Water heater, check valve, accumulators revisited

[US1Fountain]

I'm replacing all my plumbing with ProPex and having a dilemma on the water heater configuration. Currently a standard system design; water pump, (non variable speed) followed by an accumulator, then splits to cold water supplies and water heater. It has the check valve right at the inlet to prevent hot water migrating back thru the cold lines. Have read the concerns about hot water expansion and the need for a expansion tank/accumulator between the check valve and the water heater inlet for safety. Can this 2nd accumulator be installed on the outlet side of the water heater in the hot water line circuit instead of in the inlet side? This would make my replumbing 100x easier.

Thanks

[Lepke]

Yes. There's no restriction or check valve between the inlet and outlet.

[Uncle Bob]

Originally Posted by US1Fountain

I'm replacing all my plumbing with ProPex and having a dilemma on the water heater configuration. Currently a standard system design; water pump, (non variable speed) followed by an accumulator, then splits to cold water supplies and water heater. It has the check valve right at the inlet to prevent hot water migrating back thru the cold lines. Have read the concerns about hot water expansion and the need for a expansion tank/accumulator between the check valve and the water heater inlet for safety. Can this 2nd accumulator be installed on the outlet side of the water heater in the hot water line circuit instead of in the inlet side? This would make my replumbing 100x easier.

Thanks

Hi I don't understand the need/desire to install an expansion tank at the heater. The heater should have a combination pressure/thermal relief valve built into the tank which prevents over pressure problems, with marine hot water systems the volume is so small any lost water is inconsequential in volume.

[a64pilot]

I have no accumulator, and have no issues, the T&P valve does not open.
Maybe there is enough line flex? Or maybe we just don’t go from full cold to full hot very often without using some water at some time.
There is no check valve in my water system, and hot water doesn’t mix with the cold, how could it?
My house had no accumulators either, except when we were on a well, and then the accumulators function was to keep the pump from short cycling, not to allow expansion for the heater.
I suppose that water doesn’t expand as much as we may think, and as has been stated there is a T&P valve that will protect the system from excess pressure if it were to occur.

[Tessellate]

I was working on our water heater just yesterday (a Seaward 6 gallon).

I don't see why an accumulator tank would be necessary at the water tank. I do think check valves have some purpose (in some water system setups) though. I had a check valve at the water heater cold inlet, and removed it for 2 weeks (it's leak prone, and I wanted to simplify the system).

We had no hot water migration issues that I could tell, but it did have one unexpected consequence - it made the water pump take longer to activate upon pressure drop when we open the faucets. We don't have an accumulator, so check valves may have some role in bisecting the water system for more convenient actuation of the water pump.

My guess is the way it works is when we open a water faucet, if we didn't have a check valve at the water heater, some water would flow downhill from the tank into the cold water line - and therefore the pump doesn't detect a pressure drop as quickly. By having that check valve we get faster actuation of the pump without requiring an accumulator tank. Now I just need to find a check valve that doesn't leak.

[Cadence]

Originally Posted by a64pilot

I have no accumulator, and have no issues, the T&P valve does not open.
Maybe there is enough line flex? Or maybe we just don’t go from full cold to full hot very often without using some water at some time.
There is no check valve in my water system, and hot water doesn’t mix with the cold, how could it?
My house had no accumulators either, except when we were on a well, and then the accumulators function was to keep the pump from short cycling, not to allow expansion for the heater.
I suppose that water doesn’t expand as much as we may think, and as has been stated there is a T&P valve that will protect the system from excess pressure if it were to occur.

If memory serves me, the only way to make water expand is to make ice. I would guess and expansion tank may serve the same as you described for a well pump cycling.

[Tessellate]

Originally Posted by Cadence

If memory serves me, the only way to make water expand is to make ice.

That's not true: http://lmgtfy.com/?q=volume+of+water...t+temperatures
Ice is simply a large expansion (and abrupt, like a step function).

[Cadence]

Originally Posted by Tessellate

That's not true: LMGTFY
Ice is simply a large expansion (and abrupt, like a step function).

I doubt we are speaking of those extremes in temps.? 100C to 20C in one. One was 360F which would require it being under pressure. Hardly practical. But thanks, I learned something.

[Mike Banks]

I used a Bosch LPG heater on my tub. Never had a problem with it--turn on the tap and it lit up evety time.

However, for those using storage type heaters (I am unfamiliar with the one in question) It makes sense to me to have a high pressure release valve in the system venting into a sump or over the side. One might even vent it back into the water tank, since it is the same water as came from it.

These are just a spring loaded valve--but even the smallest duration of expansion at of high pressure will slowly stretch the copper or alloy tanks or heat exchanger tubing over a period of time until ultimate leaking.

