Draining expansion tank at start of season

[joe1575]

Hi Guys
I have a conventional expansion tank that is about 15 gal. in size and was told to drain off about 5 gal. of water at the start of the heating season. This is to be done so the tank doesn't get waterlogged and creates the air pocket for expansion. Is this sound advice? Or will this cause problems.
Thanks for any help.
Joe1575

[NJ Trooper]

Draining 5 gallons isn't going to help.

You may occasionally need to COMPLETELY drain that tank though. If it's an extremely well designed system, you may NEVER have to drain and refill that tank.

The reason that draining 5 gallons won't help is because you won't replace that 5 gallons with air when you close the valve, you will replace it with WATER, which gains you nothing.

In normal operation, that tank will be 1/2 to 2/3 full of water the rest of the space is a compressed bubble of air.

The proper maintenance is to COMPLETELY drain the tank so that it is 100% full of AIR, then refill... the air will compress to the top of the tank.

It is difficult to completely drain the tank because a 'vacuum lock' will be formed in the tank. Like when you put your finger on the top of a soda straw and pull it out of the drink...

So when the flow stops, you need to take measures to break that 'suction' ...

[gilmorrie]

I never have to drain/refill my conventional expansion tank (equipped with a ~60-year-old Bell & Gossett airtrol system). Is your tank becoming water logged? Is the system's pressure control going haywire or the relief valve lifting? If not, then I would leave it alone.

If you are having such problems, I would suspect something is wrong with the air-removal device that is supposed to direct air into the exansion tank.

Unlike conventional tanks, bladder-type expansion tanks do slowly lose their air charge over time.
Doug

[joe1575]

Thanks Guys
Now I see what your talking about I do have a B&G Airtrol at the bottom of the tank. And didn't have any problems with system pressure or relief valves and haven't heard air going into the rads. So I'll just leave it the tank alone or drain completely, about 3 yrs. ago I did drain it completely and got about 9-10 gallons out of it so it must have had the air cushion at the top.
Thank you very much for the advice!!!
joe1575