Lennox Pulse fan temp. set points faulty?

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Old 01-09-11, 08:50 PM
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Lennox Pulse fan temp. set points faulty?

G14Q4-60-2, for natural gas, Dallas, Tx. area, 26 yrs old, various A/C parts replaced over the years, but never any problem with heater, digital thermostat replaced mechanical about 5 yrs ago, set on 67 during winter.

Over the last few years, I've noticed the plenum is not allowed to heat up much before the fan cuts on, (5-10 seconds, this winter) and when the gas cuts off per the thermostat setting, it takes a longer time to cool the plenum sufficiently to cut the fan off.

My curiosity aroused, I removed the cover on the temp. sensor gizmo to watch the dial in action. The set points are: Fan off-90, Fan on-125, and top limit-195.
Thinking that the plenum must be heated to 125 before the fan cuts on, I found this not to be the case; No matter whether the plenum starting temp. is 75
(in the case of infrequent cycling) or 115 (in the case of frequent cycling), the fan cuts on in 5-10 seconds, before the dial even starts to move.

With the fan on that early, it takes a long time for the plenum to heat up to and stabilize at 155, if it's cold enough to require a long of a heat cycle.

Conversely, when the gas cuts off, the plenum must cool down PAST the low set point of 90 to about 75 before the fan cuts off. Once off, the plenum heats back up somewhat, I guess from residual heat. On occassion, the re-heat will trigger the fan back on at 125 (the set point) and then will go back off at the actual low set point of 90.

So, it would seem that the fan is controlled by these set points SOMETIMES, and by another control at OTHER times.

What gives?
 
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Old 01-09-11, 09:03 PM
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It has been soo long that I've been around this.. If I recalled, it has a small heater build into it and will start the fan in about 30-45 sec.

So the sensor may be worn out, and starting way to early for you. Cut off should be set to 90˚.
 
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Old 01-10-11, 10:05 AM
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you need to have this heat exchanger pressure tested by a lennox tech! these are positive pressure heat exchangers and lennox had problems with them. Risk of Carbon Monoxide poisoning! As far as your current problem, it sounds like you need a fan limit.
 
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Old 01-10-11, 07:49 PM
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thank you gentlemen, I will take the pressure test under advisement, and consider a new fan limit device. Is the presence of two fan-switching methods done for safety reasons? (ie., "fail-safe" in that if one fails the other will still operate)? Or is the heater to insure fan startup in a certain length of time regardless the plenum temp?
 
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Old 01-10-11, 08:16 PM
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it is to speed up the fan limit. yours may not have a heater
 
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Old 01-10-11, 08:29 PM
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Hvac- I looked at the Lennox manual on this model, and it does have a heater to the fan "ON' in about 30-45 sec.
 
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Old 01-12-11, 08:13 PM
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Thanks Jay, does the heater also control fan shutoff? Because after gas shutoff and as the plunum cools,the fan does not shut off till it reaches 75 which is 15 degrees below the set point.
Then, with the fan off, the residual heat turns the dial back up to around 95. If I then manually turn the dial to 125 so as to start the fan, then let it go to watch it cool, the fan shuts off at the exact set point of 90.
 
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Old 01-12-11, 08:43 PM
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I didn't see that in the manual.
 
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Old 01-13-11, 09:19 PM
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Thank you kindly, for the link to the manual...I will study it when time permits...I might learn something!
 
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Old 01-16-11, 04:14 PM
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Jay11J, The manual was a most interesting read, revealing to me that the combustion process is the same principle of operation of the similarly-named
"Pulse Jet" early jet engines of the German unmanned "Buzz" bombers used to bomb England during WWII......but back to the furnace: I can only conclude that the heater is faulty in the fan limit unit, requiring less-than-normal heat to make and break, so my question is, can the heater element be replaced by itself, or do you just replace the entire fan limit unit as a whole?
 
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Old 01-16-11, 04:22 PM
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I'd say you are best to replace the whole thing.

I have to admit, I enjoyed the sound of these furnace, I wouldn't mind having one if Lennox still made them, but I'm sure my neighbor wouldn't like it!

As hvac mention, may not hurt to get a Lennox dealer out there with test kit to make sure the heat exchanger is fine.
 
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Old 01-17-11, 12:27 PM
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I'm actually in need of replacing the whole kit & kaboodle (furnace, A/C, lines, ductwork) and hoping to do that this year.....

Yeah, the Pulse has a nice throaty rumble to it...when was it discontinued, and why, do you recall?
 
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Old 01-17-11, 01:48 PM
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Then I wouldn't spend the money on the new control if you can live with what you have now.

In 1997 Lennox had a recall of all Pulse furnaces made from day one (1982 I believe). They stopped making them in '99.

They stopped due to the recalls on the heat exchanger, and fan asst system that you can get now are far quieter than the pulse, so people were going with them instead of the Pulse.
 
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Old 01-17-11, 03:45 PM
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the pulse furnace uses a positive pressure heat exchanger, therefore when a crack or hole forms it with out a doubt will put CO in the living space
 
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