Reznor XL blowing cold air

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Old 01-06-10, 01:15 PM
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Reznor XL blowing cold air

Hey folks, I have a overhead forced heater running natural gas that is acting up.

Before I start let me answer these to make things clear-er...

1. In which area you live and ambient temperatures you usually experience.
chicago, right now it's 12F outside, about 30sh inside
2. House style and construction details.
1400^2' shop, 15ft ceiling
3. Make, model and age of equipment related to the problem.
Overhead forced air heater, running natural gas, Reznor XL200-3 (200k btu)
4. Fuel type.
natural gas
7. Thermostat type.
always on

Here is the sequence of things that happens.

Thermostat is shorted so the heater should always be on
I light the pilot, and turn the gas valve to on
Main burners light up, flame is blue about 1-1.5" high
Heat exchanger gets up to temp (takes about 1-3minutes)
Fan turns on
After about 15seconds the fan turns off
Main burner stays on
After about 1-2minutes the fan turns back on and the cycle continues

I have already replaced the limit switch and the overheat switch but I didn't think that was even the problem (I had spare parts from another heater)

What (i think) happens is that the burners cant put out enough heat to keep the heat exchanger up to temp and this causes the fan to cycle.

Now to troubleshoot this I have looked at the flame when it burners start up. That part looked pretty good, flame is blue, 1-1.5 inches in height.

Then I waited for the fan to turn on, the flame tips/outline became a little more erratic and had a yellow hue to it.

Now this leads me to believe that the burn process is not working the way it was designed to be.

One possible cause for this would be a cracked heat exchanger, especially that the draft from the fan should not affect the heat exchanger; although I am not sure if it would make the heater so inefficient that it would not put out ANY heat. (it blows luke warm when i put my head in front of the heat exchanger)

Another possibility is that the gas pipe is clogged somewhere. In hopes of finding that I disassembled the trap right before the gas valve but it seemed spotless. There is a ball valve upstream from there but I did not get to that part yet.

I was going to obtain a manometer to check the gas pressure before I decide to get a new heater...

Any ideas/thoughts would be great

Tom
 
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  #2  
Old 01-06-10, 04:33 PM
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How cold is your shop when you turn the heat on? If your ambient air temp is low the blower will cycle like, you explain, until the ambient air temp rises to somewhere near normal.
 
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Old 01-06-10, 04:51 PM
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Does the flame color or size change abruptly when the blower comes on? If yes, it may indicate the heat exchanger is cracked or has hole in it. Unless, by some design of this heater, you can see that the blower can outwardly affect the burner. ???

A furnace/heater that even has a proper flame can have the problem you are experiencing, if you have a blower that comes on and stays on only when the heat exchanger is hot enough. You already aluded to this. After all 30-some degree air is trying to enter and cool the heat exchanger, and the burner flame cannot totally overcome this temp drop.

About all you can do is to set the blower fan switch's OFF setting to some real low temperature. Since you are bypassing the thermostat anyway, I guess you could do this with the fan-limit OFF setting, since it sounds like you never need it to shut off. That is - if you have say one of those Honeywell fan-limit controls that you can adjust. If you have a thermodisc control, you would not be able to adjust it. However, you could jumper that I suppose, if that is the type control you have. If you have several thermodiscs, it will likely be identified with an F(for fan) number in front of the temperature range - where a limit thermodisc will have an L in front of the temp range. A limit thermodisc might say something like L200-40, and the fan switch might say something like F120-90, or something like that.

If I might ask - why is that you have no thermostat? Is it because the furnace can never, under any circumstances, when you use that heater, warm the shop up enough? If that is the case, what about when the temp is say 55+ in the shop?: Can't the heater even make the shop's temp rise to say 70+, under that circumstance? Is it an uninsulated shop ?

.........................................................................

There were no responses prior to my typing . I see Skip also alludes to the same issue. That makes it unanimous amongst the 3 of us.
 
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Old 01-06-10, 06:07 PM
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I'm removing an earlier post since Ecman's suggestion that the room air is cooling off the fan switch enough to cause it to shut off is plausible.

Of course a lot of unit heaters like that had heaters on the fan switch to keep them on under such conditions. If the fan switch has such a feature the heater coil might be burned out.
 

Last edited by SeattlePioneer; 01-06-10 at 07:57 PM.
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Old 01-06-10, 06:19 PM
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Does your furnace have a summer switch. Most reznors had a chain and string. If yes turn on fan manually for a while, until shop has heated somewhat. As stated above, cold air going across the fan control is the most likely culprit.
 
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Old 01-07-10, 09:52 PM
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my heater does not have a summer switch ( that I can see )

The shop is uninsulated, but litterally I need to leave the heater on for days (constantly running) to get the temperature up to where it's confortable... (high 50s/low 60s)

I can have the thing on for 8h and there is almost no difference in the ambient temp.

What should the temperature change me across the heat exchanger? lets say ambient temp is 40F, should I be seeing 50s?60s?70s? right out of the heater? should the air coming right out of the vent be hot to "touch"?
 
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Old 01-07-10, 10:29 PM
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Heck, a 200K BTU heater ought to get the air noticeably warm.

It may be that you aren't getting enough gas pressure to get the rated input to your heater. Has this always been a problem during cold weather like you are having or is this a new condition?

And do you have natural gas or propane? What other gas appliances do you have, and are they operating normally?
 
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