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Lennox Pulse 21 Fan Control John 12-16-2006
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Posted by John on December 16, 2006, 4:52 pm
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...


Posted by daytona° on December 16, 2006, 5:41 pm
Plugged secondary
Plugged filter
Over firing
Weak Fan/limit
Plugged evaporator
Plugged (I think it's called a blow by filter) internal



> 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...
>



Posted by John on December 16, 2006, 5:48 pm
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=B0 wrote:
> Plugged secondary
> Plugged filter
> Over firing
> Weak Fan/limit
> Plugged evaporator
> Plugged (I think it's called a blow by filter) internal
>
>
>
> > 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...
> >


Posted by Bob_Loblaw on December 16, 2006, 6:00 pm



> 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.

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

Posted by John on December 16, 2006, 6:45 pm
> Do a little more reasearch and you'll find there were problems with the
> Pulse heat exchanger.

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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