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  1. #40
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    Quote Originally Posted by beenthere View Post
    I don't think there was ever a genreal concenses on it.

    I'd have to read the rating procedures again to tell you.

    I prefer to set temp rise to the mid point of the listed allowable rise. that gives room for the filter to get dirty and still provide a safe temp rise across the HX.

    If your allowable temp rise is 70°, and you set up a 54,000BTUs output firnace ,for a 60° rise, thats 833CFM. The air filter only needs to restrict a little to drop that air flow to 714CFM(or 14%), and now your at 70° rise.
    If you set it for a 55°, thats 909CFM, which eans air flow can drop almost 22% before the rise exceeds manufacturer specs.

    As trivial as that may sound. Many people with 4" media filters hold off as long as they can before changing that media.(heck, so do people with standard air filters)

    Plus, higher CFM from ceiling registers gives better through for heat. Lower the CFM you could lose throw, and make the house uncomfortable. Or as I said in one of the other post, just increase run time.
    Good point about dirty filters potentially bringing you closer to the danger zone...

    Keep in mind that many (most?) VS furnaces adjust airflow to a pre-determined number up to (usually) 0.5" esp (well I shouldn't technically say esp - it will compensate for the filter which is often in the furnace, or at least near it). This alleviates that fear somewhat - until the filter is so dirty it drives the static above 0.5".

    Maybe I'll measure the temp/o2/co of the exhaust with the different fan speed settings on my furnace one of these days if I get inspired. I suspect efficiency stays the same irregardless of airflow (within the temp rise variation set forth by the manufacturer).

  2. #41
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    Many VS blowers can move set CFM up to .6" TESP.(Atleast York can)
    After that they drop off. When is the last time you measured .5" or lower on a VS system. Too many times, VS blowers are used for a cure all for poor air flow.
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  3. #42
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    Jul 2003
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    Philadelphia Pa.
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    Quote Originally Posted by beenthere View Post
    Undersized ducts would be another variable in the equation.

    But, with under sized ducts, setting the blower to a lower speed also tends to leave the rooms at the end of the duct system several degrees cooler then the rest of the house.

    Is your furnace 95% efficient at a 60°, or a 40° temp rise.

    As far as a lower speed saving a noticablr amount of money in heat mode, its a wash. At the lower blower speed, the blower often runs longer to deliver the heat.

    124 watts in a 12 minute run time, cost just as much as 124 watts, in a 10 minute run time.

    But if your burner runs 2 minutes less on every heat call at a lower temp rise, it adds up on 1000 plus heat calls a year.
    The thought about the blower running longer on low speed and so the burner cycle lasting longer to deliver the heat is what prompted my question. Seems like hotter air but longer to arrive and perhaps more shallow distribution throughout the rooms.

  4. #43
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
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    14,915
    On consideration, at least in my cooling dominated climate, our ducts and supply diffusers are sized for cooling, and the supply outlets are at the ceiling level.
    So lets say we have a duct system designed for moving 1600 cfm in the cooling mode for a 4 ton AC. The duct system and diffusers are oversized for the typical 80kbtuh 80% furnace that would be found on a 4 ton AC system around here. The heating capacity would be over sized too, but we have to pick a furnace with a blower that will handle the AC requirement.

    This leaves us with a furnace that doesn't run for very long at a time, even on the coldest days. Since the ducts and air diffusers are oversized for the heating mode, we often have some issues with poor air mixing in rooms, and poor throws from the ceiling diffusers in the heating mode.

    The only real viable way I'm able to combat that problem is to jack up the heating airflow as high as I can and still have the temperature rise through the furnace in spec. We also install a 2 stage furnace better than 90% of the time, which helps a lot due to the longer run times, but makes the velocity problems from the ceiling diffusers even more of an issue.
    If more government is the answer, then it's a really stupid question.

  5. #44
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    Jan 2008
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    85
    Quote Originally Posted by larobj63 View Post
    Good info.

    So the answer to the question: Will slowing down the blower increase or decrease heat loss from duct in an un-conditioned space?

    It depends. It depends on the leakage, and it depends on the exposure, and it depends on the temp variation in said unconditioned space, among other things.

    Like most things in HVAC - no straight-forward answer for all cases...
    Duct heat losses via leakage vary little with CFM. Conductive heat losses in ducts on the other hand are always greater with lower CFMs. It's all explained here and is a real good read.

    http://www.osti.gov/bridge/product.b...osti_id=812518

  6. #45
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    Jul 2008
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    Quote Originally Posted by House View Post
    Duct heat losses via leakage vary little with CFM. Conductive heat losses in ducts on the other hand are always greater with lower CFMs. It's all explained here and is a real good read.

    http://www.osti.gov/bridge/product.b...osti_id=812518
    That is a good read - I will finish reading the whole paper later.

