GTI DV Fireplace Problem

[DAKSY]

OK, DV (millivolt systems) experts...I've got a baffler.
Got a customer with an older Gas Technologies, Inc gas fireplace. This company was the precursor to Heat n Glo...
Not a very aesthetic unit...Flames are all the same size & the logs are concrete...
Anyway, I digress...
Like I said, it's a millivolt system T-couple/T-pile & the unit works fine, burns nice & steady
Mv readings are 550 +/- on the pilot & 220+/- with the burner on...
Perfect, right?
Once the blower comes on, the millivolts on the t-pile drop like a rock...
I mean quickly...
Within 2-3 minutes, the burner will shut right down...
If I turn the blower rheostat off, the mvs climb back to normal...
This unit has a spill switch wired into the valve/t-pile/rocker switch system, so I rewired the the system
to eliminate any potential grounding due to the deteriorated sheathing...
Still the same results...
I took the thermo-disc out of the blower system & still the same results...
I unplugged the fan & turned the rheostat on & the mvs held true...
I turned the rheostat off & plugged the fan into the "hot" (unswitched) receptacle,
& the mvs held true...
I don't have the know-how to test the rheostat or the blower to determine which one is (or BOTH ARE) the culprit...
Any ideas where to check next?

 

[trafick]

I'm no expert but it sounds like you are somehow grounding thru the rheostat. It only happens when the blower unit is plugged in because I'll bet the blower cord has a three prong plug on the end and when you plug it in you ground the system. Is there any way to seperate the rheostat from the blower motor and have the blower motor run by itself? Rheostat completly removed?

That would eliminate either the blower motor or the rheostat. Also think of a rheostat as a variable resistor. If you can get it out of circuit you can check from one side to the other and from one side to the wiper. Also check to case ground to see if it has shorted out.

 

[DAKSY]

Hey, trafick...

I'm no expert but it sounds like you are somehow grounding thru the rheostat. It only happens when the blower unit is plugged in because I'll bet the blower cord has a three prong plug on the end and when you plug it in you ground the system.

The blower unit has a 2-prong plug

Is there any way to seperate the rheostat from the blower motor and have the blower motor run by itself? Rheostat completly removed?

Yep, I did that...These units have a switched (thru the t-disk) outlet & an unswitched (Remote/Auxiliary)
The fan runs fine & the mvs hold true...

That would eliminate either the blower motor or the rheostat. Also think of a rheostat as a variable resistor. If you can get it out of circuit you can check from one side to the other and from one side to the wiper.

I agree with the first statement...but WTF is a wiper?

Also check to case ground to see if it has shorted out

How the heck do I check the ground? I'm a half-assed electrician - I wired my entire house - but the theory part has a tendency to confuse me...

 

[trafick]

The wiper is the variable part of the rheostat. It's the thing that changes the resistance, therefore the voltage dropped by the rheostat. The more voltage dropped the slower the blower motor runs.

Think of a rheostat as a horseshoe with a wire to the middle. If you put 120v to one side of the horseshoe and the wire is in the middle, you will get 60v on the wire meaning the blower will be at half speed. The closer the wire gets to the end of the horseshoe that power is applied the more voltage it gets (less voltage drop) making the blower motor go faster. Likewise the farther the wire gets from the end (more voltage drop) the slower the blower motor goes.

Now that you are totally confused, to check the rheostat read across it with an ohm meter. If its two wire the resistance will change as you turn the rheostat knob. If it's three wire one of the wires will be the wiper and reading from it to either of the other two will give you a resistance that can be changed by turning the knob. The rheostat should be sealed in a metal housing (maybe) and if it is check from any wire to it and make sure it is not shorted to the case (ground).

 

[DAKSY]

Hey, trafick,

The wiper is the variable part of the rheostat. It's the thing that changes the resistance, therefore the voltage dropped by the rheostat. The more voltage dropped the slower the blower motor runs.

With ya so far

Think of a rheostat as a horseshoe with a wire to the middle. If you put 120v to one side of the horseshoe and the wire is in the middle, you will get 60v on the wire meaning the blower will be at half speed. The closer the wire gets to the end of the horseshoe that power is applied the more voltage it gets (less voltage drop) making the blower motor go faster. Likewise the farther the wire gets from the end (more voltage drop) the slower the blower motor goes.

Kinda still with ya...

Now that you are totally confused, to check the rheostat read across it with an ohm meter. If its two wire the resistance will change as you turn the rheostat knob. If it's three wire one of the wires will be the wiper and reading from it to either of the other two will give you a resistance that can be changed by turning the knob

I've got the rheostat & my cheapo multimeter with me as I type this & jumping the two wires, I'm not getting squat, other than "OL"

The rheostat should be sealed in a metal housing (maybe) and if it is check from any wire to it and make sure it is not shorted to the case (ground)

It's totally enclosed in a plastic housing, so I jumped from either wire to the threaded shaft in the middle, where it would make contact with the Fireplace frame...Once again, nothing...
At the price I can get another fan rheostat for, I mizewell just buy one to add to my tool kit...Ya never know...
Thanx for the Electricity 101!

 

[trafick]

Hey DAKSY,

Is this an auto-ranging meter? Set to read ohms? If it is AND you have two wires from the rheostat AND putting the leads of the meter to each lead of the rheostat gives you an OL reading no matter how you turn the knob, then I would say you have a bad rheostat. The only problem is that if the rheostat is open then the blower shouldn't come on at all. Now were back to square one...Glad I could help!

Please let us know what you find out.

 

[stoveguy13]

seal around pilot assembly it cloud be pulling air from around that spot cooling the tp.

 

[Inside Guy]

I think I've seen this before. Does this unit have heat tubes inside the fire box? Inspect the top back fireplace. There may be a leak. I think the gasket/sealant is bad. Reseal top back. I think that might be your problem.

 

[DAKSY]

Well, I think I figured it out...
I decided the cheapest option was to start by replacing the rheostat...
I got everything - fan, rheostat, wiring & t-disk back into the correct position,
fired the unit & kept my fingers crossed...
T-pile mvs held steady (Yay! I win one)...
I don't know why it worked, but it may have had something to do with the OEM rheostat having a brass
boss that the retainer nut threaded onto..
I'm thinkin the rheostat was on its way out & may have had an internal short that was grounding thru the brass
to the fireplace shell & that was causing the mvs to drop...
Anybody care to chime in here & tell me if my logic is correct/incorrect?
It's been running strong since Friday nite w/o issue...