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  1. #1

    Furnace Amp Draw vs Blower Consumption

    How much power does a typical furnace draw beyond what the blower requires? I'd assume not much...

    Here's the situation: As many know I recently had dual 80k XV95's and 3t XL16i HP's installed in my house. (I know, I still have to post pics. This weekend!) On the first floor, I replaced an 80% furnace and an XR12 AC (that is being re purposed.) After receiving the first new electric bill covering the first floor only, it's approx 60% higher than the same period last year. I checked weather data, and we had 156 cooling degree days both years. There have been no significant changes to my consumption patterns other than this equipment change. (Actually, I used to have a server that lived on 24/7 that is no longer in service...and I was out of town for 7 days this year...so if anything usage should have decreased!)

    I've been attempting to narrow down where the power drain is coming from by checking electricity usage of everything I can think of, using either a Kill-A-Watt type meter for plug devices, or a clamp meter for hard wired. The XL16i came in pretty close to its performance data, around 6-7 amps on stage 1, 10-11 amps on stage 2.

    The only surprise I had was the furnace, which appears to be drawing around 5-6 amps itself. I know there's a power factor which I don't know, but that seems awfully high to me. There is a CleanEffects in there, and it's difficult for me to find exact power draw for those, but it seems to be pretty low.

    So back to my question: Should I assume that number is almost entirely blower consumption, which would imply very high static pressure (based on the blower performance data) or some other anomaly, or is it normal for the furnace to use a few amps in addition to the blower?

    This is not a DIY. Believe me, for what I spent I have ZERO intention on messing with anything in the system lol. I just don't want to point fingers at the HVAC without some evidence that there is an issue there (even if doesn't explain the whole increase.) The contractor also redesigned/replaced the first floor duct system.

    Thanks!

  2. #2
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    The furnace transformer uses very little elctric.
    Around .3 amp.
    So your blower is whats using most of that amp draw.
    Undersized ducts can cause that with VS blowers.
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  3. #3
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    Here's an example of current draw for a 1/2 hp variable-speed fan motor as a function of static pressure, where air flow is constant at 1000 CFM:

    IWC Amps
    0.12 0.32
    0.18 0.36
    0.20 0.39
    0.27 0.48
    0.46 0.82
    0.55 0.93
    0.74 1.13

    At a constant air flow, current draw varies as a function of SP.

    Best regards,

    Bill

  4. #4
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    Ok.

    So whats the amp draw at 1.1" blower set to 1000 CFM, but only able to deliver 900 CFM.
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  5. #5
    Quote Originally Posted by beenthere View Post
    The furnace transformer uses very little elctric.
    Around .3 amp.
    So your blower is whats using most of that amp draw.
    Undersized ducts can cause that with VS blowers.
    Nuts. That's what I was assuming, but grasping on a little straw of hope that I was wrong. The performance data tops out at .9 for static pressure, with 3 ton unit set at 400 CFM/TON, drawing 420w. Taking a wild guess at a power factor, seems like mine might be pulling ~500

    I'm thinking the return is where my issue lies. All of the returns (7) feed into two fairly large trunks (don't have measurements, at work,) but those feed into an 8x24 drop to the furnace. The elbow at the furnace does have turning vanes (at least I have one thing going for me, lol.) The supplies are 9 6" runs off of two 8x16" trunks.

    I guess it's a Monday call to the installer.

  6. #6
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    9, 6" supplies for 1250 CFM is also under sized.
    That would average out to about 139CFM per supply. And 6" is too small for that CFM. That would come out to a FR of .17"(not to be confused with static pressure)

    Your installer should take individual static readings of both the supply and return.
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  7. #7
    Quote Originally Posted by beenthere View Post
    9, 6" supplies for 1250 CFM is also under sized.
    That would average out to about 139CFM per supply. And 6" is too small for that CFM. That would come out to a FR of .17"(not to be confused with static pressure)

    Your installer should take individual static readings of both the supply and return.
    Thanks beenthere. I actually questioned the size of the supply lines before they were installed and was assured they knew what they were doing, and they were the correct size to ensure good air mixing in the rooms. I looked at the guy skeptically, but figured "Eh, what do I know, really" and obviously conceded...

    Wouldn't it be 1200CFM if it's 400CFM/TON and a 3ton unit? I'm just curious as to whether that was a math error or if there is another factor in there I'm not aware of!

    I have no concrete reason yet to think they won't be responsive to the issue, but the fact that it happened at all leaves me very uneasy....

  8. #8
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    Quote Originally Posted by beenthere View Post

    So whats the amp draw at 1.1" blower set to 1000 CFM, but only able to deliver 900 CFM.
    Don't know. For the example above, I believe the fan 'curve' is constant only through 0.50". I thought the example would still provide good illustration, without actually including a few footnotes, to keep it simple.

    Best regards,

    Bill

  9. #9
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    Quote Originally Posted by a0128958 View Post
    Don't know. For the example above, I believe the fan 'curve' is constant only through 0.50". I thought the example would still provide good illustration, without actually including a few footnotes, to keep it simple.

    Best regards,

    Bill
    The example is ok. If the blower is delivering set CFM.
    But misleading for VS blowers that are working against static beyond what they can deliver set CFM
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  10. #10
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    Quote Originally Posted by beenthere View Post
    9, 6" supplies for 1250 CFM is also under sized. That would average out to about 139CFM per supply. And 6" is too small for that CFM.
    But wouldn't 6" be fine if it's round galvanized pipe versus flex? I.e., 175 cfm for 6" pipe (at 900 fpm)?

    Best regards,

    Bill

  11. #11
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    Quote Originally Posted by skunkiechris View Post

    Wouldn't it be 1200CFM if it's 400CFM/TON and a 3ton unit? I'm just curious as to whether that was a math error or if there is another factor in there I'm not aware of!
    LOL.. Don't ask me why I used 1250. I think I was thinking of 350 per ton, and just added the 50 to 1200.

    But that only drops it to an average of 133 per supply. Which is still about .145" FR.

    As far as good air mix. They didn't mislead you, its probably blowing out fast at most if not all registers. Which does give a good air mix.
    Just it can be loud, and use more electric then need be.

    WAG on my part. Your supply is exceeding .7" static.
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  12. #12
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    Quote Originally Posted by a0128958 View Post
    But wouldn't 6" be fine if it's round galvanized pipe versus flex? I.e., 175 cfm for 6" pipe (at 900 fpm)?

    Best regards,

    Bill
    I've been basing the FR on metal pipe. If he has flex its even higher yet.

    175 CFM in 6" metal, would be .22" FR. way too high, and too loud for resi.
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  13. #13
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    Quote Originally Posted by beenthere View Post
    I've been basing the FR on metal pipe. If he has flex its even higher yet.

    175 CFM in 6" metal, would be .22" FR. way too high, and too loud for resi.
    Thank you. Looks like max CFM for 6" metal should be about 118, assuming a desired fpm no higher than 600. This would be equivalent to a FR of about 0.12, I believe.

    Best regards,

    Bill

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