Low water cutoff conundrum

yz613

Good morning,
I'm hoping some of the experts here can help me with my problem.
For the last few weeks, my steam boiler has been intermittently ignoring calls for heat. What I discovered is that as soon as I blow down the low water cutoff for even 2 seconds, the boiler turns on. The low water is a McDonnell & Miller 67.
Being as the pressuretrol is mounted on top of the cutoff, I thought maybe the it was the pressuretrol blocking the heat, and the blowdown was somehow clearing that, but I checked the pigtail, and it looked clear.
This led me to assume that the cutoff float was getting stuck in the off position, and that I need a new cutoff.
What's confusing me though, is that it hasn't turned on my automatic water feeder even when the system is not turning on. I know the feeder works, because it does turn on when I drain the boiler.
Is it possible for the LWCO to block a steam call without turning on the feeder?
Is there a 3rd option I could be missing here?
Thanks for all your help!

Fred

You are assuming it is the LWCO but it could be the Pressuretrol. The pigtail is clear. Did you check the tapping in the top of the MM67 to make sure it is not plugged? Also, the MM67 float may be dragging on the side walls inside the LWCO. They need to be scraped out every couple years. The MM67 is designed to add water when the level falls below the very top line embossed on the cast iron sides of the unit.
Also, Next time the boiler won't fire, lightly tap the side of the Pressuretrol. The microswitch may be sticking.

JUGHNE

Or the pressuretrol may be set too low and not clicking until nudged by pressure of the LWCO blow down.

yz613

Thanks for the replies, Fred and Jughne.
Fred, to scrape out the sides of the LWCO - I assume it would need to be disassembled?
Also, it is possible at all for the MM67 to shut off because it thinks there's low water, but not trigger the feeder? Are there 2 separate triggers inside of it?
Thanks again

clammy

That 67 lwco is mounted off of the sight glass assembly . Aside from dis assembly of the low water cut off and cleaning out the inside and a good flush I would suggest the sight glass assembly and pressuretrol pig tail be cleaned also . For cleaning the lwco it would be good advice to have a new float and switch assembly and a spare gasket kit on hand . When was the last time it was das assembled and thoroughly cleaned and inspected for proper function and do you regular blow it down while the boiler is operating to ensure of it functionality . Peace and good luck clammy

Jamie Hall

Yes, the LWCO has separate switches for the cutoff and the feeder -- and it's quite possible one or the other is not working. If it hasn't been disassembled and cleaned in a few years, it should be, as @clammy said. But you should also make sure the pressuretrol isn't the culprit.

JUGHNE

On the 67, which switch is activated first......water fill or burner off?

EdTheHeaterMan

water feed is first NO contacts then Low Water second NC contacts. unless the actuator has been damaged or the switch is somehow ******- up

EdTheHeaterMan

Fouled -up. What were you all thinking?

So to be clear, on a working 67 there is a point where the NC and the NO contacts are both closed at the same level.

Fred

@yz613 , yes, the MM#67 will need to be taken apart to clean/scrap it out. You need to order a set of gaskets, available from several sources, including, I think Supplyhouse.com. The boiler will also have to be drained to do the cleaning. It is a fairly simple job. I would not order a new float. They rarely are bad and since yours does operate albeit intermittently , it is highly unlikely that the float is waterlogged. It is of a copper or brass construction. If, by chance you find you need a float, you can probably find a complete new MM#67 for just a few dollars more than the float and gaskets.

Do make sure the problem isn't the Pressuretrol first though.

EdTheHeaterMan

If you have an electric meter, you can find the source of the problem the next time you experience the problem. You first place one lead on common (white wire for line voltage or C terminal on the transformer for low voltage) Then start at the source of power, then follow the wire to the first stop, (maybe a switch or the wire to the thermostat) then to the next stop (wire returning from the thermostat or switch) then to the next stop, (maybe the pressure switch) then the next stop, (NC contacts on the low water cut off switch) until you get to the gas valve or burner primary

Koan

the float on my MM 67 failed by leaking. That failure mode was to stop the call for heat due to the sinking float. It was also hanging on the sides of the a very scaled housing causing intermittent issues. Inspect the float carefully for leaks. You should probably remove the float when cleaning the housing so you don't damage the float.

yz613

Thanks everyone for your help!
I ended up taking apart the LWCO. It was entirely rusted and crud filled inside. I tried to clean it up a bit, with no success. I ran to my local plumbing supply, bought a new one, and stuck it on.
I also took off the pressuretrol and cleaned out the orifice on the bottom. Put everything back together, and it's working for now.
Will post again if the problem recurs.
This site - with all it's amazing people - is really the best!

