Installing Main air vents

Jerry Withers

Tomorrow I am having my annual steam system maintenance done and in addition to that some main air vents installed.



I spent a lot of time in determining where they should go. I am hoping that the contractor reaches the same conclusion.



I have 13 radiators. Nine of these are one pipe with counter flow and 4 are two pipe without an F&T traps or thermostatic vents on the returns. these 4 are all off of the same main and in essence seem to function as one pipe radiators. I have been getting at times steam into the returns so here is what I am planning to have done.



Install main air vents at the end of the mains for the "normal" one pipe radiators.



The 4 two pipe radiators all have thier returns pipes flow into a single return pipe (they all feed from a single supply and end up in a single return), so I am planning of having a main air vent installed right where the 4 returns reduce to a single return and between that and the boiler a 5 foot water seal as described on page 106 of "We Got Steam Heat". I figure that way all the radiators will have a main vent and the water seal will prevent steam from backing into the returns.



Al it is, with all the supply valves open on these 4 radiators I do not have any problems (no water hammer), but if the supply valve is closed a bit I did notice the radiators will heat from both sides and be cold in the middle. So, obviously I am getting steam through the returns. With the supply valves totally open and the air vents on the radiators (I know... not sure if they should be there with this 2-pipe hybrid ) set to vent very slow I am getting good heat.



What does everyone think?



Thanks!

Jamie Hall

Whether it was meant to operate

this way or not, it sounds as though those four radiators are operating in much the same way as some vapour systems do -- depending on a restriction in the supply to admit just as much steam as the radiator can handle, but not more than that.



There are two factors here: maximum steam pressure (set it as low as you can if you have a pressuretrol; try 8oz. max with a 6 oz. differential if you have a vapourstat) and valve opening.  The latter should be adjusted so that you do NOT get steam into the returns -- close it gradually to experiment.



Having done that, having the main vents on the returns as you suggest should work great.