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I'm a new homeowner (1.5 years), and there was a new gas furnace installed just before we moved in.

A month or so ago, the furnace started making a whistling noise for a second or two just at the beginning of a heating cycle, and then the noise would go away for the rest of the cycle. I checked the filter and it was clean. After a couple of days of whistling, the high limit switch opened, which I presume (based on my reading) indicates some sort of overheating, likely (?) due to an airflow issue. A manual reset of the limit switch gets the system working again for another day or so (still with the whistle), and then the limit switch opens again.

I've had three different HVAC techs look at the unit, and there is no consensus on what will address the issue. Here is a summary of their suggestions:

  1. Clean the evaporator coil. He said a dirty evaporator coil could be the cause of the airflow restriction. I'm a little confused here though, because the AC works fine (no whistle), which I'm guessing means that there is no restriction from the ev coil.

  2. Add more return vents. Currently there are two return vents, both about 2ft X 1ft. One is directly under the unit, and the other is down the hallway, connected to the unit under the house through the crawl space. Both are unobstructed, and can keep a piece of paper lightly sucked up against them when the unit is on. The one under the unit has stronger suction. It's a 60000 BTU furnace for a 1350 square foot home, so from what I've read it seems like the two return vents should be sufficient.

  3. Check the inducer motor and/or flue pipe for a restriction. He noticed that when the unit whistles for a second or two at the beginning of a cycle, the inducer motor visibly slows down. He also said the motor may be a bad part, but the furnace is fairly new.

With no consensus, I'm left with a difficult decision. Do I get a fourth opinion? These diagnosis fees are adding up though. I'd greatly appreciate any advice or teaching anyone can provide here. I'm happy to provide pictures/more info. I could also try to pinpoint where the whistling is coming from if that would help (my guess is the flue pipe right where it comes out of the unit). Thanks in advance.

Edit: I went on the roof today, and heard the whistling coming through the exhaust vent very loudly. Is this further evidence of a clogged flue pipe?

Also, if it is helpful, the brand of the unit is Day and Night, and the model number is N/R80ESU.

Another edit: Here are some pics/video that could help? Here is the inducer motor gasket - it looks pretty worn out. Is this normal for an 18-month old furnace?

enter image description here

Here is the description of the fault code I'm seeing (4 flashing red lights)

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Here is a video of the cycle starting up. The whistling noise occurs a little after the minute mark.

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  • I will take a SWAG: Take the filters out of the vents!
    – Gil
    Commented Jan 22, 2022 at 16:22
  • Are there any electric dampers of any kind? Flue dampers or duct dampers? All air registers fully open? Air flowing from all registers? Have you checked combustion air vents, both intake and exhaust, for obstructions? (Bird or rodent nest? Dead critters?) Is the blower multispeed or variable speed? Might be helpful to add the brand and model number of the furnace to your question.
    – MTA
    Commented Jan 22, 2022 at 16:33
  • @MTA: I don't think there are any dampers (how could I check?). All air registers are fully open, and air is flowing. The intake vents seem clear - they both provide good suction. It looks like I have a multispeed blower. Notice the edit to the post, which provides brand and model, and indicates the whistle is coming from the flue. If I wanted to clean it, would I take the vent off at the roof, or take the pipe off at the unit inside the house?
    – Jared
    Commented Jan 22, 2022 at 21:35
  • @Jared A gas furnace flue should never need to be cleaned, though it may be obstructed by a foreign object at the roof end or at the first bend from the roof end.
    – MTA
    Commented Jan 23, 2022 at 16:19
  • @MTA: I see. I checked the flue for obstruction (all the way from the roof to the unit), and found nothing.
    – Jared
    Commented Jan 23, 2022 at 23:26

1 Answer 1

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I'll make several suggestions that you can evaluate before you call for another diagnosis:

  1. The whistling sound from the flue that only lasts a few seconds suggests that there is an electric flue damper left over from the previous furnace. Look for a device attached to the flue somewhere near the furnace. It might look like one of these. A proper flue damper should prevent the furnace from operating unless and until it opens fully and operates its "damper open" switch. The furnace installer may not have connected the new furnace to that switch, and the damper may not be fully opening. The furnace user manual says that inadequate air flow through the flue can cause overheating.

