[oldfurnace]

Hi, I have an old gravity furnace that uses an old general controls magnetic valve controller. The pilot is working and the voltage between the screw and the wire nut is a totally reasonable 600 millivolts. But the furnace isn't working. The pilot doesn't light up the flame, which means the gas isn't entering the furnace for some reason. Can anyone recommend troubleshooting steps to get my furnace up and running again? Thanks so much.

Best,
Victor

 

[user_12345a]

By gravity furnace, I'm assuming you mean a central hot air unit without a blower.

Are you talking about something else?

Check the voltage being applied to the valve (with the valve in the circuit) with the stat calling for heat, then compare it to the output from the thermopile.

If it's the same or similar, power being applied to the valve - likely bad gas valve and not worth fixing something so old. Parts likely unavailable, existing setup likely doesn't have a pilot shutoff and would have to be updated to a modern combination valve setup.

Wouldn't have safety approvals so if anything happens you may not get insurance coverage.

If there's no voltage or it's not the same, have to check the thermostat circuit. disconnect wires go to the stat and with it calling for heat, ohm it out.

mv systems are very sensitive to resistance and a marginal connection can cause failure to operate.

If you have the money, really, best option is to get a new heating system even you can fix it for now. What you have is probably 50% efficient at most and not very safe.

 

[user_12345a]

this thread may be of use to you: https://www.diychatroom.com/f17/help-magnetic-gas-valve-type-b-60-6b-cat-53r108-620434/

 

[oldfurnace]

Thanks for the recommendations; I appreciate it. You're right....a central unit without a blower. Replacing makes sense, but it'll have to be something we do in a few months after we've saved the funds. I'll try fixing it for now, since we'd like to have heat!

I checked the voltage between the screws, and that's zero. I even bypassed the resistance and turned up the thermostat, and still no gas flowing to the furnace. So the valve isn't opening. I think that means I have a bad controller. Unless someone tells me how stupid I'm being, I'll try getting a new controller/valve. I know that requires adjusting the gas input, but my goal is to figure out how to do that.

Picture of the setup are below (assuming I did it right).

Thanks,
V

 

[user_12345a]

To my knowledge, there's no controller.

Power comes for the power-pile heated by the pilot and goes to gas valve - thermostat switches power to the valve on and off. There may be a safety/overheat limit in series but unlikely.

You'll have to trace the wiring and use your meter. You need voltage applied to the main valve -> if you don't have that, look upstream.

If you do have that, bad regulator or valve.

--------
As far as replacement goes, keep in mind it's a huge job; your air ducts and vents are unlikely to be suitable for a forced air furnace. Sizes wrong, in wrong locations, etc.

The conversion could be north of 10 grand.

 

[oldfurnace]

Yeah, the conversion might have to wait. There was no voltage between the screws when the pilot was on, but there was sufficient voltage between that wire nut and the screw with the wire attached. Maybe the valve is bad, but that just seems odd given that it's worked well for so long (I realize that could also be a reason why it's *not* working!).

Thanks again.

-V

 

[user_12345a]

I don't know which screws you're testing - the connections on the valve?

No voltage to valve = good news because it means the problem is upstream of that.

 

[oldfurnace]

Apologies for being unclear. I've labeled it in the attached. Between screw #2 and the wire nut I get about 600mV when the pilot is on (and nothing, of course, when the pilot is off). Between screw #1 and screw #2 I get nothing. The gas company guy told me that meant I had a bad valve.

 

[user_12345a]

The problem is upstream.

 

[surferdude2]

It wouldn't hurt to clean the pilot valve assembly and the thermopile.
I know you said it was putting out pretty good but I've seen it make a world of difference to clean it up anyway.

Also make sure the vent on the gas valve isn't plugged by a spider nest of something.

Looks like an old reliable B-60 valve. When you bypassed the thermostat (I'm assuming you jumped between the wire nut and terminal 2), what voltage did you get on the gas valve terminals? If it was 350 mv or more or if it went down to about 1/2 of what you measured when it wasn't in demand mode, and the valve still didn't open, the valve is electrically ok but the pilot solenoid is probably stuck. Did you hear the solenoid click? I've taken those apart and cleaned the solenoid with a solvent with good results before. You may lose the gasket but you can make a new one.

