Munchkin T80M Boiler low fire induced rumble

BigBlue

Munchkin T80M Boiler running on propane lite's reliably but occasionally there is a low fire induced rumble. The rumble immediately disappears when the boiler goes to hire fire mode to supply hot water tank and stops rumbling even after turning off high fire but returns about 20-30 minutes later. I have two other Munchkin T80M units that work fine in the same exact installation.

Adjusting Gas Valve Throttle screw in either +/- no affect other than making the rumble louder. Seems like the burner chamber needs more air?

Ironman

Was the burner setup with a digital combustion analyzer?

How long has it been installed and has PROPER maintenance and cleaning been performed? By "PROPER", I mean according to the book. Most "techs" are clueless about cleaning a Gianonni heat exchanger.

JUGHNE

What does the inside of the heat exchanger look like?

BigBlue

All three boilers are about 10 years old but are in a second home used on weekends and only used about 20% of the time so age is less about wear out and all three units were all cleaned and services recently. When the rumble occurs I can see the flame burning but fluctuating and smell some propane residue. This one unit has exhibited these symptoms off and on for a few years. I only recently want to find root cause and fix.

plumbum

( note ; we have a complex with over 100 x T 80 m boilers) had the same issue, unit fired and ran for domestic hot water, but would die on heat call. error code F 10.Using the display board d6 and d7 you can check fan speed and probe current. We have about 7 micro amps on d7 normally.
check swirl plate and gas valve adapter plate first for deterioration.
ultimately we also changed the gas valve. also note we are having issues with blowers and m boards in general and thought this was the cause but don't believe so.

BigBlue

The home is located at 8300 ft. elevation so most issues are related to lack of oxygen. The problem is still present with cover on or off. The other two boilers operate just fine at the same altitude and setup. The unit displays no error code. Both Swirl plate and gas valve adapter was replaced for deterioration and this corrected for failure to reliably ignite on first attempt last year and are still in good condition. My next step was to order replace the Dungs gas valve but what would cause this to always work accept for certain intermittent conditions at low fire? Something is marginal not just failing or it would be easy to isolate and fix. I was hoping it would just stay in failed state.

Ironman

Is that the high altitude model Munchkin? It will have hot surface ignition if it is.

DZoro

What are your combustion numbers CO, CO2, O2? Where is the flame current #'s during high and low fire?

BigBlue

None of the units use a hot surface igniter. Further you can read above none of three units have any problems igniting and the flame can be seen so igniting is not the problem. Will obtain flame current readings and report back.

HomerJSmith

I assume your cleaning consisted of removing the front boiler plate and scrubbing the HX and cleaning the probes. It also includes replacing the burner tube gasket and any damaged refractory.

Concentric venting is a potential disaster for the blower and gas valve with gas reversion. The case insides should be shinny aluminum, no pitting or corrosion.

By the way, D7 should be about 4 micro-amps.

Look, these boilers need to be set up with a recently calibrated combustion analyzer, without exception. If there aren't 3/8" holes in the exhaust pipe near the boiler, then a combustion analysis has never been done. I always do a combustion analysis when I do a major cleaning, the gas valve parameters do change.

If you are having trouble falling within HTP guide lines for combustion, you may have to adjust the offset screw on the gas valve. Don't do this without instructions and a combustion analyzer. Make sure your input propane pressure are within guide lines.

You can also call HTP at 1-800-323-9651 and talk to a tech person, but do this after a combustion analysis.

HomerJSmith

plumbum, you have concentric venting, I bet.

SuperTech

Concentric vents are terrible for these boilers. I strongly suggest having separate terminations for the intake and exhaust. I've seen several munchkin and Peerless pinnacle/purefire boilers destroyed by exhaust gas recirculation from the use of concentric vents.

BigBlue

We are narrowing in on the problem:

Boiler 2: High fire mode (works fine)
d6 = 9.8mA probe current
d7 = 4250 RPM fan speed

Boiler 2: Low fire mode (measured when boiler was rumbling)
d6 = Jumping around between 1.5 to 3.5mA probe current. Sometimes the boiler will recover on its own and the current climbs over 7mA after a few minutes and rumbling stops.
d7 = 1270 RPM when failing but some occasions it will recover gradually increase speed until rumbling stops at higher RPMs.

