My place has a steam heater. On its right side, there is something similar to radiator vent

It has a pin movable anywhere in the middle slot which is about a half circle. I was wondering what it is used for, and how it is used? When the pin is on either of the two ends, what does it do?


2 Answers 2


According to the manufacturer, Heat-Timer Corp. The item pictured is a "Varivalve" air vent.

"The function of the radiator air vent is to provide a means for air to be driven out of the radiator as steam enters. At a given pressure the fill time of a radiator varies proportional to its heat capacity and inversely proportional to the venting capacity of the radiator air vent. It is possible to control the speed at which a radiator will fill with steam by installing a radiator air vent with a certain venting capacity. Used in this manner, radiator air vents are helpful in evening out the heat distribution in a building."

The pin is an adjusting lever that controls how fast the air is released from the radiator which, in turn, affects how fast the radiator fills with steam. By setting the pins correctly you can prevent all the steam (heat) from running to one end of a room or building.

I don't have any first-hand experience with these specific devices, but here's a link to the manufacturer's instructions for the product.

  • When I set the pin to either of the two ends, what consequence should I expect? Is it dangerous?
    – Tim
    Dec 21, 2019 at 5:26
  • That's your adjustment for, essentially, how fast the radiator fills with steam. I don't suppose it would be dangerous, but I can't say how to adjust it without knowing the particulars of your building and so on. Maybe set them all at about "half" and adjust accordingly. Consult the website and/or the manufacturer for specifics.
    – gnicko
    Dec 21, 2019 at 16:49

That's a thermostatic valve, a mechanical device which closes when exposed to heat and opens when cool. As the hot steam enters the radiator the air inside escapes through the valve. Once the hot steam reaches the valve it shuts, trapping the steam inside. As the steam cools it condenses and the resulting water returns through the pipe, while the now open (because it's cool again) valve allows air back into the radiator.

The trick to that particular valve is that you can adjust how large the vent opening is. Typically you have multiple radiators in sequence, which means the one closest to the boiler fills and shuts off first. Increasing venting on the end of the line (and/or restricting it at the start) allows the radiators to fill at the same rate, hopefully balancing out the heat. (Or unbalancing it, if you want a particular room cooler.)

It's not dangerous because steam radiators use only thermal rise and gravity to move the steam around. Somewhere in the instructions it will state that it's for use only in low pressure applications.

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