Modine Hot Dawg issue

[jgd]

Good afternoon. I have a Modine Hot Dawg 75AH0134 that is around 20 years old, that has never given us any problems, until recently. When there is a call for heat, the unit will go through its start up cycle, including energizing the heating element. No gas is released from the valve. The unit will cycle 5 times and then go into lockout. The valve will flash 6 times, indicating a lockout. The 2 rollout switches and the limit switches all are closed, showing 14 ish volts across them. I watched the low side of the transformer and see about 25 - 26 volts to the circuit board. The low side voltage is steady. The pressure switch also shows 14 volts. I expect 24 volts. I replaced the circuit board and have the same problem. I unplugged and jumped around the limit and roll outs. The venting is wide open. It’s hard to confirm, but I can only assume that the new board is transferring 24 volts to the valve. Also, I did purge the gas line, for fun.

Has anyone ran into this issue? After chasing the usual suspects, I’m now suspicious of the gas valve.

Thanks

 

[chinboys]

Try cutting the problem down to halves... All of the furnace's safeties are in series. Jump or bypass half of them with an alligator clip jumper returning to the circuit board. If there is a problem, continue cutting the problem again in half. You will find the faulty component.
Lastly, you can jump all the safeties but remember to reconnect the whole string again once you have now determined it isn't the safeties.
Check the thermocouple that creates current when the ignitor turns on and gas flows to create the fire.

Are you positive you have gas pressure going into the unit's gas manifold? Might it be clogged? Pressure regulator pressure check?
Ideally, you need a gas manifold pressure check on the inlet and outlet side.

 

[jgd]

Thanks for your reply.

I did systematically jump through the safeties, finally omitting them all, no change.

I do have inbound pressure and did purge the line. I’m not a fitter and don’t have a way to confirm the pressure, but it seems to be quite strong, as it blasted out for a couple minutes. There hasn’t been any line work. I’ll hunt up a gauge, if all else fails and confirm in/out as you suggested.

I’ll look closer at the thermocouple, as you noted and reply here tomorrow.

Again, thanks for your input.

 

[jgd]

Thanks for your reply.

I did systematically jump through the safeties, finally omitting them all, no change.

I do have inbound pressure and did purge the line. I’m not a fitter and don’t have a way to confirm the pressure, but it seems to be quite strong, as it blasted out for a couple minutes. There hasn’t been any line work. I’ll hunt up a gauge, if all else fails and confirm in/out as you suggested.

I’ll look closer at the thermocouple, as you noted and reply here tomorrow.

Again, thanks for your input.

… the thermocouple was cleaned up. How do I test it? Thanks

 

[PoorUB]

If you have 24V to the valve, the valve is stuck shut. Confirm the voltage at the valve and go from there. If it has power after the ignitor has been in for a few seconds, then the valve is bad.

Once in a while a light hit on the valve or the gas pipe leading to it, when the valve is powered will shock the valve open. After that, it might run fine forever, or until the next cycle.

With that heater, IIRC, the thermostat calls for heat, the inducer starts and closes a pressure switch, then the ignitor starts, then shortly after the gas valve opens. When the burner lights the board looks for proof of flame and either keeps running or fails. If it fails it tries again. It looks to me that the gas valve isn't opening, but make certain you have power at the valve. It could be a control not sending power to the valve.

 

[fitter30]

Any switch that is closed will not show voltage across it. Only a open switch will.

 

[fitter30]

Replace the hot surface igniter. They can glow red and the board is looking for a minimum amperage to open valve. If there is a appliance parts house near you take the igniter with you they will probably have one sincere there only a few different ones that are used.

 

[jgd]

Test...

 

[jgd]

Is there an admin?

 

[jgd]

OK, more data
Mounting a gauge on the unit’s valve I see approximately 5 psi inbound. Gauge mounted on outbound side of unit’s valve, I see zero psi. I monitored the output side throughout the start up cycle and never saw the gauge twitch. Thermocouple/flame sensor; not sure how to check it but saw a steady 1.9 volts. I see 14.x volts on each side, of each switch. I did jump around all switches (3) with no change. To be clear, I ultimately deleted all safeties. 26 volts on the low side of the transformer, to the board. Board is new.
I did bang on the pipe and valve throughout startup, with no luck. I’m not sure how to confirm power to the valve, as these are sealed connections? The new board is getting the low side voltage. The system is 24 volts, but yet I’m getting 14 volts in the safety circuit loop?. The valve is doing this? I can swap out the surface igniter and see what happens, it's worth a shot. I see the logic. I’ll make some calls tomorrow. I appreciate all the fitter expertise and guidance, as I muddle through this!

