expansion tank sizing

jrc2905

Is there a site here that has a formula for sizing expansion tanks? I ask because I have a customer who used steam radiators when he installed the hot water boiler in his home. Prior techs have replaced every component as it relates to boiler pressure or expansion, and it is still over pressurizing and blowing the relief valve. I have tested the indirect and it is not the problem. I suspect the #30 expansion tank is too small. I also moved the water feed to the point of no pressure change.

Terry

http://www.amtrol.com/

bob young

tankless

i got a feeling the radiators are not the problem. if you use steam radiators on hot water chances you you won't even need an expansion tank at all. do you have a coil in the unit ? could also be a bad feed valve seeping.

Ron Schroeder

Hi JRC,

The size of the expansion tank is based on the TOTAL volume of water in the whole system and the operating temperature. Radiators hold a lot of water and I suspect when that water heats up, it expands too much for a #30 tank.

Dave Yates (PAH)

sizing

If you know the existing expansion tank's volume and initial air charge, you can determine system volume by observing temperature rise and pressure rise. (That's providing the system has no air pockets or components that will expand - like PEX.) Boyle's Law from a dyslexic point of view!

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John McArthur_2

Based on John Siegenthaler’s article in PM Magazine:

To calculate the correct air-side pressure of a thermal expansion tank:

Pa=Hs(Dc/144)+5

Where:
Pa = Airside pressure of the expansion tank prior to filling system. (psi)
Hx= Height of liquid above the inlet of the expansion tank. (feet)
Dc= Density of fluid in the system when cold. (lb/ft3)
5= Allowance for 5psi static pressure at top of system.

To calculate the volume of a thermal expansion tank:

Vt=Vs[(Dc/Dh)-1][(Prv+9.7)/(Prv-Pa-5)]

Where:
Vt= Minimum expansion tank volume. (gallon)
Vs= System volume. (gallon)
PRV= Pressure at which the relief valve opens. (psi)
Pa= Correct airside pressure. (psi)
Dh= Density of fluid in the system when hot. (lb/ft3)
Dc= Density of fluid in the system when cold. (lb/ft3)


To calculate the amount of fluid contained in a length of pipe:

Gallon = 0.0408 * diameter of pipe squared * length of pipe

jrc2905

thanks for the info but now I am back to square one. wouldn't a larger expansion tank fix several of the posible problems.

bob young

call the plumber

If it is not the tank , a larger one will fix nothing. you got all this information but understand none of it. you need a plumber. period.

Glenn Harrison_2

Here's a link to a chart I always use...

http://flexconind.com/pdf/htx-sxht_sizing.pdf

If you look, a number 30 tank is only good on cast iron radiators if the boiler is 50,000 BTU's or less.

An example I can give you also is I had a 175,000 BTU boiler on mostly cast iron radiators, and an Unico Hydronic air handler was added on the third floor for a boost in a poorly heating room. Between the boiler size, the water volume, and the 18 psi operating pressure to get water to the air handler, it was figured that three #60 tanks were needed. Something to think about.

rich pickering

Why btu's?

Glenn, can you, or anyone else, explain why btu's matter? I've had suppliers ask how big the boiler is, but a 50,000 btu boiler can get a system to 200, just like a 250,000 btu boiler, it just takes a little longer.

Shouldn't the fluid expansion be the same?

jrc2905

Glenn, thanks for the chart if confirms what my first reaction was, too much heat and water, it needs a larger tank.
As far as calling in a plumber goes, they are the ones who put the system in and won't return.

hr

Without knowing

the entire system volume, it will be hard to get an exact size required.

It doesn't hurt to oversize, or manifold tanks together. Even tanks of different sizes can be piped together.

You need to look at acceptance volume, not just tank size. Best to consult a rep for sizing help if you have a question. Their sizing software goes further than the online ones shown here.

And, be sure to check the precharge pressure. Rare to fine one from the factory at exact 12 psi, most are way over charged. If 12 psi is you desired fill pressure.

hot rod

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Tom R.

Xtank

5% of the SYSTEM capacity always works for below 240 Deg.

Ron Schroeder

Hi Rich,

BTU of the boiler is just an inaccurate "rule of thumb". Temperatures and system water volume is the data that you need to calculate expansion.

Glenn Harrison_2

Rich, you would have to ask someone smarter than myself

A person like Ron Schroeder for instance I think Ron is right in that somebody figured out that a certain BTU boiler with a certain type of heat emmitor on it will carry an certain average of water, and this certain average will expand a certain amount at a starting pressure of 12 PSI, on average.

I will say I have used this chart for years and I can't remember a time that it failed me yet, although I think there was one time I upsized the tank one model from the chart because there was huge piping in the system.

Other than that, I don't know exactly how the BTU's comes ointo play on the chart.

Glenn Harrison_2

Your quite welcome

Glad to give you someplace to start.

As said by others, the best way is to calculate water volume. But, when I'm doing service calls, I don't have the time to size all the pipes and rads to figure out the tank size needed, so I have to go with the "rule of thumb" charts.

rich pickering

Glenn and Ron

Thanks. On large systems, I've always done the calc's, especially when troubleshooting.

Once fired up a 2000Sq ft infloor system, Trinity boiler,no buffer tank,without the exp tank hooked up. I was just curious what would happen. Pressure went from 12 to 15. Decided to put in a smaller tank than planned.

Ron Schroeder

"Other than that, I don't know exactly how the BTU's comes ointo play on the chart."

The BTU's only determine how quickly the T&P valve will open when the expansion tank is too small ;-)

Ron

Ron Schroeder

Hi Rich,

Radient systems with PEX pipe are an example where you can often get by with a smaller expansion tank than expected because the PEX itself will expand with the heated water.

It was wise of you to put in an expansion tank anyway to establish the "point of no pressure change".

Ron

Ron Schroeder

Hi Glenn,

I see at the bottom of that chart that it says "Sizing recommendations based on average sized systems".

My gut feeling would be to use either the BTU rating of the boiler or the BTU rating of all of the Radiation, whichever is LARGER.

Ron