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  1. #14
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,052
    Quote Originally Posted by caterpillar View Post
    I did. Pacific NW. The other thing worth mentioning is that my house lacks any insulation and has single pane windows (typical for my area). This will get corrected eventually, but I am nowhere near as tight as a new house.
    If you intend on improving insulation & windows soon, I'd want that factored in the sizing.

    How cold do you get? We're similar to been's climate. 2000 with average insulation and windows probably would do just fine on 60K 90+ here too. Sis has 2400 sq ft 2 story, decent insulation, leaky windows, uninsulated crawl and heats to about -10° out with 75K 90+. Below that it loses ground. Bigger isn't better.

    Also dealers need to look at duct sizing. Today's 90s do not like a high temp rise. They are quick to shut down on their limit switch with a borderline sized duct system. These $3 limits tend to stick open if they have to open much.

  2. #15
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,877
    Quote Originally Posted by Poodle Head Mikey View Post
    So why isn't this an important consideration with a dual fuel system?

    PHM
    ---------------
    It is. But your talking a different context.
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  3. #16
    Quote Originally Posted by BaldLoonie View Post
    If you intend on improving insulation & windows soon, I'd want that factored in the sizing.

    How cold do you get? We're similar to been's climate. 2000 with average insulation and windows probably would do just fine on 60K 90+ here too. Sis has 2400 sq ft 2 story, decent insulation, leaky windows, uninsulated crawl and heats to about -10° out with 75K 90+. Below that it loses ground. Bigger isn't better.

    Also dealers need to look at duct sizing. Today's 90s do not like a high temp rise. They are quick to shut down on their limit switch with a borderline sized duct system. These $3 limits tend to stick open if they have to open much.
    We're most often between 45 and 75, although the rare historical lows are in the 20s. We had a handful of those this past year. High is not a consideration since there will be no AC. The ducts are being reused, so I am stuck with those.

    Can you explain the "temperature rise" comment?

  4. #17
    Quote Originally Posted by seatonheating View Post
    Hey caterpillar, you need a proper load calc done....bar-none. Those city forms aren't that accurate.


    If you are in the Seattle area give me a call, I can be of great assistance to you.

    Info in profile.
    So, I decided to do my own load calculation, since this will also allow me to see the efficacy of insulation in terms of BTU dollars.

    I bought a copy of HVAC-Calc Residential since it seems to have gotten some good reviews.

    The software put my house at just under 69,000 BTUs heat loss, which is a little less than my previous calculation, and exactly the effective output of a 75K furnace.

    So, installing a 75K would put me at the furnace's rated performance limit with no "safety", but insulating would lower my BTU loss by 10% or so, eventually giving me a small margin.

    Installing an 90K would give me an immediate 20% margin, but that would increase to 30-40% after insulating.

    What would you guys do? Am I looking a significant effciency/cost difference? In both cases, we are talking about a multistage furnace, so presumably a large one would be firing on the lower stage/lower speed more often than the smaller one.

  5. #18
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,877
    I would go with the smaller furnace.

    2 stage furnaces provide more comfort then single stage. They do not save fuel by being in first stage.
    The 75,000 will need to move 1250 CFM to have a 50° temp rise across the heat exchanger in second stage.

    The 90,000 will need to move 1500 CFM to have a 50° temp rise across the heat exchanger in second stage.

    Your duct system will be quieter with the smaller lower BTU furnace.
    And it doesn't take much to knock a couple 1000 BTUs off a houses heat loss.
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  6. #19
    Quote Originally Posted by beenthere View Post
    I would go with the smaller furnace.

    2 stage furnaces provide more comfort then single stage. They do not save fuel by being in first stage.
    The 75,000 will need to move 1250 CFM to have a 50° temp rise across the heat exchanger in second stage.

    The 90,000 will need to move 1500 CFM to have a 50° temp rise across the heat exchanger in second stage.

    Your duct system will be quieter with the smaller lower BTU furnace.
    And it doesn't take much to knock a couple 1000 BTUs off a houses heat loss.
    Wow, I did not realize that the smaller one would be quieter. I just assumed an automotive analogy - an idling V8 is quieter than a screaming 4 cylinder.

    So, the 75 is available with 3/4HP or 1/2HP blowers - the former giving more CFM than the latter. Which one will run quieter?

  7. #20
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,877
    You don't pick the blower that way.
    You pick it by the amount of air you need to move.
    And the duct work it has to work against.
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