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Formula for pre-calculating radiator vent size?
Is there a formula for pre-calculating the ideal size for a radiator vent, or is trial and error the only way? Specifically, I am curious if a formula might exist to indicate the maximum vent size usable on a radiator based on its volume to prevent the dreaded rush of steam across the top or bottom and the closing of the vent before all the air has been pushed out.
I am doing some re-balancing to warm up a first-floor bedroom addition from the 1940s that is over a crawlspace (rest of house is over basement). This room, despite the radiators heating, has always been five or more degrees colder than all the other rooms, since the crawlspace is colder than the basement and this bedroom addition is just one story (rest of house is two story plus attic) and there is more heat loss through the roof than the other first-floor rooms experience. I put Vent-Rite #1 vents on three radiators closest to the thermostat in the main part of the house, and I have them set between 0 and 1 to slow them WAAAAYYYY down. I did that to increase the time required to satisfy the thermostat so there is more time for the colder bedroom radiators to continue condensing steam to warm that room to a temperature equal to the other rooms in the house. I use Maid-O-Mist throughout the rest of the house, including that colder first floor bedroom. On all the MOM vents, I am using the smallest orifice, except for in the colder bedroom addition, where I am using the third-from-smallest orifice. This has helped quite a bit so far in warming the cold room to within a few degrees of the other rooms.
The Vent-rite vents at their minimum setting vent a good bit slower than the smaller orifice.
In the cold room, I can easily increase the Maid-o-mist orifice size up another size or two, but I wonder if that will cause steam to race to the vent and close it before all the air has been pushed out. I would be interested to know if a formula exists which would show a maximum venting rate for a radiator of a given volume at a given PSI of steam above which the steam would race too fast and close the vent prematurely.
I should mention that my mains are very well vented with Big Mouths, all are insulated, and I have my vaporstat set to cut out at 9 ounces. My boiler is 50% oversized, so I confess before you, my brothers and sisters, that I do cycle on pressure. I'm one-pipe steam. 10 year old Peerless. Despite the cycling, my gas bills look great compared to neighbors' bills. Water is clean. No surging. Needs makeup water pretty much only when I do a blowdown.