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  1. #27
    Join Date
    Oct 2009
    Location
    Raleigh, NC
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    776
    Quote Originally Posted by beenthere View Post
    The 1.6 to 2.2" and 3.2 to 3.8" are the allowable adjustment range. This is so you can allow for the BTU content of the gas your supplier provides. Yes, your probably under fired.

    Set both stages up by clocking your meter for the BTU content of your gas. And then make minor adjustments as a CA and temp rise may indicate are needed.
    No one has really commented on the fact that I am within range for temp rise (30-60). I am running about 40 temp rise measured about 6 feet downstream from the furnace. My primary concern is to make sure that I am not having condensation in the primary HX. I don't really care that I might be underfired if I am not hurting anything. Is it not true that as long as I am making the published rise that I am good as far as condensation is concerned?

  2. #28
    Join Date
    Feb 2010
    Location
    In a boiler room
    Posts
    6,915
    Quote Originally Posted by MicahWes View Post
    I don't really care that I might be underfired if I am not hurting anything.
    You don't really care if you have to buy more gas than you would have to?? Because I can assure you positively that it will use more gas at those numbers than if it was properly fired.

    And just because you are in range for temp rise is no guarantee that there won't be any condensation in the primary.

    You seem like some of my customers, they only want to believe what they want to, and not necessarily what is true. Good luck man, the gas company should thank you.

  3. #29
    Join Date
    Oct 2009
    Location
    Raleigh, NC
    Posts
    776
    Quote Originally Posted by chuckcrj View Post
    You don't really care if you have to buy more gas than you would have to?? Because I can assure you positively that it will use more gas at those numbers than if it was properly fired.

    And just because you are in range for temp rise is no guarantee that there won't be any condensation in the primary.

    You seem like some of my customers, they only want to believe what they want to, and not necessarily what is true. Good luck man, the gas company should thank you.
    I know exactly how much gas I am using, and it sure isn't much.

    So, are you saying that if I burn 100 cu ft of gas at low fire, and 100 cu ft of gas at high fire, that I getting less heat from the furnace per cu ft of gas consumed when using low fire?

    If this is positively true, please explain it to me. Something to do with the temp difference between the HX and the air passing over it maybe?

  4. #30
    Join Date
    Jul 2005
    Posts
    5,153
    Quote Originally Posted by MicahWes View Post
    I know exactly how much gas I am using, and it sure isn't much.

    So, are you saying that if I burn 100 cu ft of gas at low fire, and 100 cu ft of gas at high fire, that I getting less heat from the furnace per cu ft of gas consumed when using low fire?

    If this is positively true, please explain it to me. Something to do with the temp difference between the HX and the air passing over it maybe?
    For low fire your CB drops both the blower and inducer speeds to maintain a correct hx temp and efficient air/fuel ratio. Manually reducing the input rate results in a different outcome. Excess air will be too high and the primary hx will run too cool. It's the increase in excess air that causes the drop in efficiency. The cooler hx actually acts to increase efficiency, but not enough to offset the losses due to the higher excess air.

  5. #31
    Join Date
    Oct 2009
    Location
    Raleigh, NC
    Posts
    776
    Quote Originally Posted by hvacrmedic View Post
    For low fire your CB drops both the blower and inducer speeds to maintain a correct hx temp and efficient air/fuel ratio. Manually reducing the input rate results in a different outcome. Excess air will be too high and the primary hx will run too cool. It's the increase in excess air that causes the drop in efficiency. The cooler hx actually acts to increase efficiency, but not enough to offset the losses due to the higher excess air.
    Oh, so you are saying that the furnace being underfired in general makes it less efficient, not that running in LOW STAGE in a properly fired furnace will make it less efficient. I think I have gotten confused between the posts saying that I am using more gas because I am (might be) generally underfired, and the ones that say low-fire is just intrinsically less efficient than high fire (when input is within range). I do not believe the latter.

    I measured my flue temps just now at 123F, and temp rise is still about 40F.