I would just fit a HP valve and take the outflow back to the water tank--or into a vessel such as a plastic jerry can, so any release can be monitored.

[US1Fountain]

From all of my researching, here and RV forums, it appears there is no set conditions where a check valve is needed. Those that complain of having a rush of hot or warm water coming from the cold side for a few seconds are always informed to install a check valve and usually respond that fixed it. Then there are just as many that have no check valve and no issues. If I do not reinstall the check valve, then no accumulator is needed in the hot side as the accumulator at the water pump will address any expansion. But to be honest, the suggestion to install a accumulator is only on this board. I never ran across that suggestion on the RV forums.
Granted the T&P relief valve will come into play if needed, I'm just not of the mindset to use a safety item as means of controlling the expansion as my experience with them at home, once they are opened, its a crab shoot that it will seal again.
I think I'll just leave the check valve out and forgo the tank, keep it simple.

Thanks

[a64pilot]

Water does expand when heated, that is of course why we have expansion tanks on our engines, but if you look, it doesn’t expand much, and the old way if you left just an inch of air in the radiator, it was all that was needed.
So there is expansion as water heats, but water lines do expand slightly, and if you ever open any water outlet whether hot or cold, it releases the excess pressure if any.
Water is not compressible of course so very little water would need to be released.
Ever notice that even when the pump is off, a significant amount of water will flow from the facet? Well since water isn’t compressible that pressure and flow has to come from line flex, or trapped air somewhere.
I suspect the lines swell if you will, more than we realize, but there are dozens of feet of line too, so it doesn’t take much.

[Brewgyver]

Originally Posted by Mike Banks

One might even vent it back into the water tank, since it is the same water as came from it.
(Snip)
These are just a spring loaded valve (snip)

They are definitely NOT just a spring loaded valve. They are called T&P (Temperature AND Pressure) valves,and they open if either limit is reached. Also, the output CANNOT be piped back into the water tank. That is a recipe for disaster: if the supply tank is full, and the T&P opens due to over-temp or over-pressure, it wouldn’t be able to relive the condition, possibly resulting in a catastrophic failure of the heater tank, i.e. an explosion.

[sailorchic34]

Generally a check valve is not needed in the inlet of the water heater for US heaters. Most marina water heaters are horizontal. Vertical water heaters have a dip tube to near the bottom of the tank.

Rather then a check valve, installing a u loop of pex (or hose) at the inlet, say 12-16" high will create a heat trap that will prevent hot water from migrating too far back the cold water side.

Another option is to drill a 1/8" hole in the check valve clapper. That was the method used to relieve thermal expansion up to about 2000. After that the codes changed and if a check valve was installed that a thermal expansion tank was required.

Note relying on a T&P relief for thermal expansion is generally not a safe thing to do with a cruising boat. It's actually against all current us plumbing codes. (not that it matters on a boat) First the T&P may not open (it happens) which sometimes does not end well. Second once opened the seat can be clogged from mineral buildup and it might not close all the way. That could in some cases dump all the water to the the bilge.

As to hot or cold side, it does not matter, either side of the water heater works just as well.

The issue of course is water is not compressible at normal pressures (<2000 psig). As water is heated say from 50 degrees to 120 degrees a 70 degree rise, 6 gallons would expand an additional 0.2 gallons. That water will go somewhere or something will break. It might leak back to an accumulator, but it will go somewhere.

[hellosailor]

I can see that an expansion tank "on" the water heater could be a good thing. Less pressure on the overflow valve, less pressure on the tank walls/seams as well, so at least in theory it has a purpose. On a small boat with less room for less stuff...maybe not worth doing.

But I've also been on a larger boat with a very generous hot water tank, where we lost 100 gallons of fresh water in one afternoon. The safety valve (temp & pressure) on the hot water tank failed and dumped 100 gallons of fresh water before it was caught! Not fun.

If putting in an expansion tank took some of the pressure off that valve and eliminated potential problems, it might be worth adding. Even if it meant "more stuff" and more potential failure nodes.

[boatpoker]

Originally Posted by Mike Banks

I used a Bosch LPG heater on my tub. Never had a problem with it--turn on the tap and it lit up evety time.

However, for those using storage type heaters (I am unfamiliar with the one in question) It makes sense to me to have a high pressure release valve in the system venting into a sump or over the side. One might even vent it back into the water tank, since it is the same water as came from it.

These are just a spring loaded valve--but even the smallest duration of expansion at of high pressure will slowly stretch the copper or alloy tanks or heat exchanger tubing over a period of time until ultimate leaking.

I would just fit a HP valve and take the outflow back to the water tank--or into a vessel such as a plastic jerry can, so any release can be monitored.

All marine storage type water heaters have a TP (temperature/pressure) valve installed from the factory.