Replacing dying Munchkin 199 with what?

Marc_19

17-year-old Munchkin 199 is finally biting the bullet. I'm looking for a replacement of similar size. Seems like there are many 199s combis that are nearly identical. We would remove the 30 gallon DHW tank that was installed with the Munchkin. I believe the Munchkin was properly sized for our house, an 1830s 4000 sq ft brick with plenty of old radiators. 3 zones.

Just the two of us--long hot showers after farm work and maybe 1 tub a week. In non-COVID years maybe fill the house for a week with family. Are some of these combination boilers better than others? Rheem, Laars Mascot, UFT, Westinghouse......

Original Munchkin installer would do the work (very good). Thanks for ideas.

HomerJSmith

Why is the Munchkin dying?

BillyO

Laars Mascot if it where my house. Just my opinion

Marc_19

Munchkin--coils are leaking water into chamber.

Zman

It sounds like you have already done the hard part which is finding the right contractor.
What are they recommending?
I would make the decision based on parts availability and local support. A boiler with a high turn down ratio will perform better for you in heating mode. I suspect your boiler is a bit oversized. You will need that extra capacity if you go with a combi, DHW on demand takes lots of energy.

nosirra1Arrison

Marc_19 said:

17-year-old Munchkin 199 is finally biting the bullet.

That is a good lifespan for a Munchkin. Did it receive yearly cleaning/service? Looking to see what you did to get that many years from your boiler.
The majority here recommend a separate indirect and boiler over a combi. I will leave that for others to detail.
Go with the brand your installer recommends. The installer is the important factor here and you have already found one you trust.

mattmia2

I'd go with another indirect for the hot water. Lots of the combis are good. The munchkin has a grandchild if HTP support is good in your situation.

HomerJSmith

What's your water quality? The life of a HX has a lot to do with water quality.

PerryHolzman

If I recall correctly the Munchkins used a HX where the individual tubes were sealed to the header with o-rings. It may be the o-rings that are leaking. I'm not sure if Munchkin ever offered an o-ring rebuild kit or not.

Something to ask about. It may save you from replacing the boiler at this time...

Perry

Marc_19

In answer to other questions about the old Munchkin:
Good soft water supply.
Occasional--not annual--maintenance. Many parts replaced like an aging human with many joints replaced. F9 shutdowns have been an ongoing issue for a long time.
We're ready for a new unit.

hot_rod

4000 sq feet, call it a WAG high btu/ sq ft of 30, so a 120,000 boiler would maybe be adequate.
A load calc would tell you the best boiler size.
A 150 combi could maybe cover the heating load and get you endless DHW at about 4 gallon per minute.

JUGHNE

I was also thinking that is a lot of boiler for a house.
If you have the convenience of time it would be worth doing a load calc.
I have a Munchkin that size heating 4100 sq ft. of a solid masonry power plant, (insulated roof),
steel frame single pane industrial type windows.
It heats the cooling jackets of diesel engines.
It still cycles in Northen Nebraska to maintain 65 degrees.

HomerJSmith

Elevation has to be taken into account, too.

Geosman

Sorry but I could not help answering a few questions on this thread...The original Munchkin is less than 1/2 the capacity of the oil system it replaced. Load calcs on this home with all its hidden features and catacomb crawl spaces is/was a bit like pissing in the wind. The munchkin was sized by looking at previous oil consumption and boiler efficiency vs weather conditions to arrive at an "occupied heat loss" that would also reflect leakage from infiltration air....so I'm more comfortable with something that can provide upwards of 160,000 when needed in severe sub zero weather. When the Munchkin was installed the system was installed with a Y strainer as indicated by HTP. It was flushed, cleaned and filled using softened house water and Zurn Corrosion inhibitor. It ran without difficulty or pressure loss until 2-3 years ago when it had a slow pressure loss over time with makeup from the house supply. Disassembly of the boiler, monitoring the condensate and PRV showed no loss from the boiler. In the meantime with loss of inhibitor due to raw make up water, rust had begun to clog the Y strainer restricting flow in the Munchkin. It began popping like a rock tumbler when making domestic hot water. Cleaning the Y strainer became almost a monthly activity. Disassembly of the boiler proved heat stress damage to the HX with a seepage leak but not so severe that leak stop might help limp it through until better timing for a replacement. Finally last summer a leaking valve was located deep inside a crawlspace catacomb and repaired by a local plumber before Christmas. Here in Early January more Y strainer clogging from debris and loss of circulation re-opened or created a new HX leak which is now misting system fluid into the combustion chamber. Ignition can only be maintained by removal of the ignitor; heating it to dry and quickly inserting it for a start-up. Once running the boiler is OK but if off for an hour or more the process needs to be repeated. A replacement will go in with a sediment trap and magnetic separator along with boiler trim valves to facilitate a repeat system clean-out flush and fill and corrosion inhibitor. No more Y strainer to clog and give problems and no more restrictive passage heat exchangers as with the Munchkin to cause issues. The Mascot and others from the same source appear to now have low water level sensors directly into the upper level of the fire chamber which is better assurance for operation if fluid levels are not sufficient. Navien had issues with their 3 way valves in early combos and have made changes. I suspect this series may have taken notice and may be using a digital motor to change valve positions....More to learn. Any input is appreciated.

mattmia2

Water softened with an ion exchange type water softener is not good for a boiler.

The oxygen in the makeup water will cause ferrous parts of the system to corrode. If you keep adding fresh water because of a leak it will corrode the system

JUGHNE

The power plant had the same challanging construction. The same method was used to determine the boiler size.
That Munchkin is 18 years old. However it uses a tube in shell heat exchanger between boiler and engine cooling/heating jacket water. Engines are cast iron, 60-70 years old with a lot of raw water introduced when generating. So there is limited water in the boiler system.

(I realize now I should have gotten a larger HEX to give more temp drop on the return water temp which would give more condensation/efficiency)

You could consider the HEX for your new system if a lot of iron pipe, I would still use strainers and mag pick ups on both sides of the HEX.

It is a moot point now but my Munchkin has a water pressure safety switch which acted as a Low water cut off. Less than 10 PSI would produce a PrO code.

HomerJSmith

I would never put a strainer on the secondary boiler piping. I don't want anything that may plug up and slow down the flow thru the boiler HX. Overheating the HX is asking for a shorter life span. If one insist on a strainer, one need a pressure gauge on the input and output in order to observe flow thru it.

Boiler water that has minerals in it will conduct electricity thru it. Not a desirable condition. Also, the PH of the water is a most important consideration. The old cast iron boilers were bullet-proof. The new mod-cons are sensitive to the water environment.

Caleffi has some good publications on water quality in their Idronics series that can be found on the Web. Bless, Caleffi, for their continuing education, although I will miss their hard copy of Idronics.

Geosman

All well understood. This is why a firetube boiler with lower pressure drop will better serve this application. The original Munchkin had very tight passages for the HX and mandated a strainer on the supply side which was critical for use with older cast iron systems. As this industry has progressed we now have magnetic sediment traps and better ways to deal with debris in the system. The repipe will have a Caleffi Dirt Mag on the main line ahead of the boiler tees and a boiler isolation and bypass for system flush and fill. I like Fernox or Sentinel for clean up and inhibitor. With this project, finding a valve leak deep under the home back a hand excavated dirt trench used to install the original boiler piping took seemingly forever. Remember this home dates to 1830. The original home used fireplaces and did not have boiler heat until sometime in the 1900s. Just one of the challenges working on older systems.