LWCO won't trigger auto-filler - bad switch or bad float?

iamn8

Hello to all. My first post here. I found this forum when I first began looking into my problem over the summer.

My boiler has the common McDonnel Miller LWCO.

Starting last March, as the water level was boiled off, the LWCO would no longer trigger the auto-feeder to add water. I've been manually adding water since that time. EXCEPTION - if I open the yellow blow down handle, the auto-feeder IMMEDIATELY comes on and refills to it's usual level, but not enough to reopen the burner circuit.

I did a thorough cleaning over the summer with the help of this great thread: https://forum.heatinghelp.com/discussion/153834/low-water-cut-off-replace-or-clean When we started using heat again ~6 weeks ago, I found the same issue still existed.

Surely, the problem has to be either with the float OR the switch. Does this behavior indicate one vs. the other to anyone here? I'm leaning toward switch.

I also shot this 2 minute video:
https://youtu.be/AdRbOk_8QiU

It shows the morning water level low enough to shut off the burner, but NOT allow the feeder to come on, followed by it being triggered by a brief blow down, followed by the boiler coming on as I manually top it off.

Input greatly appreciated.

mikeg2015

I think those switches tend to be a little touchy. Mine doesn’t work reliably and I just fill manually at this point and have a second electronic LWCO connected to the water feeder.

When if the last time you removed it, took it apart and cleaned it out really well? The MFG recommends replacement something like every 7 years. Probably excessive, but they probably need a complete rebuild every 15-20. Depends if its been blown down regularly and if the boiler is kept full in the off season. Keeping is submerged probably helps a lot on longevity. By flooding the boiler all summer, the only exposure ot air is the outlet and the wet return. Without oxygen, iron won’t corrode.

EBEBRATT-Ed

MM does recommend replacing that control I think it's every 10 years (not 100% sure)

That being said you really shouldn't be running this boiler if you KNOW the low water cutoff doesn't work. You need to fix this or call someone that can. It's a safety issue that could lead to a boiler failure .......or worse.

If the feeder turns on when you blow it down and the low water cutoff doesn't shut the burner off I suspect a wiring problem or a bad or mis wired switch.

terminals 1 & 2 on the lwco are the burner shut down switch. 3 & 4 are the water feeder.

Sometimes there is a jumper between terminals #2 &#3 if the feeder and the boiler are the same voltage. These need to be wired properly and power needs to be connected so the burner will shut down and the feeder will work properly

Fred

The problem could be the switch, the float or the inside of the LWCO may have crud built up on the side walls of floor of that unit, preventing the float from fully engaging the swing arm, on the end of that float. As @EBEBRATT-Ed said, McDonnell Miller recommends the LWCO (MM#67) be replaced every 10 years and to be honest, the cost of a new float, switch and the gasket set are almost as much as a complete new unit. You can take it apart and scrap it out but you need to order the gasket set, if you go that route.

Fred

BTW, The LWCO should shut the boiler down when the water gets to that top cast line on the casing. Where do you fill the boiler up to? Should fill it up about half way up the sight glass. The real question is why are you loosing that much water every day??? There has to be a leak somewhere in the system. Also, that MM 101 water feeder has a screen in it that gets clogged up and it may not be letting the water flow through it like it should. The filter is located in the bottom of the water feeder, four screws and a gasket but if you haven't replaced that scree in a long time, that feeder may be toast. They really get gunked up, almost beyond cleaning. Find out why you are losing the water to begin with. Fill the boiler to half way up the sight glass and then watch it. The way you are filling the boiler (if just up the that black rubber, you are just filling to the minimum water level and anything below that will shut the boiler down.

iamn8

Thanks to all for your thoughts.

mikeg2015 said:

When if the last time you removed it, took it apart and cleaned it out really well?

Back in September. It wasn't terrible, but due for a clean out.

EBEBRATT-Ed said:

MM does recommend replacing that control I think it's every 10 years (not 100% sure)

That being said you really shouldn't be running this boiler if you KNOW the low water cutoff doesn't work.

I downloaded install and troubleshooting instructions and it does say replace every 10 years. I bought this home 8 years ago and the LWCO looked pretty new.

The cutoff does work. It's just not telling the feeder to add water when it's low. Instead, the boiler just remains off until I manually fill. If I open the blow down valve while the boiler is running, it DOES shut it down.

Fred said:

the float or the inside of the LWCO may have crud built up on the side walls of floor of that unit, preventing the float from fully engaging the swing arm, on the end of that float. As @EBEBRATT-Ed said, McDonnell Miller recommends the LWCO (MM#67) be replaced every 10 years and to be honest, the cost of a new float, switch and the gasket set are almost as much as a complete new unit. You can take it apart and scrap it out but you need to order the gasket set, if you go that route.