Why do d6 & d7 sync together and track each other? The only difference I notice in low fire mode the fan speed starts out low and gradually increases. But why does it work always work fine in high fire mode and intermittent in low fire?

BigBlue

Never heard back from anyone on the below values point to electrical issue not combustion?

Boiler 2: High fire mode (works fine)
d6 = 9.8mA probe current
d7 = 4250 RPM fan speed

Boiler 2: Low fire mode (measured when boiler was rumbling)
d6 = Jumping around between 1.5 to 3.5mA probe current. Sometimes the boiler will recover on its own and the current climbs over 7mA after a few minutes and rumbling stops.
d7 = 1270 RPM when failing but some occasions it will recover gradually increase speed until rumbling stops at higher RPMs.

Why do d6 & d7 sync together and track each other? The only difference I notice in low fire mode the fan speed starts out low and gradually increases. But why does it work always work fine in high fire mode and intermittent in low fire?

Zman

It sounds like your poor combustion is giving you some undesirable flame current readings.
How is it you don't think you need a combustion analyzer? You are just playing a guessing game without one.

SuperTech

Combustion analysis is critical. Without testing you might as well just try swapping parts to see if that fixes it.

BigBlue

You guys crack me up. Please explain how a problem that is intermittent points to combustion not electrical! I do not know of any mechanical mixture of air and propane that will fix itself do you, then break and repair itself? Approximately 80% of the time the unit always works. The other two identical boilers work perfectly and all match settings. Suspect unit lites and stays lite just fine. Adjusting the fuel mixture will just cause the unit not to lite. It is already tuned correctly.

HomerJSmith

First, mA is milliamps and uA is microamps.

You say,"Please explain how a problem that is intermittent points to combustion not electrical!" O.K., what, Big Blue, you are measuring is the rectification current. It is called flame rectification. An ac current is passed thru a flame and is rectified to a dc current which the controller recognizes as a proof that there is a flame present and then turns on the gas valve. If the flame is erratic on low fire, it is going to interfere with the rectification current. Up to and at high fire you have a good flame present, so rectification is smooth (which is why D6 & D7 track. The boiler has to be set up for both low fire and high fire. That's why a combustion analyzer is necessary. Without one, it is anyone's guess what is going on. Altitude, BTU's/cuft, and gas type all affect combustion. Wear on the gas valve affects combustion. When combustion is corrected to manufacturer specs, all variations are rectified.

Do a combustion analysis with a recently calibrated analyzer and then report back if you still experience rumbling.

Jamie Hall

Sorry we crack you up. We don't intend to. Rumble is almost always -- if not always -- caused by unstable combustion. The odd electrical measurements are the result, not the cause. There are a number of possible causes for unstable combustion, of course but finding out what the problem is without using a combustion analyzer is a fool's -- or knave's -- game., never mind fixing it.

HomerJSmith

By the way were there any fault codes with your T80M?

BigBlue

We are sort of going in circles. Once again there are no fault codes present. All three identical boilers are set up the same. Only one boiler has the low fire rumble what is called an intermittent failure. Which means it is NOT always present works fine during low fire the majority of time. Can we all agree you can only have a stable flame with the correct oxygen and fuel mixture and the boiler is a closed loop system? The only adjustment I know I have is the Gas Valve Throttle which is a fixed setting after setup which either you can set the propane fuel too rich or too lean. The only other variable that controls combustion is air flow which is from the electronic control board to the fan. It is my opinion the flame chamber needs more air flow. It is not so hard to believe the controller board has an electrical open/short that is intermittent? No one has provided any mechanical failure modes that can explain how mechanical items like valves, fittings, chambers can fix themselves?

As I stated in my first posting adjusting the Gas Valve Throttle screw while the unit was in the rare failure state (low induced rubble) had no affect. If it was a fuel mixture issue adjusting this should have momentarily made it stop rumbling regardless if it was in spec or not and I would be all over the theory of measuring combustion numbers next.

Is there some sensor that is in the loop that is not working why the fan is not speeding up? Thirty-five years of trouble shooting complex systems it usually two or more problems that are present.

Jamie Hall

Since you seem to have all the answers to your problem, far be it from me to make a suggestion. I will anyway. Is it remotely possible that the gas valve throttle is slightly out of adjustment, and the mixture is, in fact, slightly off? It wouldn't take much...

