Adding a back flow preventer or a check valve to a domestic water
supply will create a closed system.
Usually the issue of thermal expansion in a close system is handled by
installing an expansion tank on the cold water side of the water
Could the thermal expansion in the system be handled by installing a
pressure relief valve; with a relief pressure lower than the relief
pressure of the T/PO valve of the water heater?
Of course this relief valve would dump a small amount of water every
day...... approx the amount of expansion of the cold water heated to
water heater temp.
Is the expansion tank solution "better" than the relief valve
solution? or is a relief valve a better solution?
I'm just curious........... since I can see if the relief valve is
working but the behavior of the expansion tank is internal & thus
hidden.....so I have to go on faith as to whether the expansion tank
is doing it's job.
The standard solution is the expansion tank. I would guess that the TP
relief valve is for emergency use and is likely to fail after too many uses.
Water heater manufacturers tend to use real cheap junk on them. In addition
you are loosing the warning that the system gives you anytime the TP valve
goes indication something that should not be happening but making it as
In theory, a relief valve set at an appropriate pressure would work.
However, you then have have 3 problems. You now have to deal with
sending the released water somewhere. It wastes water. And the
valve could eventually fail to close completely. An expansion tank
is the better solution, which is why it's used.
I looked at expansion tanks today, and in the instructions it plainly states
that the tank WILL eventually fail. Given that both of my hot water heaters are
inside my home and in places where a flood of water would be destructive and
potentially VERY expensive, I think that I could dump a little water every now
"bd" wrote in message
replying to trader4, bd wrote:
I looked at expansion tanks today, and in the instructions it plainly states
that the tank WILL eventually fail. Given that both of my hot water heaters
are inside my home and in places where a flood of water would be destructive
and potentially VERY expensive, I think that I could dump a little water
now and then.
Actually the expansion tank ios also used in some homes with an
anti-backflow valve. If the water heater is filled with cold water and
then heated, the water expands and has to go somewhere. Without a
check-valve it goes back into the water main (small amount - but
enough to build incredible pressure in a closed system) With check
valve it can't get out so an expansion tank is used. A thermal
pressure relief valve on the water heater should NOT be depended on to
bleed off the pressure due to expansion - what happens if and when the
water heater is shut down and the system cools??? Also, the valves
tend to leak after a few operating cycles. If the relief valve has
been activated it SHOULD be replaced.
I would not be using the water heater T/P valve to relieve expansion
pressure but an addtional relief valve elsewhere in the system.
The water heater T/P valve would not cycle (it's rated at 150psi) &
the additional relief valve would be 125 psi
Frankly I don't know of a T/P valve designed for regular use like that.
I suspect the life cycle and failure rate would make you wish you had use
the conventional expansion tank. Maybe you have come up with a great idea
that will save lots of people money, but I suspect, not. That it just will
not work out.
This is more of a thought experiment.....plus I do not intend (I'm not
using currently) a T/P valve
it's just a pressure only relief valve.
I doubt that this is a "great idea" given the fact that expansion
tanks are the norm.
I'm just curious about alternatives....relief valves aren't all that
cheap (good ones)
The relief valve I'm currently using was an expensive one obtained
The only time the system pressure would reach (&stay at) the relief
is when the water heater had re-heated cold water AND no water was
used in the system.
As soon as the water heater is back at temp & ANY water is used (hot
or cold) then system pressure will drop back to regulated pressure.
Maybe I'll add a pressure gage to the system & take periodic
but it sounds like adding an expansion tank is the way to go.
If you have a closed system and heat water from, say 50F to 130F, I think
that you would be amazed at what the potential system pressure can be
without the protection of an expansion tank or relief valve. The answer is
what makes an expansion tank the way to go because you don't want a stuck
relief valve to be the cause of a component failure--water tank, washing
machine hoses and the like.
Water is not incompressible. A term called Bulk Modules defines the
compressibility of water.
The equation that will calculate the change in pressure in a closed system
is as follows:
Delta P=(Delta V)/V*(BM)
Where Delta V is the change in volume due to its change in density. For 50
to 130F, the density (D) decreases 1.3%
.---V=W/D since D decreases 1.3%, the volume will increase 1.3%--since the
weight, W, of the water doesn't change.
In system with about 45 gal that's a potential increase of about 0.6 gal.
