Non bladder expansion tank

Precharge the tank first. This will reduce the initial amount of water entering the tank.

Ideally speaking, you'd precharge both the expansion tank with air and the system with water to 12 PSI each, then when you open the valve separating the two the air and the water will both stay put. Until you heat your system - then the water will expand into the expansion tank, and go back into the system when it cools off. At least I'm hoping that's what will happen. I'm thinking though there will be some absorption of air into the water over time (or else there wouldn't be a market for bladder tanks), but not really sure until I go through it for myself in a week or two.

maple1 said:

Ideally speaking, you'd precharge both the expansion tank with air and the system with water to 12 PSI each, then when you open the valve separating the two the air and the water will both stay put. Until you heat your system - then the water will expand into the expansion tank, and go back into the system when it cools off. At least I'm hoping that's what will happen. I'm thinking though there will be some absorption of air into the water over time (or else there wouldn't be a market for bladder tanks), but not really sure until I go through it for myself in a week or two.

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The key will be to size the air bubble large enough to handle the expansion of your system. Often times those non bladder tanks have a sight glasses so you can know and keep an eye on the air bubble size.

Yes over time air will be re-absorbed back in to the water and additional air will need to be added. Some tanks had a schrader (tire stem) on that sight glass to re-charge the air bubble.

Another method was to add a B&G tank fitting that would push and vented air from the boiler to the top of those tanks.

Often those non bladder tanks were mounted horizontal between the joist.

So the bladder tank was developed to keep the air and water apart and eliminate the potential to waterlog which results in a pop of the boiler relief valve.

Maple1 your response is dead on. My expansion tank is a used 50 gallon electric water heater. Each year it will absorb less than a gallon of water. Total system volume is about 600 gallons. This will be it's 5th year.

TheSteelFabricator said:

Do you precharge the tank with 12 psi before you open the valve to let water into the tank, or do you just open the valve and let the water into the tank untill it reaches system pressure.

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I have filled a few hydronic systems over the years and usually fill the entire system (cold), add chemicals, and circulate. This is for the FINAL fill after flushing and cleaning. I let the expansion tank fill to the top with the mixed water-chemical and then pump the air into the top of the tank. This is intentional so the expansion tank walls get an exposure to the anti-oxidizing chemicals in the water. Then the air pressure and water level are adjusted so the system cold pressure is higher than atmospheric (so pumps don't cavitate) and lots of air-space in the tank exists. Heat is slowly applied until operating temperature exists throughout the system and then the final expansion tank water level can be set by bleeding out air from the exp. tank or water from anywhere in the system. It is always prudent to re-visit the system and check water / air levels some months after the system is in service as pockets of air can work their way out over long periods of time. This is a good time to top up the chemicals as some (moly-based) will have plated out onto the (new) pipe inner walls and/or hooked up with free air (O2 scavenger)

Any ideas on using CO2 or nitrogen instead of air. No O2, no corrosion???

Any ideas on using CO2 or nitrogen instead of air. No O2, no corrosion???

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I thought about using CO2 since I have a tank and regulator but when CO2 dissolves in water (and it does eagerly when pressurized) it creates an acidic solution. That just doesn't sound good.

Nitrogen would not be very reactive when in solution in water. And it's cheap once you have the tank and regulator.

Any REAL chemists out there to back that statement up?

DaveBP said:

I thought about using CO2 since I have a tank and regulator but when CO2 dissolves in water (and it does eagerly when pressurized) it creates an acidic solution. That just doesn't sound good.

Nitrogen would not be very reactive when in solution in water. And it's cheap once you have the tank and regulator.

Any REAL chemists out there to back that statement up?

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Bladder tanks are pre-charged with nitrogen at the factory. A bladder tank filled with air will over time lose it's charge thru the bladder, as epdm is not an O2 barrier.

Tire stores now offer Nitrogen as a fill option for the same reason, I believe.

Bob Rohr said:

Bladder tanks are pre-charged with nitrogen at the factory. A bladder tank filled with air will over time lose it's charge thru the bladder, as epdm is not an O2 barrier.

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Does this hold true with the butyl type bladders as well?

Floydian said:

Does this hold true with the butyl type bladders as well?

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I'm not sure about different types of rubber. Here is some info from the Tire Rack on why nitrogen is used. I suspect same reason as expansion tank pre-charges?

www.tirerack.com/tires/tiretech/techpage.jsp?techid=191