Oh and by the way, there is no such thing as three identical machines of any kind. Very very similar, yes, but identical, no.

Ironman

@BigBlue
Can we be realistic here? You came looking for answers to your problem, but everything we've suggested you've rejected and essentially implied that we don't know what we're talking about. In all honesty, how can we be expected to help you if know better than us how to diagnose the problem?

ced48

I believe your gas valve also has a pressure differential adjustment which may well be causing the problem. However, you can't make any adjustments to it without a combustion analyzer. Time to buy one, or hire a professional.

HomerJSmith

BigBlue, for your information the gas valve has an offset adjustment that can move combustion to the wrong side of the combustion curve. It is not the +/- adjustment. Change that without a meter at your own peril.

You didn't have any fault codes so I don't think this is the problem, but you may have a loose burner tube in the combustion chamber.
Since you are hooked on a mechanical solution. You might try unplugging all the molex connectors and re-plugging them back in.

You say, " It is my opinion the flame chamber needs more air flow." Indeed, it may need more oxygen, at your altitude, or less fuel to balance the combustion as your oxygen is what it is and the only change you make is with the fuel.

When you do a yearly inspection, you clean the spark and rectification probes. You can take them out and clean them with wet and dry sand paper. I use a stainless wire wheel on a dremel tool. Be careful of harming the gaskets. A dirty probe can affect the rectification current. But this doesn't address the rumbling.

Switch the control boards between a good boiler and the bad one if you are so inclined.

Giving you advice is like pulling teeth. I feel I'm playing the game of "Yes, But".

BigBlue

Homer - Those are all good suggestions and easy to try while I wait for my analyzer to return.

For the negative critics - This is a 12,000 sqft. mountain home in the remote part of Colorado that I can't obtain professional service from any knowledgeable service technicians on boilers. I have already fixed and repaired my Wolf appliances and the same boilers for the past 10 years not because I want to, I had to.
Either they do not have the expertise, diagnostic equipment, or will not drive 2-3 hours one way to repair because no one carries ANY possible spare parts that maybe required once they arrive. They can all make easier money working in a good economy near the cities. They pretty much all said diagnostic is all good but you still have to prove by replacing parts at some point and rarely turning an adjustment screw 1/16 of a turn fixes much of anything on these boilers. Since I have three fit, form, and functional identical boilers I can use one for parts and back order replacements like I have done in the past. Of course no two snow flakes are identical but if we start assuming no two parts are functioning the same then all that is left is madness.

Jamie Hall

You do have a difficult situation -- but not an impossible one. What about learning to use a combustion analyzer? Yes they aren't cheap -- but neither is getting service in you location cheap, as you have noted -- and with a house that size I dare say that you probably can afford it. They aren't hard to use, once you get the hang of it. Besides, perhaps there are others in your area who could use the service you would then be able to help with.

DZoro

Your low flame current is telling you that in that state you have bad flame/combustion. A good running/happy boiler will always have a flame current higher than 6 (8+ is optimal fire).Like others said will need combustion analyzer. Then go to low fire setting adjustments per instructions for your boiler.

BigBlue

What is a good brand and model number combustion analyzer one should use?

DZoro

I'm very happy with the Testo brand. Been using this brand for over 10 years. Good price, good quality. #310, 320 they may have newer models also.....

NY_Rob

If you're not using it professionally and you just need CO and CO2 readings the UEI C20 will work.
Reasonably priced, and it's got a 5yr sensor life expectancy (you still need to get it calibrated/certified every couple of years $99).

HomerJSmith

BigBlue, if you do, do you own combustion analysis, you should have some idea of what it is all about, so search youtube for information on combustion.

Also, you should call HTP technical and ask them to email you a copy of the offset adjustment for your T80M if you need to make that adjustment. If you can't meet the HTP guidelines for combustion, then you might have to make the offset adjustment.

Don't buy a used analyzer on Ebay. Make sure the seller guarantees a recent calibration and provides the documentation from the calibrating company to prove it. An uncalibrated meter is as good as none.

4Johnpipe

I've had this happen with a bad fan...The fan speed may be off causing a bad burn / combustion...