Thermocouple

Vwbuser

Hi, I have an older vented propane wall heater, and I’m looking to replace the thermocouple. The part is no longer available per the manufacturer. I’ve been looking online for a similar looking one, but thread size may be an issue as well as the fact that most say they’re for ventless heaters. Main question is what’s the difference between a thermocouple designed for a vented and ventless heater.

EBEBRATT-Ed

Not sure there is a difference. Most thermocouples are "universal" and come with some different fittings for thread in, push in etc. The usual difference is in the capillary length, so you just buy one that is long enough.

EdTheHeaterMan

Actually there is a specific thermocouple for ventless appliances that are precision made to fit in the opening at an exact point of contact for the pilot flame. It is known as an ODS thermocouple. ODS stands for Oxygen Depletion Sensor. The idea is that since the appliance is not vented, it could use up all the oxygen in the room. If the oxygen in the room starts to be depleted, the pilot flame will get smaller. Then the thermocouple will not touch the flame anymore and shut off the gas.

I don't think your vented space heater has that part. I believe that you may have a standard thermocouple or a power pilot generator . Take a look at these pilot sensors and see if they look like what you have:
https://www.supplyhouse.com/Robertshaw-1950-001-36-Thermopile-Generator-w-PG9-Pilot-Adaptor
https://www.supplyhouse.com/Resideo-Q390A1061-36-Thermocouple

This is one type of ODS thermocouplehttps://www.supplyhouse.com/Robertshaw-1960-027-27-Low-Mass-Thermocouple

EdTheHeaterMan

Now for my nonsensical reply to your Discussion Title

Thermocouple = a couple that is having a heated argument.

EBEBRATT-Ed

@EdTheHeaterMan never herd of a ODS thermocouple. learn something new every day. thanks

EdTheHeaterMan

EBEBRATT-Ed said:

@EdTheHeaterMan never herd of a ODS thermocouple. learn something new every day. thanks

I learned about it the hard way. Before I knew about them, I tried to fix one in an oil burner customer's enclosed front porch. Knowing how a standard thermocouple works, I looked at the space heater and saw that the pilot was not touching the tip of the thermocouple. Since the connection end of the thermocouple was different from the ones I had on the truck, I just made some modifications to the bracket (NOT EASY) and got the pilot closer to the business end of the existing thermocouple. Problem solved, Right?

NOT

I wanted to have one of those thermocouples on my truck in case I ever encountered one of them again. The local Johnstone did not carry them so I went to the fireplace store in the Strip Mall down the road. I asked about the part and they had it in stock. Then the parts counter guy explained how they work (as did I above). OH SHIP! (without the P) I just disables the safety feature of that customers heater. I need to go back and fix it. Knock on the door... No answer. Call on the phone, No answer... Did this for a week and never got in touch with the customer, so i sent a registered letter explaining that the heater was not safe to operate and that I would make the necessary repairs for free. I got the return reciept from the customer.

However, I never heard from that customer again. I wonder if all the oxygen was sucked out of the room while they all were sitting on the porch? Didn't see anything in the newspapers. Fingers crossed! Oh well!

By the way, the correct fix would have been to blow the dust out of the pilot burner combustion air intake with a plastic straw. A 3 second fix.

JUGHNE

I recall reading about this safety feature on un-vented FP's.

This should be a reminder to all that the ODS is possibly the only thing guarding against CO poisoning.

Bob Harper

The ODS is simply a finely tuned pilot burner that responds to ambient O2 levels. As the O2 begins dropping from 20.9% down to 18.5%, the flame speed slows such that the flame no longer engages the thermocouple so the voltage decreases until the pilot drops out. ODS thermocouples are quite specific as Ed said. ODS pilots have a heavy bracket to maintain the relationship between the pilot burner flame and tip of the TC. The bracket, TC, angle of attack- it all affects the dropout rate. Also, ODS pilots have their own mini poppet regulator so you cannot adjust the pressure. If you have the requisite minimum inlet gas pressure under load, you cannot overfire the pilot. Note that VF logs in a fireplace with old-style glass doors can trap enough CO2 to snuff out the pilot by displacing the O2. Also, for those tempted to drill or ream out an ODS pilot orifice, don't. It's an industrial ruby that has been shot with a laser so it resembles a spider web. If you poke it with a drill bit or probe it will shatter and the whole pilot assy. must be replaced. Keep the primary air hole clear by using a pipe cleaner or canned 'air' (pilot flame off).

Tim McElwain

I thouight by now all those ODS units would have disappeared.