnace by observation of current one?

    Thank you HVAC-Talk for having this forum.

    I'm trying to educate myself as I near the time to having a new furnace & A/C installed. The current furnace is 100k BTU (I think - its a Rheem Criterion 4 burner) and the A/C is 3 tons.

    Based on everything I've read here, the furnace is probably oversized by a good bit (i.e. the guys in Canada and Minnesota possibly oversized with 60k BTU).

    Here is what I come up with on sizing:

    1) Observation of current furnace:
    At 32F outside (per HVAC-Calc the low temp for our area is 10F) and inside set to 69F, it runs for 5-6 minutes, waits 13-15 minutes, then does it over again.

    2) HVAC-Calc estimates:
    Infiltration set to 'average': 25.5k cool and 44.2k heat
    Infiltration set to 'excellent': 23.1k cool and 34.6k heat
    (house is standard vinyl-sided split-foyer built in 1992 with a vapor barrier, but a few leaky spots.)


    Can the real-world observation be extrapolated to indicate proper sizing or validate the HVAC-Calc result?

    How does a consumer know if the load calc done by a contractor is correct? Even with my (pretty good I think) data in HVAC-Calc, just the change of opinion on infiltration makes a big difference in result. I'm not trying to be a PIA customer, just improve my odds of getting the right thing.

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,250
    I'd do a 2 ton, 2.5 ton if 2 story for a little more ooomph upstairs. I'd probably do about a 45-50K 95%er.

  3. #3
    Join Date
    Feb 2012
    Posts
    12
    Thanks a bunch.

    If it matters, the house is 1.5 story-ish - main living level over unfinished, partially below-grade lower level and garage. The furnace is in the attic.

  4. #4
    Join Date
    Dec 2011
    Location
    PA
    Posts
    98
    Don't forget duct sizing... you may need to change current duct work to larger trunks or take-offs too.

  5. #5
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,703
    The 2 ton is probable okay, but I would probably go with a little larger output because the furnace and ductwork is in the cold attic.

    I would suggest a single stage furnace so when the heat is on ur getting good airflow out of the lower level registers.

    Or go with a variable-speed furnace and leave the blower run all the time. You can set the blower on a higher speed (continuous fan setting) when the burners are off.

  6. #6
    Join Date
    Feb 2012
    Posts
    12
    Quote Originally Posted by George2 View Post
    I would suggest a single stage furnace so when the heat is on ur getting good airflow out of the lower level registers.

    Or go with a variable-speed furnace and leave the blower run all the time. You can set the blower on a higher speed (continuous fan setting) when the burners are off.
    Thanks. I was convinced I definitely wanted a variable-speed, 2-stage furnace, but apparently this isn't entirely the smart idea I thought it was. I figured a variable speed would run for a longer time in low mode, making up for the reduced heat & CFM, and keeping the lower level heated roughly the same as it is now.

    I don't think the continuous fan idea will work since the wife can't stand cool air blowing on her, and even the slightest bit keeps her awake all night.

    I was kind of excited about the super coziness we would get out of a VS furnace, and appreciate you preventing me from insisting on one and being disappointed with the lower level heat.

  7. #7
    Join Date
    Aug 2011
    Location
    Eastern Shore, MD
    Posts
    798
    Quote Originally Posted by messer View Post
    Thanks. I was convinced I definitely wanted a variable-speed, 2-stage furnace, but apparently this isn't entirely the smart idea I thought it was. I figured a variable speed would run for a longer time in low mode, making up for the reduced heat & CFM, and keeping the lower level heated roughly the same as it is now.

    I don't think the continuous fan idea will work since the wife can't stand cool air blowing on her, and even the slightest bit keeps her awake all night.

