Any HVAC techs here? Modine Hot Dawg garage heater short cycle problem...

[BGCarrera32]

Hi:

I have a 60k btu natural gas Modine Hot Dawg garage heater with a hot surface ignitor. When the wall thermostat calls for heat the HSI will glow, the burners will fire, fans come on (exhaust and circulator), burners run for about 4-8 seconds and then cut out. Fans stay running as the furnance will attempt to refire. Sometimes it stays lit for the cycle, sometimes it tries 4-5 times short cycling and then goes into lockout and needs to be powered off to reset.

I have replaced the flame sensor, switched the AC polarity (recommended to try by Modine), and bypassed the limit switch to rule those out. It has 2 flame rollout switches and neither are tripped. The limit and rollout switches are in series on the same circuit and all have continuity when checked by a meter as this nonsense is going on. The unit is clean inside and the flue is not blocked.

Thoughts? What am I missing?

Thanks
BG

[Cajundaddy]

My guesses not knowing that particular unit:
Failed flue sensor, low gas pressure.

Does this have an ignition control module with led fault codes? Often you can describe the problem to tech support and they will narrow the possibilities. This discusses the TS codes:

https://www.youtube.com/watch?v=UNR1qBOHe1Y

Fault codes:
http://www.modine.com/techsupport/com/modine/spl/portletImpl/documentList/5H79749_UT_Ign_Control_Troubleshooting_1-16-09.pdf

[billybek]

Clean the flame sensor with sand cloth.
Check all the sta-con connectors from ignitor/sensor to board.
Check all connectors to/from the board.
Check voltage across the main gas valve when energized.
Check to see if the pressure switch is closing. Tap gently on this during the ignition cycle with the back of a screw driver.
Edit: Make sure the fitting for the hose connection to the pressure switch is not plugged. Pull the hose off and clean the opening off the ventor fan motor. Those products of combustion are wet and can plug the port.
Check for a good ground to the unit.



Sounds like it is not sensing a flame after ignition.
Flame sensor shouldn't be too much money and I bet the ignition board is not all that expensive.
As mentioned above there should be a flashing code led on the ignition board and usually a chart of codes on the burner section panel.
Trouble shooting at a distance is kind of like throwing parts at the problem but start with the easy stuff and go from there.
If polarity wasn't the problem, I would switch it back.
You have the installers guide and wiring diagram?

[dad911]

Copper gas line with Natural Gas? Was it working or is this a new install? If you bought used, any chance it's converted to propane?

[DerkPerk]

Billybek is spot on. Sounds like a flame sensing issue.

One thing I’d clarify is that the draft motor runs before the ignition sequence, correct?



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[BGCarrera32]

Thanks for the feedback.

As mentioned, the flame sensor is brand new. The previous one that I replaced was spotless after 15-18 years, but I cleaned it anyway with 600 grit and it didn't fix the problem so I just replaced it. Bad flame sensor was my initial thought too.

The pressure switch is tripping when the exhaust fan kicks on, you can visually see it retract. Exhaust fan starts before ignition sequence. Board has no LEDs. Only LED is on the Honeywell gas valve and flashes a system lockout code (5 or 6 flashes, I forget) when the furnace won't stay lit after a few tries.

Will add more info later...it does act like its running out of fuel. I put the unit in new in 2003 or 2005 and it is natural gas.

What sort of voltage am I looking for across the main gas valve and what is this telling me? I'll check that and probably get a manometer on next to check gas pressure.

Thanks~!

[billybek]

Originally Posted by dad911

Copper gas line with Natural Gas? Was it working or is this a new install? If you bought used, any chance it's converted to propane?

In some Canadian provinces (example BC) copper pipe is permissible with natural gas.
Joints are brazed, not soldered. Silphos is typically the brazing alloy of choice and I believe type G tubing.

I don't know if this is permissible in the states for natural gas but it is fine for propane installations in both countries.

[billybek]

You should see 24 VAC across the gas valve when energized.
Low voltage indicates a switch problem before the valve creating a voltage drop.
Take a photo of the wiring diagram and if you have the original installers guide have a look through that...

[DerkPerk]

Originally Posted by billybek

In some Canadian provinces (example BC) copper pipe is permissible with natural gas.

Joints are brazed, not soldered. Silphos is typically the brazing alloy of choice and I believe type G tubing.



I don't know if this is permissible in the states for natural gas but it is fine for propane installations in both countries.

Brazed type L copper, soft and pipe is allowed for natural gas here in MN.


