McDonnel & Miller #67 Low Water Cut Off. How do I test the valve is working?

CoachBoilermaker

You should put a bucket under it while the boiler is firing, open the valve, be prepared for hot water/steam. Keep it open until the burner shut down……there is a electrical switch inside that will shut the burner off in the event your water level gets too low……very important that this functions correctly.@JUGHNE

I just want to make sure I don't mess anything up.

McDonnel & Miller #67 Low Water Cut Off
How do I test the valve is working?

1. Boiler is firing
2. Open LWCO valve to drain
3. Keep draining until boiler shuts off
4. Refill to middle of sight glass

What if float is broken, BUT the boiler just happened to stop b/c it was ready to stop firing anyways? It usually runs for a short duration when not heating the house. In fact, during normal operation, how does boiler know when to stop firing?

How much water should I expect to drain before LWCO shuts down the boiler? 1 gallon? 10 gallons? How large a bucket do I need?

How often should I perform this test?

https://imgur.com/a/oil-boiler-tankless-coil-steam-heat-general-motors-delco-db-4s-WFbadvv

Jamie Hall

If you open the blowdown valve fully, the water level in the sight glass should drop rather quickly. The float in the float chamber will drop even faster, and you will notice the initial very fast flow from the blowdown should slow (not stop, but slow). At or before the level in the sight glass gets to the bottom the switch should open and the burner should stop. If you then close the blowdown valve, the water level should come back up in the sight glass some — and in many cases the burner should refire. Not always.

Now to make sure you didn't have some weird coincidence going on, to get the thing to fire in the first place, simply turn the thermostat all the way up, or at least to some outrageous value. That will eliminate the possibility of the thermostat turning it off. The other safety which might turn the boiler off is, of course, the pressure control — but if you do this test as soon as the boiler starts to steam, it shouldn't have built enough pressure to trip that.

How much? My Cedric is a pretty good size boiler. I've never had to drain more than a gallon to trip the low water cutoff doing it this way. Nor have I ever had to add more than a half inch to an inch to the sight glass, once everything has settled down.

And ideally you would do this once a week — though not everyone does.

PEvans

Jamie, wouldn't it be better to drain some water from the boiler (that is, not via the LWCO) to make sure the LWCO is responding correctly to a low water condition without messing with the valve itself?

IMO doing this weekly has the benefit of getting human eyes on the sight glass.

ethicalpaul

I want to see what Jamie says, but in my mind, this isn't "messing with the valve", it's "flushing the LWCO" in addition to testing it at the same time. Flushing it is important.

Furthermore, to drain enough water from the actual boiler to trigger the LWCO would be several gallons and you don't want to drain that much because of the unnecessary introduction of fresh water.

EBEBRATT-Ed

You can test it either way but testing with the LWCO valve has the advantahe of flushing sludge and much fro the LWCO and sight glass.

CoachBoilermaker

https://forum.heatinghelp.com/discussion/comment/1811015#Comment_1811015

Yea, but I am draining some water from LWCO weekly anyways.

So, I just want to do a clean test of the shutoff. I'll just wait until winter, set temp to 85, and drain from LWCO until I drain a few gallons or it stops. Water should cut off soon after level drops below sight glass bottom level?

Jamie Hall

Actually, if you are draining from the LWCO's blowdown valve, which is what you should be using, the burner should cut off when the water is still visible at the bottom of the sight glass — and the water level in the sight glass should recover to almost where you started very quickly without your adding any water when you close the valve.

Now there is a gimmick here. Or rather, two. First, I'm assuming that the LWCO is connected to the sight glass, rather than having its own independent tappings into the boiler. Second, I'm assuming that the sight glass tappings into the boiler are free and clear.

With the first gimmick, you may find that the sight glass doesn't respond all that much. It depends on the nature of the sight glass connection to the boiler. On the other hand, you may find that you have to drain a LOT of water to get the sight glass to respond. The LWCO should cut off anyway.

If the sight glass tappings aren't free and clear, the LWCO will cut off or certainly should — but that may have little resemblance to the actual water level in the boiler. That depends on just how clogged they are.

In any event, watch that sight glass — it should be actually pretty lively. A quiet sight glass (look how stable the water is when it's boiling!) always worries me.