Results 1 to 12 of 12
  1. #1

    Default Temperature reading from gas furnace

    While taking temperature readings at heater vents, what are acceptable temp readings for furnace?
    With thermostat set at 90 degrees, I usually expect my reading to be 105-120 degrees.
    Last couple of houses I've been getting 160 - 180 degrees readingss What do you consider normal and what would could cause such hi readings?

    Thanks
    Mike

    Similar Threads:
    Inspection Referral SOC

  2. #2
    Join Date
    Feb 2008
    Location
    Chicago IL
    Posts
    1,969

    Default Re: Temperature reading from gas furnace

    Doesn't really matter what you set the Stat at. As long as the furnace is running it will create whatever heat level in the plenum that it normally creates (outside of being a modulating furnace of course)
    160-180 Plenum temp would not worry me. At the high end temp excess heat should shut the unit off via high limit so I'm not sure why you'd be worried about it during an HI. Granted the high limit could fail but so can anything else. 105-120 at the plenum would make me wonder about what's going on, that would be on the low side.
    I don't know you, I don't know what you do or your experience level so hopefully don't be offended.
    While checking temp at the furnace is nice its also kind of useless unless you have suspicions about the unit not functioning properly. If I suspect a furnace isn't working / firing properly I'll check temp at the unit.
    More importantly though it tends to be more productive to monitor heat gain throughout the house and temp at room registers. Unless the furnace is an old clunker HVAC problems aren't going to be at the furnace. Problems will likely be related to crappy duct work, duct work that isn't sealed so more air goes into cavities rather than actual rooms, poor register placement, undersized registers, etc. Those are the kinds of things a buyer will have to spend money on to actually get various spaces to heat / cool properly.
    Various manufacturer's make a Anemometer that you can use to check CFM. I use the Fieldpiece unit. Works great for sticking it to a flipper / developer. Room doesn't get warm and based on initial measurements installed duct work isn't providing sufficient CFM based on room size. Please fix mr. scumbag flipper.
    Hope that helps.

    www.aic-chicago.com
    773/844-4AIC
    "The Code is not a ceiling to reach but a floor to work up from"

  3. #3
    Join Date
    Mar 2007
    Location
    Plano, Texas
    Posts
    4,144

    Default Re: Temperature reading from gas furnace

    Yes, 160 to 180 discharge temperature is a little too high for my taste. Typical high limit setting is on the 165-180 F range so you would be bumping up against the limits during normal operation. The thermostat setting does not matter but the return air temp does. If your house temp is above normal (80 F or so) then your discharge temp will also be skewed above normal also.Che
    ck for clogged filters or other reasons for restricted air flow like blocked returns or bedroom vents shut down. Another possibility is overfiring due to excess gas pressure or improper conversion from natural gas to propane.

    Jim Luttrall
    www.MrInspector.net
    Plano, Texas

  4. #4
    Join Date
    Mar 2007
    Location
    Near Philly, Pa.
    Posts
    1,643

    Default Re: Temperature reading from gas furnace

    Look for the furnace rating plate which should list the maximum design supply temperature. Also, Ch. 16 in the IRC states above ground ducts should be have a maximum design temp. of 250F which is the same temp. referenced in NFPA 90B.

    Keep the fire in the fireplace.

  5. #5

    Default Re: Temperature reading from gas furnace

    Quote Originally Posted by Bob Harper View Post
    Look for the furnace rating plate which should list the maximum design supply temperature. Also, Ch. 16 in the IRC states above ground ducts should be have a maximum design temp. of 250F which is the same temp. referenced in NFPA 90B.
    Thanks for the input!
    Mike


  6. #6
    Join Date
    Mar 2010
    Location
    Prescott, AZ
    Posts
    85

    Default Re: Temperature reading from gas furnace

    Checking for a dirty filter and/or closed registers is very good advice. Two other things I've found frequently in my area is a very tight bend in the return air duct (most furnaces in my area are in the attic). And the insulation falling off in the blower compartment, falling on top of the fan and cutting airflow by 50%. I've found that twice in the last week. The fan tries to pull air through the insulation, I attached a photo of the insulation I pulled out of a furnace. The round dark area is where the fan was pulling air through the insulation. I noticed in newer furnaces this insulation is not just glued, it's secured with metal strips and screws. But I've also found that insulation coming loose.

    ***IMPORTANT*** You Need To Register To View Images ***IMPORTANT*** You Need To Register To View Images
    I have not failed. I've just found 10,000 ways that won't work. (Thomas Edison)

  7. #7
    Join Date
    Feb 2008
    Location
    Caledon, Ontario
    Posts
    5,005

    Default Re: Temperature reading from gas furnace

    No age of furnace, which leads me to ask if the blower is direct drive or belt driven. If belt driven, it is possible the belt could be loose, thus inadequate airflow over heat exchanger, thus higher temps.


  8. #8
    Join Date
    Jan 2011
    Location
    Mississauga, Canada
    Posts
    42

    Default Re: Temperature reading from gas furnace

    Interesting topic. I always check the temperature at the registers, but don't really check it at the furnace. If I was to check the temperature at the furnace where would be the exact location to capture that reading and what is the best tool for that?

