Water level drops when burner shuts off, then returns!

jim_40

When burner shuts off the water level momentarily drops out of the sight glass and the main vent squirts water. Doesn't happen everytime, about one out of five times. When this happens the temperature in the Hartford loops climbs rapidly. Once the water level returns, the loop cools. Sometimes the water feeder opens so over a period of days the boiler eventually floods.
System: 2 pipe steam, gravity return. Mains and returns check for slope. OK - some spots just make it - about 1"/20'. F&T traps at the ends of each main - OK. Pipe are insulated. Running system at 1.25 psi, .5 psi differential. Have 42" between water level at boiler and end of lowest main. Geometry of building precludes any changes to pipe slope. We've imroved the situation (and reduced the banging) by dropping the pressure, new traps and rehanging the pipe. What would cause this problem?

Gordo

Water line

Just a few questions: What kind of main vent do you have? Could there be a slight vacuum forming in your system return? Do you have dry and wet return(s)?
It sounds like you also could use a vaporstat to run at a lower pressure. 1.25psi may be too high.

Big Ed

Skim

Did you clean and skim the boiler water ?

DanHolohan

Jim,

what brand of boiler is doing this?

jim_40

Peerless, about 15 years old.
FYI: returns are open, flowing

jim_40

No. We drained the boiler, water is clean. Little sediment.
Returns are open, flowing.

jim_40

There is a single vent as the main dry return drops
down into the Hartford loop. The vent is located 15"
back from the drop elbow, about 16" up from the main.
I suspect it could be a vacuum but I don't understand
how/where it is forming.
There are two smaller (3/4, 1") return which drop down
and turn into wet return about 15' away from the
boiler.No apparant reason why they were piped this
way.
FYI: we checked and flushed all returns.
Thanks for your interest and input!

powerhead

Ooops,
stepped on someone elses post.

Alan(CaliforniaRadiant)Forbes

Was anything

in the boiler room changed; i.e. near-boiler piping? How about the dry returns or radiators?

If this house is in Piedmont, I think I know where you are.

Alan

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jim_40

Nope. No changes recently.
It's actually an apartment building with 25 units, 3 stories.

Big Ed

Skim

Shut the lower valve on the gauge glass assembly and open the pet cock on the bottom. Run the boiler up to steam . If large droplets of water form on glass and or water spits out pet cock , then skim the surface of the water. Draining the water from boiler drain will not remove oil. Any time you replace iron pipe or fitting you can introduce oil and grease. Some pipe dopes are also a oil base....

Unknown

Do you know what size

the equalizer pipe is ? If that pipe is undersized you could be pushing the water out of the boiler into the dry return when some pressure builds up . I see this situation happen every so often on the Peerless ECT line , and we pipe the equalizer to spec . I should start doing them all in 2 inch minimum .

Oops forgot to ask - does the dry return drop to the ground , the come back up into the Loop tee ? And what height is the Hartford Loop tee , in relation to the normal water level ?

jim_40

Equalizer is 2". Boiler is 600K in.
I'll check photos of the dry return tommorrow and reply to you.
The tee (connection of the return) is about 4" below the water line. There's a 4' nipple between the tee and the elbow.

jim_40

OK - will try. Thanks!

Unknown

There is a

4 foot horizontal run between the Hartford Loop tee and the elbow going to the dry return ?

jim_40

Sorry - 4". Yes 4' would be a problem.