Vermont Plumber

Snapper

I have a steam boiler with gravity return.  On 2 of the supply mains are 2 modine fan unit and return with main vents with no traps.  On the last building supply is a control valve with main vent and no trap.    When the control shuts down the boiler water backs up that return line and shuts down the boiler on low water.Now what do I need?Boiler runs at 1/2 to 2 pounds

nicholas bonham-carter

returns sucking up water

are you sure that this happens at shut down? maybe it is the pressure while steaming, which is pushing the water too high in the returns. when you measure the pressure, don't rely on the useless, but unfortunately required 0-30 psi gauge. instead get a good low-pressure gauge [gaugestore.com 0-3 psi].

if this is a problem with unwanted vacuum, then are the main vents working in both directions-air out, and air in? a hoffman vacuum vent looks just like a straight one.

look for any horizontal part of the returns which are at the same height as the boiler waterline, no matter how far upstream. we had such a problem, and it drove us crazy, but when the returns were repiped straight down to the floor, our 55 rad, gravity system now has a constant waterline.--nbc

Snapper

snapper

This small boiler has 2 zones one on a house valve and the other 2 are gravity return and the boiler run on pressure setting all the time and when the house valve closes the water up it's return line.

Charlie from wmass

Zone valves need drips

you need to  allow for the  condensate to return to the boiler. and for the condensate from the radiators of each zone to return to the boiler. Traps are not needed if you can tie in below the water level of the boiler and maintain the enough height  so you do not have the steam short circuiting back through any of the returns.  

Dave in QCA

Zone valves create a "B" dimension

When the zone valve shuts, you are going to have some vacuum, but if your main vent, and the radiator vents are working correctly, (I am assuming that this is a 1-pipe system) they should relieve the vacuum quickly.  However, when zone valves are used on a steam system, the end of the main, ie, the lowest part must have an adequate A dimension (28" above the water line) when the valve is open.  When the valve is closed, it becomes a "B" Dimension, and therefore must be 30" for every pound of pressure.  You indicate that you are running at 2 PSI, so that is enough to push the boiler water backwards up into a vented main which has 0 PSI pressure to a level that is 60" above the water line.   If your main is less than 60" above the water line, it will fill with boiler water.  Can you reduce the pressure?  Try getting it down in the range of 8 oz - 1 psi max.  This should fix the problem as long as your vents are relieving and vacuum.  Swing checks in each return might be helpful as well.

Snapper

Vermont Plumber

Yes you were right about jthe running pressure of the boiler and I have now set the working pressure to 4oz on and off at 18oz working great now.      Thanks