    Mike Rodney
    Ontario, Canada

  9. #9
    Join Date
    Mar 2010
    Location
    Prescott, AZ
    Posts
    85

    Default Re: Temperature reading from gas furnace

    Quote Originally Posted by michael Rodney View Post
    Interesting topic. I always check the temperature at the registers, but don't really check it at the furnace. If I was to check the temperature at the furnace where would be the exact location to capture that reading and what is the best tool for that?

    In my area furnaces can be in the garage, crawlspace or a closet, but more commonly are in the attic or on the roof. I usually do not take temperatures at the air handler. I use a high quality infrared thermometer to check the temp at all supply vents (and another high tech device- an extending pole with a piece of surveyors tape on the end, which serves as my airflow tester, smoke alarm tester and Rottweiler Repellent).

    I know that the supply temp will be a little higher at the air handler than at the hottest temp I find at a supply vent. You have to consider other things. If the home is vacant in the winter I will run the furnace longer before testing the temperatures, but still have to consider the supply ducts are in a very cold attic.

    If the highest temp I find is a higher split or max than on the furnace label I document it and recommend an hvac contractor.

    I don't try to check the temperature at the air handler with an infrared thermometer. To get a truly accurate reading you would need to insert a probe through the return and supply ducts as close to the air handler as possible. And then of course patch the holes.

    A couple times a heating contractor made negative comments about my method of testing, but they also found something wrong. I figured they didn't want to waste an opportunity to 'correct' the home inspector.

    I have not failed. I've just found 10,000 ways that won't work. (Thomas Edison)

  10. #10
    Join Date
    Mar 2007
    Location
    Ormond Beach, Florida
    Posts
    25,976

    Default Re: Temperature reading from gas furnace

    Quote Originally Posted by Randy West View Post
    A couple times a heating contractor made negative comments about my method of testing, but they also found something wrong. I figured they didn't want to waste an opportunity to 'correct' the home inspector.
    That's because the home inspector is not measuring the air temperature, the home inspector is measuring the temperature of the metal and the inside of the ductwork, which is related to the temperature of the air ... but is a distant cousin of the temperature of the air.

    Being as you already have your extension pole, Velcor(TM) a digital thermometer to the end of your pole and actually measure the air temperature.

    The HVAC tech "didn't want to waste an opportunity to 'correct' the home inspector", the HVAC tech was actually trying to "help" the home inspector to keep the home inspector from repeating the incorrect measuring and reporting of the 'return "air" temperature' ... just sayin' ... the HVAC tech may have just tryin' to be helpful.

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  11. #11
    Join Date
    Mar 2010
    Location
    Prescott, AZ
    Posts
    85

    Default Re: Temperature reading from gas furnace

    Quote Originally Posted by Jerry Peck View Post
    That's because the home inspector is not measuring the air temperature, the home inspector is measuring the temperature of the metal and the inside of the ductwork, which is related to the temperature of the air ... but is a distant cousin of the temperature of the air.

    Being as you already have your extension pole, Velcor(TM) a digital thermometer to the end of your pole and actually measure the air temperature.

    The HVAC tech "didn't want to waste an opportunity to 'correct' the home inspector", the HVAC tech was actually trying to "help" the home inspector to keep the home inspector from repeating the incorrect measuring and reporting of the 'return "air" temperature' ... just sayin' ... the HVAC tech may have just tryin' to be helpful.

    I understand what you're saying and agree with you regarding the temperature readings. However, what the contractor said was the temp needs to be taken at the air handler, and that the "supply air temp does not matter", which I don't agree with. The IR thermometer is my basic/general test. I don't pull out my gas leak tracer at every inspection, only if I suspect (eg smell) a leak. I don't pull out digital thermometers or probes on every inspection, only if I suspect a problem, which the IR thermometer can help with.

    I am a 'generalist'. I do not claim to be testing the temp rise or split. In fact my report states that taking the temperature at the supply vents is NOT a measure of a furnace temp rise or an AC temp differential, but MAY help indicate the overall performance of the system.

    If no visible defects were noted (kinked duct, dirty filter) and I detect a very high temp I usually double check with my digital thermometer before recommending a contractor. And if feasible I may check the temps at the air handler.

    But just using the IR thermo, if the temp rise says 40 to 70, and I pick up a 100 degree difference at the return/supply registers, there is very likely something wrong. And if I pick up a 5 degree (or 50 degree) split with the AC using this method, there is likely something wrong. In either case I will be looking for something visibly wrong, and/or will do more accurate testing.

    Personally, I have found the IR to be less useful when testing the ac then the heat. Perhaps it's because of the lower temp difference, or maybe the register/duct cool more from cold air than they heat from warm air. And there is no exact range, like the temp rise on a furnace data label. I've heard 16 to 20 and 18 to 22 degree splits as 'ideal'. But as you have stated in other posts it can be outside that range when there's nothing wrong.

    I have not failed. I've just found 10,000 ways that won't work. (Thomas Edison)

  12. #12
    Join Date
    Feb 2008
    Location
    Caledon, Ontario
    Posts
    5,005

    Default Re: Temperature reading from gas furnace

    Randy

    According to this article you are correct in measuring the return air in formulations that may be required for Delta T and air flow..

    Airflow Calculation - HVAC Training Solutions

    See-
    7. Measure and record the return air temperature at the filter rack.
    8. *Locate a duct approximately 36″ away from the supply air plenum. Measure and record the supply air temperature at this point.


Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •