unexpected drop in efficiency (Munchkin)

Chuckles_3

I've been monitoring my new Munchkin's gas use in terms of therms/degree-day, having learnt on this board that this is the right thing to calculate.

For the October and November billing periods, it consumed 0.20 therms/dday after subtracting a baseline gas usage (I know this number depends on the house, but the point is that the number has changed, see below).

In December it started consuming 0.225, although the supply temperature was almost always below 130. Now, over the last week (using my own readings and dday information available on the web) it is using 0.26 therms/dday. (My previous boiler, 40-year-old cast iron, did 0.29 in cold weather, only 10% worse.)

Of course the weather is now quite cold, the water temperature is about 155 and the return is about 137. Condensate is still being produced, but probably not much condensate. I haven't collected and measured it.

To put it in different form, IF the October/November gas usage corresponds to the AFUE (92% condensing efficiency), the current gas usage indicates a drop to 70% efficiency non-condensing. In spite of getting a smaller boiler, outdoor reset, side venting, modulation, condensing, low-mass, blah, blah, I am not doing much better than my old boiler!

Is this reasonable, or could there be something wrong?

PS The condensate line wasn't blocked, but I blew it out anyway following instructions in the manual. The stuff that came out had some black powder in it, not a lot.

Constantin

Well, let's think about this...

... if condensate is still being produced, chances are that the boiler is still condensing. By definition, that entails a thermal efficiency over 89%, according to this source. I think the real question is, how much condensate are you producing? According to the radiant heat web site FAQ, you should be producing about 1.15 gallons of condensate per therm burned whenever the boiler is condensing fully...

Anyway, your results still seem to fall into the realm of the possible, considering that your old boiler may have been capable of reaching 80% thermal efficiency in the colder winter months.

Furthermore, the inadequate emitters in your home are causing some of the grief, as the return water temp is approaching the temp of condensation for combusted natural gas. If maintaining combustion efficiency is a requirement, I would upsize the emitters in you home to heat it adequately while being supplied with 140°F hot water on a design day.

Lastly, I would treat AFUE numbers with a bit of suspicion. They are published, IIRC, with 140°F water temps, etc. so appliances hooked up to low-temp heat emitters can do better. My Vitola has a steady-state rating 3% higher than its AFUE rating when its hooked up to a RFH system.

Leo G_99

hey Leo

could you please change your handle, as we already have a Leo G, that would be me!

thanx!

Leo G

George_10

Was this system cleaned and treated ?

That black power could be iron corrosion if there is iron in the system componants.

Let it settle in the botton of a plastic bottle and see if you can move the setiment with a magnet.

If you have further questions, give me a call at
800-543-5975.

Boiler Guy

Another question

might be; is it possible this brown powder iscarbon - indicating a possible combustion problem Is the interior of the vent perfectly clean?

Chuckles_3

Yes, the condensation is on the burner side, not the water side, so it's not iron. It's probably carbon as you said.

Isn't this normal? Isn't this what we homeowners pay to have cleaned every year? I guess it could be a sign of incomplete combustion. I'll check the inside of the PVC vent when I get home.

Chuckles_3

OK, sorry! I changed it.

jalcoplumb_2

Was the unit ...

set up with a combustion analyzer at install? I have a few old installs with radiators and they are doing fine.

If your numbers are correct than I think you should start by checking the burner combustion. Then look at your system.

Was the old system purged of debris? Is there any rumbling from the boiler?

Just a few thoughts.

Chuckles_3

> set up with a combustion analyzer at install?


No. I am trying to have that done, but my contractor is either busy at this time of year or does not have an analyzer, I can't tell which.

> Was the old system purged of debris?


No.

> Is there any rumbling from the boiler?


No.

JohnWood1

BROWN POWDER

Boiler Guy;

Please tell me what you know about the "brown powder isocarbon" as I have need of that info if you would be so kind as to share it. Email would be fine

Thanks

jw

hr

Where is the intake air

coming from? I had one fill up with brown grainy sludge from the dust it was inhaling. Sealing the case better seems to be the cure to prevent it from sucking room air.

hot rod

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S Ebels

Why unexpected?

Any boiler will drop in efficiency when producing higher temp water. You wouldn't think a 40* elevation in the temp of the water side would matter against the 1,700* fire side of system, but it does. I've seen it on every boiler that I ever did a combustion test on.

Example: We changed an oil tank today and tuned up the boiler along with the job. G215-6 Buderus oil fired with a Riello. After the 3-4 hours off time, the boiler started dead cold, like about 60* water temp. We let it get to about 100* and proceeded to test.
At that temp the raw combustion efficiency was 89.1%. By the time it hit 170* water temp it was running 86.8%.

My nat gas fired Vitola will run on the far side of 88% when it's seeing water temps below 100* and show an honest 90%+ when starting from room temp. It settles at around 86 when temps hit 140.