  6. #32
    Join Date
    Jan 2013
    Location
    Champaign Illinois
    Posts
    97
    Sounds to me, if I'm understanding the situation properly. That you may have an air flow issue, duct size, plenum height, filter size or return size. If your gas valve is set at minimum range yet your temp diff. Is near max then I would think your not moving enough air if you want or need to up the gas pressure. On the other hand if your within all ranges provided by manufactur I would assume that the chance of condensation in the primary Hx would not be an issue. Again the best thing to do is to use the CA. And make adjustments as needed. If rising the gas pressure is required and this causes TD to be out of range look at air flow.

  7. #33
    Join Date
    Oct 2009
    Location
    Raleigh, NC
    Posts
    776
    Temp rise is not near max. It is about 40 degrees for both stages. Published range is 30-60. I have the manifold pressure set to the minimum specified because I wanted low output temps, not because raising it puts the temp rise out of range. If I set the manifold pressure at the nominal published pressures, the temp rise is about 58 degrees.

  8. #34
    Join Date
    Jan 2013
    Location
    Champaign Illinois
    Posts
    97
    I would say your good then. I would think if everything is within range it wouldn't condense in primary. If your still worried about it let it run for a month or two, take the collector box off and inspect for signs of condensation. I am still a little confused about the posts on efficancy though, I always thought things worked more efficantly when things were low and slow. Hence the use of 2 stages. In my mind 1st stage should be most efficantly and the only reason for 2nd is that in a high load situation the 1st stage would not be sufficient to maintain comfort. Maybee there's something I don't know about so if I'm wrong please let me know.

  9. #35
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,808
    Quote Originally Posted by MicahWes View Post
    No one has really commented on the fact that I am within range for temp rise (30-60). I am running about 40 temp rise measured about 6 feet downstream from the furnace. My primary concern is to make sure that I am not having condensation in the primary HX. I don't really care that I might be underfired if I am not hurting anything. Is it not true that as long as I am making the published rise that I am good as far as condensation is concerned?
    Under fired like that you can still condensate in the primary. Plus, your efficiency is a lot lower because its under fired. Your excess air will be very high. You have very little IR heat transfer because of the low firing rate.
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  10. #36
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    66,808
    Quote Originally Posted by Jkb79 View Post
    I would say your good then. I would think if everything is within range it wouldn't condense in primary. If your still worried about it let it run for a month or two, take the collector box off and inspect for signs of condensation. I am still a little confused about the posts on efficancy though, I always thought things worked more efficantly when things were low and slow. Hence the use of 2 stages. In my mind 1st stage should be most efficantly and the only reason for 2nd is that in a high load situation the 1st stage would not be sufficient to maintain comfort. Maybee there's something I don't know about so if I'm wrong please let me know.
    1st stage will be less efficient. People save money by getting 2 stage units usually because they can set their stat set temp 2 degrees or so lower and feel just as comfortable as they did with the old unit at the higher indoor temp.
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  11. #37
    Join Date
    Oct 2009
    Location
    Raleigh, NC
    Posts
    776
    WHY is 1st stage less efficient???

  12. #38
    Join Date
    Jan 2013
    Location
    Champaign Illinois
    Posts
    97
    I guess I understand that in 1st stage that the furnace would not have the full capacity of the entire house, and I can see how if 1st stage is allowed to run for a very long time how it would be less efficant than at full fire. I guess maybee my understanding of the fire efficacy between first and second stage is incorrect. My view point on beeing able to drop the temp a degree or two because I have a 2 stage system could only come from the fact that when the furnace is in first stage it does not remove as much humidity from the air as compared to the old single stager. So if it is less efficant to have first stage on then why don't I just install a humidifier on the singlet stage furnace and keep humidity up which would allow for a degree or two drop in the stat due to the fact that the more humid air would make me feel warmer. I'm not trying to make anyone mad or anything I'm just trying to learn. Please help me to understand thanks.

  13. #39
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    5,821
    Warm air coming from vents for longer time rather than hot air for 15mins then off back on ......off.....

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