 

[jgd]

OK, more data
Mounting a gauge on the unit’s valve I see approximately 5 psi inbound. Gauge mounted on outbound side of unit’s valve, I see zero psi. I monitored the output side throughout the start up cycle and never saw the gauge twitch. Thermocouple/flame sensor; not sure how to check it but saw a steady 1.9 volts. I see 14.x volts on each side, of each switch. I did jump around all switches (3) with no change. To be clear, I ultimately deleted all safeties. 26 volts on the low side of the transformer, to the board. Board is new.
I did bang on the pipe and valve throughout startup, with no luck. I’m not sure how to confirm power to the valve, as these are sealed connections? The new board is getting the low side voltage. The system is 24 volts, but yet I’m getting 14 volts in the safety circuit loop?. The valve is doing this? I can swap out the surface igniter and see what happens, it's worth a shot. I see the logic. I’ll make some calls tomorrow. I appreciate all the fitter expertise and guidance, as I muddle through this!

I tried repeatedly tried to format my reply above, but kept getting swatted with errors, so I apologize for the one long paragraph.

 

[The Cobbler]

I see approximately 5 psi inbound

that's way too much pressure . you sure it's not 5"water column?

 

[The Cobbler]

Is there an admin?

moderator here, what do you need?

 

[jgd]

moderator here, what do you need?

Struggling to understand why the server was throwing errors when I format my posts with bullets. I had to remove all formatting and create one long paragraph to get my post through the up rights. Just trying to do better going forward. Thanks

 

[jgd]

that's way too much pressure . you sure it's not 5"water column?

My analog gauge is reads PSI, but not very detailed. It’s probably 3 - 5 psi. The unit has run on this pressure for 20 years. Thanks

 

[The Cobbler]

not sure on your editing issues, sorry...

but I still maintain 5 psi for gas is way high. propane should be in the range of 9 inches of water column, natural gas in the area of 3.5 inches of water column ... ounces of pressure in both cases .

 

[jgd]

not sure on your editing issues, sorry...

but I still maintain 5 psi for gas is way high. propane should be in the range of 9 inches of water column, natural gas in the area of 3.5 inches of water column ... ounces of pressure in both cases .

Yeah, my psi number can’t be right. I’ll measure in wc and get back with you. My gauge does not measure psi at low levels well. Thanks

 

[PoorUB]

If it is actually 5 PSI you need a regulator right before the heater.

I have seen it before where the gas supply was higher than required and ran for sometime. The regulator in the gas valve will meter pressure too, but it isn't supposed to be that high to the valve. The valve will lock out on over pressure.

 

[jgd]

If it is actually 5 PSI you need a regulator right before the heater.

I have seen it before where the gas supply was higher than required and ran for sometime. The regulator in the gas valve will meter pressure too, but it isn't supposed to be that high to the valve. The valve will lock out on over pressure.

Thanks for that. I hope to know more tomorrow, if all goes according to plan. Thanks

 

[jgd]

Good afternoon. Fitter30, I slapped in a new hot surface igniter and the unit fired up immediately ! The old one glowed red hot, but apparently was bad. The error flash cadence gave no clue either. Good call.

PoorUB, I did get better gas measurements. I’m seeing 49 inH20 inbound, 46 inH20 running and 2.x after the valve, running. 49 is quite a bit higher than the preferred 3.5 number. The unit has been running this way for 20 years, do I let’er buck, or do you feel that I’ve just been lucky and should insert a regulator?

Thanks to everyone for your input! I was getting pretty frustrated…

 

[PoorUB]

PoorUB, I did get better gas measurements. I’m seeing 49 inH20 inbound, 46 inH20 running and 2.x after the valve, running. 49 is quite a bit higher than the preferred 3.5 number. The unit has been running this way for 20 years, do I let’er buck, or do you feel that I’ve just been lucky and should insert a regulator?

50 inches is like 2 PSI. Yes you need a regulator! I am not certain why you got along this far without one. Valves usually lock out before that pressure.

You should have 3.5 inches after the valve. There is an adjustment on the valve for the gas pressure.

 

[jgd]

Yeah, I’ll be adding a pressure regulator up stream, I have it now. I’ll also tune the valve for 3.5

Thanks

 

[PoorUB]

Set the valve for 3.5 inches, and after the regulator, before the valve at 7 inches.

I am assuming natural gas?

 

[jgd]

Set the valve for 3.5 inches, and after the regulator, before the valve at 7 inches.

I am assuming natural gas?

Yes, natural gas. I’ll set up with those values. Thanks