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subcool 01-23-2013 07:40 AM

lp furnace not igniting
Hey I have lp ruud furnance that will not light. I have a call for heat, the inducer motor starts I am getting 24V out of the centrifugal switch, and ground out of the roll out switch.I am getting 24V to the gas valve on the "PV" wire coming from the pilot relay control. There are two other wires going to the gas valve that I am not sure of, but they do not have any voltage to the gas valve. they are labeled MV and MV/PV. I assume PV stands for pilot valve and should be at 24V, but what do the other stand for and should there be voltage there? I'm thinking I have a bad gas valve, but wanted to make sure before I go buy a 100 valve.

I forgot to say it is tring to light I can hear the "tick tick tick" from the igniter and can see the arc.


Doc Holliday 01-23-2013 08:00 AM

main gas valve

pilot gas valve

flames sensor/thermocouple first, control board second. can you take 24 volts from anywhere else to the gas valve? listen for the distinct click as it (if it) opens.

subcool 01-23-2013 08:20 AM

But since I am getting 24V to the PV shouldn't that cause the pilot to light?

Where do I take the 24V to? all PV, MV/PV , or MV? or do I take it to all of them.

I did put a manometer on both side of the valve, 11 coming in but nothing on the other side.

for the flame senser to work there would have to be a flame right?

Doc Holliday 01-23-2013 08:27 AM

I'd start with MV but you can try each one. if there's no gas pressure than you're simply clarifying what we already know, the valve is not opening. it isn't opening due to the absence of voltage.

what we're trying to find out is IF the valve will open, why you take 24 volts to it.

subcool 01-23-2013 08:36 AM

So if it does open when I take 24 to it, that would mean the board or (relay igniter control) is bad right? since I have 24 coming in. If it doesn't open after taking 24V to it, does that mean the gas valve is bad?

Doc Holliday 01-23-2013 08:38 AM

make sure there's not a break in the valve circuit such as a roll out or something that's breaking the voltage to the valve first but mainly yes, if the power is supposed to be sent directly from the board to the gas valve but is not present from the output of the board than the board is bad.

if you know the board is being sent 24 volts but still is not opening than the valve is bad. Well, not the valve itself but the coil on the inside, just like that of a contactor on a condenser when on call for cool. 24 pulls the coil in allowing for continuity. If that coil breaks, it never can pull in.

subcool 01-23-2013 09:02 AM

thanks for the replies! I did check the rollout and centrifugal switch. Those are the only two I see in front of the board.

How do I know if I am supposed to have 24V coming out of board? I mean doesnt the PV need to go live before the MV does? I do have 24V at PV

so even if I take 24 volts to the MV and it opens that doesn't tell me whats wrong. could be the valve not opening on PV, right? Maybe they all have to have 24V then that would be a bad board.

How do I know what should be?

The wiring diagrams are in tact, but there are no expected voltages.

Once again thanks!

subcool 01-23-2013 01:23 PM

I went and checked again, and I have 24v between "pv" and "mv/pv". i checked for continuity across the all three and all read open.

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