Payne 100H48 11 Furnace low R-W voltage or high W voltage

[PEvans]

Not an appliance but I have seen some other furnace posts.

This is a 70s vintage standing pilot furnace with a Robertshaw combination gas valve. Right now the heat does not come on.

Measuring voltage at the terminals on the furnace I see:

R - G: 29.6 VAC

R - W: 14.4 VAC

G - W: 13.7 VAC

As I understand it, R - W (heat) and R - G (fan) voltages should be about the same.

Also if I tie R, W, and G together the fan comes on but the main burner does not light.

The pilot is lit.

[WOOKIE]

Whenever you are working on a gas furnace you take the voltage readings from the common (LOW VOLTAGE SIDE) of the transformer to the terminal you want to check.   If you have a call for heat you would read 24 to 29 volts from transformer (low voltage side)  to "W"  and then at the gas valve. You Should read 24 to 29 volts at the R terminal  (all the time). You will read 24 to 29 volts at the G terminal when you have a call to bring the fan on.  If you have 24 volts at the gas valve check you low voltage common on the other terminal. This is just a rough sketch.  Send me a model number and I can get you the information you need. .  I am not sure if that unit has a auxiliary fan switch  which will control the fan in the heating mode.  Check any fusible links and roll out switches.   Sorry about the picture , it is a rough sketch this late at night. 

 

         

[WOOKIE]

Sorry just noticed the model number .  You have a 100,00 BTUH furnace with a 4 ton blower motor.  I will look for any other information on it.  Message me with any questions. Or email me at DandGService@aol.com.

[PEvans]

It would be nice for testing to know if this unit has an internal switch that controls the fan during heating. The white wire from the thermostat runs directly to the gas valve.

When I call for heat (strictly, jump from the hot side of the transformer to the terminal on the gas valve) nothing happens. If I also call for fan (jump from the hot side of the transformer to the green terminal on the control box) the fan comes on.

In that state, relative to common on the transformer I read about 28V on the hot side of the transformer, 28V at the gas valve, and 0V on the other side of the gas valve.

[AccApp]

I’m guessing your gas valve has failed. I’d also bet you have a fan/limit switch of some kind. The way these classic furnaces work is the burner fires and when the fan/limit gets hot enough it turns the fan on.

As Wookie said, you want to check all voltages relative to the low voltage side of the transformer. If you have 24v + going into the gas valve and it’s not firing then you know the gas valve is no good. Test the coil for continuity to further confirm.

[AccApp]

More detail about a fan limit here: https://inspectapedia.com/heat/Fan_Limit_Switch_L4064B_Settings.php

[WOOKIE]

Absolutely agree. 24 volts jumped directly to the gas valve should release the gas.  Make sure the pilot is lit and, the gas valve is set to ON.

The auxiliary fan limit controls the fan allowing the heat exchanger to heat to a certain temp so the unit is not blowing cold air on startup.  In addition, the auxiliary fan limit will keep the fan running  at the end of a call for heat to allow the heat exchanger to cool down  before shutting the fan off.  

[PEvans]

For sure the pilot is on, I can see it.

My furnace has a fan limit control that looks just like the one under the link.

How could I distinguish between a bad gas valve and a bad pilot generator keeping the valve from opening?

The resistance across the coil (the two 24V contacts) is about 32 kOhms, so quite high. Isn't it supposed to be around 350 Ohms?

The gas valve is marked "Unitrol" and Robertshaw. It is just like the one in the video below. Are replacements available?

 

 

[Terry Carmen]

Before you get too deep into this, check the heat exchanger. The unit is 40-50 years old and there's a good chance that it's cracked. 

Even if it isn't cracked, a new one would take about 25% right off the top of the gas bill and a big chunk off the electric bill too.

[PEvans]

Would it even be feasible to find a compatible HX replacement for a unit this old?

[Terry Carmen]

1 hour ago, PEvans said:

Would it even be feasible to find a compatible HX replacement for a unit this old?

That's a really old unit. I can't even find a parts list for it. Maybe check with a distributor for that brand.

I just wanted to mention the heat exchanger before you ran out and bought an expensive valve, then discovered the unit was shot.

[WOOKIE]

Their a many  replacment universal valves available at the HVAC supply house.  Cost is approx $70. The thermocouple is heated by the pilot which intern generates a mili volt to energize the PILOT valve .  If the coil in the pilot valve was bad then the pilot lite  would not stay lit( same if the thermocouple is bad).  once the valve selector is turned from "pilot " to "on " the only thing that is needed is 24 volts and a common to energize the main valve releasing the gas.  Alot of times I find the gas valve is stuck from the additive in the gas to make it smell. A small tap on the side will release it. 

Before going any further , take the burners out and inspect them.  Check the area above where the pilot was for a burn thru .  Sometime you can press your finger thru the metal above where the pilot is lit ( if so condemn it)  .  Vacuum out the exchanger and inspect the chambers with a mirror and flashlight. If you have a inspection camera you can look at it closer. 

If everything checks out and you install a new gas valve  then, upon start up you need to adjust the gas pressure to spec's.  Watch the flames make sure they are blue and not lifting off the burners which is a sign the gas pressure is too high.  Also watch the flames  when the blower motor turns on .  If there is a crack or burn thru the air from the blower will cause the flames to change and wave back and forth.  In some cases it could cause the flames to roll out of the chambers.

ABOVE ALL WITH THESE OLD SYSTEMS. IT IS CRITICAL YOU INSTALL A CARBON MONOXIDE ALARM IN THE HOME. 

[PEvans]

Thanks all of you for your comments and guidance.

Is the resistance reading I got on the coil confirmation that the gas valve has failed?

This unit was built into a loft above my family room before the roof was put on. So replacing it is nearly infeasible.

I have CO alarms. 

[Budget Appliance Repair]

That is a really basic valve so doing an OHM'S check across the two terminals should be just reading the magnet coil that opens the valve.   Yes 32,000 Ohms is way too high and indicates a bad valve coil, also you jumped direct low voltage from the transformer to the valve and it didn't work so that pretty much confirms it.

[PEvans]

Thanks, everyone.

[PEvans]

Assuming the gas valve is dead, is that a Robertshaw 700-40x valve (that is, 700-series 24V combination valve, non-2-stage)? If so, how can I tell which one?

 

[PEvans]

There is no nameplate on it that I can see, just a label that says

24 VAC 0.2A

50/60 Hz

12VDC 1.8A

I measure the outer diameter of the incoming gas line as about .85" so I think that is nominally 1/2"; the valve incoming port is female.

 

[PEvans]

More info: The valve is marked 7000BER, and has a 1/2 inch inlet. The outlet looks larger but I couldn't measure it. It is plumbed straight through and does not appear to have a side outlet. So based on this could I be sure it is a Robertshaw 700-400?

[PEvans]

I had a Robertshaw 700-400 installed. It appeared to be identical to the old valve. The furnace seemed OK on visual inspection and no need to adjust the gas pressure.

Now the furnace is working great. Yes I have CO detectors around the house.

Thanks again for all of your input.

[ECtoFix]

3 hours ago, PEvans said:

Now the furnace is working great. Yes I have CO detectors around the house.

Thanks again for all of your input.

Good to hear!