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  1. #1
    Join Date
    Jun 2010
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    Charlotte, NC
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    155

    Interpret my static air pressure numbers

    Today I ran a system check on my furnace for the first time, a Carrier 80% two stage (54k btu output on high and 36 k output on low, oversized I know, but about as small as I could get, and probably does not matter much as the heat pump generally runs except during morning warmup) installed with a two ton heat pump for dual fuel. It has an infinity thermostat and reports air flow cfm, motor rpm, and static pressure.

    First, are the numbers reported by the system generally reliable? Do ya'll go by them when they are available on the thermostat, or do you always take your own readings?

    My numbers were

    Low heat: Air flow 633 cfm, motor rpms maxed out at 654, and static pressure bounced between .27 and .28.

    High heat: Air flow 1033 cfm, motor rpms 1133, and static pressure of .78.

    Are these normal, acceptable numbers?

    Thanks

  2. #2
    Join Date
    Jul 2009
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    Columbia, MD
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    they are good. you only need 800cfm for high fire. i would have a professional lock it in hi fire and lower airflow to ~ 800cfm. this will save you in fuel consumption and heat your house quicker.

  3. #3
    Join Date
    Jun 2010
    Location
    Charlotte, NC
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    Thanks, I did not know I could do that. It sounds like good advice.

    I understand why locked into high fire would be economical, but why do I want to lower the cfms? Just wondering what the theory is behind it.

    Thanks

  4. #4
    Join Date
    Jan 2004
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    Lancaster PA
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    Quote Originally Posted by gravity View Post
    they are good. you only need 800cfm for high fire. i would have a professional lock it in hi fire and lower airflow to ~ 800cfm. this will save you in fuel consumption and heat your house quicker.
    Heating the house quicker doesn't save money on the heating bill. most people find the constant heat from low stage keeps them feeling warm at a lower thermostat setting. And saves them money on their heating bill.
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  5. #5
    Join Date
    Jan 2004
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    Quote Originally Posted by Maikerum View Post
    Today I ran a system check on my furnace for the first time, a Carrier 80% two stage (54k btu output on high and 36 k output on low, oversized I know, but about as small as I could get, and probably does not matter much as the heat pump generally runs except during morning warmup) installed with a two ton heat pump for dual fuel. It has an infinity thermostat and reports air flow cfm, motor rpm, and static pressure.

    First, are the numbers reported by the system generally reliable? Do ya'll go by them when they are available on the thermostat, or do you always take your own readings?

    My numbers were

    Low heat: Air flow 633 cfm, motor rpms maxed out at 654, and static pressure bounced between .27 and .28.

    High heat: Air flow 1033 cfm, motor rpms 1133, and static pressure of .78.

    Are these normal, acceptable numbers?

    Thanks
    Second stage blower sped is a bit high, lowered it would provide a better static.
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  6. #6
    Join Date
    Jul 2008
    Location
    Rochester NY
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    4,702

    Thumbs up

    Quote Originally Posted by beenthere View Post
    Heating the house quicker doesn't save money on the heating bill. most people find the constant heat from low stage keeps them feeling warm at a lower thermostat setting. And saves them money on their heating bill.

    Quote Originally Posted by beenthere View Post
    Second stage blower sped is a bit high, lowered it would provide a better static.
    .78 is terrible. .28 is awesome. Going a step further, probably easiest to simply lock out high fire. Slowing airflow at a stage reduces transfer efficiency X heat exchanger.

    Gravity may have recently taken a course that obsesses on combustion efficiency. These courses emphasize combustion efficiency as if it is the prime determinant in efficiently replacing heat loss. Usually takes about a year to correct the dogmatic and myopic perspective indoctrinated by these classes.

    Efficiency is maximized by keeping occupants comfortable, not by obsessing on a point or two of combustion efficiency (at steady state). High fire is often impediment to achieving comfort.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  7. #7
    Join Date
    Aug 2010
    Location
    West Monroe, LA
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    1,529
    If the system is oversized for heating. I would not expect the 2nd stage heat to come on unless you raise the temp on t-stat 10 degrees above set point. That is a very advanced system with many options and settings.

    Best thing to do is let the installer review over it and make changes for you. It could just be a simple setting that needs to be changed for better comfort. I agree with Beenthere, 2nd stage numbers look high by reducing the cfm down will allow for more even temps and want run such high static.

  8. #8
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    so you have a 60,000btu furnace. 54,36

    If the furnace was tuned to get the max btus then you would have ~ 54,000 output. 54,000/70,000input = 80% efficient
    now low fire; 36,000/70,000 = 51% efficient.

    high fire runs at 5 mins to heat the house
    low fire runs at 15 mins to heat the house

    i wonder which one is cheaper to run?

    now if you like longer run times with lower temp coming out the registers then go ahead with the 2 stage.
    if you want to save money and have hot air coming out, then keep it in high fire or get a single stage furnace.

