Domestic hot water, hydronic heat? Open or closed loop? Temperatures and pressures? Sketches/schematics always help.

You need to set it to the system pressure before connecting to water. You need to account for the height difference (if any) between the expansion tank and the RPZ or fill station or whatever you use to maintain the system pressure.

So if your fill valve is set to 20 psi, and the ET is at the same height, set it to 20 psi. Subtract 1 psi for every 2.31 ft the ET is higher (or add 1 psi for every 2.31 ft it is lower)

HVAC-Novice
What do you mean by set to system pressure..Normal operating pressure or highest operating pressure system might see ?

It can be surprisingly complex.

First off you need to know what is the min pressure required at the connection point which is usually at min temperature. Min pressure is usually the pressure required to maintain pressure > 10-15 psi at the highest point in your circuit when you have no flow.

Then you need to know max pressure, usually at max temperature. Knowing the volume of the water in the system gives you how much water the expansion tank needs to contain. Most work on the basis of being between 25 to 75% full min to max pressure.

Then you can work out what initial pressure it needs to be when completely empty.

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Normal operating pressure. As system pressure increases, it pushes the membrane to the air side and also increases that air pressure. How to determine that is a different thread/discussion. but once you have that and the elevation difference to the ET, you do what I said above.

don't just use some rules of thumb for system pressure. It depends on the elevations of all components and their pressure ratings etc.

ET are sized to NOT let pressure rise to within 5psi of the relief valve setpoint. BTW, you also need to height-adjust the pressure at relief valve if that is at different elevation from ET and fill station. when sizing ET, assume the maximum temperature the boiler can achieve. this should be way above your system design temperature.

Setting the pressure without knowing the volume in the expansion tank and how much expansion is required is meaningless.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Littleinch
Do you mean the entire water system (client’s hot or cold water system) that the expansion tank is exposed too?

Thanks

Yes.

I'm assuming these are closed loop systems??

Too small and you will hit relief valve pressure when it warms up and then when it cools will need ito refilling to avoid low pressure.

Too big and it costs more than it should and take up more space than you need.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

You are asking about setting pressure.....how did you select these tanks without knowing the pressures? You say one system has it on supply and the other return. Where are the tanks in relation to the system pumps - suction side or discharge? Sketches would help.

A typical installation might be to set the air charge (before connecting to the system) at 5 psig higher than the static fill pressure. You see many tanks pre-changed at 12 psig. That is figuring a 2-story building. In that case, maximum height from expansion tank connection to highest point in the system would be no more than 16 feet. 16 feet is roughly 7 psig. You want to have a least another 5 psig at the high point, so 12 psig is the setting for that case. The fill valve is set at 12 psig.

That's just one scenario. Each situation is different. We need more info in order to help.

Little inch and Novice hit it. Think of it this way, and I'm thinking of a closed loop system (not domestic). The fill pressure is the amount of pressure to lift the water to the highest heating/cooling element in your building with no pumps operating. I then add what I call a flood pressure, making sure I don't suck in air anywhere. I typically use 5.0 PSIG, this number can vary depending on the engineer. (nobody tell me this is wrong because I have 25 years of 'cplaining to my clients if I'm wrong). For example; You have a 40' elevation to terminal unit, then I would charge the tank at 17 PSIG (40/2.31) + 5.0 PSIG or 22.0 PSIG. This assumes a specific gravity of 1.0 which even glycol systems are pretty close to that.

You did mention one is on the supply line and one is on the return line, and I'm hoping on different systems? You only want one expansion tank in your system, think of it as an electrical ground. Normally the expansion tank is located on the suction side of pump(s). I'm not saying what you have is wrong, just curious and believe me I've designed stuff that had tanks tied to EVERYTHING. (Won a bet on that one). People get crazy on that it has to be right next to the pump, it really only needs to be hydraulically close to the suction side of the pump. Not right next to it. A pump with an expansion tank on its suction side will add its pressure to the system pressure. An expansion tank on the discharge side of a pump will subtract it. If placed on the suction side the system will be operating w/ a positive pressure, even at the highest point. If the tank was on the discharge side, it would draw down the pressure to obtain its pressure differential, possibly sucking in air at the high points.

Hope this is helpful.