I actually soaked the float in CLR when I cleaned the unit. There was a bunch of crap in the bellows the I carefully cleaned away. I was sure that was going to do it, but unfortunately no.

Cost of float vs. switch is almost the same. I'm hoping to isolate the culprit and only purchase one of them if I can.

Fred said:

BTW, The LWCO should shut the boiler down when the water gets to that top cast line on the casing. Where do you fill the boiler up to? Should fill it up about half way up the sight glass. The real question is why are you loosing that much water every day??? There has to be a leak somewhere in the system.

It's shutting it down @ the cast line. I'm filling to the blue tape mark before I got to bed and it's back to the cast line in the morning. I don't feel like it's a lot of water for an 8 hour run with outside temps in the low 20s. When the feeder was working properly, the water was never above the black donut.

It's also hard for me to say where the water line SHOULD be. This isn't exactly a modern burner. It's a 1932 coal furnace converted to oil in...god knows when. The top of the site glass is effectively at the top of the unit itself. Someone determined this was the correct level for the cut off several decades ago.



Concensus seems to be replace switch, yeah?

Fred

That is a lot of water to lose in an 8 hour window. Are there return lines buried under the floor? If you don't see any water at or above the floor, steam is either going up the chimney or water is running out under the floor. Those old boilers have very large capacities so the water loss is substantial.

Also, per my earlier post, check the screen in the water feeder. If it hasn't been serviced in a number of years, the water flow is likely very restricted.

iamn8

Fred said:

That is a lot of water to lose in an 8 hour window. Are there return lines buried under the floor? If you don't see any water at or above the floor, steam is either going up the chimney or water is running out under the floor. Those old boilers have very large capacities so the water loss is substantial.

Also, per my earlier post, check the screen in the water feeder. If it hasn't been serviced in a number of years, the water flow is likely very restricted.

No, everything above grade. With only 5 radiators in the system, I can verify none of them are leaking.

How could I verify if there's steam going up the chimney?

I'll look into the feeder screen tonight, thanks!

ethicalpaul

> Concensus seems to be replace switch, yeah?

I'd say replace the boiler! Wow

iamn8

ethicalpaul said:

>

I'd say replace the boiler! Wow

The service techs are always surprised by it's efficiency, but yes, its day is coming.....along with a crippling abatement bill when that day comes.

ethicalpaul

I saw that :sadface:

I wish you luck with that

Fred

@iamn8 said: How could I verify if there's steam going up the chimney?
When the boiler is running , go outside and look at the chimney and see if there is a cloud of steam rising up into the air.

EBEBRATT-Ed

Your switch could be wired wrong as well. It needs to cut the burner off as it is doing but when the burner shuts off it could be killing the power to the feeder if not hooked up right

iamn8

Fred said:

@iamn8 said: How could I verify if there's steam going up the chimney?
When the boiler is running , go outside and look at the chimney and see if there is a cloud of steam rising up into the air.

Thanks Fred. I've monitored it closely over the past few days. No steam from chimney, even on a morning in the low 20's. No condensation in the flu pipe or excessive wetness in the cleanout. I can't find any evidence to say steam is going anywhere but out the rad vents.

EBEBRATT-Ed said:

Your switch could be wired wrong as well. It needs to cut the burner off as it is doing but when the burner shuts off it could be killing the power to the feeder if not hooked up right

I went back and checked it today Ed. Wiring is correct. Switches confirmed to open and close correctly with an ohmmeter.

However, my testing reveals a potential problem. In the schematic below, I'm getting "normal water level" and "LWCO operating level", but when the water level is in between, both switches are OPEN, not closed. Both burner and feeder are off. I have to add 10 seconds of water through the feeder manually to close the circuit between "1" and "2".

According to the schematic, there should be a period when both switches are closed and power flows both to the feeder AND the burner. I've got the exact opposite condition.



Even with that in mind, I could make a case for either float or switch being the issue. There is a rocking mechanical connection between the two pieces in which clearances may be outside of spec. Safe money replaces them both as paid I guess.

EBEBRATT-Ed

Yes, the feeder should start feeding with the burner still operating. Check to see how steady your water level is. A bouncing water level can drive the control nuts. If the water level is steady it could be the float or the switch

I am guessing switch

Parts for those ain't cheap

wcs5050

Float types MM’s aren’t so bad to replace. Can you get an electronic LWCO retrofitted in.

iamn8

UPDATE - I changed the switch and float as a unit about 2 weeks ago and have been monitoring them since. Everything is now working properly even through a cold spell with nights @ zero.

Price for float+switch was less than buying the 2 pieces separately. Since I wasn't positive it was the switch, I thought I'd hedge my bet. Cost wasn't excessive and, if you've cleaned the unit on your own, you already know the simple replacement procedure. Given that there are no other mechanical parts on these LWCO's, I've essentially got a brand new one with no pipe fitting required.

Thanks to everyone for their input and assistance.