Expansion Tank

[kaldag]

I have a Weil Mclain Ultra 105 boiler that has been weeping water from the relief valve. Pressure gets to 30 PSI which I understand is to high. My home also has cast iron baseboards and a two wall radiators. C urrently there is a Amtrol 30 expansion tank. From these pictures can you tell me if this is plumed correctly and would a Amtrol 60 help with the pressure.

Thanks

 

[Dana]

The expansion tank needs to be pre-charged with air to the intended system pressure. If it leaks air the available expansion volume diminishes. It's not possible to accurately measure or check the amount pre-charge while fully plumbed into a pressurized system. If it's possible to isolate the tank from the system with valves, unscrewing it enough to allow water to escape, equalizing the pressure from the water side to the air side of the tank can work, or removing it fully. If the tank has failed internally water will dribble out of the air valve if you poke it with something to open it.

When the system is cool/tepid, the system pressure should be 12-15 psi for a boiler installed in the basement, unless it's a fairly tall building. What pressure does yours indicate when it's cool?

Another common cause of this symptom is a seeping auto-fill valve (aka pressure reducing valve) slowly overfilling the system causing the pressure rise over days/weeks.

The volume of cast iron baseboards isn't large, not dramatically more than cheap fin-tube, but some. If by "wall radiators" you mean something like an Arco SunRad or Burnham Radiant installed inset into a wall, they run about 0.15 gallons for every 2.25" (the width of an individual section.) Using standard plumbing volume tables, try to get a reasonably good estimate the total water volume of the system before up-sizing the tank. I suspect upsizing it not really needed here.

< Arco SunRad

Burnham radiant

With the water volume and the water temperatures and pressures you can use Amtrol's handy web calculator to see if you're close to the margin for a 30 psi pressure relief valve with the existing expansion tank.

 

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[kaldag]

The expansion tank needs to be pre-charged with air to the intended system pressure. If it leaks air the available expansion volume diminishes. It's not possible to accurately measure or check the amount pre-charge while fully plumbed into a pressurized system. If it's possible to isolate the tank from the system with valves, unscrewing it enough to allow water to escape, equalizing the pressure from the water side to the air side of the tank can work, or removing it fully. If the tank has failed internally water will dribble out of the air valve if you poke it with something to open it.

When the system is cool/tepid, the system pressure should be 12-15 psi for a boiler installed in the basement, unless it's a fairly tall building. What pressure does yours indicate when it's cool?

Another common cause of this symptom is a seeping auto-fill valve (aka pressure reducing valve) slowly overfilling the system causing the pressure rise over days/weeks.

The volume of cast iron baseboards isn't large, not dramatically more than cheap fin-tube, but some. If by "wall radiators" you mean something like an Arco SunRad or Burnham Radiant installed inset into a wall, they run about 0.15 gallons for every 2.25" (the width of an individual section.) Using standard plumbing volume tables, try to get a reasonably good estimate the total water volume of the system before up-sizing the tank. I suspect upsizing it not really needed here.

< Arco SunRad

Burnham radiant

With the water volume and the water temperatures and pressures you can use Amtrol's handy web calculator to see if you're close to the margin for a 30 psi pressure relief valve with the existing expansion tank.

 

[kaldag]

Thank you for the info. Only have 2 of the wall units. I will remove the tank and charge to 12 PSI. Not sure on the tepid pressure.

I have had one HVAC guy look at system and he questioned the tank placement. What is your opinion on the plumbing placement of the tank??

Thanks again.

 

[Dana]

In the ideal system the expansion tank would be located fairly close to the intake side(s) of the pump(s) which makes it something of a shock-absorber evening out pressure fluctuations, reducing the likelihood of impeller cavitation. :

Cavitation occurs when the pressure on the pump intake intake is fluctuating at too low a pressure, taking a toll on pump efficiency & lifespan. The expansion tank can dampen the fluctuations.

The tank is usually mounted with a short piece of pipe directly below an air-purging device, ( such as the Spirovent about a foot from the pump in your picture.) It looks like there maybe wasn't enough room to mount it there, so they build a pipe-sculpture several feet long to hang it somewhere else(?), possibly on the fill-pipe coming from the auto-fill(?) (It's hard to see the relative locations of things in the pictures, which don't include all of the relevant components.)

So the plumbing to the tank is less than the ideal, but it's probably not a disaster, since the rest of the lead-in to the intake side of the pump looks decent. The air purging vent is in a good place relative to the pump, with a nice straight section of higher diameter pipe for lower velocity, lower turbulence, even if the "equivalent length" of the plumbing from the vent to the expansion tank is several feet rather than several inches, which would have protected the pump even better. If the pump seems extra noisy from cavitation you can increase the pressure a few psi, but I suspect it's fine. The pump is still something like 1.5-2' of straight pipe section from the nearest turbulence-inducting ell, and by-design the Spirovent component imparts little or no turbulence to the water passing through.

Check the system pressure after the boiler hasn't fired for an hour or so and write it down. If it's 20psi or something, bleed water from the system (there has to be a port somewhere- usually a hose-thread compatible valve near the boiler, possibly multiple locations), until the pressure reads 12 psi.

Tap on the top of the expansion tank- it should be a somewhat dead thud, then tap the bottom, which should ring a bit. If it doesn't ring, see if you get water rather than air out of the air valve. If it does ring, check the air pressure, and write it down.

Go ahead and pre-charge the tank to 15 psi after fully depressurizing the water side of the tank, but set the system pressure to 12 psi as a starting point.

 

[DewPoint]

It looks like your expansion tank is valved off of the system by that green handled ball valve. That valve, if it is between the air vent and the expansion tank, must be open for the tank to accept any expansion.

Also, it looks like the pipe that connects the air vent to the expansion tank continues onward to the right ... where does it go? I’m assuming it leads to a fill valve and backflow preventer and a shutoff valve, in that order, if not then something is piped wrong.

It looks to me that the circulator, air sep, and expansion tank are piped correctly, though I admit I’m leary of disagreeing with Dana. It is my understanding that the length of the pipe that connects the expansion tank is irrelevant; the pressure will not change where the expansion tank connects no matter the length of the pipe.

 

[Dana]

Good observation about the green handled ball-valve- probably 100% of the problem if it's still closed!

An extra-long length of the run to the expansion tank can matter in terms of it's ability to protect the but it's a second (or even third) order issue. The fat straight section into and out of the Spirovent to the pump looks nearly textbook perfect, and the volume of water in the those fatter pipes probably exceeds that of the twisted path to the tank.

 

[kaldag]

Yes the valve was off but used that to isolate the expansion tank. Valve is now open. Had ad Amtrol 30 replaced with a 60 and replaced the pressure reducing valve. Fired boiler up reached temp and was around 20 ish PSI. Will monitor. Thanks for your input.

 

[Dana]

If you pre-charged the -60 to 12-15 psi and the system is still hitting 20 psi it's an indication that the -30 was marginal for your water volume and temperature swings, though it still might have worked.

If it's not going over 20 psi, sleep well- you're good to go!