Problem inspecting a Lennox

Yes. All the time. What happens in a power loss event? I have 4 relatively expensive thermostats for my boiler zones and I have thrown the switch plenty of times with no issue ever.

While I agree that he could have taken different steps and followed a different procedure I still see no flaw in operating existing means of disconnect.

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Everybodies entitled to their own opinion. You now have mine. Have a good weekend!

Thanks Jeff.
I have not been doing step 2. I am adding it to my procedure now!

@ jjonas The reason you don’t kill power to a computer is to prevent data loss. The hardware is not susceptible to sudden power loss. Any thermostat that can’t handle sudden power loss is a POS and I’m hard-pressed to believe it was designed with that vulnerability.

@ snadeau I would be contacting the manufacturer’s tech support department to find out from them if their thermostat is designed in a manner that leaves it vulnerable to sudden power loss and if there is a specific shutdown procedure that is supposed to be taken before powering down the furnace/air handler (I highly doubt there is). IMO someone should have checked (tech/homeowner) the warranty status of the thermostat. I would need to see the documentation that powering down the furnace with the service disconnect is “improper procedure” before I tolerated any abuse or forked over money for it).

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Who can anticipate everything. %^&* happens.

The receipt said “t stat was faulty needs to be replaced”,

Everyone assumes the T-stat really needed to be replaced.
Could be an example on an overzealous tech, looking to install parts that aren’t needed.
I’m sure most of us have seen this happen to many clients.


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Why even remove the filter in the first place? Just to check if it’s dirty? You usually can open the door or cover to make sure there is a filter and to note type and size, but Regardless if it’s dirty or new I always put in a note saying “Changing filter on a regular monthly basis is always advised”. Unless they have an electronic type filter or another type other then disposable don’t mess with them. Note if there is one, size and type and be done with it.

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It could have been as simple as the batteries in the t’stat for backup power were dead. When you killed the power at the disconnect, the t’stat reverted to the factory default heat setting (usually 55°). I don’t know if you checked the t’stat setting after you restored power at the disconnect; or whether you just assumed that the t’stat had retained the 76° heat setting.


      [David Williams, NACHI 18092410](

      InterNACHI®️ CPI

    April 15

It could have been as simple as the batteries in the t’stat for backup power were dead. When you killed the power at the disconnect, the t’stat reverted to the factory default heat setting (usually 65°). I don’t know if you checked the t’stat setting after you restored power at the disconnect; or whether you just assumed that the t’stat had retained the 76° heat setting.

The stat had no replaceable batteries, and was just a blank screen after the power cut. It was a sealed case, powered from the four wires in the thermostat low voltage bundle.

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It could have blown the 3-amp automotive-type fuse at the fan-control board, if there was a loose wire there. Just another possible explanation. And any HVAC tech always carries a couple in his pocket. I just can’t go with re-powering at the the disconnect causing the t’stat to fail. Could have even been a failing low-voltage transformer. They will cause the low-voltage fuse to blow sometimes. This is another cheap item an HVAC tech will have on his truck. / If there’s no automotive-type fuse present (for future reference) look to see with the power turned off, of course, if there is a little reset switch or button on the side of the LV transformer.

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After the access panel to the filter was slid back, the filter was recessed and held in place with a spring “bar”. The only way to note the size was to remove it in this case. But in every case on my inspections, I show with photos where the filter access is and how the filter should “look” when installed properly. If possible I show the client in person how and why to replace the filter. In this case, my client was a single mom with 3 knee-high kids and I want to make sure she knows how and why to change it. I also note whether the filter is clean or dirty, as that in itself is an indication of the condition.

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Steve, You did not do anything wrong and I would not change your procedure except maybe turning the service switch off first, then raising the thermostat, then switch back on. Watching it start up can reveal a lot more than getting to it while running for a while. Not a HVAC tech, but have some of a electronic engineering background. Chuck and David Williams have the correct responses imo. thermostats should be designed with multiple power shutoffs at the prototype testing stage.

More often than not, by pulling a slammed filter out, you are actually saving your customer from having to call an HVAC tech for a service call that might cost them hundreds of dollars. This is especially true when the house has undergone recent carpet replacement or drywall repair or interior spray painting. If the power is on when these crews are in there doing one of these dusty jobs, you can bet they’ll be taking advantage of the heating or cooling… And if you do find one that is slammed, it’s best to take it out, show the customer how to replace it (if they don’t have any new filters onsite) and advise them to leave the old filter out of the unit until they get a new one today or tomorrow (or whenever). A slammed filter blocks the transfer of heat to the evaporator coil’s vaporized refrigerant, allowing refrigerant to return to the compressor as more of a colder liquid instead of a warmer gas. This will destroy the compressor crankcase bearings in a matter of time, as the compressor oil is displaced from the crankcase and made to circulate throughout the lines and system until the oil eventually migrates back to the crankcase.

I hear that if you flip them over when they’re like that, they breathe much better

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Dirty filters are best with 3 pimento olives - for the record. If they are merely dirty, I put them back in - I’ve suffered the wrath of sellers for leaving them out. If they’re dished or looking like an alley cat, I’ll leave them out with a note for the seller.

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after reading all. one small point killing power by service switch, what would happen if power went out. cut power should not do it

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Exactly, Steve. That’s why the disconnect is there at the unit: so you don’t have to depower the t’stat first, when opening the air handler for whatever reason. Digital and analog t’stats alike were built so they would reset and run automatically when power is restored. You just have to keep in mind that the digital t’stat can revert to the factory default settings- and especially so, if it doesn’t have batteries as a backup power source to retain the newest user settings.

Steve as a past HVAC tech, the service switch is at the unit SO a tech can set it to heat or cool at the t-stat in the house … go to the basement, attic, crawl, etc … check the unit out, service or repair it by turning off the service switch WITHOUT crawling back out of the attic, crawl or basement to do so.

Some units when turned off at the service switch OR if the door interlock is disengaged WILL require reset at the tstat BUT do not cause the unit to kill the tstat.


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Yes that was my point. I’m also HVACR

There is absolutely nothing you can do to damage the t-stat when turning off the power (under any sequence), unless you short out a wire on the control circuit (ie.removing wire nuts, or shorting a terminal to ‘common’).

As for dirty filters; I am currently looking for an R22 scroll compressor because a tenant didn’t change a ‘high efficiency’ filter. She figured she bought a better filter and didn’t need to change as often. It flooded the compressor with refrigerant and broke the shaft to the scroll (something that is uncommon in a scroll).
Flooding a scroll generally does not do damage like in a reciprocal, but washing out the oil from the compressor will kill it.

If I see a dirty filter I can’t see through, it stays out.
You should always document this because it may fail as soon as you walk out the door.

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