Munchkin Diagnosis and Replacement Size Check

SENorthEast1895er · 10:39AM

Hi All,

I'm a homeowner with an old Munchkin that I've been nursing along. It recently started dripping off the bottom of the condensate drain tube. As far as I can tell, this is only happening when it's condensing (pressure steady w/ auto-fill valve closed, puddle bigger in the morning).

I opened it up to take a look and didn't want to disturb all the fiberglass without knowing what was going on. So I called my plumber… who also didn't want to disturb the fiberglass and just gave me a quote to replace.

So my first question here is about the drip: Any advice on tracking it down? Or is it, indeed, time to finally replace?

Second question: My current boiler is 140k BTUs and I strongly suspected it was waaay oversized (which I told my plumber). He gave me a quote for a Burnham ModCon that was something like 195k BTUs and said:

it is only a few hundred dollars of price difference
it doesn't matter because it has a 10-stage step down and will always be able to fire at the appropriate level
- which just sounds nuts: why would they make smaller models if that was the case?

Are these statements accurate?

My ChatGPT-assisted estimate of our heating needs (via a few cold months of gas use, degree days, and some math) is about 60k. I want to sanity-check that with a local retired plumber I know and probably redo the math and post it here. But I can't see a reason to get a boiler that's 3.3x larger than I need it to be. And I can see some reasons not to.

Edited to add:

I need to rerun the calculations (which I did; chat gpt just gave me the formula), but it was:

((BTUs used * efficiency) / (degree days x 24) ) x (target indoor temp - design temp)

I'm somewhat familiar with manual Js, having done my best to do it myself when we bought the house. This Munchkin had been running @ 180 for 15 yrs or so (due to a truly terrible install job; my plumber finds something new wrong with it every time he looks closer). I ended up using a Taco ZVC to work around the piping issues to be able to use the outdoor reset (see post history for more info).

And please don't forget my first question: Can I track down this condensate leak? If so, how? (or should I just replace?)

Thanks

Jamie Hall · 11:23AM

Lots of reasons not to go with a boiler which is too big. And the idea of replacing a 140K unit with a 195K unit, when you know (I presume) that the 140K unit is plenty big enough — yo do stay warm don't you? — is just nuts.

In fact yu may be able to go smaller — but to determine that you need to know at least vaguely how big a boiler you really do need. There are several ways to do that. There are estimators which aren't goo bad which can give you a good number from your previous fuel usage and where you are located. It is possible to calculate the heat loss of a structure — this is often called a Manual J calculation. In any case you don't need a boiler bigger than what your radiation can handle, usually taken as being around 500 BTUh per foot of baseboard, for fin tube baseboard. For standard radiators, there's another approach.

So find someone who will at least take a pass at actually estimating what size is needed and go from there.

PC7060 · 11:29AM

Definitely don’t get an boiler. I heat a large house with a 80 K modulating condensing boiler, which often runs at its lowest level.

I wouldn’t be comfortable using ChatGPT based heat loss without verifying the calculations.

What’s the average usage on your gas bills for coldest month for last three years? This give you a ballpark estimate of minimum size but would need to confirm with heat loss report. In my area, temperatures can vary 30 degrees F between night and day / week to week so averaging can be off significantly.

SENorthEast1895er · 12:29PM

@PC7060 I edited my original post to include the formula that ChatGPT gave me to use. It all made about enough sense to me. The major questions I had were:

what efficiency I should use: I used 90%, which is tiny bit under what my boiler was spec'd at from the factory. I think it read that on last fall's servicing report. But maybe I should lower to 88% and rerun
the "Base Temperature" setting on the degreedays.net report. I left it at default (65)

This year was the coldest winter I can remember; I used the two coldest months (I did match degree days to billing dates), but thanks for the reminder that I should check historical data.

How do you know what level your boiler is running at?

pecmsg · 12:57PM

You don't size for last winter. That was extreme. ASHRAE or any local weather station will have your local design conditions.

pedmec · 12:59PM

If the boiler is leaking from underneath check that the trap. It is either plugged or cracked. it's just a molded rubber trap. no need to change your boiler of a stupid trap. its not easy to get too but if thats it you'll save a little money

You said that you are running at 180 degrees all the time. Sounds like the outdoor reset sensor was not installed. Install the outdoor reset so you can lower the boiler supply temperature. When Munchkins first came out the didn't supply the ODR sensor with the boiler.

pedmec · 1:10PM

i reread your question and i couldn't understand what fiberglass you were talking about and then it dawned on me that its most likely the heat exchanger wrap. If that is the case then don't be afraid to move it. i would move it out of need and check the supply and return sensors plus the pressure sensor. they are leak points in that boiler and a lot of MOD/CONS that use a water tube heat exchanger.

I had the misfortune of having a corroded supply sensor blow off and destroy the inducer and flood the finished basement. ruined the 925 control. its really the only thing that i have had go wrong with that boiler.

SENorthEast1895er · 3:03PM

@pedmec Thanks. It was running at 180 when I bought it (due to incorrect piping), but I installed an ODR and use a hacky setup with a TAco ZVC to run at 140 for design temp.

I'm quite concerned about flooding… now you've got me ancy. Thanks for the tips.

pedmec · 3:13PM

No problem. I don't want to happen to anybody else what happened to me (in reality my ex-wife has the house but i always get the call to fix. The issue is i don't live there anymore so when the crap hits the fan i get the call. Because i don't live there anymore it turns into a major fiasco).

Its a shame you bought the Taco ZVC with the Taco PC700. You could have just bought the 10k sensor and wired it into the 925. It's just a 2 conductor. I think this is one of the things that HTP failed on when they introduced the Munchkin on the market, was not supplying the sensor with the boiler. Once you connected it to the 925 all you had to do was adjust the parameters in the 925

hot_rod · 3:46PM

Size the boiler close to the actual calculated heat load. But oversizing a bit with a mod con is not as big as an issue as a fixed output boiler. 10 or 11-1 is a common turndown, so possibly a 120 or even 80K would be adequate. About the lowest turndown you will find is 8,000 btu/hr on a 50 or 80,000 mod con.

I think all the mod cons have ODR onboard now. Most companies have a basic model and a more $$ version with more control options, possibly a different heat exchanger design.

What size home are you heating? That can give you a ballpark number to start with.

SENorthEast1895er · 4:13PM

@pedmec The Taco ZVC was because it was piped with only 1 circ. Vision 1 requires 2 circulators, so the TACO is used to control the single circ and passes the HW and CH calls to the boiler. Pretty hacky, and can have some issues, but works.

@hot_rod 3 floor victorian twin (1895) with roof insulation and a mix of modern and original+storm windows. 2800sq ft. My retired plumber friend said that the 60k sounds very low, so maybe I did the math wrong. Design temp 14 degrees.

Munchkin Diagnosis and Replacement Size Check

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