The common test back in the day was to use a weak AA battery to see if the valve would open. I say that for historical humor and not to encourage you to do anything dangerous.

If worse comes to worse, you can get a cross referenced replacement valve for about $160. You have a regulator in line I see so a straight gut valve will not need any adjusting.

 

[user_12345a]

If worse comes to worse, you can get a cross referenced replacement valve for about $160. You have a regulator in line I see so a straight gut valve will not need any adjusting.

Never knew they still make the same style valve.

Would be infinitely safer converting it to a combo millivolt valve with pilot safety, but ironically would change the appliance and could get in trouble with insurance if it causes any property manage.

 

[surferdude2]

I got locked out by the timer. I meant to add that the vent will have a small piece of copper tubing running to the burner area near the pilot. If it gets plugged with soot, that an cause the valve to not open.

I also wanted to explain that the pilot solenoid I mentioned doesn't refer to the gas pilot light but to the pilot control system built into the valve. The pilot armature or plunger is the part that sometimes sticks due to buildup of foreign material in the gas. It's called a pilot operated valve that uses a solenoid to open a small flow of gas to the the area above the operator diaphragm that in turn opens the main flow orifice.

If the pilot flame is not healthy, I mean yellow or not totally encompassing the thermopile, cleaning the assembly may be all that is needed. The orifice may be partially plugged.

It's a good time to start putting some dough back to capitalize a replacement for this old dog. It's about my age although I still pass my gas pretty well. My valve is leaky at times though. :biggrin2:

 

[oldfurnace]

Ha! Thanks for the advice, all. And for the laugh-out-loud quality of the last post. I'll put in some work tomorrow or the next day and report back soon.

 

[surferdude2]

Good luck with the project there. I don't know where you're at but I hope it isn't too cold... going without heat in Feb. where I'm at would be tough.

I think your thermopile can do better if cleaned. I'd like to see a good 750 mv out of it. I'd put a little extra heat on it with a propane torch for a test to see if a good pilot cleanup might do the job.

Here are a couple links to gas valves and a thermopile you might find useful if it comes to that:

Open Box E-bay

Walmart (30 day returnable to store)

Thermopile

From the picture it looks like you have a swing joint so the valve change out will be easier than most.

 

[6t7gto]

Had this same problem with a rental.
Our HVAC guy converted it to a modern 24v gas valve.
Cost $200.00 in Feb. 2018.

 

[surferdude2]

Had this same problem with a rental.
Our HVAC guy converted it to a modern 24v gas valve.
Cost $200.00 in Feb. 2018.

*
*
Hang on to that HVAC guy... he's a keeper in this day and age of high priced service calls!

As a side note, you have to consider that there are areas that have frequent power failures, some long term. Millivolt systems on non-blower gravity furnaces, floor furnaces and wall furnaces still deliver heat under those circumstances.

Also there are lots of wall furnaces with millivolt systems that were popular in the 50's still going strong in areas like Socal where winters are mild. There will be a demand for thermostatically controlled millivolt valves for a while yet.

 

[surferdude2]

The OP could likely cut his gas usage in half by replacing his furnace with a forced air system. Depending on the weather where he's at, the ROI might be pretty short.

 

[user_12345a]

There are millivolt combo valves, not need to put a 24v transformer in to convert.

 

[oldfurnace]

OP here...thanks again for all the pointers. I'm in northern California, where it's currently 61 degrees during the day and about 50 at night. Not comfortable, but tolerable with space heaters. The desire to fix my problem myself is (perhaps stupidly) pushing me to take care of this myself, even though I'll likely call a professional to double-check my work. My wife's pregnant and I'm a bit more paranoid these days!

 

[user_12345a]

you'll never get the money back upgrading to forced air in that climate.

really, gas furnace is huge overkill in places that don't get real winters.