Tried everything I can think of (which isn't much...)

subaru400

Came home yesterday to a cold house and quiet steam boiler. Thermostat set to 20C - room temp 14. Went downstairs and found a little bit of water on floor and under the boiler. Looked like high pressure relief valve may have opened, but not 100% sure (going by vague splatter pattern almost - but not quite - directly underneath it). Less than ~100ml on floor. Brown water in sightglass - drained down a few liters until clear. Water auto-filled to line and then the burners kicked in, but house was cold. Went upstairs to see how it heated. It got up to 16 and shut off. West half radiators hot - East half cold (they split off at boiler). Thermostat still calling for heat. Went back and checked pressuretrol - seemed OK @ .5. Boiler cycled again, damper opened, but gauge on pressuretrol never moved. Shut her down and removed pigtail. Clean as a whistle. Took pressuretrol apart and it looks new. Reset cut in to just under 1. Checked chimney - clear. For whatever reason, bumped thermostat to 20 and went to bed. Same thing this morning, but thankfully, water had all dried up. Pressure gauge still pinned to the peg @0. Smart thermometer showed overnight low of 14, high of 16. Thus far, boiler calling for heat continuously, but damper closes and burners shut off before set temp achieved. Very cold week here, but interestingly, I checked my gas usage online over the last 3 weeks and it has been between 1.2 and, 1.5mfc every single day regardless of outside temp. So this may have been going on and I just noticed it due to the cold snap. Thinking it's a pressuretrol issue? Does failed gauge matter to system operation? Any other ideas on what I can check out?

Jamie Hall

First off, it's unlikely the gauge will show anything, even if it's working properly, assuming it's the normal 0 to 30 psi (200 kilopascals). If you really want to know what pressure the boiler is running at, you need a 0 to 3 or 0 50 5 psi gauge in addition to that 0 to 30 gauge.

OK. That's out of the way. Something, however, is shutting off the boiler early. First thing I'd check at this point is the thermostat. They have been known to fail. Does it have it's batteries? Are they fresh (assuming it's battery powered)? Have you tried turning it up when the boiler quits to see if it is lying to you (that it holds the structure at a more or less constant -- but wrong -- temperature makes me mighty suspicious).

Second, if it turns out not to be the thermostat, it is possible that the pressuretrol is not functioning properly (though unlikely -- but it happens). To check that, next time you are playing with it, try cranking the cutin pressure (if ti's the blue grey box type with one small scale on the outside) or cutout pressure (if it's the clear face type with two scales) up to... say 5 psi (you're not going to leave it there -- this is a test, only a test!) and see if the burner kicks on. If it does, there's something amiss with the pressuretrol

If neither of those moves help, you'll have to start trying to figure out what else is shutting the burner off. Which, I realise, is not a particularly helpful comment...

JUGHNE

If adjusted too low some pressure control linkage parts come apart and will not activate the boiler.

Also do you have a Cycleguard low water cut off? It will shut the boiler down to check the true water level and then restart. Many find it annoying.

In addition to the thermostat possibly shutting off prematurely.

mattmia2

It sounds a lot more like something in the burner safety chain to me. Is it gas or oil? I'm thinking gas from the vent damper. Pics of the burner and valve and controls will tell us what you have and where it can fail.

HVACNUT

Post a pic of the wiring diagram. It seems to be intermittent if the damper does open and the boiler fires occasionally. 

EBEBRATT-Ed

If your loosing water and steam out of the relief valve the ONLY thing it can be is the pressure control, or plugged pigtail

You may have other issues but that is the first one. Check the linkage as @JUGHNE mentioned.