  9. #9
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    Jan 2004
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    Lancaster PA
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    Quote Originally Posted by gravity View Post
    so you have a 60,000btu furnace. 54,36

    If the furnace was tuned to get the max btus then you would have ~ 54,000 output. 54,000/70,000input = 80% efficient
    now low fire; 36,000/70,000 = 51% efficient.

    high fire runs at 5 mins to heat the house
    low fire runs at 15 mins to heat the house

    i wonder which one is cheaper to run?

    now if you like longer run times with lower temp coming out the registers then go ahead with the 2 stage.
    if you want to save money and have hot air coming out, then keep it in high fire or get a single stage furnace.
    Using voodoo math there. Since in low fire it doesn't input 70,000BTUs to have an output of 36,000BTUs.
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  10. #10
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    Columbia, MD
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    Comfort and Efficiency are two totally different things.

    Efficiency is maximized by inputting and outputting what the appliance is designed to do!

    Comfort is about keeping a homeowner comfortable.

    You could have a very undersized furnace that is putting out its maximum efficiency but not keeping the customer comfortable.

    Give it time. People will start to become aware of these energy wasting furnaces.

    Or people who monitor their fuel bills will notice the bill getting higher with their new 98%AFUE furnace. Which is nearly impossible to get 98% efficiency out of a furnace. 88% is the maximum you can get out of a furnace.

  11. #11
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    Jun 2010
    Location
    Charlotte, NC
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    In this case and for my specific application, I agree with Gravity, even over Beenthere and Tedkidd, both of whom I respect very much and have learned a lot from.

    The reason is that I like to do a night setback and drop the temp from 71 to 65 overnight. Most nights the temp only drops to 66-68 overnight, though on very cold nights it will go down to 65 by morning.

    When I have the system kick back on, at 5:30 in the morning, 15 minutes before my son has to get up for high school, I want the house to warm up fast and that is when the heat pump is at its least efficient, it being cold outside. So I would be glad for the furnace to come on high fire to get it quickly back to 71. Once it does, the two ton heat pump will take over for the rest of the day and provide slow even cost-effective heat all day.

    Right now the furnace runs about twice as often in low stage as in high, and I would prefer it to be locked in high to get maximum efficiency for that short period of time in the morning the furnace runs.

    My manual J had my heat loss at design conditions to be 34,000. So it is true I could get by with only low stage, but why use the furnace at its least efficient?

    On the other hand, I would like to have low stage furnace come on during those coldest nights when the indoor temp does dip below 65 overnight. Wonder if I can program it to do that while always going to high fire when during morning warm up.

  12. #12
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    Quote Originally Posted by Maikerum View Post
    In this case and for my specific application, I agree with Gravity, even over Beenthere and Tedkidd, both of whom I respect very much and have learned a lot from.

    The reason is that I like to do a night setback and drop the temp from 71 to 65 overnight. Most nights the temp only drops to 66-68 overnight, though on very cold nights it will go down to 65 by morning.

    When I have the system kick back on, at 5:30 in the morning, 15 minutes before my son has to get up for high school, I want the house to warm up fast and that is when the heat pump is at its least efficient, it being cold outside. So I would be glad for the furnace to come on high fire to get it quickly back to 71. Once it does, the two ton heat pump will take over for the rest of the day and provide slow even cost-effective heat all day.

    Right now the furnace runs about twice as often in low stage as in high, and I would prefer it to be locked in high to get maximum efficiency for that short period of time in the morning the furnace runs.

    My manual J had my heat loss at design conditions to be 34,000. So it is true I could get by with only low stage, but why use the furnace at its least efficient?

    On the other hand, I would like to have low stage furnace come on during those coldest nights when the indoor temp does dip below 65 overnight. Wonder if I can program it to do that while always going to high fire when during morning warm up.
    Get a Honeywell IAQ thermostat then. It can control both stages of your gas furnace. Coming out of set back it will use second stage. On normal days/nights when the heat pump and first stage can't keep up, it will know to use second stage. Giving you long run times in first stage, and going to second stage if need be. No sudden temp swings. All other times it will use the heat pump or just first stage of the gas furnace.

    If you use AI recovery, it will start to heat teh house up early, and may not even have to use second stage.
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  13. #13
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    Quote Originally Posted by Maikerum View Post
    The reason is that I like to do a night setback and drop the temp from 71 to 65 overnight. Most nights the temp only drops to 66-68 overnight, though on very cold nights it will go down to 65 by morning.
    Why do you like to do that?

    Quote Originally Posted by Maikerum View Post
    Right now the furnace runs about twice as often in low stage as in high, and I would prefer it to be locked in high to get maximum efficiency for that short period of time in the morning the furnace runs.
    Why would you prefer that? What do you mean by maximum efficiency?

    My manual J had my heat loss at design conditions to be 34,000. So it is true I could get by with only low stage, but why use the furnace at its least efficient?
    Why would you think that's least efficient?

    What is your static pressure at low, and high?
    What is your duct leakage at low? At high?
    What pressure imbalances occur in your rooms at low? At high?
    How much pressure induced leakage is occurring at low fire? At high?
    How much heat gets delivered to occupied space per therm burned on low? How much on high?
    And at what temperature is comfort achieve with a low fan rate? How much warmer must the setting be for a rapidly cycling high blowing fan rate?
    You aren't buying temperature, you are buying comfort. http://bit.ly/comfortcalculator

    If you look at this just looking at one tiny slice, it's like your nose is pressed up against a work of art. Would you really do THAT and think you were seeing the whole picture?
    Last edited by tedkidd; 12-01-2012 at 10:42 PM.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

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