    I was kind of excited about the super coziness we would get out of a VS furnace, and appreciate you preventing me from insisting on one and being disappointed with the lower level heat.
    Sounds like you may have a ducting issue if your wife is comPlaing about the air from the registers.i disagree on oversizing bc ur ducting is in the attic. Fix the duct and Properly size

  8. #8
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,703
    Quote Originally Posted by messer View Post
    Thanks. I was convinced I definitely wanted a variable-speed, 2-stage furnace, but apparently this isn't entirely the smart idea I thought it was. I figured a variable speed would run for a longer time in low mode, making up for the reduced heat & CFM, and keeping the lower level heated roughly the same as it is now.

    I don't think the continuous fan idea will work since the wife can't stand cool air blowing on her, and even the slightest bit keeps her awake all night.

    I was kind of excited about the super coziness we would get out of a VS furnace, and appreciate you preventing me from insisting on one and being disappointed with the lower level heat.
    Have you considered zoning? That would solve the low airflow problem to the basement area.

    The reason I thought a larger btu was because of the duct heat loss you'll have. Just a thought to keep in mind.

    If you decide to go with a single stage get the X13 motor.

  9. #9
    Join Date
    Mar 2006
    Location
    The South
    Posts
    2,209
    Messer

    I think a two stg 95%+ eff var speed furnace is your best choice.

    Of course, properly sized with a good ductwork inspection.

    Just for information, Rheem makes a nice mdl both in 60K and 45KBTU models.

    Link below.

    IMO

    http://www.rheem.com/documents/rgrm-specification-sheet

  10. #10
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Sizing by observing yoru current furnace and looking at the gas bil form a couple months and ocorrelating that to average monthly temeprature compared to design temperature will give you a good calculation of system sizing. Actually, I think it's better than load calculations because it uses real world data.

  11. #11
    Join Date
    Feb 2012
    Posts
    12
    Quote Originally Posted by George2 View Post
    Have you considered zoning? That would solve the low airflow problem to the basement area.
    Actually I thought about the possibility of dampers and/or auxilliary fans, but didn't realize that was called 'zoning'. My only experience with zoning is my wife's office where there are 4 zones (with four thermostats) and is a 'mess' according to my HVAC guy. It has a very large control box and I assumed it was pretty expensive. If it can be done simply and not crazy expensive, I'm all for it. It makes total sense to me.

  12. #12
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,703
    Quote Originally Posted by messer View Post
    Actually I thought about the possibility of dampers and/or auxilliary fans, but didn't realize that was called 'zoning'. My only experience with zoning is my wife's office where there are 4 zones (with four thermostats) and is a 'mess' according to my HVAC guy. It has a very large control box and I assumed it was pretty expensive. If it can be done simply and not crazy expensive, I'm all for it. It makes total sense to me.
    I've done a lot of zone systems with great success. Cost will depend on how the existing ductwork is run.

    Example: I've done two-story homes with 3 dampers and I've done two-story homes with 15 dampers.

    My older 2-story home has a 3 zone system I installed. What I noticed as a big improvement (from my manually balancing) was the increase in the airflow to the zone calling.

    My wife loves it and it reduced my utilities about 10%, maybe more.

  13. #13
    Join Date
    Feb 2012
    Posts
    12
    Quote Originally Posted by tigerdunes View Post
    Messer

    I think a two stg 95%+ eff var speed furnace is your best choice.

    Of course, properly sized with a good ductwork inspection.

    Just for information, Rheem makes a nice mdl both in 60K and 45KBTU models.

    Link below.

    IMO

    http://www.rheem.com/documents/rgrm-specification-sheet
    Rheem is what I've been leaning towards.

    But is 95% really OK in an attic with the condensate? If the condensate freezes in the attic, as some have stated it can, what is the result? Does the furnace just stop working or does it overflow?

    There is 10-12" of cellulose insulation, and last summer when the A/C overflowed, it soaked up enormous water (possibly for days) before finally bulging multiple ceilings down. I also don't know where the condensate would / can go, but draining outside there is no option. I was told (years ago) that it can't go into the house drain, but clearly the neigbhorhood doesn't have ice patches everywhere.

    That reminds me of another real-world observation - last summer when the A/C overflowed, two 5200 BTU window units, running continuously with blinds & curtains closed, kept the house sufficiently cool, even at 95F outside.

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