Radiator with no air vent?

mbowes80 · March 16

We have a radiator that gets somewhat warm, but not as hot as others, doing a somewhat subpar job of heating up the room. One of the steps I was going to take to try to remedy this was to replace the air vent … upon inspection, from what I can tell, both locations where an air vent might be have plugs in them.

I'm wondering if I'm missing something, or if there's any use case for a radiator to be completely without an air vent? Or is this just an obvious mistake?

I appreciate any input!

Jamie Hall · March 16

You are missing something: the trap on the outlet from the radiator just to the right of your feet in the pictures.

That trap is the way air — and condensate — get out of the radiator so steam can get in. The pipe going down through the floor should connect to a dry return near the overhead in the basement, and that, in turn, should go to a vent somewhere near the boiler.

If that trap is failed closed — which happens — the radiator won't heat. It also won't heat if the pipe in the basement isn't connected to a dry return.

So that's what you want to go after. Check first the piping in the basement, and if that looks at least plausible the odd are on the trap. Replacement innards are readily available — if you tell us what markings there are on the cover we can help find them.

You neither need nor want a vent!

Pumpguy · March 16

The trap on the radiator outlet tells me you have a two pipe system. Typically, the radiator vents you describe are found on one pipe systems.

As @Jamie Hall advises, problem is most likely the trap on this radiator has failed closed.

If you have similar traps on the outlet of your other radiators, those too should not have radiator vents.

mbowes80 · March 17

Thank you, @Pumpguy and @Jamie Hall - I'm aware of the traps, as I've replaced cage units in them before, but I didn't realize they de-necessitate the air vents. I do see that in the basement, on the returns, there are Maid-O-Mist air valves which perhaps do this air venting.

There are a few radiators in the system that have air vents … what are the consequences of having them there when they are not needed?

Appreciate your insight!

Jamie Hall · March 17

Depends on the system. On most two pipe systems odd air vents in odd places do no harm — though they often reflect bad traps somewhere in the past. On some vapour systems, however, they can defeat some of the ways in which the system is designed to maintain very low pressure differentials which they depend on for proper opeation.

mattmia2 · March 17

If there are vents on some of the radiator there are probably other issues with the system, probably failed open steam traps letting steam in to the returns keeping the radiators from venting through the returns or possibly failed closed vents on the returns themselves (the returns should never have steam in them so the vents on the returns should never close.) The traps that are failed could be the crossover traps between the mains and the returns or some some combination of both. Less likely but possible is a pocket of water trapped in the dry returns. Somewhat more likely is a connection between mains or returns that is supposed to be in a wet return below the water line of the boiler that has become above the water line when the boiler was replaced with a new boiler with a lower water line.

I almost did a clammy there…

Radiator with no air vent?

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