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Thread: Hot furnace

  1. #1

    Hot furnace

    Just received a call from a furnace guy that we had hired to perform seasonal cleaning on our Tempstar Superhigh efficiency 7000 unit which is approx.16 yrs old. He was concerned that he was getting temp reading in the heat run approximately 10 ft away from the furnace of 163 degrees. Temp reading on the door of the furnance was 190 degrees. He recommends we replace. Based on the temps and age of unit this makes sense to me. I have approx 1700 sg ft current furnace is sized at 100,000 BTU. He is recommending an 80,000 BTU unit.

    Does this seem reasonable?

  2. #2
    Join Date
    Feb 2008
    Location
    Altmar, New York, United States
    Posts
    4,318
    did he do a heat load or just guess?

  3. #3
    Join Date
    Jul 2007
    Location
    Mount Airy, MD
    Posts
    7,281

  4. #4
    Join Date
    Jul 2006
    Location
    Virginia
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    4,608
    I would want to know exactly what is wrong with my old one may be nothing ,it may be something a new furnace is not going to change .do not let him use ignorance as a sales tool,his reason for replacement is not adding up I would get a second opinion if he can not explain what is going on ,I agree that at 16 yrs it might be time for a new one. would not just do a box change as has been said demand a manual j load calculation and also ask him what the static pressure on the duct system is ,he should have checked static pressure when he found the system temperature to high to verify correct air flow .

  5. #5
    Join Date
    Sep 2002
    Location
    Northeast Ohio
    Posts
    4,842
    Do you have air conditioning installed with the furnace? I appears that the furnace is running on or close to the limit. Air flow or more correctly lack there of is the most likely culpret. This can be caused by several things, most of which can be remedied. 16 years old is getting into the golden years but depending on how well it's been maintained, there should still be some life left in it. If you decide on going with a new unit, require that you be provided with a heat loss calculation aka Manual J instead of a WAG as to what size furnace you require to meet design contions.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  6. #6
    Central air is installed with the furnace. Concerning the lack of airflow - would that be a lack of outside air drawn directly into the furnace or a need for additional cold air returns within the house?

    Also, what does a heat loss calculation tell me or what am I looking for with a heat loss calculation?

  7. #7
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,058
    Quote Originally Posted by Jrees17 View Post
    Central air is installed with the furnace. Concerning the lack of airflow - would that be a lack of outside air drawn directly into the furnace or a need for additional cold air returns within the house?

    Also, what does a heat loss calculation tell me or what am I looking for with a heat loss calculation?
    The lack of air flow could be on the return or supply or both.

    The manual J load calculation is based on area,windows, doors,insulation in walls,ceiling etc. It will tell you how many BTUs of heat or cooling is needed to maintain a design comfort level at an outdoor design temperature.

  8. #8
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,829
    Heat loss is the ACCA method of determining how many Btu's it will take to heat or cool your home at the selected design outdoor temperature. Based on the heat loss, the contractor selects the furnace that is most near the Btu output capacity indicated by the heat loss total, without going under the Btu's needed. Once the Btu needs have been established, the duct system can then be addressed as to proper sizing for the furnace/AC needs. Many manufacturers, for example, build mid-range furnaces in multiple different models, each having the same heating Btu's but with different blower assemblies producing different airflows for cooling purposes. It's important that the duct system be able to handle the cooling load airflows, which are typically higher than those for heating.

    At 1,700 square feet of house, I'd doubt the 100,000 Btu need and even doubt the 80,000 Btu need, unless the place has no wall insulation or some other uncommon today, anomaly. Please insist on a load analysis form any company proposing to install a new furnace and please understand that the furnaces of today are NOT built with the strength of materials of your old furnace. If the furnace installed is too big, you will suffer a premature failure of the primary heat exchanger. Sized properly, it should perform equally as well as your current furnace, albeit more comfortably.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

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