Boiler Pressure

[Engine1]

Hot water boiler pressure is at 18 cold, 25 or so when hot. The new regulating valve is adjusted to the lowest possible setting. Expansion tank has 15PSI. The two zone baseboard loop is on a first floor.

I don't know if this important, but the feed is into the return line and the pumps are on the supply side.

Any reason for concern here? The manual says the pressure should be at 12.

[jadnashua]

If the expansion tank is properly sized and the pressure is adjusted correctly, AND if there's no additional water being inadvertently being introduced to the system (autofill valve leaking, a leak in the heat exchanger with the WH, etc.), AND, you've gotten all of the air out of the pipes, the pressure should be stable, hot or cold. The only way to correctly measure the pre-charge in the expansion tank is when there is no water pressure applied, otherwise, it will read the same as the water pressure.

Assuming you have an autofill valve, it may no longer be shutting off properly. Once the system is filled, you could shut off the autofill entirely as the system should be closed and never lose any water. If it does, the autofill valve will mask the leak problem and keep the pressure in the safe to operate range (unless it is leaking, then the pressure will rise).

12psi is probably the minimum pressure otherwise the safety pressure switch would prevent the boiler from firing. A common residential boiler relief valve is often 30psi, so the usual desire is to operate somewhere above the minimum and less than max...a figure of 14-16psi is fairly typical. You don't want it to be bumping off the minimum because it might fail to fire up. Too high, and it would be dumping water. Adding fresh water to a typical boiler is not a good thing. It works best when it is in there for the duration as it reaches an equilibrium with the metal parts.

[BadgerBoilerMN]

Jim is right.

I would add that the typical residential boiler pressure is 12psi and that the pressure in a diaphragm tank should be equal to the design static system pressure all at
room temperature. The proper pressure is determined by the measured height of the highest radiator. A slight increase in pressure is common
and acceptable (5psi or so), but if you are approaching the PRV max of 30 you have a problem with the size or condition the expansion tank.

As for safety pressure switches; they are generally found on all high efficiency condensing boiler and never found on low efficiency non-condensing cast iron
boilers.

[Engine1]

Was wondering about that pressure safety switch. There isn't one (that I know of) on on this cast iron 85% system. Thanks for clarifying.

I removed the diaphragm tank and it had a pressure of 13PSI, connected it earlier read 15PSI or I'm just off a bit or inadvertently let out 2 - either way it appears OK. The baseboard hot water loop supplies a typical first floor, so all indications are that it shouldn't need much more than 12 PSI when cold. The pressure valve adjusting screw is unscrewed as far as it can be which sets it for the lowest possible pressure. The nut holding it is snug - not tight. The device is a Watts B1156F and the label reads that it is set for 14-17PSI. Is that simply the wrong valve?

Right now it's 2.2F outside, it's running with a setpoint of 195 (likely in boost mode) and at 25PSI.

Will anything bad happen if it's running at a higher than called for pressure?

[jadnashua]

The spring in the autofill valve may no longer be able to adjust to where you want it and a replacement may be good.

You don't want your boiler dumping water from the safety valve. If it never exceeds 25psi, it should be okay. Just like the radiator cap on your car, the minimum pressure is to prevent the water from flashing into steam, which can be dangerous. There'll be some max your boiler is designed to handle, often indicated by the safety valve, but not necessarily...you'd have to read the spec sheet as to the max safe pressure. The higher you need to pump, the more pressure you need in the boiler to keep it all liquid. Steam in a hot water boiler is the bad thing, and maintaining at least the minimum pressure is required. Too much, and it may leak. How high that is, depends on the equipment.

[Tom Sawyer]

In any case the auto-feed valve should not be left open. fill the system to 15 lbs and shut the auto fill off. Once the system is pressurized there should be no need to add water ever unless there is a leak somewhere. Go here www.heatinghelp.com for a detailed discussion of autofill valves and whether or not to leave them on. Also, because new code regulations prohibit dumping the drain port from any back flow or check device with an atmospheric vent on the floor we no longer install auto feed/bfp valves on any hydronic systems. We fill the system with either a hose or a pump, set it and leave it. Depending on system head pressure and the high limit setting, on a cold day the system will fluctuate 2 to 3 pounds normally and, depending on where your circulator is mounted it may show more variation than that. Installed on the return, which is pretty normal for packaged boilers and having the expansion tank on the feed side, the system will experience larger pressure changes than if the circulator was mounted on the feed with the expansion tank before it. In any case though, if the pressure relief ain't weeping and the pressure is below 25 lbs then forget about it.