Mr Heater MHU 75 - Eratic Ignition

[Joelk]

I installed an MHU 75 in my Garage last fall. A Gas Co Tech installed the Propane lines and checked the flue and all other aspects of the installation. After I became aware of issues, he inspected again and verified that the gas pressures are correct.

When it was first installed, the unit fired up and seemed to heat properly, but this was in very warm 70-80 degree temps.

I only use the shop occasionally and during the fall I noticed that the heater would often start burning for a few seconds, then go out, then about 30 seconds later would start burning again for a few seconds, go out again, and then on the 3rd cycle it would usually start and continue burning normally for as long as it took to get the shop up to thermostat temp.

I did not think this seemed normal, but it always seemed to light eventually, so I did not worry to much about it.

Since it has gotten colder, it now sometimes fails to light. It seems that if it is 50 degrees or warmer in the shop it would start(after the sequence described above) pretty much every time.

When it is below 50 degrees in the shop, it frequently will not light(even after the sequence) and the control light will flash 2 times which is supposed to indicate a system lock out.

I took the end panel off of the unit and observed the burner operation.

When the ignitor starts clicking the bottom 4(of 5 burners) light almost in unison, the top burner often will not light, and after about 10 seconds the ignitor will stop clicking and the gas will turn off. About 30 seconds later another ignition cycle will start again and the bottom 4 burners will always ignite and the top burner may or may not light. IF the top burner does light it is ALWAYS after some delay. If it has not ingited after about 10 seconds the ignitor and gas will stop and after another 30 seconds or so another ignition cycle will start again. Again the bottom 4 burners start almost in unison and the top burner will sometimes ignite. If the top burner does not light the indicator light goes to two flashes and it will not longer attempt to light.

It often does start on the third ignition cycle and I suspect that this is because the previous ignition attempts have warmed up the temperature in the unit somewhat.

I removed the #4 and #5(#5 on top) burners to see if there are any defects in them. I found nothing abnormal about the #4 burner. The gap on the "flame channel" on the #5 burner looked very similar to the #4 burner, except there was a spot weld that was not holding and allowed a very slight gap at the back of the burner. I welded this gap shut and reinstalled. This did not seem to have any effect on the operation, and as before, the #5 burner ignition is always delayed, if it ignites at all.

Any suggestions on what I can do to get consitent ignition of the top(#5) burner?

Would opening, or reducing, the gap of the flame channel help?

 

[dmsween]

I had a problem with my Mr Heater natural gas heater. My heater was not lighting at all. I would light it by hand with torch to get it going but it could not re-ignite itself. I called the manufacturer and it turned out to be a bad main control board. They had a problem with the boards not sending enough voltage to the ignitor

I would call the manufacturer (Enerco, i think) My heater had a 3 year warranty on it.

 

[Joelk]

Thanks for the reply.

I have called the manufacturer, but they did not seem to know what is needed to fix the problem. They said they could send a control panel, but I doubt the control panel is the problem since it always lights the bottom burners, it is just the upper burner that sometimes does not light.

Since the bottom burner always lights, I assume(please correct me if I am wrong) that the ignitor is getting enough voltage. It appears that the bottom burner is the only one that is directly ignited by the igniter and the others are then lit by the burner below.

For some reason, mine does not consistently transfer the flame from burner #4 to burner # 5.

 

[nehog]

What size gas tank are you using?

 

[Joelk]

Three 100 pounders connected together.

 

[dave67fd]

Just a thought but when you removed the burners did you check the burner heads for proper alignment?
You can also clean/check filter screen and check/confirm spark electrode gap (0.110- 0.140").

 

[philjafo]

Most heaters come set up for natural gas (runs 3.5 in/wc) and need to be converted to lp (runs 10 in/wc), check alignment of the burners, make sure nothing is blocking the gas orifices. If you were a HVAC tech with all the proper test equipment I would ask what the combustion analyzer readings are, but it would be cheaper to hire a service tech to fix your problem then to buy a $2000 dollar tool to use one time. Sometimes things are just impractical for the diyer, sounds like it hasn't ran right from day one so maybe don't call the guy you had run the gas line and check the venting he should have turned it on at least once to check it was working correctly.

Just reread your post and you said the gas co verified the pressures were correct, is that the pressure incoming to the appliance or output of the gas valve. Around here the gas co will only work on the gas piping outside the wall, anything inside they only check for leaks and they won't fix them if they find any they just shut you off and tell you to get it fixed.

 

[Joelk]

The Gas Co Tech. is a good friend of mine and assited with the installation and did the testing, as a favor to me.

It was installed over a period of a couple weeks. I initially hung the unit from the ceiling and installed the flue. He than ran and connected the gas lines and I later installed the thermostat and connected to 120V. He was not there when I initially started and ran the unit.

When it was originally installed, he checked the line pressure to the unit, before the valve.

Since the cold weather and occasional failure to start, he checked the manifold pressure with the burners lit and it was about 10.5 in/wc.

The burners are in good alignment and the screens are totally clear.

When the burners are all lit, they seem to have a good appearance and all of the burners look very similar to each other when burning.

 

[Joelk]

I think I got my heater fixed and thought I would post the "solution" in case someone else encounters the same symptoms.

I modified the top burner, by closing and pinching off the "upper flame channel". I think this had the effect of making more gas go to the "lower flame channel" and making the top burner light more reliably.

It has been through quite a few starts without going through more than one ignition cycle so I hope it is permanently fixed.