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  1. #27
    Join Date
    Jul 2001
    Location
    Gold Coast of Connecticut
    Posts
    4,624
    I am going to stay away from this. I do not believe for a minute that a 97.5 furnace will run at 50%. I do not believe a combustion test can show proper readings on modulating furnaces if thats the case.

    The new York. Luxaire vary the blower motor on all models, not just using 2 or 3 speeds either. Also the induced draft motor varies as well.

    Do not believe this thread.................!
    Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced

  2. #28
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,248
    Quote Originally Posted by Freezeking2000 View Post
    I am going to stay away from this. I do not believe for a minute that a 97.5 furnace will run at 50%. I do not believe a combustion test can show proper readings on modulating furnaces if thats the case.

    The new York. Luxaire vary the blower motor on all models, not just using 2 or 3 speeds either. Also the induced draft motor varies as well.

    Do not believe this thread.................!
    Well if I had not seen this very thing occurring in the field I would agree with you 100% freezeking.

    Problem with that is I have seen it.

    A lot of the issue boils down to how those efficiencies are actually derived in laboratory conditions.

    If we sell a customer a furnace that is rated at 90,000 BTUs output and this same piece of equipment combined with the duct system it's attached to only delivers 45,000 BTUs output into the building envelope; have we done our job?

    Looks to me like they are only getting 50% of what they paid for.
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  3. #29
    Join Date
    Oct 2008
    Location
    Michigan
    Posts
    29
    Looks like I stumbled onto a highly debated subject in asking what I thought would be a simple question with an answer like they both work about the same and you'll be happy with either...

    Since I have not signed with any contractor or decided on a brand but merely know that I want a high efficiency 95+ Variable speed unit to go with an Air Sorce Heat pump. I assume that the furnace won't be used except when the HP can't keep up.

    Would a typical installer setup the system, for GAS heat only in the high stage assuming the outside temp has already passed the need for 30KBTUh?

    One of the concerns I had was when the system changes over to Gas, will the furnace start at low fire and take awhile to figure out it really needs to be medium or high....

    Also, since the Sales people who have sent quotes don't really beleive in Manual J I did my own 36K gain / 88K loss Should I expect the HP to be sized higher than 3 Ton?

  4. #30
    Join Date
    Jan 2008
    Posts
    85
    Quote Originally Posted by davidr View Post
    Well if I had not seen this very thing occurring in the field I would agree with you 100% freezeking.

    Problem with that is I have seen it.

    A lot of the issue boils down to how those efficiencies are actually derived in laboratory conditions.

    If we sell a customer a furnace that is rated at 90,000 BTUs output and this same piece of equipment combined with the duct system it's attached to only delivers 45,000 BTUs output into the building envelope; have we done our job?

    Looks to me like they are only getting 50% of what they paid for.
    I had an 80% efficient Armstrong builder's special replaced with a 92.8 efficient Rheem mod last December. My seasonal savings based on heating-degree days was ZERO. My house consistently uses 0.083 (+- 0.02) Therms/HDD over the past four years.

    As far as I'm concerned, the 13% savings I was supposed to get was lost in the ductwork due to the lower average airflow.

  5. #31
    Join Date
    Oct 2008
    Location
    Michigan
    Posts
    29
    Since I caused the bickering, here's a link to a document I found on a government website on the subject. Looks like Captian CO has a basis for his statements. Now if only I could figure out the best way to buy a new system to keep on of you local guys employed...

    http://www.osti.gov/bridge/product.b...osti_id=804678

  6. #32
    Join Date
    Aug 2002
    Location
    Westlake, Ohio
    Posts
    2,492
    You are a consumer that asked a simple question and I answered with the truth based on real field testing and experience. A two stage variable speed 90% is the best selection but then the furnace needs to be tuned or adjusted for high fire only. Manufacturers have already started adding bypass switches or making thermostate bring on high fire first for some reason. Find a contractor that know the difference and actually measures system performance after he installs it.
    captain CO

  7. #33
    Join Date
    Jul 2006
    Location
    NW burbs of Detroit
    Posts
    6,058
    Man, this is turning into BS. i am not as versed as Robin or Davis, but when I set a furnace up to a temp rise in the middle of btw 45 and 75* and try to get as close to a 120* supply as I can, and clocking the meter tells me my furnace is putting out at the GAMA rating I know I have done my job.

    And five years worth of crunching customer gas bills against how many degree days over the period, I know my customers have saved money on their gas bills from the 90%ers I have installed.

    There is some dgree of self promotion here but I have never known Robin to a uninformed person.

    Hey Jim Davis are you ever going to answer that PM I sent you?

  8. #34
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,962
    Quote Originally Posted by Jim Davis View Post
    Correct, a btu is a btu, but the actual operating efficiency determines how many btus are needed to heat a house. A furnace running at 50% efficiciency needs 100,000 btus input to accomplish this. A furnace running at 75% efficiency only need 75,000 btus input to accomplish the same thing.

    Low fire operates below 50% efficiency on all equipment tested in the field to date regardless of its rating.
    Agreed. Also, the heat transfer rate is slower and less in low fire. I have abolutely had complaints that a new two stage furnace cost more to operate then the old furnace did. Every one of these furnaces were oversized.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  9. #35
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,962
    Quote Originally Posted by cpetku View Post
    Since I caused the bickering, here's a link to a document I found on a government website on the subject. Looks like Captian CO has a basis for his statements. Now if only I could figure out the best way to buy a new system to keep on of you local guys employed...

    http://www.osti.gov/bridge/product.b...osti_id=804678
    Captain CO is correct in the things he states. Unfortunately Capt. CO's technically accurate scary stories do not answer your question. Either of those furnaces, properly sized and properly installed will do you well. The modulating furnace has the capability of providing more comfort, but may not be able to be noticeable to the average person.

