Thermal Expansion Tank

nycpa

Does anyone know if I need a thermal expansion tank for a house that has steam heating?  If so, do I also need to install a backflow preventer before the cold water line meets the thermal expansion tank?  Currently, I use a 50 gallon hot water tank.  Thanks

Jamie Hall

No

steam heat certainly does not need a thermal expansion tank!  That's hydronic only.  Nor does it need a backflow preventer on the feed lines -- in general.  I can see some overzealous building inspector possibly requiring though, come to think of it.



What are you using the 50 gallon hot water tank for?

nycpa

Expansion Tank

The 50 gallon hot water heater is what provides the hot water to my house, the typical gas powered tall cylinder.  I asked because I read if you have hydronic heat, then you need a thermal expansion tank but wasn't sure if this included heat expansion coming only from the hot water tank. 

Is the consensus that steam boiler feed lines do not need their own back flow preventer?  I thought it was common practice to install a backflow preventer to boiler feed lines.  In my case, i would have two backflow preventers, one for steam boiler feed lines, one for the main water line right before the expansion tank. 

Only a hot water heating system needs a thermal expansion tank?  The hot water produced from the hot water heater will not make the water expand and cause problems? 

thanks

Jamie Hall

Good thinking...

but not quite!  First, you don't need an expansion tank on domestic hot water because the system isn't closed -- that is, the hot water tank is connected to the water supply at all times, so if expansion does occur (and it does) the pressure doesn't increase.  In principle, a very small amount of water is forced back out of the hot water tank into the rest of the domestic water supply system.



You do need one on a hydronic heating system, though, because the system is closed -- even if it is connected to your water supply, as most are, the connection is through a pressure reducing valve and a backflow preventer, so when expansion occurs there is nowhere for the water to go, and the pressure rises instead.



I'm not sure what a consensus would be on backflow preventers on steam boilers.  Certainly on power boilers and many process boilers, you would need a backflow preventer (if they were left connected at all; power boilers usually aren't) as the pressure inside is high.  But for residential boilers, the pressure is very low -- never more than 2 psi -- so unless the domestic water line dropped essentially to a vacuum you couldn't get backflow anyway.  That said, some authorities do require a backflow preventer.  It varies.  It won't hurt anything, anyway, on the boiler.



If you do indeed have a backflow preventer on your main domestic water line, then yes you would need an expansion tank for the hot water. 

Paul48

You

absolutely do need an expansion tank for DHW, if there is a PRV present.

Jamie Hall

Quite true

a pressure reducing valve or a backflow preventer or a check valve -- anything that prevents flow back from the system.

ChrisJ

Backflow preventer

I believe most areas require a backflow preventer on the water supply to a steam boiler.  In fact, my inspector called me a few hours after he left just to double check that I had one, which I do. 

nycpa

Thanks

Thanks for the advice