anyone w/similar problems on MUA?

[relay320]

Got a greenheck m#VSU MAU that fails to light intermitantly. Have done everything suggested by manufactures, no help. Now suggest to convert to 120v ignition at xxxx dollars just for parts. Existing fenwal ignition system,amplifier,maxitrol mod valve,& honeywell main valve. Have verified gas press.,press. drop across burn,adjust low fire bypass on maxitrol. Guess my question is is there something I might be missing that somone else might catch or suggest to resolve problem without spending xxxx $ in just parts and end up with same problem afterward?

[simux]

I've had issues with the transformers being under sized from the factory on boilers. Not sure if this will help or not.

[chillerguy81]

You don't have another similar unit onsite that you could start swaping ignition components with, and see if the problem follows do you?? You probably would have already thought of this, just throwing it out there!

[ascj]

Got a greenheck m#VSU MAU that fails to light intermitantly. Have done everything suggested by manufactures, no help. Now suggest to convert to 120v ignition at xxxx dollars just for parts. Existing fenwal ignition system,amplifier,maxitrol mod valve,& honeywell main valve. Have verified gas press.,press. drop across burn,adjust low fire bypass on maxitrol. Guess my question is is there something I might be missing that somone else might catch or suggest to resolve problem without spending xxxx $ in just parts and end up with same problem afterward?

What options you got? Is is 100% fresh air? 80/20? Does it have a burner bypass damper? Is it setup for variable airflow?

You have a 24v ignition system? All the ones I have ever worked on where 120v spark ignition with a RM7800 primary with flame rectification. What exact ignition is in there?

[DPinst]

Have you been able to varify airflow across the burner?
You may have a air pressure switch that is not set correctly and causing you problems.

[Moose]

Is the burner clean?

[relay320]

thanks for replies. I have already swaped out the fenwall ignition control,ignitor,cleaned burner,ohmed out ignitor wire, & thought I had it. Following day got word was locked out. It is 100% FA. Has VFD and is there for easy start. When it fails, it fails on ignition start. Did replace flame sensor also. Have followed manufactures' installation instructions to the T, and they themselves have been helpfull, but would like to cover every base before I convince owner into spending xxxx $ and still have same problem afterward!

[ascj]

thanks for replies. I have already swaped out the fenwall ignition control,ignitor,cleaned burner,ohmed out ignitor wire, & thought I had it. Following day got word was locked out. It is 100% FA. Has VFD and is there for easy start. When it fails, it fails on ignition start. Did replace flame sensor also. Have followed manufactures' installation instructions to the T, and they themselves have been helpfull, but would like to cover every base before I convince owner into spending xxxx $ and still have same problem afterward!

So what was your velocity across the burner and what size unit do you have?

[relay320]

drop across burner .75" . 206,200 max in & 16,000 min I believe it's a VSU-110-H20. guess to answer I did not check actual velocity. believe velocity would be right if drop across burner is in close paramaters of manufacture which is 0.65, or am I wrong.

[james mo]

Are there any other safeties that reset that deenergize the gas and pilot valve?
A pair of cambridge units I work on will have a limit switch that will open and lock out the heating relay that allows the pilot and main gas valves to operate. This causes the fenwal 24vac hsi ignition control to lock out on flame fail when it tries to relight.

[relay320]

the only thing is since direct fired it only has single try and if unsuccessful it is locked out on flame fail & then goes out on freeze protect. It does have hi-temp limit, but it modulates and runs fine. Just not when it sometimes fails to light.

[burto]

had similar problem with old rapid unit and it was a dirty uv scanner.

[emcontrols]

I inherrited an account with a fire tube boiler, that for years went out on flame failure sporadically, I went through every possible situation, after days of studying the situation I began noticing on extremely cold mornings it would trip, on mild mornings it would not. I began checking inlet gas pressures on various temped mornings and finally caught the inlet pressure supplied by the city on extremely cold mornings dropping low enough to cause flame failure, of course after speaking with the city, it became a gas demand issue, they ended up replacing the city block regulator and increased the supply pressure and in turn "NO MORE FLAME FAILURES" As a temporary fix I increased my gas regulator pressure to max allowable inlet pressure which was high enough on most cold mornings to keep flame failure from occuring. Sometimes looking at the condition the system is in such as manufacturing etc... find out when its tripping what other loads are sharing gas line etc.... Outside the box so to speak.

[relay320]

I know the gas supply is an issue that has occured prior on other units around this facility, that is still in the process of getting resolved. This unit would not fire at times when I still had 8-9" inlet press. and about 8" between honeywell and maxitrol valve. Is there a setting on downsteam side of the modulating valve (manifold) I should be looking for? Was informed to not even pay attention to this. Was this legitament advice?

[fxb80]

I don't pay any attention to manifold pressure if airflow is correct and unit will make rated rise at high fire.

If the honeywell valve has a fluid power actuator; I have seen a few actuators intermittently run slower than rated when cold and of course if it loses fluid, which could cause a flameout.

I also wonder what the flame current is pilot only and also main flame high and main flame low. Should be well above minimum when things are right.

If gas pressure is remotely a problem and hard to catch, I've installed temporary manual reset low gas safety switches to prove or disprove the possibility. Not wired, just connected to inlet gas.

And then there's the flame sense lead and sensor, if either starts to fail, fine hairline cracking, it can work ok when dry but not when wet. Check the flame sense lead and sensor on high megohms. I use a 60 megohm meter for this, should show infinity if good.

[pairman]

What condition is the ground wire going to the burner. Corrosion will cause sensing problems

[emcontrols]

If this is a direct fired make up air unit the manifold pressure operates in a negative and typically needs much higher inlet pressure to account for required btu output. manifold pressure on these type of burners is super critical. especially on low fire which is what will prove your flame snensor.

NOTE: On these type of burners you can still have your inlet pressure but not the volume of gas output.

[ascj]

If this is a direct fired make up air unit the manifold pressure operates in a negative Maybe in low fire some will.....but most end up in positive manifold numbers and typically needs much higher inlet pressure to account for required btu output.What does that means...please elaborate. manifold pressure on these type of burners is super critical.I beg to differ...manifold pressure's are less critical on direct fire units as they are on indirect burners. Not saying ignore them.especially on low fire which is what will prove your flame snensor.That's only true on units with interrupted pilots. Must are intermittent pilots.(which I don't like...but it is what it is.

NOTE: On these type of burners you can still have your inlet pressure but not the volume of gas output.

You keep referring to proper inlet pressure but not volume of gas. What do you mean by that?

[ascj]

I know the gas supply is an issue that has occured prior on other units around this facility, that is still in the process of getting resolved. This unit would not fire at times when I still had 8-9" inlet press. and about 8" between honeywell and maxitrol valve. Is there a setting on downsteam side of the modulating valve (manifold) I should be looking for? Was informed to not even pay attention to this. Was this legitament advice?

I wouldn't worry about it at this point. A couple questions;

Are you able to see the pilot being lit?
What is you pilot regulator pressure?
Is this a interrupted or intermittent pilot?
Do you know what manufacture of burner is it? I thought those ran Maxon burners.
If it is a Maxon burner......was the ignitor ever replaced?
If it was.....make sure the right ignitor is in there. They come in a couple of different lengths, depending on the end plate.

[ahc&r]

looking forward to ascj's questions, I had issues with dirty pilot burner at ignition source and igniter points