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[pwd72s]

When we had a gas furnace installed, it would click on & off much to rapidly. Turned out a young intern tech had mistakenly grabbed a thermostat made for a heat pump...

Might check the wall thermostat for a malfunction?

[GH85Carrera]

Our central heat system failed this fall. The unit would kick on but shut off in seconds, and do that three times, then stop. If I unplugged it, the same three try cycle repeated. I had cleaned the flame sensor. It was beyond my talents to figure out what to try next.

I called a local father son repair shop recommended by a friend. They tried several steps, and even replaced the flame sensor and some relays. In the end they had to replace the entire board. It is a Carrier central heater and AC system. They had talked to Carrier tech support and they first thing the tech support guy said 'wow that is an old unit" as it is 25 year old.

The new board was the cure. I could not see any burned capacitors or any obvious failed parts on the old board. I was just really happy it failed in the fall, and it was a warm enough to survive the weekend until they could come out.

Good luck with your heater. Please let us know what fixed it.

[cabmandone]

You're cycling off of your limit switch.

Put one lead of your meter on the wire that sends signal from W on the thermostat the other on common. You should read 24 volts while the valve is energized. If you're breaking W before the stat ends the call for heat, you're going off on one of the flame roll out switches or the limit switch. If you still have 24 volts at the valve and the gas shuts off, you're main valve solenoid is bad and the valve needs replaced.

[ddinham]

I have the same unit in my garage and had the same problem last year. I ended up removing the limit switch thinking it was open. It was coated with a fine dust. I gently cleaned and reinstalled and it solved the problem.
Hope your solution is as simple.

[BGCarrera32]

Thank you guys. I will check it out further.

[abisel]

There is a troubleshooting guide in the download area, right side of this web site:
https://www.modinehvac.com/web/products/residential-gashydronic-garage-heaters-hot-dawg/hot-dawg-power-vented-hd.htm

Also, if your unit is the HDB, the installation manual says to mount the unit 6 inches from combustible material. Your picture doesn't show 6" at the top. Just saying.

[billybek]

Some of that info will be exactly the same.
The op has an older unit with a Honeywell smart gas valve.
Some of the control circuitry resides in the valve.
He mentioned his board didn’t have an led. The led is on the valve itself.
It may not have the same flash codes. Those should be on the panel door or in his installation operation maintenance info that would have come with the unit.

[BGCarrera32]

Ah hah!

So, I bypassed the limit switch and the 2 flame rollout sensors last night as they are on the same loop. It was still exhibiting the problem so those were not the issue.

Standing and observing the unit upon firing, HSI lights up, gas comes on...but not consistently and "cleanly" in the lowest tube or port where the flame sensor resides (see attached image). So the flame sensor is not bad, but its not sensing flame either since there's none there, intermittently. The 3rd time it tried to refire, I happened to give a couple quick blows of air with my mouth into the lower port and it fired up (blowing from a safe distance of course).

So- I am guessing whatever that jet or nozzle is I've circled in the bottom pic is bad. Do those go bad? When the furnance is running its pumping heat, so I guess I don't think it's low gas pressure at this point, even though that misfiring port is the lowest tube away from the gas input off of the control valve. But...

How best to clean? I can pull it apart this weekend. Unfortunately I don't have a gas pressure tester.

edit-> I should clarify, the top photo was snapped within about 4 seconds of the unit firing and then the control cut the gas flow as it should.

[abisel]

Looks like 2 screws at the top and 2 screws at the bottom, and however it is attached to the gas valve would free the entire assembly from the unit. Then you can take it to the bench and maybe a shot of compressed air from the air compressor will blow out whatever is blocking the nozzle. And can you verify the orifice is correct for natural gas?

There may be a couple youtube videos on cleaning out the gas distribution lines/nozzles.

[Brian 162]

If you blew at the burner and it lit I doubt the orifice (jet) is blocked. There may be a blockage in the flame spreader. Once you remove the panel that holds the burners in place you will have better access to the burners. Using compressed air should do it.

[billybek]

Shut off power and gas.

Use a toothpick, nothing metal that might change the size or shape of the orifice.

Try doing that in place.

After try to blow the orifices (I didn't just type that, did I?) with a computer duster. Something that you can't pressure up the manifold with and potentially damage the valve.

You might have a rust or even dead bugs inside the manifold. Try the above first and give it a try. You might need to disassemble further it if that doesn't work.

Edit:
I missed the part where he had mentioned that it would light if he provided some air movement to encourage it.

Blow down the burner tubes with compressed air.
Tap on the burners gently with the back of a screwdriver when you do this.

Try firing it again.