    The most important factor is that the furnace not be oversized. If the furnace is oversized, it will operate too often only in low fire and will be inneficient overall.

    Don't let the scary Captain upset you. His expertise is best put to use after the install if there is a problem that standard set up parameters won't address.

    I will say that if the Captains companies training were employed by every contractor, the world would be a better place. That does not mean that companies not employing the Captains training methods are doing anything wrong and that proper manufacturer set up will not provide comfort and efficiency.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  10. #36
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,962
    Quote Originally Posted by Jim Davis View Post
    You are a consumer that asked a simple question and I answered with the truth based on real field testing and experience. A two stage variable speed 90% is the best selection but then the furnace needs to be tuned or adjusted for high fire only. Manufacturers have already started adding bypass switches or making thermostate bring on high fire first for some reason. Find a contractor that know the difference and actually measures system performance after he installs it.
    The new algorythmic control boards that control the staging by furnace run time are set up to bring on the high fire more often, I believe for this very purpose along with burning off possible condensate in the primary heat exchangers. We are learning how to build better mousetraps.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  11. #37
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,962
    Quote Originally Posted by hvaclover View Post
    Man, this is turning into BS. i am not as versed as Robin or Davis, but when I set a furnace up to a temp rise in the middle of btw 45 and 75* and try to get as close to a 120* supply as I can, and clocking the meter tells me my furnace is putting out at the GAMA rating I know I have done my job.

    And five years worth of crunching customer gas bills against how many degree days over the period, I know my customers have saved money on their gas bills from the 90%ers I have installed.

    There is some dgree of self promotion here but I have never known Robin to a uninformed person.

    Hey Jim Davis are you ever going to answer that PM I sent you?
    Jim's specific technical knowledge could make me seem like a first grader on the subject of furnace combustion and I respect his teachings very much. Where Jim and I split is on how much of a buffer needs to be considered with furnace combustion set up and how much tweeking is actually worth the cost on all but special issue systems.

    Jim's intent is to have every technician be perfect on every job in the assumption that the HO will provide proper maintenance and that no oddities that occur often will occur with furnaces set up to his teachings standards.

    My intent is more practical in that I want technicians to leave a furnace in a manufacturer's specified condition to provide comfortable, efficient heat for the HO with enough leeway to account for problems caused by closing of vents, not changing the filters properly, installing too restrictive of filters, gas Btu values changing etc.

    What your parameters are for leaving a furnace are what I prefer technicians to do. Just don't put too much credence on that leaving air temperature. With a 60 degree temperature rise and 60 degree return air temperature, you will still have 120 degrees leaving air temperature. Raise that return air to 72 degrees and your leaving air will be 132 degrees.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  12. #38
    Join Date
    Jul 2006
    Location
    NW burbs of Detroit
    Posts
    6,058
    Quote Originally Posted by RoBoTeq View Post
    Jim's specific technical knowledge could make me seem like a first grader on the subject of furnace combustion and I respect his teachings very much. Where Jim and I split is on how much of a buffer needs to be considered with furnace combustion set up and how much tweeking is actually worth the cost on all but special issue systems.

    Jim's intent is to have every technician be perfect on every job in the assumption that the HO will provide proper maintenance and that no oddities that occur often will occur with furnaces set up to his teachings standards.

    My intent is more practical in that I want technicians to leave a furnace in a manufacturer's specified condition to provide comfortable, efficient heat for the HO with enough leeway to account for problems caused by closing of vents, not changing the filters properly, installing too restrictive of filters, gas Btu values changing etc.

    What your parameters are for leaving a furnace are what I prefer technicians to do. Just don't put too much credence on that leaving air temperature. With a 60 degree temperature rise and 60 degree return air temperature, you will still have 120 degrees leaving air temperature. Raise that return air to 72 degrees and your leaving air will be 132 degrees.
    Thanks Robin. 120* is an ideal I shoot for with a right sized 90% but it is not always atainable because of under size ducts. But I am trying to get a 45* rise which at 120 supply gives me 75* return. Gotta good ductsizing for that.

    Just want the ht ex to last longer than the warrany.

  13. #39
    Join Date
    Jul 2006
    Location
    NW burbs of Detroit
    Posts
    6,058
    Quote Originally Posted by RoBoTeq View Post
    Jim's specific technical knowledge could make me seem like a first grader on the subject of furnace combustion and I respect his teachings very much. Where Jim and I split is on how much of a buffer needs to be considered with furnace combustion set up and how much tweeking is actually worth the cost on all but special issue systems.

    Jim's intent is to have every technician be perfect on every job in the assumption that the HO will provide proper maintenance and that no oddities that occur often will occur with furnaces set up to his teachings standards.

    My intent is more practical in that I want technicians to leave a furnace in a manufacturer's specified condition to provide comfortable, efficient heat for the HO with enough leeway to account for problems caused by closing of vents, not changing the filters properly, installing too restrictive of filters, gas Btu values changing etc.

    What your parameters are for leaving a furnace are what I prefer technicians to do. Just don't put too much credence on that leaving air temperature. With a 60 degree temperature rise and 60 degree return air temperature, you will still have 120 degrees leaving air temperature. Raise that return air to 72 degrees and your leaving air will be 132 degrees.
    Thanks Robin. 120* is an ideal I shoot for with a right sized 90% but it is not always atainable because of under size ducts. But I am trying to get a 45* rise which at 120 supply gives me 75* return. Gotta have good ductsizing for that.

    Just want the ht ex to last longer than the warranty.

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