Radiator vent loudly sucking air INWARD after boiler turns off

CoachBoilermaker

Boiler starts. All rads hot. Room is warm. Boiler stops. All is well.

A few minutes later, the living room Radiator vent loudly sucking air INWARD after boiler turns off. Lasts about 2-3 minutes, then stops. The only noise is inward sucking after the boiler shuts off, no hissing, no spitting, no weird behavior during the heating part.

I do not recall this happening last winter. And I don't think it does it every heating cycle this year.

EBEBRATT-Ed

The volume change between steam and water is tremendous.

If you took a box that measures 28" x 28" x28" and filled it with water, when that water turns to steam the size of the box to hold that steam would be 28'x 28' x28'.

The same volume change holds when the steam changes back into water.

So in your house when the boiler is steaming all the pipe and radiation is full of steam. When the steam is generated it pushes air out of the vents. When the boiler shuts down the steam collapses and turns to condensate. The air expelled previously has to get back in the system and the system goes into a vacuum until the air come back in.

So sucking air in through the vents is normal when the boiler shuts down

Jamie Hall

All true. However if it is doing it this year but not last, or only does it now and then — some other vent somewhere (likely the main vent) is sticking closed when it shouldn't or didn't.

ARobertson13

Sucking in air loudly for 2 or 3 minutes is not normal operation for a properly functioning steam system. This really should not be happening if your pressure is below 2 psi and or the vents are functioning properly.

gfrbrookline

Try smacking the vent if it isn't closing. Sometimes they get out of alignment. It has worked for me for years, especially with Gordons.