Lennox Pulse21 Question

tchp

Early one morning about three weeks ago our Lennox Pulse21 80,000 btu furnace, controlled by a Nest Thermostat, was running the fan like normal for a cold morning, but we noticed it was blowing room temperature air. I let it continue for about 15 minutes to see if anything changed, but when nothing did I shut the furnace down at the service panel, then waited a couple minutes and turned it back on. Initially, it produced normal heat for probably about 20 minutes or so, then blew room temperature air again.

I then made a service appointment with the original installer of the furnace.

In the meantime, since I suspected that the flame sensor might be responsible, I removed it and cleaned the sensor rod with 1200 grit wet/dry sandpaper, then re-installed it. The furnace then seemed to operate normally, so I cancelled the service call, and had it changed to a routine annual maintenance call. I also ordered a new flame sensor online. When the service tech came out, he also removed the old flame sensor to inspect and clean it. He found the furnace to be in good operational condition with no failure of the heat exchanger and no carbon monoxide leaks.

About two days after the service call, the furnace repeated its same behavior of initially blowing warm air, then losing combustion and blowing room temperature air. So, I replaced the old heat sensor with the brand new one I had ordered, but the issue was not resolved. As things stand, the furnace will heat the house, and it is just that there are these extended periods of it blowing room temperature air. You initially notice it because you can really feel the coldness when you walk past a floor register. Eventually though, the furnace will produce heat and get the interior temp up to the normal 70 degrees we have the Nest thermostat set at.

So, at this point I have ordered a New Old Stock control board for the furnace and a new auto spark plug style igniter that the Pulse21 uses, but am wondering if the problem might be something else, like a bad high limit fan switch, etc. Thanks in advance for any suggestions.

JUGHNE

Has this always had a Nest tstat?

Ironman

Well, it could be several things, but throwing parts at it randomly is not how we diagnose something.

Thats an obsolete furnace that the manufacturer recommends replacing unless a pressure test is performed to confirm the integrity of the heat exchangers.

That being said, it may be difficult finding a tech who’s knowledgeable on it, but that’s what I’d recommend.
If you have the instruments and tech savvy to follow the diagnostic charts for it, you could attempt it yourself, but I don’t recommend that you do that if not.

GW

Pulse is an odd bird. Very few techs want to touch this unit. When I did some pulse classes 10 or 15 years ago, the big deal was making sure the flappers were replaced every several years. Also, pressure testing, half the people that do touch this furnace don’t know that it’s supposed to be pressure tested every four years (special tools needed to plug off various intake and exhaust orifices). This is the one and only positive pressure combustion furnace ever made. If the heat exchanger does crack, which is rare, and combustion goes south, not good. I tip my hat to Lennox, this furnace was ahead of its time.

EdTheHeaterMan

first condensing furnace.  A year or two later Carrier has a condensing furnace that did not Pulse to compete with Lennox.  They called it a Plus furnace.  

Enough history 

is the fan set to ON somewhere?  Sometimes it is just that simple.  

SuperTech

That's definitely an obsolete furnace.  Lennox doesn't support that product anymore.  I would recommend finding a Lennox dealer to service it, most other companies will not have the kit to pressure test the heat exchanger.  Checking combustion and testing for CO in the supply air is NOT adequate to make sure the heat exchanger is safe. I worked for a Lennox dealer and when we started pressure testing our customers Pulse furnaces probably around 2/3 of them failed. More so the G14s then the G21s but we did find some bad exchangers on the G21 furnaces as well. 

I would consider wether it's worth throwing money at this furnace and swapping random parts when it's problems are well documented. In the 80s this was the best gas furnace available, its time to consider something that would be a safer, more reliable option.  

EdTheHeaterMan

first condensing furnace.  A year or two later Carrier has a condensing furnace to compete with Lennox that did not Pulse.  They called it a Plus furnace.  

Enough history 

is the fan set to ON somewhere?  Sometimes it is just that simple.  

Oops. I think I already said that.  

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