V=trapped system volume
BM=Bulk Modules, which for water is 312,000 psi
So, Delta P=.013*312,000= 4,056 psi
Total pressure P+4056 or approx 5,000 psi.
An expansion tank is designed to absorb the change in volume as the water
heats up thus preventing such a significant increase in pressure. You don't
want the health of your system to be dependent on when a relief valve
decides to open.
Think auto cooling system as an analogy--water temp goes from say 60 to 190
F. The radiator cap cracks at 15 psi and the expanding fluid fills your
overflow tank. What do you think would happen if the radiator cap didn't
I fully understand the concept of bulk modulus & had already done the
calc for the increase in water volume based on the operating
conditions in my home.
your comment >>>>>You don't want the health of your system to be
dependent on when a relief valve decides to open <<<<<
but isn't this what we already do .....we depend on the T/P to prevent
water heater explosions?
adding another (pressure only) relief valve with a lower relief
pressure seems like it would be improving the system.....the relief
valve with the lower setting will let go before the T/P
valve.........the expansion tank only has so much volume.so a stuck T/
P.would be real trouble.
some plastic component in the cooling system would fail or a hose
would blow :)
I've been watching my relief valve & it only releases a few ozs per
day .......The worst case of using all the hot water doesn't seem to
generated all that much "dump water"
I'm wondering where all this expansion is going
In case it got lost in the discussion, only those that have a closed system
(valve that prevents backflow into the city supply) have the potential to
reach the level of destructive pressures noted in my post. Without this
valve at the inlet, I think that the T/P valve would rarely, if ever, come
into play since the city inlet essentially acts as a giant sized expansion
tank.. With respect to putting another relief valve in the system at a lower
pressure than the T/P----how does that improve the system? True, you've
dropped the max system pressure from maybe 125 psi to "whatever"---but it's
redundant and serves no useful purpose other than having one valve as a
backup the other. T/P and relief valves come into play so infrequently that
they are notorious for leaking once they are cracked open.
The expansion tank is a better solution since it has the capacity to handle
changes in volume associated with the heating of the water without having to
vent overboard. Worst case scenario (and this is a big stretch) is that the
expansion tank uses it's total volume--then the T/P valve is there as a
backup to limit system pressure.
seems like a good idea excpet..
1) I would have the outlet outside (but the valve itself inside so it
won't freeze) so that in case the valve fails open, it does not flood
2) use a pressure only valve, not a temperature/pressure valve
It is amazing to me that there are not more problmes casued by this
closed system / thermal expansion situation.
And I think you are right, expansion tanks with a bladder eventually
fail, and those without a bladder, eventually get water logged as the
internal air gets absorbed into the water.
I live where freezing is not a problem so the valve can be exposed.
It is where I can see it & discharges into a flower bed.
The relief valve is pressure only
I guess I was just being lazy & cheap since the relief valve was way
cheaper than the expansion tank & easy to install.
There was one on the market that dumped excess water/pressure into the
toilet. I don't know if it is still available.
My pressure reduction valve has a backflow valve on it, so if the pressure
downstream exceeds the pressure upstream you get water flowing backwards to
relieve the pressure. Of course, if you have a backflow prevention valve
that won't help you any.
My supply is 90psi and I have the valve set to 55psi. When I let the hw
tank cool off and then turn it up higher than normal, the pressure in my
house never exceeds 85psi. I figure that is the worst case senerio, so I
don't see I will ever need the backflow valve and certainly not an expansion
You might want to try testing it before doing anything drastic.
I just re-did my domestic water service entrance and all the interior
fresh water plumbing.
I installed a pressure reducing valve (that has back flow bypass
capability) but I installed a check valve upstream of it to protect
the municipal supply from any back flow.
The city supply is about 75 psi but I've seen it as high as 90 so I
have my PRV set to 60.
With the closed system I needed some way to handle the expansion
pressure......I opted for the relief valve because I didn't like the
rubber bladder idea (why I don't know)
The plumber working on this with me was ok with the relief valve
instead of the expansion tank.
I guess we'll see how this all works out.
If you have hard water eventually it will stick either open or shut.
open will waste water and leak, closed stress everything till th
HWtanks T&P valve opens.
here we NEVER touch the T&P valve since once disturbed they always
I was told expansion tanks should be replaced when you replace the HW
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