Expansion Tank Pre Charge Pressure

Tom Blackwell_2

The precharge pressure should be a small amount below the system fill pressure. Some diaphragm tanks will damage the bladder if the bladder is forced against the inlet for long periods. Ideal is for the bladder to be approximately 90% extended in normal operation. If the charge presure is higher than system pressure, then the bladder will be fully inflated and the tank will take on no water until the system pressure rises above the pre-charge pressure. Going in the other direction, if the precharge pressure is lower than the system fill pressure some of the acceptance volume is wasted; could have used a smaller tank.

Unknown

Expansion Tank Pre-charge Pressure

Good day Wallies - I have a rather simple question.

While training the other day I was asked why the expanxion tank pressure is set lower than the system pressure. Busted!

I'm sure there is a good, logical reason for this - can one of you please offer a bit of insite as to why this is? And what is the rule of thumb as to how much lower is the set pressure than the system pressure?

Thanks folks...

John Starcher_4

I have always...

....set the expansion tank pressure to MATCH the system pressure. I've never heard of setting it below the fill pressure.

Starch

WaterHeaterGuy

I have never...

... set pre-charge under system pressure, if you think about it it makes no sence.

If you set a pre-charge pressure to 10# and then fill the system with 15# of pressure. The 15# system would compress the 10# diaphragm in the expansion tank. A compressed diaphragm cannot move and accept any future expansion on the system.

DanHolohan

I'm with Starch.

That's a new one on me.

WaterHeaterGuy

I have never...

... set pre-charge under system pressure, if you think about it it makes no sence.

If you set a pre-charge pressure to 10# and then fill the system with 15# of pressure. The 15# system would compress the 10# diaphragm in the expansion tank. A compressed diaphragm cannot move and accept any future expansion on the system.

Come to think of it I think I recall someone giving me an arguement for setting pre-charge for 2# over system pressure. I don't remember it, maybe someone else has heard it.

Brad White

I agree with Starch

The expansion tank gas charge should match the system cold fill pressure which in turn should be enough to fill the system with a 4 to 5 PSIG reserve pressure at the top of the system.

If the expansion tank were under-filled it would have less available acceptance volume (and would take on the system pressure anyway). By extreme example, say it had zero pressure- you then have no tank.

If the expansion tank were over-filled, the system itself would take on that pressure. Call it "push back". Less room for thermal expansion before relief valve lift-off, depending on the tank starting pressure.

When filled to match the system cold fill pressure, you have preserved your full tank potential to absorb thermal expansion. Goldilocks confirmed this when she checked out the Three Boilers.

Paul Fredricks_3

Does it make sense that an bladder tank coming out of a cold truck would have less pressure then when it was factory charged?

Brad White

Yes it would.

Boyle's Law. Pressure will rise with temperature in a captive gas. As opposed so Simmer's law which says, keep below boiling until potatoes are tender.

Steverino

And lastly...

just because the box that the expansion tank came in might indicate that the tank is precharged to 12# doesn't necessarily mean it is. Always check the tank pressure before installation.

Plumdog_2

and even lastly more

I recall someone mentioning that high altitude would affect the pre-charge, or that the pre-charge should be adjusted slightly?

Brad White

Mmmm. Interesting

In a fixed vessel filled to atmospheric pressure at sea level, (14.696 PSIA for absolute) if moved to Colorado (the national reference for altitude it seems), if the temperature held constant, the internal pressure would be the same. The relative pressure to the outside would increase (by about 3 PSI at 5,000 feet elevation.

But because the expansion tank is not a fixed vessel, (it has a flexible diaphragm), the pressure would drop to equilibrium, say 12 PSIA.

A system filled at altitude would start at that 12 PSIA but being a closed system, measurement reverts to gauge pressure. There is not atmospheric effect. Water still weights about the same for all practical purposes.

So I would say, no adjustment is necessary because both start from the same "zero" point, that being atmospheric pressure at the fill location. What do you think?

Paul Fredricks_3

Picture this

A demo trailer from a boiler manufacturer takes his system from New York to Pikes peak for a seminar. Doesn't realize that his expansion tank bladder has ruptured and someone put a plug in the relief valve as a joke. Half way to Pikes Peak he hears a BOOM from inside the trailer...

Just thinking. Must be a slow day.

Brad White

There goes the fixed

vessel volume part of the equation!

Hope the driver bought a change of clothes.

Tony_23

Every book/lit

Everything I've EVER read, from any manufacturer, says equal to cold start pressure. If I were to vary from that, I'd go a bit higher, not lower. Lower decreases expansion capacity.

If I had to worry about a bladder rupturing because of it's contact with the inlet of the tank I wouldn't buy that brand, period.

I also check EVERY SINGLE TANK for pre-charge, including the built in ones in the Baxi boilers. About 1 in 4 are short or flat.

J.C.A._3

Tony,

I agree. Who... or what,is filling these tanks?

I bought a special 0-20 psi gauge for just this purpose, and I've got to say I find LOTS of tanks under charged from the box.

I also agree that the best time to check is BEFORE installation, if you have the chance. If you know the height of the radiation, there should be no guessing. Chris

Tony_23

Chris

A couple of years ago we had to make return trips to 3 calls because of relief valves dripping/blowing afetr we'd changed them and the true culprit, the ex-tank. This was in less than 2 weeks.

We now carry 12 volt compressors in the vans and 0-50 gauges. EVERY tank gets checked. Assume NOTHING. Callbacks cost money AND customer confidence !

Couderay

A couple of pounds under

The mentioned 2 lbs. under is for the bladder tanks on a deep well system.

John Starcher_4

AHHHHHHH!!!

I never considered that he may be referring to a pump sytem PRESSURE tank for domestic water!!!!!!!!!

If that's the case, then yes you must set the air charge a minimum 2 psi lower than the pressure switch's "cut-in" pressure setting. This is so that you don't blow all the water out of your pressure tank before the pump kicks on, causing wierd fluctuations at the faucet. Personally, I like to set the tank charge at 5 psi under cut-in.

Unknown

Precharge Pressure

Thanks guys for your input - The Wall once again proves it's the greatest resouorce of heating information on the planet! Getting advise from real world folks that actually install systems is invaluable - thanks to all.

Checking the tank perssure prior to installation is a very important point, one I have preached for years - but I assumed (a bad word in our industry) it is a common practice. I will continue to "preach" this proceedure.

Having the tank pressure the same as cold fill for thermal expansion tanks makes sence (I confirmed this with Amtrol). Don't know where I came up with the idea it needed to be slightly below fill pressure (might have been from the old days back when I used slide rules!)

Pressure tanks for water systems require 2 psi below the pressure switch cut in pressure, but obviously that is a complpetely different application.

Thanks again guys - have a great heating season!