Venting Question

JohnW2

I feel like I'm starting to understand steam systems a little better, but some things still confuse me. An adage that I see on here a lot is vent your mains fast and your radiators slow. Can someone explain to me the theory behind venting the radiators slowly? Wouldn't venting them faster allow the steam to fill them faster, thus allowing the system to work less hard and shut down sooner? Or does venting them fast make things too hot too soon? Thanks.

nicholas bonham-carter

Vent your mains quickly, and rads slowly

This fills all the mains with steam first(because of lower resistance to the escaping air).

After the mains have all filled, the steam can then overcome the slightly higher resistance of the rad vents to flow into the rads--filling all simultaneously.

If you had no main vents, and large radiator vents, then the radiator closest to the boiler, in pipe feet would get steam first, and the radiators would fill sequentially in order of their distance. Before the last one had filled, the thermostat may have been satisfied, and the last rooms would still be cold.

This really a matter of balancing the resistance of the vents to the escaping air. A 0-3 psi gauge should show a back pressure of 2 ounces if you have enough main venting. When the main vents close, the pressure will rise, as the rad vents are more restricted. Air will leave each radiator at the same rate, ensuring simultaneous steam arrival.--NBC