MM67 LWCO life expectancy

danmayer175

The McDonnell Miller 67 LWC manual says to replace the whole unit after 10 years. Is that for real?

Mad Dog_2

The cast iron body doesn't go bad...it's the float.Mad Dog

ethicalpaul

https://forum.heatinghelp.com/discussion/199876/mm67-lwco-life-expectancy

Yes they really do say that. Whether ignoring that directive is smart, that's between your insurance company, potential liability suits, and luck. But MM will be able to say "we told them to replace it after 10 years so our hands are clean"

Intplm.

Surely many of us here have seen them last for many many years. Even decades.

Jamie Hall

It's their recommendation. There are life limited parts in there, as there are in any mechanical device. They are all replaceable, and the body of the thing is cast an pretty well bulletproof. So… if one does one's maintenance and replaces the switch contacts and gaskets and, very likely, the float and its linkage either on condition or at failure, they'll go for decades. If one doesn't, they won't. There is a cost/benefit equation in here too: is it cheaper to just replace the whole thing after a certain time, or is it cheaper to take it apart and replace worn out stuff from time to time…

EBEBRATT-Ed

If they are maintained, they can last 40 years. Most times they are not maintained.

The CI body will last almost forever. The float and switch and blow off valve not so much.

Clean it, test it and maintain it and keep using it. If you have issues replace the needed parts.

Problem is the parts and a new one will break the bank. (they used to be like ($75) for a new control.

Buying parts will quickly exceed the cost and extra labor of a new control unless you can do it yourself.

I agree the 10 year thing is a CYA

If not maintained its junk in 10 years.

If you need a new one I would consider a Hydro level probe type.

Long Beach Ed

Check out what these cost today.

When you recover from that, check the price of a 47.

danmayer175

Hmm. I'm not sure my MM67 has been cleaned out in the past 10 years. I've had the boiler serviced by the oil company, but I didn't know enough to ask about all the parts and pieces.

Intplm.

Unfortunately this is not something that is done unless asked for or it breaks. (If It aint broke don't fix it) comes to mind.

If it's been a long time it's time to get done. Sort off like having the spark plugs on the car replaced type of service.

109A_5

Hello danmayer175,

My M&M 67 is at least 50 years old as near as I can tell. And it was mostly ignored except when someone remembered to do the blow down. When I first replaced the blow down valve (leaking) it had the round knob and slotted screws. I can't remember if it is now on the the 3rd or 4th OEM type valve now (I realize I can put a 'Dirt Pocket Cap' on and a cheap full port ball valve and/or just add cheap full port ball valve below the OEM valve). I stock replacement OEM valves.

Much more recently have had the float assembly removed twice, both times it would not shut off the burner. The first time the chamber was full of rust and crud (as expected from the age). The second time the float was stuck to the top of the chamber. Probably could have nudged the float loose from the pressuretrol tapping but I wanted to inspect it inside.

For me it is easy to take apart and clean. Be careful of the sylphon cleaning and tightening the screws that secure the switch it is plastic or Bakelite and will break, barley snug is plenty.

Dirty, messy job but so much cheaper that having someone replace it for $$$$ (although I would just do it myself if needed, still expensive). I've even made my own gasket from silicone sheet material for easier service.

Many Youtube videos if you want to watch someone else disassemble it and here is a cutaway video here.

https://youtu.be/HSZLaqm9TXE