What is the purpose of this second expansion tank on our boiler?

[Audra Caler]

Hi all,

We just bought a house with a Weil Mclain WTGO-3 115k BTU Output Gold Oil Boiler w/ Tankless Heater and baseboard heating and have been struggling to understand why the system has two expansion tanks. The smaller expansion tank adjacent to the boiler is a Therm-X-Trol ST-5 (2 gallon). It appears to feed into the boiler (pipe feeds into an inlet named "in" on the boiler. The larger expansion tank above the boiler is a Extrol Model 60 (7.6 gallon). The system also feeds the domestic hot water supply and after reading through the boiler manual, I can't find anything that talks of 2 expansion tanks being needed (unless being used with a refrigeration system - which it is not).

The smaller tank is currently full of water and sprays pressurized water when trying to test it with a pressure gauge. As a result, I'm thinking this is why the boiler is reaching 33psi during operation and leaking from the boiler relief valve.

I would greatly appreciate any help with the following questions to try and help me understand the importance of this smaller tank.

1, What is the purpose of this smaller tank if a larger one exists on the system?
2, What is the purpose of the line the smaller tank is on that feeds into the boiler?
3, If replacing the smaller tank (2 gallon), can I use a 4 gallon (designed for boilers) or should we upgrade the larger tank?
4, What PSI should the replacement tank be set at? Should it match the incoming water pressure or be left at 12 PSI pre-charge?

Thank you!

 

[Reach4]

I don't know the answer to all of your questions.

The Therm-X-Trol ST-5 (2 gallon) has failed. Going bigger would probably be a good idea. There are calculators and tables to help with sizing. Basically the bigger the WH is and the bigger the temperature rise of the water when heated, the bigger the thermal expansion tank needs to be. The size required is a minimum. As long as you can mechanically hold the tank properly, bigger than minimum is good. The Therm-X-Trol brand is aimed at domestic hot water. Domestic hot water would be at a higher pressure than boiler water. The air precharge would be set to the setting for the pressure reducing valve (PRV) sometimes used with city water, or a tad higher. I presume you have city water. I suggest that you get a pressure gauge to let you see the pressure after the PRV.

The expansion tank for the boiler is for non-potable water at a lower pressure. Its precharge is set to the pressure that the water is at when cold. There may be a valve that automatically adds water when the boiler water drops below that pressure. When the heat turns on, the water expands. The tank gives the water space to expand into.

Both are for accommodating the expansion that occurs when you heat water.

 

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[Audra Caler]

I don't know the answer to all of your questions.

The Therm-X-Trol ST-5 (2 gallon) has failed. Going bigger would probably be a good idea. There are calculators and tables to help with sizing. Basically the bigger the WH is and the bigger the temperature rise of the water when heated, the bigger the thermal expansion tank needs to be. The size required is a minimum. As long as you can mechanically hold the tank properly, bigger than minimum is good. The Therm-X-Trol brand is aimed at domestic hot water. Domestic hot water would be at a higher pressure than boiler water. The air precharge would be set to the setting for the pressure reducing valve (PRV) sometimes used with city water, or a tad higher. I presume you have city water. I suggest that you get a pressure gauge to let you see the pressure after the PRV.

The expansion tank for the boiler is for non-potable water at a lower pressure. Its precharge is set to the pressure that the water is at when cold. There may be a valve that automatically adds water when the boiler water drops below that pressure. When the heat turns on, the water expands. The tank gives the water space to expand into.

Both are for accommodating the expansion that occurs when you heat water.

Oh! Thank you for your great response! I was initially thinking the 2 gallon tank you mentioned was the lower PSI for the boiler. This makes much more sense now, so it would be the expansion tank for the potable water side feeding into the boiler and out.

Thanks again!