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  1. #27
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    Quote Originally Posted by beenthere View Post
    That would be an indication that more attention should have been given to the duct work, then the blower speed.

    If you have that much leakage at 50° rise. Then sealing the ducts will do far more then just slowing the blower down to a 60° rise.
    Well, true enough! And that's exactly my point...

    Every duct system leaks something - which is why your original statement is risky IMO. The leakage loss in a duct system outwieghs the heat loss from radiation most of the time. Every joint and every connection must be considered. So: more airflow = more leakage = more heat loss; or, less airflow = less leakage = less heat loss.

    I don't think it's safe to say that slowing down the blower will decrease system efficiency becasue you increase heat loss in un-conditioned spaces where ductwork is. I'm saying it's quite the contrary in most cases. If ductwork were sealed perfectly, your statement would be true. In practice, ductwork leaks air...

  2. #28
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    Interesting point.

    So lets say, slowing the blower to go from a supply duct temp of 115° to 125° is a 32% reduction in static pressure.
    Is the leakage reduced by 32%, or just 3.2%.

    Going from 115 to 125 increases supply duct loss by 5%.
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  3. #29
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    Quote Originally Posted by beenthere View Post
    Interesting point.

    So lets say, slowing the blower to go from a supply duct temp of 115° to 125° is a 32% reduction in static pressure.
    Is the leakage reduced by 32%, or just 3.2%.

    Going from 115 to 125 increases supply duct loss by 5%.
    I have analyzed this in the past by considering leakage a percentage of total airflow, independant of static pressure.

    I just did some rough calcs - and I think you're right. Duct leakage would have to be 20% or more to make my assertion correct...

    At 115* supply, 1,000 cfm delivers 48,600 btu
    At 126.25* supply, 800 cfm deleivers the same btu

    Difference of 200 cfm. "Savings" of 40 cfm at 20% leakage rate. 40/800 = 5% = 2,430 btu

    20% leakage is very high. If the leakage rate were 10% (more typical), you would only "save" 20 cfm, which would not out-weigh the 5% increase in supply duct loss from radiation. 20/800 = 2.5% = 1,215 btu

    Good excersize - and I aplogize for being wrong!

  4. #30
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    Not a thing of being right or wrong.

    Its more a thing of being closed or open minded.

    A closed mind believes what they are told is true, an open mind seeks the answer.
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  5. #31
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    Quote Originally Posted by beenthere View Post
    Not a thing of being right or wrong.

    Its more a thing of being closed or open minded.

    A closed mind believes what they are told is true, an open mind seeks the answer.
    That's why we do peyote in the desert - expand the mind and seek the truth.

    Seriously though, I agree, and I will never shy away from admitting I could be wrong (or closed minded ) - I have a habbit of questioning everything until I can prove out the answer to myself...

    The above approach is very simplistic - there are many variables (as you know) - duct insulation, sq footage of exposure in unconditioned space, airflow in unconditioned space, etc, etc.

  6. #32
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    Quote Originally Posted by larobj63 View Post
    20% leakage is very high. If the leakage rate were 10% (more typical)
    If only that were true.

    At least in my area, leakage rates of >20% are more common than leakage rates of 10% or less.

    I've even found a couple of flex duct systems, in 3-4 year old houses, that were "sealed" with mastic, but still had >20% leakage because the RNC installers around here seem to only put mastic on the outside of the vapor barrier where the inspector can see it, and none at all on their sloppy start collar, duct and plenum connections where the air actually leaks.
    If more government is the answer, then it's a really stupid question.

  7. #33
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    Quote Originally Posted by mark beiser View Post
    If only that were true.

    At least in my area, leakage rates of >20% are more common than leakage rates of 10% or less.

    I've even found a couple of flex duct systems, in 3-4 year old houses, that were "sealed" with mastic, but still had >20% leakage because the RNC installers around here seem to only put mastic on the outside of the vapor barrier where the inspector can see it, and none at all on their sloppy start collar, duct and plenum connections where the air actually leaks.
    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...

  8. #34
    This is interesting discussion, and it seems to be focusing on gas use. What about electricity use?

    As another poster mentioned, higher speeds will have to operate against higher static, which makes the motor work harder, increasing the electric consumption. This would be especially noticable in houses with undersized ducts, which the consensus seems to be is the majority of homes. Around here at least, electricity is much more expensive than gas, so savings on electric might overshadow BTU losses.

    Having read my Goodman instructions, they recommend the lower speeds:

    "In general lower heating speeds will: reduce electrical consumption,
    lower operating sound levels of the blower, and increase the
    outlet air temperature delivered to the home. The speeds available
    allow the blower performance to be optimized for the particular
    homeowner’s needs."

    They go on to recommend the lowest speed setting, as long as temp rise limits are met:

    "Example: The GMV95704BXA is set for 990 CFM on
    cooling, the “ADJUST” is set to “-” (minus).
    The four heating speeds available are “A
    Minus”, “B Minus”, “C Minus”, and “D Minus”.
    “A Minus” has a rise of 56°F for both stages
    which is within the 30-60°F rise range for the
    GMV95704BXA. This setting will keep
    electrical consumption to a minimum. Set the
    “Heat” speed DIP switches to “A”."

    Thoughts?

  9. #35
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    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.
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  10. #36
    Quote Originally Posted by beenthere View Post
    124 watts in a 12 minute run time, cost just as much as 124 watts, in a 10 minute run time.
    Wouldn't the first use 20% more electricity than the second?

    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.
    Why would your burner run time depend on the blower speed? Not counting losses (and maybe this is where you are coming from), total heat rate delivered is what the burners produce, and that is fixed. So if you need X BTU/hr to heat the home, you can achieve that with either (High cfm * low delta T) or (low cfm * high delta T), but the run time should be unchanged.

    Since the heat being generated by the burners is fixed in the furnace, the question is which approach minimizes losses - low delta T reduces duct losses, but low cfm reduces motor electrical consumption. Perhaps it's a wash, as you said .

  11. #37
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    Quote Originally Posted by beenthere View Post
    Is your furnace 95% efficient at a 60°, or a 40° temp rise.
    I know this has been discussed before - what is the general concencus?

  12. #38
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    Quote Originally Posted by mark johnson View Post
    Wouldn't the first use 20% more electricity than the second?

    Why would your burner run time depend on the blower speed? Not counting losses (and maybe this is where you are coming from), total heat rate delivered is what the burners produce, and that is fixed. So if you need X BTU/hr to heat the home, you can achieve that with either (High cfm * low delta T) or (low cfm * high delta T), but the run time should be unchanged.

    Since the heat being generated by the burners is fixed in the furnace, the question is which approach minimizes losses - low delta T reduces duct losses, but low cfm reduces motor electrical consumption. Perhaps it's a wash, as you said .
    The input by the burners remains the same.
    But how much is being delivered to the rooms at different blower speeds/CFM's is the question.
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  13. #39
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    Quote Originally Posted by larobj63 View Post
    I know this has been discussed before - what is the general concencus?
    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.
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