Random malfunction on Rheem furnace.

[Raindem]

I don't understand where I am supposed to be connecting the DVM leads to check for voltage at the gas valve. There are only 2 wires going to my gas valve, a blue and a brown. I've checked it from the connected blue wire terminal to common (0.1v), from one terminal to the other with the wires connected (< 0.1v), and blue wire to common with it disconnected (15v). I get the same results when the furnace is running normally versus when it does the failed start-shutdown thing. I attached photos of the gas valve and info tag if that helps clear anything up.

Guzzle, unfortunately I can't reproduce the problem. The furnace will work-not work on its own without anyone touching it, so I doubt anything I did triggered it (later on in the day it started working again on its own). I won't be able to get back up on the roof until Saturday. There's a couple more things I want to check but if I don't get a resolution I'm giving up. This obviously takes someone with more skills than I possess. Ideally, I'd like to find an HVAC tech who's willing to accept a flat rate fee for fixing it, not just a service charge for showing up and looking. But people have a business to run and I doubt anyone would be willing to do that, especially since I live out in the boonies and the nearest HVAC repair establishment is 40 miles away.

 

[BuzzLOL]

I can't make out much in the wiring diagram. Does it show one or two solenoids on the gas valve? Does it show a separate 3rd return 24 volt path for the red and blue wires? Or do the red and blue loop to each other? The gas valve may sometimes open and sometimes not open with only 15 volts showing...

 

[Raindem]

I don't know how to read schematics. I was hoping someone else could make sense of it.

 

[BuzzLOL]

I don't know how to read schematics. I was hoping someone else could make sense of it.

Even on my large 27" monitor, I can't make out what the schematic has written on it... maybe you could post a picture of just the gas valve part...
While the furnace is operating, are you able to take a voltage reading on the 24 volt AC terminals of the 24 volt transformer? (if this style) (Set meter to read AC, not DC voltages)
https://images.thdstatic.com/produc...rdware-doorbell-transformers-40tr-64_1000.jpg

 

[Raindem]

Do you want a closeup of the gas valve from the connection schematic or the ladder schematic?

I previously checked the transformer and it's putting out 24v (more like 26-27), whether the furnace is working or not.

 

[BuzzLOL]

How about a picture of both?
So transformer is 24 volts but only 15 at gas valve? I get 25.5 volts at my gas valve...

 

[BuzzLOL]

Do you want a closeup of the gas valve from the connection schematic or the ladder schematic?

I previously checked the transformer and it's putting out 24v (more like 26-27), whether the furnace is working or not.

How about a picture of both?
So transformer is 24 volts but only 15 at gas valve? I get 25.5 volts at my gas valve...

 

[BuzzLOL]

View attachment 27421

Your gas valve has a place on the left to plug two wires onto it, but there is nothing there...

 

[Raindem]

Your gas valve has a place on the left to plug two wires onto it, but there is nothing there...

That is correct. There's never been anything connected there.

 

[maxdad118]

That looks like a place for a ground.

 

[BuzzLOL]

That is correct. There's never been anything connected there.

I was wondering if that was supposed to be a better return path for the 24 volts...

 

[Raindem]

Dunno. The unit had worked fine for several years as currently wired. And the two HVAC techs that came out didn't seem to have an issue with it.

 

[BuzzLOL]

Dunno. The unit had worked fine for several years as currently wired. And the two HVAC techs that came out didn't seem to have an issue with it.

The solenoids may be needing more voltage as they age... eventually they won't work at all...
A lot of 'techs' are just 'parts changers' and don't really know what they're doing...

 

[Raindem]

A lot of 'techs' are just 'parts changers' and don't really know what they're doing...

I guess that qualifies me then. LOL.

So let's assume for a second that the unit is wired correctly, let's further assume that the circuit board is sending correct voltage to the gas valve (I checked it again with the unit running the other day and it was 25v). With the no-start problem generally appearing in the early morning, and with it almost always working later on in the day, the problerm would appear to be ambient temperature related.

Could this cause a sticky gas valve to not function? Could I test this theory by heating up the gas valve with, let's say, an electric heat gun when it is not working to see if that fixes it?

 

[BuzzLOL]

Yes, you could try that... being careful not to over heat a spot on it and melt the rubber parts inside... although theres not much to 'stick' inside... just some neoprene rubber parts being pressed together...
You get 25 volts with it running, how about when it refuses to run?

 

[Raindem]

You get 25 volts with it running, how about when it refuses to run?

It's hard to check voltage in that case because the system shuts down the instant it starts to spark. I tried checking it this morning but could not get a good voltage reading (< 1v with the meter jumping around). But at this point I don't if that is a circuit board problem or a gas valve problem.

 

[Raindem]

Tried the heat gun this morning. No good.

OK, I was able to get good voltage readings on the gas valve today, both when it was working and not working. The meter probes were at the blue wire at the gas valve, and the brown common wire. Here are the results...

When unit is working: At rest voltage is around .4v. Call for heat comes and the inducer kicks on. A couple seconds later igniter starts sparking. Gas voltage goes to 1v. Once the burners light, the gas valve voltage increases to 25v and remains continuous while running.

When unit is not working: At rest voltage is around .4v. Call for heat comes and the inducer kicks on. A couple seconds later igniter sparks one time and the unit shuts down. Gas valve voltage never changes from .4v.

So still not sure where the problem lies. The gas valve has to close to complete the circuit, right?

As requested, I took a couple more pictures of the schematic. The first 2 are closeups of the gas valve from the ladder diagram and connection schematic. The other 2 are the same photos a little further out.

 

[BuzzLOL]

Gas voltage goes to 1v. Once the burners light, the gas valve voltage increases to 25v and remains continuous while running.

+ I would expect the voltage goes to 25v first, then the burners light... maybe a delay in the meter showing the 25v... * +

So still not sure where the problem lies. The gas valve has to close to complete the circuit, right?

+ No, the gas valve solenoid circuit is always complete. + but current has to be supplied to it for it to work +

As requested, I took a couple more pictures of the schematic. The first 2 are closeups of the gas valve from the ladder diagram and connection schematic. The other 2 are the same photos a little further out.

+ Lots of things in that gas valve circuit that have to be closed for current to make it to the gas valve. +

+ * My gas valve assembly has 3 wires, white neutral, red for pilot flame solenoid, black for main burners solenoid+

+ My ignitor starts sparking and then gas pilot flame is turned on and lights and when sensor sees it lit, then turns on main gas valve for burners which light off the pilot flame... your furnace may skip the pilot flame step... +

See comments inserted above...

 

[maxdad118]

How are the vacuum hoses going to the inducer switches? If they don’t close I don’t think you will get 24v at the control. Have you pulled them off to check for any cracks?

 

[Raindem]

Yes. I checked the vac hoses for cracks, and made sure the vac port was clear. I verified that the pressure switch was in fact closing when the inducer kicks on.