Domestic expansion tank question

bill_105 · June 2011

So I need to replace a water logged tank.

Why is it that a domestic tank comes with a pre-charge of 45 lbs.? Shouldn't such a tank have it's pressure adjusted to the incoming water supply? Water pressures vary wildly from city to city. I've never really heard of any talk about it.

The boilers' tank gets all the attention, but the lowly domestic tank seems to get no respect. Any help appreciated.

icesailor · June 2011

Domestic Expansion:

Heating systems are closed systems. Domestic hot water (Potable) water systems are considered open. Every time you "open" the system by opening a faucet, you drop any residual pressure. Not so in a closed heating system.

Water heater Temp. & press. Relief valves open at 210 f degrees and open at usually, 150#. 45# precharge is a comfortable number for an open system. Some air is compressed in the precharge but there is room for more. You need the tank on domestic hot water systems if there is a check valve on the water service and the water can't back out into the city system. Or, the street pressure is above 75# and you need a pressure reducing valve which will act as a check valve and you have no place for expansion.

Make sense?

Mark Eatherton · June 2011

Follow the manufacturers instructions...

The ones I have read state that the air charge needs to be adjusted to the normal operating pressure of the system. So if your system runs at 60 PSI, then air up the diaphragm to 60 PSI.

The actual sizing of the tank is also done with proper calculations. It is based on the total volume of water, starting temperature, ending temperature, starting pressure and maximum allowable water pressure (relief valve threshold).

Here's a link to Watts sizing calculator. You can play a game of "What if..." Try leaving the pressure where it is pre-charged , and then adjust the pressure to the actual incoming water pressure and see what effect it has on the actual size of the expansion tank.

http://www.watts.com/pages/support/sizing_DET.asp

ME

icesailor · June 2011

Tank Pressure

What Mark E. and the manufacturer say is the correct way to set the pressure.

In my real world experience, you may want to set the pre-charge a bit lower because if you set it to the static pressure (stand-by state), and the flow pressure drops considerably below the static pressure (Flow Pressure), you may get a "Bump" when the bladder flattens out on the tank. This is a common problem with well extrol tanks and customers find it annoying. You may too.

I consider it more important that the tank be sized large enough that it will take care of thermal expansion and not make relief valves leak that having "correct" pre-charge.

Mark Eatherton · June 2011

So Chris...

What if they are on a well and running at 30 to 35 PSI operating pressure. Should they still leave the diaphragm at 45 PSI, or should they adjust it downward, to 80% of operating pressure.

Leaving it at 45 PSI would render it virtually useless unless there were a check valve on the inlet to the cold water tank that would isolate it from the potable water pressure tank, no?

You have to have SOME kind of formula for diaphragm adjustment. Can't just leave it as is and assume that all applications will be well above the pressure setting of the diaphragm...

ME

icesailor · June 2011

Pressures:

Mark,

I'm not sure what Amtrol Et-Al say about pre-charge pressures but I find that one should NEVER set the precharge pressure at the cut-in pressure. The pre-charge pressure must be low enough below the cut-in pressure to keep the water flowing while the pump is starting. If not, you will get a complaint of the water stopping or pausing for an instant while the pump starts. On a 40# cut-in, I set the precharge to between 36# to 38#.

Last week, I was asked to help out on a troubled pump install. This was the main complaint, the pausing and that although a 2 stage 1HP jet pump, it would barely pump over 40# and very little pressure. It turned out that someone had replaced the venturi tube with one for another pump and the nozzle and venturi were not matched for this pump. Once corrected, the pausing went away.

The bigger the tank, the more the storage, and the more room for expansion.

icesailor · June 2011

Tanks:

Mark,

I always deal with PS104 pressure switches. I always deal with a 20# differential. 20#-40#, 30#-50# or 40#-60#.

They have constant pressure set-ups but I don't do any and I have never installed one. I don't understand how they work and with a constant pressure set-up, you could deal with the 45# situation you ask about. But with a differential pressure controller, I would say that you must set the pre-charge pressure below cut-in pressure.

Charlie from wmass · June 2011

Unless you have a check valve at the inlet

to the domestic tank an expansion tank is not needed with a well system that is functioning properly. If it is functioning properly and you have a check valve the the expansion tank is only there to give the t&p a break and allow for the tank volume to expand as it heats. If there is enough volume in the expansion tank when the water has reached it highest volume well below relief pressure the starting pressure is a moot point. I would set the pressure in an expansion tank for the top range of the water cycle if I did need to install one for a well system so it did not act as a second well tank. If running 40-60 I would set it at 60. if 20-40 then at 40.

icesailor · June 2011

Pre-Charge on Extrol:

Charlie,

You can not set the pre-charge pressure on a bladder well tank higher than the cut-in pressure, but you must set it lower than the cut-in pump pressure. Over-inflating the pre-charge decreases the amount of expansion you will have and it makes the water stop while the pump is starting. If it is a 40/60 PS104 and you set the precharge at 35, the water will still be flowing out of the tank while the pump is starting at 40#. If the pump can not keep up with the 40# pressure, the transition from stored to pumped is smooth and no quick drop in water flow at the faucet. Customer seem to get upset when the water pauses for an instant.

Mark Eatherton · June 2011

So what you are saying then, is...

set the diaphragm of the tank to be lower than the minimum operating pressure of the system. Correct me if I am mis-stating your position.

And the reason behind this decision is to keep the diaphragm from completely discharging its volume and acting as a hammer on the water pressure?

If so, sounds reasonable, but it will have the effect of lessening the "acceptance" volume of the tank. As long as THAT is taken into consideration (requiring an upsizing or oversizing of the expansion tank) then it should work fine.

ME

RobG · June 2011

P.E.T.

I think that the answer to Bill's initial question "why is the potable expansion tank pre-charged to 45psi" is that potable pressure reducing valves are pre-set to 50psi from the factory. This way an installer throw in a PRV and exp tank and not even own a guage to test his work and still be pretty safe on his install. I'm not saying that the installer (note that I said installer and not plumber) is doing it correctly, but that it is a way for the manufactuer to cover his a**.

Rob

Charlie from wmass · June 2011

CHris I am talking about the expansion tank

for the domestic hot water not the tank for pump control.

Domestic expansion tank question

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