  2. Confirm that air from both returns is passing through a filter. The manual suggests that two air filters may be needed if return ducts are attached in a particular way. If one of the returns is passing unfiltered air, your evaporator coil may well be clogged with dust as one of the techs suggested, resulting in overheating.

  3. Your new furnace came with 3/4" thick washable filters, according to the manual. If these have been replaced with higher efficiency pleated filters that are 3/4" thick, you may be restricting air flow so much that the furnace overheats. The manual says that the only approved filters are 3/4" washable or 4-5/16" (100 mm) thick pleated filters. Those 100 mm filters have very deep pleats that allow much greater air flow than 3/4" pleated filters.

  4. Is the furnace in a closet? Or a small room with a tight fitting door? If so, leave the door open and see if the overheating is resolved. The manual warns against this, which will result in inadequate combustion air. You can add a large grille to the door or an adjacent wall if this solves the problem.

  5. You went on the roof and heard whistling from the flue. Did you take a flashlight and look down the flue? There may be something blocking the flue. If there is a flue cap of some kind, the cap itself may be clogged by dust or insect nests.

  6. Your furnace is designed for natural gas only. If the installer made a hack modification for propane, there's your problem!

If none of these suggestions pan out, you might consider adding photos to your question, concentrating on the ductwork, filters, flue inside and outside and anything else associated with the furnace that you are not familiar with.

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  • Forgot to mention: (7) Check the combustion air (inducer) blower intake screen for dust or lint, and (8) The main blower is variable speed. Is it running faster / slower during a heating cycle? Ramping up speed as the heating cycle progresses? Some of these can be pre-set by the installer for a single speed. If yours is pre-set and that speed is too slow, that can cause overtemp. Your gas burner has only one setting, which is full-on, so if the blower is not ramping up as the heat exchanger temperature rises, it may overheat.
    – MTA
    Commented Jan 23, 2022 at 15:58
  • Thanks so much for this thorough response! I don't see a damper, and both returns feed through the same (reusable) filter, which I clean regularly. I've tried running the unit for a few minutes without the filter to see if that's the cause, but even with no filter I hear the whistling. The unit is in a closet, but it still whistles with the door open (I'm using the whistle as a proxy for the overheating, because the two symptoms appeared together). The furnace is connected to natural gas. As far as I can tell, the blower is not speeding up or slowing down during a cycle.
    – Jared
    Commented Jan 23, 2022 at 23:33
  • I added some pictures and a video above if they are helpful. I'm not too sure what an inducer blower intake screen is, but a google search doesn't return anything that looks familiar on my unit. I took the inducer motor off yesterday, and to my untrained eye nothing looked obviously wrong (save for the gasket - pic above).
    – Jared
    Commented Jan 23, 2022 at 23:40
  • You should forget about the "whistling". From here it just sounds like a resonant vibration that only occurs at one particular RPM of the inducer. Prove it to yourself: tap the outer edge of the round spinning housing (when it's stationary) with the handle of a screwdriver. Does it ring like a bell? With the same note as the whistling? If so, it's just a resonance and not related to the overtemp.
    – MTA
    Commented Jan 24, 2022 at 0:02
  • Once you've gone through the 8 points and eliminated them as possibilities, you're left with (9) the overtemp cutout or its sensor could be bad. If you drill a small hole through the main duct as close to the furnace as possible, you can insert a meat thermometer into the airstream and see what the temperature is. And (10) follow up on the main blower speed question, perhaps with a call to tech support at Day & Night. Is the blower speed supposed to ramp up or is it designed to be constant speed? Can it be set to a higher speed without too much fuss?
    – MTA
    Commented Jan 24, 2022 at 1:16

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