Expansion tank vs relief valve

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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 heater.
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.
cheers Bob
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BobK207 wrote:

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 usual event.
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Joseph Meehan

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wrote:

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.
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replying to trader4, bd wrote:

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 and then.
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On Tuesday, August 16, 2016 at 8:14:04 PM UTC-4, bd wrote:

Do you even have a closed system? Most people don't because they are either connected to a municipal water system with no backflow valve or similar or a well that has a pressure tank.
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"bd" wrote in message
replying to trader4, bd wrote:

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 and then.
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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.
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wrote:

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
cheers Bob
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BobK207 wrote:

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.
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wrote:

Joseph-
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 cheaply.
cheers Bob
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125 psi is double the recommended pressure for residential use. Over 60 psi causes undue wear on fixtures.
Harry K
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Harry-
The only time the system pressure would reach (&stay at) the relief pressure level
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 readings.......
but it sounds like adding an expansion tank is the way to go.
cheers Bob
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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 open? MLD
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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 :)
cheers Bob
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
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(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. Cheers MLD
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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 inside
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.
Mark
3
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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.
cheers Bob
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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 tank. You might want to try testing it before doing anything drastic.
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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.
cheers Bob
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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 leak.
I was told expansion tanks should be replaced when you replace the HW tank.
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