1995 Pulse furnace


1995 Lennox Pulse 21 furnace. Ironically I did everything from welding the heat exchanger to assembling it to installing it in my house. I am not an HVAC tech or an engineer. The other day I noticed my second problem. A couple of years ago the blower motor failed. Had to replace it with an upgraded motor as the OEM was unavailable at the factory. Very successful operation. I also replaced the air flapper as the old one was caked with dirt. Last week I noticed it was short cycling. The purge blower would run and the for about 5 seconds it would try to fire. Then it would go silent, then the purge blower would run. Then the cycle would repeat. Anyway, I found that if I have the thermostat set at 75 deg instead of the 68 deg I usually run it at, it fires and cycles just fine. Weird. Before I call a real tech, does anyone think the problem could lie in the electronic thermostat?
Thanks. :)
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On 2/25/2010 5:44 PM, zimpzampzormp wrote:

And I inspect the heating coil return bends for leaks every year. Mine is the model that had the improved brazing material.
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acbysean had written this in response to http://www.thestuccocompany.com/maintenance/1995-Pulse-furnace-427088-.htm : Anything is possible, meaning it could be the t-stat. How old is the t-stat? This is kind of concerning in many ways because you said that flap was caked with dirt, you might want to change your filter more. Could be a control board issue, could be a limit switch. Try and get it to reinact the same problem and you should see a sight class in the blower panel which you should see blinking lights. If its blinks so many times that helps diagnose what the issue is. try that and write back. zimpzampzormp wrote:

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On 2/25/2010 6:16 PM, acbysean wrote: > acbysean had written this in response to > http://www.thestuccocompany.com/maintenance/1995-Pulse-furnace-427088-.htm > : > Anything is possible, meaning it could be the t-stat. How old is the > t-stat? > This is kind of concerning in many ways because you said that flap was > caked > with dirt, you might want to change your filter more. Could be a control > board > issue, could be a limit switch. Try and get it to reinact the same problem > and > you should see a sight class in the blower panel which you should see > blinking > lights. If its blinks so many times that helps diagnose what the issue is. > try > that and write back.
There is no filter to the air box. Only a muffler on the intake pipe. I suppose some of the grime on the air flapper could be from the fiberglass insulation panels inside of the airbox. The purge blower also runs to evac the burner chamber in case of a no fire. A spark plug starts the pulse ignition and a heat sensor plug determines if the pulse is self sustaining and turns the spark off.
The thermostat is as old as the furnace. I will probably change it out first and see if that's the problem. Otherwise a furnace tech is going to make some money. That's alright as long as I can't do it myself.
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On Thu, 25 Feb 2010 19:47:30 -0600, zimpzampzormp

Natural gas buzz-bomb pulse-jet engine process. If it needs much in the way of repair you are likely farther ahead to replace it. A very large percentage of the original pulse furnaces have gone to the great scrap yard in the sky.
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On 2/25/2010 7:53 PM, snipped-for-privacy@snyder.on.ca wrote:

Well yeah. A bunch of them did. Mostly because a bean counting engineer decided to save a few pennies per unit by using a lower silver content solder in the coil brazing process. I think you'll find that most of the bad ones were of the G-14 model. That problem had been mostly solved in the G-21 model. That destroyed the rep of a finely engineered and built furnace. Still the most efficient furnace around. Soon may be a problem with getting parts but I'm going to keep her as long as possible. If the heat exchanger does go bad, lifetime warranty. Either a new heat exchanger if available or a new high efficiency furnace (which efficiency would be lower than the pulse.).
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By "purge blower" I presume you mean the blower which blows air into the combustion chamber. What happens when it "tries to fire"? Does the hot surface ignitor glow, and the gas valve open?
So, you say that when you turn the thermostat up hard and far, the furnace runs?
Of course, there are two fans. One to supply air to the fire. And one to blow heat into the building. Some of the more complicated furnace use a timer, to turn on the blower fan that blows the heat into the building. I can't comment on the Lennox, never worked with that brand. Others use a heat sensor, or other device to know when to turn on the hot air blower.
A tech with Lennox experience would be more help. Too much electronics.
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zimpzampzormp wrote:

Was anyone playing with the thermostat and changed the "heat anticipator" setting? For that matter, is the t-stat mechanical or electronic?
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wrote:

If it works fine and cycles fine set at 75 but not 68, replace the thermostat
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If it is a pulse type, trash it. My neighbor's only lasted about 3 years before he had to replace it.
I still have an old 1965 gas-fired furnace where the flame is ignited by a pilot light and burns inside a great big can. It may be old, but I have never had any problems other than oiling the blower motor and changing the filter. I also filled in the gas nozzle and then drilled it out with a smaller size drill so the gas flow is about half of what it used to be, so it doesn't burn as hot and I put less heat up the chimney. I have the blower temperature sensors set low so the blower comes on at a low temperature and runs until the air is down to about 80F. That extracts most of the possible heat available. I have provided a 6-inch diameter air inlet directly from the outside to feed into the combustion area so it doesn't use heated household air, and I have a carbon monixde detector close to the furnace as well as a couple near the bedrooms in case the big can ever develops a leak. My gas bill is way down from what is was when we first owned the house. So newer is not always better.......
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On 2/25/2010 10:37 PM, hr(bob) snipped-for-privacy@att.net wrote:

I hate to be one of those guys but I need to disagree with you. I've had this for 15 years now and very little trouble. When I installed it my gas bill was cut in half. Mine is very efficient, quiet (relatively), and reliable. I also like the feel of the heat. Not too dry. It is also the first furnace to use coil technology for higher efficiency. You might say it was a pioneering furnace for high efficiency. I do keep a carbon monoxide detector though.
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wrote:

If you take a certain amount of air and its water content and heat it by blowing the air over my can or over your coils, that does not change the moisture content unless the air is exposed to some open flame, so how can the tyoe of furnace make a difference in how the heated air "feels"? Maybe the blower/air speed changes how the heated air feels, but the type of heating does not change things at all.
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