It's just physics, the greater the T dif the faster and more efficient the heat transfer.

.

You are talking about efficiency changes of 2% to 4%. If you had read my message, you would have seen that I am talking about a 20% change.

.

Nope, doesn't look like that. Just a little fine powder in the condensate, all the same color. And while the Munchkin case doesn't really seal tightly, our basement is pretty clean.

jerry scharf_2

drop in efficiency

Leo,

You made a incorrect assumption that the efficinecy was 92% when during the shoulder months. It will run at about 92% when the supply water is 140 and the return is 120. As soon as you get return water well below 120, the efficiency increases beyond afue. With 80F return water, condensing boilers have been clocked at 98% efficiency. With 130F return water, the efficiency will be below the 92%, but not too much. Even with 150F return temps, the combustion efficiency should in the high 80s.

There's also a dirty secret in the gas industry. There is no guarantee the gas you get has the caloric content of methane. There have been reports of caloric contents anywhere from 30% below to 60% above nominal. When the utilities buys their gas, they continuously meter the caloric content and pay based on the BTUs delivered. No such luck for us! Sadly, there's nothing short of running a sample through a gas chromatograph or similar expensive technology to find out.

Getting the system set up with a combustion analyzer is a must for a munchkin. HTP's burner setup specs are based on the amount of CO generated in low and high fire. There could be something wrong, but without this you really can't tell.

jerry

Mike T., Swampeast MO

Similar Experience

As the weather cools and degree days increase, the fuel savings diminish.

I've sent my head spinning trying to compare consumption vs. degree days between a simple cast iron "cube" and a condensing/modulating.

Darin Cook_3

As Jerry said

The gas companies will add other gases to the methane depending on the amount of use. The caloric value goes up and down. We have no way to prove or disprove this in the field. There are many other variables involved in eff dropping or increasing. If this is your first season with the boiler the only fair way to see how it performed is to wait a season and then compare to the prior year. Taking into account heating degree days. Remember we have more time in the shoulder seasons than flat out cold temps.






Darin

jim sokolovic

Calorific value variations...

There is an ongoing investigation by GAMA regarding an increase in Wobbe value of Natural Gas delivered by the utilities in many states. Wobbe value is derived from the higher heating value of the gas, and expressed in the same BTU/cubic ft. units. The variations reported do not seem to vary by more than +/- 10 %, though. Variations in the range of 30 % to 60 % would definitely not even allow combustion to take place on some appliances, and would certainly cause CO levels off the scale of most analyzers in most appliances. I am assuming that the variations in this case are compared to a mean value at a certain altitude - the difference in heating value between sea level and 10,000 feet could be about 30 % (but the air density also changes by about the same proportion).

Unknown

What is the gas company

mixing now days. Well in the past they mixed propane with natural gas to compensate for "peak shaving" days. Those are days when demand was greater than purchased gas supplies. It has been my experience in the last three years that when LNG (Liquiefied Natural Gas) is being vaporized into the system that combustion is definitely affected. I believe that the BTU content is at times getting up in the range of 1100 BTU's per cubic foot. This will cause definite problems with combustion and increase Carbon Monoxide drastically. In some cases I have heard reports that BTU content may get as high as 1200 to 1300 BTU's per cubic foot.

I have had three jobs I am watching for the last three years that have had combustion problems after being set up very carefully by myself with combustion instrument. Two of these are boilers one is a furnace.

A system like a Munchkin will defintely have a problem with variables in BTU content. When it is setup with one BTU content and using CO analysis to do this and then seeing a change in BTU will change the firing condition of the system.

If propane is being mixed it can be even more drastic. Also keep in mind LP gas has problems when outdoor temps drop drastically. There may also be methanol in the gas which should not affect its BTU content but who knows.

Constantin

So...

... is your Munchkin condensing or not?

If it is condensing, how much condensate is it producing per Therm of gas?

Chuckles_3

> ... is your Munchkin condensing or not?


As I have already said in the original post, it was condensing. (That was when there were 55 ddays and the supply temp was ~155. It is warmer now, and it is now producing more condensate.)

> If it

> is condensing, how much condensate is it

> producing per Therm of gas?


Not measured at that time, but as I said, it was small.

It is possible that condensate was being produced only at the beginning of a heat call, when there's lots of cooler water returning from the radiators. If so, your rule of "if it condenses at all, the efficiency must be at least 89%" does not really apply to my numbers. I am not measuring instantaneous efficiency at the exact time when it is condensing, but rather the average efficiency over the whole day, of which only a small period is under condensing conditions.

Constantin

Well...

... in that case, I'd recommend adding some heat emitters in your home. Your return temps are too high to allow flue gases to condense out, as 137°F is the limit where any natural gas-based flue gases can condense at all.

You should still get combustion efficiency in the high 80's though. Perhaps, as others have suggested, your local gas company is playing games with the caloric content of the gas.