Munchkin Diagnosis and Replacement Size Check

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.

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.

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.

Fuel use based load calcuations are the most accurate method for heat loss for older structures unless you have something like an MCM house with walls of glass. It makes sense, the fuel the you use is burned for heat, that heat is needed to make up for the heat loss from the house.

It doesn't matter if you use a very cold month even if bellow seasonable. The math normalizes based on HDD and design temp, so the result is about the same. The only thing that would change is the efficiency of the a modcon which decreases slightly with ODR when it gets colder.

A house at 20btu/sqft is actually on the high side for older house, there is probably a lot of low hanging fruit in terms of efficiency improvement.

Of course you need (2) circulators because that's why you are required to pipe it in a primary/secondary piping method. So you need to a primary circulator to ensure proper flow thru the boiler and a secondary circulator for the heat loop(s). If its piped with just one circulator it ain't installed correctly but obviously getting enough flow to prevent flashing and hi-limit trips. Not the recommended approach for mod/cons.

And yes the red line is the trap location. it's a preformed rubber trap

@SENorthEast1895er

I am not here to bad mouth plumbers but a lot of plumbers know 0 about heating. There is no heating questions on a plumbing test.

And a lot of installers (plumbers or not) just don't want to bother with working on a MOD CON.

You best bet might be to post your location and check "find a contractor" on this site.

A lot of installers will not bother with a heat loss they see no value in it. It is early summer, so you have time on your side to look for the right installer and the right boiler.

The drain spud on the bottom of the boiler sometimes leak. Is the boiler level?

Using the guesstimate of 20btu/ ft the load is 56,000

At 25 btu/ ft, 72,000

Well within an 80,000 boiler size

is this for a combo boiler(heat & domestic hot water) or a stand alone boiler? Cause Burnham Alta comes in a 150k so there isn’t a reason to use the 180.
Unless you’re putting a Combi in and it’s being sized based on your domestic hot water needs

I came here to say that, if you make dhw with the boiler it may need to be bigger to accommodate that, ori ideally you'd size the boiler to the heating load and get the dhw required with a bigger indirect tank. Keeping the indirect cooler also lets you keep the mod con in the higher efficiency range. If you are running fin tube for heating near 180f you likely will get mid to upper 80% efficiency out of the modcon, not the 90%+ you can get with lower temp emitters.

The original munchkin piping diagram does not show one circulator. I have the original installation manual when i installed it and it does not show one circulator. they always wanted to ensure proper flow thru the boiler. i would like to see a piping diagram for a munchkin that shows one circulator.

agree, I’ve got that extent configuration. With the 40 gal indirect tank @135F/mixed to 115F, the boiler can fill a tub and keep up with two showers for 20 minutes before temp stays to roll off. I’d probably gone with a 100k if installing another for bit faster tub fill recovery.

regardless of what the piping diagrams show it is a flow dependent boiler and so must be piped in a way to have the correct flow through the heat exchanger, most replacement jobs it would be pretty hard to guarantee that without P/S piping. If you know the flow requirements for the system you can figure the required pump easy enough to direct pipe most water tube boilers. I've run into issues with direct pipe applications on occasion and the installer or owner will typically show me the diagram showing direct pipe as though that means the boiler will work in any direct pipe application. Not always the case but if it works, it works!

Munchkins serving low mass, high temperature systems tend to age better than high mass, low temperature systems. Makes sense since you will probably get little or no condensing with high temperatures. No acidic condensate to eat away the stainless steel HX.

I wouldn't be surprised if you get 25 years out of that Munchkin.

@JUGHNE Post the pic because i would like to see it. I have a 2005 and its not in my manual. single pump set just a few years ago got introduced by lochinvar on their FTXL boiler. If you have the ftxl you can have an external VFD communicate with the add on device to load match the circulator with the demand and maintain a fix delta T

SENorthEast1895er, thanks for the pic. I have never seen that. The stainless nipple has always protruded below the case. Perhaps, the boiler was installed in an incorrect position. Removing that silicone tube is problematic as the clamp is out of reach. I wouldn't be looking forward to replacing that.

If I had to do it, if I couldn't shift the HX, I would use a multi-purpose tool with the blade at 90 deg and cut the case around the tube, about 3" square to access the clamp.

it is a little bit more efficient than a ci boiler but not a lot at high temps. it can run lower temps on high temp emitters some of the time with odr in most cases so it gets a bit more efficiency that way. the efficiency bump is from condensing the moisture in the products of combustion and recovering the latent heat of vaporization. you can usually get some of that some of the time. the other advantage is that it can run fewer and longer cycles. there are some losses in the heat in the system when it shuts down that doesn't make it to the conditioned spaces and that has to be replaced when it starts up again. those are less in a mod con because of the lower mass. those can be somewhat significant in a ci boiler especially if it doesn't have a vent damper where it will pull much of that heat up the vent.

so it is more efficient but probably not enough so to pay for the increased costs of maintenance and shorter lifespan. of course if you can avoid dealing with a failing chimney by installing a modcon it can be a lot less costly.

In the winter my mod/con produces 5 gal of condensate in 3 days and in the off season it produces 5 gal in one month, hmmm. What's going on? Ya, it's working harder in winter, but… What causes condensation? Is it when the temperature falls below the dew point? Is that what happens in combustion? The temp of combustion drops? Is it because the cold water in the HX cause the combustion temp to drop, hmmm. What else causes the combustion temp to drop? There are so many BTUs in a cubic foot of NG, not a penny more. Hopefully they all go to heat the water in the HX, but, do they? Where else do those sneaky BTUs go? Why do we say that all boilers condense at some point in their heating cycle?

Homer, you're becoming un-glued.