Expansion Tank Strategy

BigRob

Hi all, so I was fighting a leak from my new Taco VR3452 over the weekend. I think it sprung a lead in the canister. The windings in the motor case were damp. Seal was fine. Anyway, I had to drain down the system a couple times and I started wondering:

What are the correct operating conditions for system pressure? For example, is a properly setup expansion tank supposed to regulate system pressure within +/- 2 psi (or whatever number) from cold, or is the idea to size and pressurize the tank so you don't pop the emergency relief valve? What is the pressure range I should see, in broad strokes, with a properly setup system?

My application is indirect water heating. I feed the tank 180F water and I pressurize to 15psi, nominally. The domestic tank set point is 145F and the boiler circuit usually cools to 135F before the boiler kicks on.

Jamie Hall

I'm not sure there is an ideal number. That said -- and I could well have forgotten that there is one -- I'd like to see a system go from cold to hot and gain no more than about 3 to 4 psi while doing it. Amtrol has a selection sizing tool -- but it should be used with a bit of caution, as it doesn't take into account odd factors such as particularly large piping...

Zman

You would normally precharge the expansion tank to the same cold pressure as the system. Do this with the tank completely isolated. You of course want to be "Pumping Away" from the expansion tank.

EBEBRATT-Ed

@BigRob

There is no "set" pressure rise when a system is heated. EX tanks are sized based on an allowable pressure rise for a particular system. Since most residential systems start at 15 psi cold and the pressure relief valve is usually 30 psi you need to keep the pressure under 25 when hot to prevent relief valve opening (lower is better) I would prefer to see 20 max.

If a system goes over pressure from cold to hot the ex tank is defective or a larger tank is needed

hot_rod

the fill pressure is determined by the height of the building. .433, call it 2/2 psi to lift water 1 foot in elevation

Determine the highest point of piping above the boilerX .5. Then add 5 osi

The tank needs to be sized correctly and ore charged to that fill pressure.

BigRob

Thanks, guys. My expansion tank is at the top of the system. I set system pressure to 15psi cold and I'm at around 19psi hot. One aspect that messed with me was the indirect water tank. It's a triangle tube tank in tank, and the pressure fluctuations in the tank were throwing me off a little. Once I figured that out, and using your comments, I was able to get it how I like it. Thanks!

JDHW

If the tank is really at the highest point in the system than 5psi is the right pressure following hot_rod's rule.

hot_rod

where is the circulator in relationship to the tank?

BigRob

Circulator is 10 inches below the tank. Very limited space in the closet. I keep the boiler circuit at 15psi so the TT tank in tank indirect is balanced a bit. Water pressure is about 45psi in the boiler closet, which is about 50ft up on a roof.