Trouble shooting a gas wall heater: thermostat? wire?


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Old 01-20-19, 03:17 PM
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Trouble shooting a gas wall heater: thermostat? wire?

Hi, I have a older (15+years) cozy direct vent gas wall heater with the original manual thermostat wall control (2 wire).

Last year, the gas valve was replaced after replacing multiple thermopiles ( pilot would go out).

Its been working fine until last week- the pilot stays on but the gas valve doesn't "open" unless I give the unit a little whack. then the gas valve will be responsive to the thermostat for a few hours then non responsive after.

I tested the thermostat with a jumper cable- no response
I tested the TH TH/TP on the gas valve with a jumper cable- fired up and then right after tested the thermostat- it fired up.

Is the issue a faulty gas valve, faulty wire or thermostat?

is it a faulty wire? the heater turned on fast when jumping at the gas valve unit
is it a faulty thermostat? it was able to "control" the unit only after jumping at the gas valve first.

I hope the issue isn't the gas valve- it was expensive to replace

thanks!
 
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Old 01-20-19, 03:30 PM
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More than likely it's a weak pilot flame or a weak thermopile. The flame heats the thermopile and it in turn generates the DC power to open the valve.

Do you have a meter ?
The output from the thermopile needs to be checked. It would be the two TP terminals on the gss valve. Without a call for heat it should be +/- 350mv. With a call for heat it shouldn't go lower than +/- 200mv.
 
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Old 01-20-19, 04:25 PM
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gas valve tp th/tp reads 220mv at rest-- so a weak thermopile again

its not responding when i try turning it on at the thermometer.

the pilot is clear and strong.

i changed out the thermopile on this unit at least 4 times last winter!!! I reused one of them on another unit- same make/model and it works great!

which is why i paid alot to change out the valve unit because i didn't know what else to do.

why would this gas heater burn thru so many thermopiles that end up working fine when placed in another gas heater?

a senior citizen uses that heater 24/7- might the thermopile go bad after being used 24/7?

off the change the thermopile (again) and thanks!
 
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Old 01-20-19, 04:43 PM
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It's more likely to be a clogged pilot orifice and the flame is too small to support the thermopile.
The pilot flame needs to be pretty large.

It doesn't matter how much the fireplace is used. The thermopile is always sitting in the pilot flame.

Does the fireplace always start up when you short the wiring at the gas valve ?
If yes.... a wire is loose somewhere. Either at the gas valve or at the thermostat.

200-220mv is on the low side of the expected voltage.
 
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Old 01-20-19, 04:52 PM
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yes, the gas heater always starts when i jump the wiring at the gas valve.

which led me to think the issue is with the 2 wire or thermostat
 
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Old 01-20-19, 05:03 PM
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Ok..... so short the same two wires at the thermostat and see what happens then. You are working with a very small voltage and it does not allow for much loss. Just the loss from using small thermostat wire can cause the circuit not to fire when the thermopile is on the low side.
 
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Old 01-20-19, 07:02 PM
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i replaced the old manual thermostat with a honeywell pro1000- has a little electric digital display- hoping the temperature regulation will be more accurate.

shorting the wires on the thermostat doesn't make the heater to fire up

BUT if i short the wires at the gas valve FIRST and then short the wires at the thermostat- it will fire up.... so does that point to a weak thermopile?
 
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Old 01-20-19, 07:06 PM
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also, can the pilot orifice get dirty or clogged if its been in continuous use?
 
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Old 01-20-19, 07:38 PM
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Yes.... your thermopile output is low.
The orifice is only a pinhole where the gas comes out.
There are artifacts in the gas that build up across the hole. Propane clogs even quicker.
 
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Old 01-21-19, 03:20 PM
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i remeasured the thermopile and it read high 600s at rest with the pilot on.... go figure.

i blasted the orifice with a pressurized can of air and cleaned the soot off the thermopile, hooked it back together and the heater seems to work off the wall thermostat for now.... its cycling on/off on its own to maintain the temperature.

i wouldn't of thought to clean the orifice since its been in continuous use.

i just wanted to report back and give a big THANK YOU!!!
 
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Old 03-09-19, 11:53 AM
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changed thermopile but pilot still goes out:(

since 1/21/19, the pilot/heater continued to work for about 2-3 days and then died.

and got harder to get the pilot to stay lit- i'd have to hold the pilot valve down more than 60 seconds and relight a couple of times to turn the valve to ON without it dying out.

i changed the thermopile and that worked great!!! for about 2 1/2 weeks

then went out 3 days ago and i've had to relight it again each day

tried to adjust the pilot flame but no matter how much i turn it clock and counterclockwise- it doesn't change the flame. with the gas off- i turned it all the way until the screw came out- to make sure it wasn't the cover

the pilot flame is blue and covers the themopile but i have another wall unit ( same model) i can actually hear the gas engulfing the pilot- while on this troublesome unit, i cannot.... so maybe the flame isn't engulfing the thermopile enough?

thoughts? thanks in advance!
 
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Old 03-09-19, 12:13 PM
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You had the gas valve changed. Was it an exact replacement ?
You should see some type of pilot change when adjusting the pilot gas pressure.

The flame needs to be pretty strong. Almost noisy.
 
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Old 03-09-19, 12:48 PM
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yes, a new gas valve was replaced. i don't know if its an exact replacement but the valve is a robertshaw 7000mvrlc 740-811-415. i just relit it and its holds- 463 mv pilot only, 181 mv when turning on but the flame is not robust and noisy.

i turned the pilot adj again counterclockwise after lighting the pilot and it adjusts from tiny to the flame it is - blue but not strong. it doesn't grow any larger than its current size. ( i found a thin but stronger driver to turn the screw)

the heater is a cozy cdv255b

the other heater ( also had gas valve replaced) is the same robertshaw 7000mvrlc and has been trouble free for several years. the pilot is robust and keeps the themopile red at the tip and is noisy.
 

Last edited by after; 03-09-19 at 03:14 PM. Reason: able to adjust flame- but still weak
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Old 03-09-19, 01:35 PM
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i just tried adjusting the pilot valve on the troublefree unit- turning it adjusts the flame, so i know i'm doing it correctly....

on the problematic unit- no matter how much i turn it, the pilot flame doesn't get any stronger, smaller-yes, larger/stronger-no

i removed the tube supplying the gas from valve to orifice and shot pressurized air through that and the orifice and steel wool'd the thermopile, orifice assembly and the attachment points on the valve.

fired it back up again, same blue but not robust flame.....
 

Last edited by after; 03-09-19 at 03:20 PM. Reason: update
 

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