tempdoc

tempdoc

one pipe steam system , commercial & residential building / has 2 zones ( 3" main w 2 motorized steam valves , thermostatically operated )
Problem : boiler starts , steam is generated , 3 lbs off ..... 1/2 lb on , zone valves close , burners are off .......... steam pressure continues rising till the water loses its energy, steam gauge registers 10 psi ..... this is the only zoned one pipe system I've seen ......... anybody else service this type of system

Jamie Hall

I can only speak from theory, not practice.

However. Fools rush in. First comment is that you shouldn't need 3 psi off, unless there is something connected which needs that much pressure. 2 psi should be fine.

Second comment is more of a question than a comment: how far are the zone valves from the boiler? This is not trivial: there is a lot of heat stored in that boiler, especially if it is cast iron. The interior of the firebox particularly will be well over 212 when the burners shut off, and the only place that heat can go is into evaporating more water. If there is relatively little volume with the zone valves closed, that pressure is going to build. To 10 psi? Dunno. But it is surely possible.

Therefore -- at least one zone valve should remain open until the boiler has cooled somewhat (zero psi, or very low anyway).

Which suggests that some control logic might be in order -- something along the lines of if a thermostat stops calling and the other is off, the boiler shuts off -- not the zone valve closes -- until the boiler pressure drops to zero. Or something like that.

Remember. Just theory here, no practice. Take with some skepticism.

nicholas bonham-carter

See if the same behavior is exhibited when the zone valves are prevented from closing, by disconnecting their wiring. I think Jaimie is on to something with the residual heat continuing to heat the water with the valves closed. The valves should never both close together.
My guess is that the valves were put in as a bandaid for a lack of balance, and main venting, and the cure would be to restore this system to its original state of operation.--NBC

Double D

If all of the radiation is connected to those zone valves, you may need to put some bypass piping across the zone valves, Nothing drastic, 1/2" pipe should be plenty. Definitely lower the pressure.

Double D

If you were to try doing what NBC suggested (prevent the zones from closing) and the pressure does not rise, try putting the bypass piping across only one of the zones.

JUGHNE

Do you have gravity return or cond pump?

I have a school with 2 zone valves; one for 1 pipe and one for 2 pipe. The gates are from 1955. So steam would seep past. The boiler used to hold 5 PSI 24/7. Windows open.
I wired so that no burner unless one zone opened.
I have pumped return so the return has no pressure from the boiler.
Just a compromise as probably 70% of the 1 pipe rad have been removed as windows removed and load dropped.
The 2 pipe has had maybe 30% radiation removed from remodeling.

1,000,000 btu boiler feeding 500,000 load. Severe short cycling.