Residential boiler with 33psi at 175 degrees, 30 lb. T&P Releiving

[Etbrown4]

Looks like 30 lb. T&P is performing as it should.

Seems odd for this residential boiler to be running much above 25psi at these temps.

Aquastat is pretty much ruled out as we just have too much pressure at this temp.

Tips appreciated.

Possibilities include the flue which appears clear, internal unit cleaning which has been done (not certain how well), and I guess there is always the chance the circ pump is delivering lower than spec'd gpm. It circulates seemingly well enough, but just have too much pressure for the temp.

 

[Dana]

The most likely causes are:

1: The expansion tank has failed, is undersized, or is improperly charged with air. The tank needs to be sized correctly for the change in water volume as it heats up, and needs to be pre-charged to the system pressure in order to work. Most residential boilers work just fine with 12psi pressure (cold/warm). The amount of pressure it gains when it heats up is a function of the high temperature, the water volume, and the expansion tank size.

2: The filler valve is seeping, letting more water into the system, causing system pressure to rise slowly over time.

3: If the boiler has a coil or indirect tank for heating domestic hot water, leaks between the potable and system side would have the same effect as a leaking filler valve.

Flues and the circulation pumps are not factors here.

If it has an auto filler valve as well as an isolating valve between the auto-fill and the potable water , close the isolating valve. Bleed the system until the pressure drops to 12psi, then run the boiler up to the hi-limit temp, and make note of the pressure changes with system temperature. Even on a high volume system it shouldn't go much above 20psi if the expansion tank is OK, and it will probably stay under 20 psi. If the expansion tank has failed completely the pressure will come up pretty fast and pretty high. If it's under-charged it may do fine until some system temp is reached, then start climbing quickly with temp.

If it all behaves itself in the initial test, leave the isolating valve closed and observe it for a few days/weeks to see if the pressure is creeping up. If it's always 12psi when cold, you don't have any leaks in the hot water coil or indirect. If it starts creeping up you probably DO have a leak. There's an outside chance that both the isolating valve and auto-fill valve are leaking, but that's less likely (especially if the isolating valve is a ball-valve.)

 

[Sponsor]

 

[Etbrown4]

Thanks Dana for the excellent post. The pressure problem exists when the water supply is turned either on or off, when the boiler is running.

The boiler is about 15 years old and the expansion tank is original to the home at about age 65.

Likely there's no rubber bladder in this old tank.

There is a 5/16 od line to the expansion tank and no obvious opportunity for air injection. Plumbing houses don't seem to sell 5/16 fittings so it's not clear how we'll be able to tee into the expansion tank to get a Schrader port. (It's also not clear how the original plumber was able to precharge the tank) The present line comes off a 1/2" copper line near the system drain, and there is a valve which isolates it.

It's a bit of a mystery as to how this ever worked.

 

[Etbrown4]

Some old timers I've talked to say that the old expansion tanks were all mounted high on a wall or at the ceiling, and merely started with ambient air in them.
If I understood correctly, once the system was filled with water, the valve to the expansion tank was opened and the system water pressure would then push against that ambient air in the tank, and giving a place for expansion during operation.

Anyone know if that's the way these old ones worked? If so, that might explain why there are no 't's' in the expansion line, and no Schrader valve?

 

[Tom Sawyer]

Yes, they were just filled with ambient air.

 

[Etbrown4]

Good luck and bad luck.

We drained the old expansion tank, reconnected the tank, leaving it therefore full of ambient air. Then opened the valve to the expansion tank. Somehow we started with 0 psi on the system gauge.
We fired the boiler and somehow this time even with the feeder valve open, the pressure stayed at zero for 10 minutes. I know that sounds like the feeder valve regulator, but I somehow doubt it.

Not trusting that was going to be the reliable result, we tried it again the next day. This time we started with 12 psi on the boiler, but did not drain the expansion tank again. On the second trial, the pressure went to about 33 although the temp never went over 190. So we still have the problem. It's hard to imagine the expansion tank could water log that fast.

Perhaps there is another step to the process other than draining the expansion tank then opening the expansion tank valve and firing up the boiler?

 

[Reach4]

There is a 5/16 od line to the expansion tank and no obvious opportunity for air injection. Plumbing houses don't seem to sell 5/16 fittings so it's not clear how we'll be able to tee into the expansion tank to get a Schrader port.

Plumbing threads are normally identified by nominal ID rather than OD. I am not saying that you have NPT threads, but maybe you do.

http://www.engineersedge.com/hardware/taper-pipe-threads.htm

I don't know hydronic heating. I do wonder why you don't buy a modern expansion tank? AMTROL EX-30 is reasonably priced and is from a leading company. I don't know if it is big enough for you, but I know that a diaphragm tank is smaller than the older style for a given application.

And I am not saying it would fix things. I am saying that it would eliminate a thing that is bothering you now.

 

[Dana]

Reach4: You need to know the system volume to spec an expansion tank.

etbrown4: Firing up a boiler at 0psi is a bad idea. Always pre-fill to at least 8-10psi even if it's stone-cold.

If the pressure gauge is sticking or way off calibration (either of which would explain 0 psi readings while filling) it's worth replacing the temperature & pressure gauge, or using a temporary pressure gauge tied onto a drain port or something for doing your system pressure pre-sets and diagnostic testing.

 

[Etbrown4]

I agree with Reach4, however we've verified the pressure is correct, and the boiler temp is accurate as well. Even the 30 lb pressure relief is hitting at its rated pressure withing a couple of pounds of the PRV's rating.

It sure is reasonable that if 0 psi is an accurate reading, that you might be starting with an imcompletely filled boiler.

As best we can tell, the only difference in the water volume held in the system is the 2 gallons or so held in the expansion tank with the feeder on and 2 gallons less if we have drained the expansion tank. We can isolate the expansion tank and just drain that, so we felt that there was not much risk of boiler damage.

Still it seems that just starting the boiler with the expansion tank full of ambient air and the feeder on, we must be skipping a step, or somehow not doing it correctly, though we can't see more than one way to do it. Perhaps there are others.

With each attempt, we end up with an obviously full expansion tank every time its run with the feeder on. It acts as if there is no back pressure against the ambient air which is supposedly there.

Obviously we have no experience with this old style tank!

At startup, we always begin with 12psi if the feeder is on.

 

[Jadnashua]

The expansion tank may have a slow leak in it, letting the trapped air out. Personally, I think that today's bladder tanks are a significant advancement. Many of the internal parts in a boiler system are made with iron. Iron and air (oxygen) don't work well together, creating rust. At some point, the water in the heating pipes will be saturated with air, and rust things from the inside. Every time you drain things, you're adding more air as well as when you add water which has some entrapped air in it, too. Those type of systems work well if they are truly sealed and in the first few days, all of the oxygen gets used up and things are then pretty inert. A bladder tank isolates the water from the air. The difference is, they wear out and need to be replaced. Course, an old style expansion tank could fail as well, but it will last longer. It could be that the iron nipple feeding the expansion tank is nearly rusted shut. The water needs to move in and out of it freely.

Dana described the ways the pressure can creep up...you need to eliminate them one by one until you get things stable. Keep in mind, once the safety release valve opens a bunch of times, sometimes they don't seal properly, so you may want to pick up a spare in case.

 

[Etbrown4]

Problem solved by installing a new tank!

Thanks to all for the suggestions!