    While I conceed that when considering changing blower speed, what can be gained from the difference in duct leakage loss is often outweighed by the difference in conduction loss - it is worth noting that our example we were talking about in here differs from the discussion in the paper in one major way:

    We were dicussing the same btu input with varying airflow. They are discussing 1/2 the airflow with half the btu input - a stage 1 vs stage 2 discussion, so to speak.

    That being said - the principals are similar enough, I think, because with the same btu input and varying airflow, the temp of said leaking air also varies. In either case it seems the conduction loss variation is more significant than the leakage loss variation, quite contrary to what I originally thought several post back...

    Anyhow - thanks for the article...always learning...
    Last edited by larobj63; 09-24-2008 at 06:34 PM.

  7. #46
    Quote Originally Posted by beenthere View Post
    Many VS blowers can move set CFM up to .6" TESP.(Atleast York can)
    After that they drop off. When is the last time you measured .5" or lower on a VS system. Too many times, VS blowers are used for a cure all for poor air flow.
    So what happens to the VS blower when it is forced to operate above .6"? Does its electricity use go up exponentially, or does it just "free wheel" at the max current draw?

    I am asking because my static is currently at .90" at 1600, but only .40" at 1200 cfm. For this case, would it be better to go for the lower speeds, as long as temp rise is within spec?

  8. #47
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    What size/BTU furnace do you have? 120,000 90%, 100,000 90%, 80,000 90%?

    The higher the static a VS blower works a gainst, then higher the amp draw.
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  9. #48
    Quote Originally Posted by beenthere View Post
    What size/BTU furnace do you have? 120,000 90%, 100,000 90%, 80,000 90%?

    The higher the static a VS blower works against, then higher the amp draw.
    It's a 90,000 93% unit, with a 5 ton AC system. What I have found is that above a certain cfm threshold, the static does not change much, leading me to believe that the ECM/fan is basically "maxed out".

    I need to make some more measurements to get some intermediate points, but so far I have seen the following:

    cfm static
    1200 0.40"
    1600 0.90"
    1800 0.95"
    2000 1.05"

    So it seems like by about 1600 cfm, my ductwork is "maxed out", and I suspect that am not getting any more cfm delivered, despite the motor doing its best.

    How can I verify the actual airflow? The light on the circuit board is supposed to blink once per 100 cfm, and it is blinking the correct number of times for the setting. But I am wondering if it is just mimicking the setting, or somehow measuring the actual cfm and confirming that number via the blinking. BTW, the furnace is a Goodman GMV90905DXA.

  10. #49
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    I wouldn't slow it to 1200 CFM for heating. AA 90,000input 93% is 83,700BTUs output, 1200CFM would give you a temp rise of 64°.
    1400 is about the lowest I would go.

    At 1.05", I doubt your getting 2,000 CFM.

    Where are youo getting your static measurements from.
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  11. #50
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    Quote Originally Posted by beenthere View Post
    I wouldn't slow it to 1200 CFM for heating. AA 90,000input 93% is 83,700BTUs output, 1200CFM would give you a temp rise of 64°.
    1400 is about the lowest I would go.

    At 1.05", I doubt your getting 2,000 CFM.

    Where are youo getting your static measurements from.
    Sorry to jump in but that may be bad news for me. My trane xv95 is 100k btu input 95k btu output and my blower speed is set at ~1200 giving a static pressure of 0.65.
    The temp rise spec range is 35 - 65.
    My unit is set to only operate in stage 1. Am i looking at an issue other than paying too much for a stage 1 furnace?
    Key
    In the land of the blind.....the man with one eye is king....

  12. #51
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    In first stage at 1200 CFM your temp rise should only be about 48°.
    So your safe for temp rise in first stage.

    If you use it in second stage, your going to need to move 1400 CFM, that would put your static at about .88", and a temp rise of about 63°.

    So, worse case, only use second stage for that once every 3 or 4 years, that it drops 10° below normal low temps.
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  13. #52
    Quote Originally Posted by beenthere View Post
    I wouldn't slow it to 1200 CFM for heating. AA 90,000input 93% is 83,700BTUs output, 1200CFM would give you a temp rise of 64°.
    1400 is about the lowest I would go.
    I'm sorry, I should have mentioned that 1200 cfm is on 1st stage, so the BTU input is correspondingly reduced, giving about 47F temp rise. I was only collecting data on that speed to create a cfm/static curve.

    At 1.05", I doubt your getting 2,000 CFM.
    So do I . How can I check? So the blinking LED is not an indication of delivered cfm?

    Where are youo getting your static measurements from.
    Negative at inlet to furnace, positive between furnace and coil.

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