Lennox Pulse 21 strange behavior

tchp

Recently my Lennox Pulse 21 has begun to exhibit a behavior where the thermostat will call for heat, the furnace will go through its safety stages and then fire up the combustion chamber (its a model that uses a Champion spark plug and combustion chamber to ignite a fuel/air mixture). The furnace will fire up like normal, but before the heat exchanger reaches temperature and the main blower comes on, the furnace will shut down. Sometimes, a few moments after shutting down, the main blower will come on. I assume that this is because an internal thermostat has detected that the heat exchanger has been warmed and needs the blower on to cool it. Then, while the blower is running, the furnace will typically fire up again, and as the blower is already running, it seems like it does not shut down this time, but continues to run until the thermostat reaches temperature, and the furnace will then shut down normally.

So, it seems that there is something that is telling the furnace to shut down after it has fired up, but before the blower turns on (the flame sensor was replaced a couple months ago, along with the ignition spark plug and the air intake "flapper" gasket)

I have watched the operation of the shut-down with the cover of the furnace removed. The furnace will fire, and then after perhaps a minute there is a clicking noise and it shuts off. Even the green LED on the control board turns off. Then, after a few seconds, you hear another click and the green light comes on solid again. There has never been an error code when these events occur, which are very frequent (several times a day). There does seem to be times where the furnace will still start and run normally, without shutting down. But those times are getting less frequent. Thanks for any suggestions.

mattmia2

You need a lennox dealer with the special pulse tools and knowledge to work on this. the second part is important, i suspect many lennox dealers won't have anyone that understands these anymore. It could just be an intermittent control board or something but there are some safety issues that only a lennox dealer can rule out.

The blower is just controlled by a thermostat, when the furnace gets hot the blower comes on. If the burner has gone out before the heat reaches the thermostat for the blower it will come on after the burner has gone out.

ratio

I don't think they've made a pulse furnace this century? & like @mattmia2 said, techs with that skill set have mostly been put out to pasture.

IOW, this is a strong indication that it's time to budget for a new system. Hopefully you won't have to do it in the middle of winter.

pecmsg

I haven’t seen one in 20 years. 

SuperTech

pecmsg said:

I haven’t seen one in 20 years. 

I still see some every now and then.  This problem sounds like the fan and limit control is shot.

tchp

We were planning to get a new furnace with AC this Spring. So, we may move that up. The Pulse 21 has a 74H6501 Honeywell limit control, but the furnace only uses the side that controls the high limit switch, which is set at about 180F. The side of the Honeywell that controls the blower motor turning on and off based on temperature is not used by the furnace. The manual for the Pulse 21says that the blower motor is operated by the control board, based on set times for when the blower turns on after a call for heat is made, and when it shuts down once the call for heat has ended. The blower can be turned on to run constantly via the house thermostat, and it spins smooth and sounds very good, so I don't think the blower motor is bad. The furnace has now begun to turn the blower off at random times, with the furnace still running, and the limit switch is what was shutting it down. So, it looks like the control board is bad. At present I am only running the furnace a couple times a day with the blower set to run constantly while I monitor it, so the faulty control of the blower via the furnace control board is not an issue. With the blower set to run constantly, the furnace ignition has operated in a normal fashion.

ratio

Is the fan single-speed? If so, I'd be comfortable just turning the fan on manually & letting it run like normal. Also, your thermostat may be able to be configured to turn the fan on with a heat call. Either of those may give you enough time to shop for a reliable installer.

tchp

Thanks. Yes, it does look like we can survive in the interim using the blower turned on manually. So far the furnace has run fine as long as it no longer controls turning the blower on or off.

elementrider87

Hey TCHP. My pulse21 has recently started doing the same thing. Originally it stopped working all together and wouldn’t kick on. I replaced the flapper media and now it works But it exhibits the behavior you stated turn off and on. It also makes a WohWohWohWoh sound the whole time pulsating. I’ve noticed my gas bill go up about $80 the last 3 months, and unfortunately I cannot afford a new system atm. What exactly did you do to fix your? 

KYOTE

elementrider87 said:

Hey TCHP. My pulse21 has recently started doing the same thing. Originally it stopped working all together and wouldn’t kick on. I replaced the flapper media and now it works But it exhibits the behavior you stated turn off and on. It also makes a WohWohWohWoh sound the whole time pulsating. I’ve noticed my gas bill go up about $80 the last 3 months, and unfortunately I cannot afford a new system atm. What exactly did you do to fix your? 

What your describing sounds like a cracked heat exchanger. You may not have no other choice but to replace. When a pulse runs like crap it could be a dirty flapper valve. You should be able to look up videos on how to clean a flapper. Try that first. If it still does it, your in trouble.

elementrider87

KYOTE said:

elementrider87 said:

Hey TCHP. My pulse21 has recently started doing the same thing. Originally it stopped working all together and wouldn’t kick on. I replaced the flapper media and now it works But it exhibits the behavior you stated turn off and on. It also makes a WohWohWohWoh sound the whole time pulsating. I’ve noticed my gas bill go up about $80 the last 3 months, and unfortunately I cannot afford a new system atm. What exactly did you do to fix your? 

What your describing sounds like a cracked heat exchanger. You may not have no other choice but to replace. When a pulse runs like crap it could be a dirty flapper valve. You should be able to look up videos on how to clean a flapper. Try that first. If it still does it, your in trouble.

So I did replace the flapper, and it didn’t get any better I have another control board on the way to test, as well as a spark plug and flame sensor just in case. If these don’t help then I’m out $200 but at least I tried. I will just have to rely on my gas fireplace and use the fan to circulate it through the house. But as if right now the heat still works, just not optimally.