Replies

1. 
   jackplane | Mar 18, 2004 04:48pm | #1

   Sorry, it didn't get posted.

     Apparently, in the DC area, a new law states that all new water heaters that are installed must have an expansion tank.I thought this was for radiator systems only, as water heaters already have a pressure relief valve.Anyone know why this would be?

1. 
   htra | Mar 18, 2004 04:56pm | #2

   I've read it's state law in Georgia. When you have backflow preventers in a system and then put a hot water heater in there, when the water gets hot it increases the pressure within the system. I don't know how much it raises it, but evidently it's enough to cause fixtures to leak because of the increased pressure.

   Expansion tanks eliminate this extra pressure cause by heating the water.

1. 
   ahneedhelp | Mar 18, 2004 05:04pm | #4

   In our area, backflow preventers are getting phased in older homes at every opportunity the town can force it and mandatory on new construction.

   They are required if a home becomes a rental and is subject to inspection.

   As for expansion tanks, they kindof slide on that issue but will probably push for it if a water heater is replaced and a backflow device is already installed.

   Then there are most of the homes that will never get one if the plumbing goes untouched for many more years, even if a waterheater is replaced.

2. 
   rasconc | Mar 18, 2004 05:02pm | #3

   Not a plumber, but check the tread about water heater leaking that is active.

   http://forums.taunton.com/tp-breaktime/messages?msg=41107.1

   In many places on city water the pressure required to provide service to all during peak demands sometimes does not get regulated down at night and will cause your temp and pressure valve to burp.  It is easy for sediment to cause them not to seat afterwards.  I believe they are designed as a safety valve, not a fulltime regulator of overpressure. 

   If the residence has a properly set and functioning regulator on the supply (not the little leverlock T&P) this should not be a problem. 

   As several others have posted backflow preventers or heat traps can cause pressure build-up.

   Edited 3/18/2004 10:05 am ET by RASCONC

   Edited 3/18/2004 10:07 am ET by RASCONC

1. 
   BillHartmann | Mar 18, 2004 05:12pm | #5

   There are two separate, but related issues here.

   High system water pressure. I think that the design goal is 60 psi, but unless we make the earth perflect flat then you are always going to have areas of higher and lower pressure. Mine runs over 100%. And high source pressure can cause the P&T safety to release water.

   And expansion tank won't help that.

   If it is that high then you should have a pressure regulator. But many pressure regulators are one way devices.

   And many cities are now installing sometype of backflow preventers. They also act as check valves.

   So if you have one of those or a pressure regulator and even if the system pressure is nomimal and you have used lots of hot water and the tank refills with cold water. As the water is heated and it expands there is no place for the water to go so the pressure increases until the T&P valve releases.

   T&P valves are not designed to "regulate" pressure, but rather to be an emergency safety system and thus they will often leak after they are opened once.

   An expansion tank will solve this kind of problem.

1. 
   rasconc | Mar 18, 2004 06:01pm | #6

   looks correct as usual.

2. 
   jackplane | Mar 18, 2004 07:47pm | #9

   Thank you Bill, and others.

2. 
   DanH | Mar 18, 2004 06:21pm | #7

   > If the residence has a properly set and functioning regulator on the supply (not the little leverlock T&P) this should not be a problem.

   Actually, a pressure regulator effectively acts as a backflow/check valve and WILL indeed cause the problem. The problem is that the cold water in the water heater expands as it heats, creating pressure in a closed system.

   An expansion tank gives the expanding water somewhere to go, so that the pressure doesn't build up.

1. 
   rasconc | Mar 18, 2004 06:41pm | #8

   Also sounds correct, solving one problem (supply pressure fluctuations) could create the second. 

2. 
   JohnSprung | Mar 19, 2004 02:55am | #10

   It depends on the regulator.  Some (Wilkins 70DU) have an "integral bypass" that lets water flow backwards thru the regulator if the pressure on the output side goes higher than the incoming pressure.

   -- J.S.

1. 
   DanH | Mar 19, 2004 06:46am | #11

   But even then, if the inlet pressure is, say, 120, then the system pressure can get at least that high, even if the regulator is set to 50.

2. 
   JohnSprung | Mar 20, 2004 02:42am | #12

   Correct, the bypass limits pressure to whatever the city supplies.  That's not ideal, but it's much better than having a closed system with no limit.  That's the explosive situation.

   -- J.S.