Lennox Pulse 21 Fan Control

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I'm trying to troubleshoot the following problem with my Lennox Pulse 21 furnace: the furnace fan will sometimes shut off in the middle of a cycle (while the gas is still on and the furnace is pulsing). Occasionally this will continue until the limit is hit. The furnace has a honeywell fan/limit control but only the limit side is used... I'm assuming that the blower motor is controlled elsewhere.
I've already tried replacing the fan relay but that did not solve the problem. Can anyone provide some insight as to how the blower motor is controlled in the pulse 21? It seems funny that it has a fan/limit control but nothing is connected to the fan side...
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Plugged secondary Plugged filter Over firing Weak Fan/limit Plugged evaporator Plugged (I think it's called a blow by filter) internal

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Did a bit more research... how about 'bad fan timer'? Seems to me much more likely that that listed below... unfortunately I can't reproduce the problem on demand. I hate to just start swapping parts out.
daytona wrote:

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Do a little more reasearch and you'll find there were problems with the Pulse heat exchanger. You need someone who is trained to work on Pulse's in order to perform a credible test to see if the exchanger has failed. IOW, you have to know what you're doing to do it properly. This should be done every year. BTW, this has nothing to do with the fan issue.
--
Respectfully, Bob

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Yeah, I know all about this. I've already performed a pressure check on the exchanger.
I'm pretty sure my problem is the fan timer control. I was a bit confused at first... it now seems that the fan relay is only use for AC, not heat. The fan timer controller has an internal relay for heat. Can anyone confirm that this is how the newer Pulse 21 is configured? If this is the case I'll order a new fan timer.
I was able to duplicate the problem once... the fan timer clicked rapidly before the fan shut down. Unfortunately I didn't have my DMM connected at the time so I couldn't verify the theory about bad timer control...
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Stick a CO tester in the exhaust and get a reading.......let me know what it is..and thing over 150 ppm is a bad heat exchanger

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Nope. Not even close. If you want to know the proper way to test a Pulse exchanger, email me.
Respectfully, Bob
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On Sun, 17 Dec 2006 19:17:26 GMT, "Bob_Loblaw"

Purchase the "plug kit" from Lennox and you fill it with water. Totally stupid design and fix just like everything they do. Im sure you've all seen their heat exchanger with the nut, bolt and washer through the eyelet at the top to keep it from separating? Now they've gone to a tubular heat exchanger just like everyone else. The "Pulse".................what a joke. Bubba
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I'm not a big fan of Lennox, but really, if you think about it, the Pulse was quite something in it's day. No ventor motor, 97% efficient, self sustaining ignition.... Once the other models caught up to it in efficiency, or close to it, there was no need for it anymore.
--
Respectfully, Bob

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"Bob_Loblaw" wrote:

by golly Bob finally posts something I totally agree with.
it must be the season. :)
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snipped-for-privacy@gonefishin.net wrote

If you're agreeing with me, then you're smarter than I've given you credit for! Cheers
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Respectfully, Bob

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On Mon, 18 Dec 2006 00:46:42 GMT, "Bob_Loblaw"

Yes they were something, werent they? Lets just quickly go over a few items of that marvelous wonder. 1) The sonz-a-biatch was heaver than a small car 2) It had 6 or 8 big bolts hiding that inducer motor you say wasnt there to allow changing of a flimsey paper flapper 3) and on the subject of those bolts.......those damn things NEVER freely turned out of there by hand. You had to ratchet ever single inch of that thread to get that bolt out. 4) How about that wonderful vibration pad the whole unit had to sit on. 5) How about those lovely canvas connectors you HAD to install on the supply and return ductwork. 6) How about that lovely flexible gas connector you had to use. 7) What did you think of that HUGE muffler that had to be installed on the pvc exhaust just to keep the noise down enough outside so that you could only hear it running 3 houses away. 8) So what did you think of all that spacious room to change the spark plug and flame rod with only a sense of feel and no visual. 9)and lastly that lovely water test of the heat exchanger. YES, that Lummox Pulse was a true marvel. It earned the recognition of placement in a museum. I've heard "Daves" ass is up for the challenge. Bubba
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Aha! Gottcha! :-) It wasn't a ventor motor. It was a purge blower. It only ran on start-up to get fresh air into the chamber, then it shut off for the rest of the heating cycle. If it was a windy day, it sometimes wasn't needed for ignition due to the venturi effect.

I never found them flimsy. Those air flappers needed to be changed every 5 to 7 years.
All the rest is true, as far as mufflers and vibration connections,but they were still pretty impressive for their day, including the ignition system, which only ignited the first couple of pulses, after which the ignition was caused by the refraction of the combustion gases. One thing that was very poorly designed was the access, or lack thereof, to the spark plug and sensor. I bought a special tool for around $50, that was no more than two socket heads welded onto a bar with a half-inch drive hole in it for your socket wrench that made the job a little easier, but what a pain in the ass it was. If you ever come across one that seems to light intermittently, and you've ruled out a bad exchanger, try removing the ignitor, then take a 1/4" drill bit and insert it through the ignitor opening, straight across to the other side of the exchanger, and ream out the gas eustation tube.It will get a build-up of crud on the lower edge of the tube, which deflects the gas upwards and away from the ignitor. Of course, all of this is done using the Braile method, because you can't see any of it inside the access panel! :-) Built to work , not to work on!
--
Respectfully, Bob

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On Tue, 19 Dec 2006 14:08:16 GMT, "Bob_Loblaw"

Vent motor, purge blower......its still a 2nd motor on a furnace that goes bad. So you're saying that on a windy day your purge motor doesnt come on? Hmmm, I seemed to miss the pressure switch that controlled that feature. Maybe it has a hidden misting feature for cool heat that I didnt read about too? :-) Bubba

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Nope.Not what I said. I said it isn't necessary for ignition if the wind is right.

Nope. It's a normally CLOSED pressure switch, the only one that I know of. I'll let you figure out the rest. :-)
--
Respectfully, Bob

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Thanks for the offer, but I don't work on the POS's....and damn glad It's easier to change sections in a boiler
wrote

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As much time as your wasting trying to figure out how to DIY when *most* Lennox parts are propriatary, you would be better off calling you local HVAC pro to diagnose and correctly repair it the first time. BTW... getting any Lennox supplier or dealer to *just* sell the part is like pulling teeth. Even as a licensed professional, I have a bitch of a time getting Lennox parts and when I do find them, the prices are seriously high.

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Why would Lennox be reluctant to sell parts to a licensed professional?? I buy Lennox parts all the time, no problem.
--
Respectfully, Bob

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Did a bit more research... how about 'bad fan timer'? Seems to me much more likely that that listed below... unfortunately I can't reproduce the problem on demand. I hate to just start swapping parts out.
John,
This group was never intended to be a homeowner forum (my opinion), however the fan is controlled by a timer and the fact that that is funny to you is reason enough for you to call a tech. You may also want to check on getting your heat exchanger checked, the Lennox guys say it needs to be pressure tested every four years. They also require the replacement of a couple of parts internal to the combustion process. FYI...
Joseph
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Yeah, this is pretty clear from the responses I've seen. "Call a tech" seems to be the standard. These things really aren't rocket science... I've been able to locate a service manual and parts source. I'll take things on my own from here.
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