Fred

Great @yz613 , I'm sure that will fix your problem. The float was probably dragging on the sidewalls of unit. McDonnell Miller recommends those units be replaced every ten years but if you clean them every couple years, I had one last almost 30 years before I had to replace it. I still cleaned out the old one and keep it as a backup.

yz613

Well, I'm back.
After I changed the LWCO, it worked well for two weeks, then went back to the same tricks. It randomly does not turn on, even when the thermostat is calling for heat. Usually, after a couple of hours of being off, it will kick in and run a full cycle. It does not shut off before reaching the set temperature.
I assumed that my only other option was the pressuretrol, so I went out and bought one. But this morning, the boiler wasn't working, so I figured I'll run some tests.
I jumped the pressuretrol wires - no fire.
I jumped the LWCO wires - no fire.
Then I jumped the gas valve wires. The boiler turned on immediately, and STAYED ON even when I removed the jumper wire. I tried it twice - same result. Once it's on, it stays on, even with the jumper removed, as long as the t-stat call for heat.
I spoke to 2 plumbers, (both of whom use this site). One suggested a faulty gas valve, the other suggested a loose wire, or a bad thermocouple (it's a millivolt system). Neither could explain how blowing down the LWCO might trigger the issue to temporarily resolve, unless somehow resetting the circuit does something.
If anyone has some help here, it would be tremendously appreciated!

Hap_Hazzard

If you take the switch off of the LWCO, you can trigger the contacts manually without affecting the pressure in the boiler. You can also blow down the LWCO without triggering the contacts. This will allow you to determine if it's the contacts opening and closing or a change in the pressure that's making the burners come on.

Fred

Did you check to see if there was voltage out of the transformer and what that voltage was when it would not fire?

yz613

Hap_Hazzard, I manually closed the circuit at the LWCO using a jumper wore. Why would it be different if I removed it?
Fred, I don't believe I have a transformer - this is a millivolt system, powered by the pilot light.
Thanks to both of you for responding!

Hap_Hazzard

If you trigger the switch manually you're toggling two sets of contacts, just like the LWCO. I just think it helps to isolate what's happening mechanically from what's happening electrically. Right now all you know is that the LWCO is doing something. You don't know what.

yz613

@Hap_Hazzard , I get what you're saying. But if jumping the wires on the LWCO did absolutely nothing, don't we know that it's not the reason the boiler is not firing? I think it's safe to assume that the LWCO is working properly, since it's brand new, and it toggles both sets of contacts when I blow it down. It does shut the boiler, and trigger the feeder, if I blow it down when the boiler is running.

yz613

The only other possibility I was considering is the thermostat. It's an Ecobee3 lite that I installed in the summer. However, everything was working perfectly for a few weeks, and it still works well most of the time, so I don't see how it could be connected. Has anyone else had a bad experience with an ecobee on a millivolt system?

Jamie Hall

I'm going to go with your plumbers on this one. I'm not going to say which one... but since it is cheaper and simpler, I'd start by checking all your wiring and connections related to the connections between the gas valve and anything else. Millivolt systems, particularly, are very sensitive to poor connections.

Chris_L

Jamie Hall said:

I'm going to go with your plumbers on this one. I'm not going to say which one... but since it is cheaper and simpler, I'd start by checking all your wiring and connections related to the connections between the gas valve and anything else. Millivolt systems, particularly, are very sensitive to poor connections.

Having a couple millivolt boilers, I fully agree with this. I'd also suggest measuring the voltage of your powerpile with the boiler both on and off. They put out less voltage as they age, and yours might be borderline. (Mine are about 500 mV when just the pilot is on, dropping to about 250 when the boiler is running.)

Also, some thermostats have solid state relays that don't play nice with millivolt systems. I don't know about the Ecobee. You need check to see if it is rated use with millivolt systems.

Finally, rather that jumping the different switches in the system, measure the voltage at each. If there is too much voltage drop across any of them, or in total, you won't have enough to keep the gas valve open.

yz613

Thank you all for your help!
I've been doing some research on the possibility that the thermostat is the issue. I just used a transformer to supply 24v to the ecobee, and hooked up the boiler wires as normal. Now I'm seeing that most tutorials recommend a separate relay in line before the boiler. Would that be to separate the 24v from reaching my gas valve?