I don't think the water level probe would cause the water level in the sight glass to bounce around as described. The other thing I was thinking of was that there could be scale buildup at the inlet pipe to the bottom of the sightglass. I've seen that cause weird visual water level vs boiler water level issues. Something in my brain says I've worked on a machine with a similar problem, but I can't suss out what the issue was.

Barry: I agree that the probe by itself won't cause the sight tube bounce phenomenon, however, if the boiler were not filling properly, you would have mostly pressurized steam in the boiler and a balancing amount in the sight glass. If you were to tap the release on the sight glass as the OP explained, it would cause a bounce in the water level as pressure on the sight glass side was released and water force into it by the back pressure in the boiler. As the release seals back up, you get a bit of bouncing around from isostatic rebound. Still an easy thing to check, imho and well worth while.

A scaled up probe would cause an overfill problem, not an underfill.

On rare occasions, the insulator around the probe can crack, resulting in constant, or intermittent (when damp) failure of the autofill circuit.

The level control box might be going bad.


I'd like to know when the pump is or isn't running in relation to the water level in the sight glass. A properly filled boiler with low water level in the sight glass points to an obstruction in the lower sight glass inlet tube. An underfilled boiler points to probe/control issues. All this can be diagnosed on a cold machine. Pull one of the fittings off the top of the boiler and look in to visually verify water level and compare with the sight glass.

barry wrote:I don't think the water level probe would cause the water level in the sight glass to bounce around as described. The other thing I was thinking of was that there could be scale buildup at the inlet pipe to the bottom of the sightglass. I've seen that cause weird visual water level vs boiler water level issues. Something in my brain says I've worked on a machine with a similar problem, but I can't suss out what the issue was.

I had a machine with the lower sight glass tube completely scaled shut. It never had water in it while it was in that condition so I don't know how the water behaves in that situation but that lower tube can and will completely close off from scale.