Is an Expansion Tank Necessary on a Fresh Hotwater System?

[Oliver L.]

Goboating,

as is often the case, "Europe" is not a homogeneous country, but a continent. We here in Germany have regularly 5 to 6 bars city water pressure, so system design pressure is 10 bars and burst test pressure 20 bars.

Ranger,

you can calculate the active expansion volume quite easy - it is simply the higher of diaphragm pre-pressure or pump-on-pressure in relation to the pump-on-pressure times the volume of the tank. So in the example above with 20 psi on and 40 psi off, if you set the pre-pressure at 20, you will have half the volume of the tank acting as expansion volume. If you have the pre-pressure at 30, it will only be 1/4 of the tank. If the pre-pressure is above 40, the diaphragm won't move and you get zilch.

If the pre-pressure is lower, you waste some volume because the pump will come on before it gets into play. For example, if you have 10 psi pre-pressure, at the pump-on-pressure the tank will already be half full, so you get only 1/4 as active expansion volume.

And yet Jabsco is not far off - as long as you have any active volume, pump cycling will be greatly reduced, and it is better to err on the low side than on the high side.

Oliver

[sailorchic34]

Originally Posted by Oliver L.

Goboating,

as is often the case, "Europe" is not a homogeneous country, but a continent. We here in Germany have regularly 5 to 6 bars city water pressure, so system design pressure is 10 bars and burst test pressure 20 bars.

Just FYI, North American water heater standard is 100 psig (6.8 bar) Max. working pressure and 150 psig (10 bar) relief pressure. Burst pressure is not listed. This is for residential and light commercial water heaters. ASME code vessels can have working pressures to 200 psig (~13.6 bar) or higher but $$$.

Some US cities have water pressures to 10 bar, so pressure reducing valves are installed at the service entrance to reduce to 80 psig. 80 PSIG is the minimum rated working pressure for all plumbing fittings/faucets. Well on the pressure side anyway.

[goboatingnow]

as is often the case, "Europe" is not a homogeneous country, but a continent. We here in Germany have regularly 5 to 6 bars city water pressure, so system design pressure is 10 bars and burst test pressure 20 bars.

well off on a tangent here, I was talking about on premisses , post pressure reduction, I understood was always nominally 3 bar , with open vented hot water at around .5 bar combi systems around 2.5bar

dave

[ranger58sb]

Originally Posted by Oliver L.

Ranger,

you can calculate the active expansion volume quite easy - it is simply the higher of diaphragm pre-pressure or pump-on-pressure in relation to the pump-on-pressure times the volume of the tank. So in the example above with 20 psi on and 40 psi off, if you set the pre-pressure at 20, you will have half the volume of the tank acting as expansion volume. If you have the pre-pressure at 30, it will only be 1/4 of the tank. If the pre-pressure is above 40, the diaphragm won't move and you get zilch.

If the pre-pressure is lower, you waste some volume because the pump will come on before it gets into play. For example, if you have 10 psi pre-pressure, at the pump-on-pressure the tank will already be half full, so you get only 1/4 as active expansion volume.

And yet Jabsco is not far off - as long as you have any active volume, pump cycling will be greatly reduced, and it is better to err on the low side than on the high side.

Oliver

Very useful, thanks! -Chris

[chala]

Originally Posted by sailorchic34

Using the T&P relief for thermal expansion has been banned by the US plumbing codes for about 15 years. Reason being is that sometimes the T&P relief becomes defective and then the expanding water will find the weakest spot in the system and cause a leak.

Curious. What good is it in having “T&P relief “ that “becomes defective”? and what happen if there is no “weakest spot in the system”?
http://www.cruisersforum.com/forums/...+water+heaters.

[sailorchic34]

Originally Posted by chala

Curious. What good is it in having “T&P relief “ that “becomes defective”? and what happen if there is no “weakest spot in the system”?
http://www.cruisersforum.com/forums/...+water+heaters.

Oh good question!

Well luckily most of the defective T&P reliefs just start dripping from corroded/wire drawn seats. There have been a few cases where the T&P relief did not open, which was somewhat annoying.

Its actually recommended that the T&P relief be tested every 6 months or so, by pulling the lever for a second and seeing if it resets. That is almost never done BTW.

As to the weakest spot; With expanding water or anything taking on a higher energy state, which by the way is actually an increase in the energy state of the atoms in the H2O molecules. So the H2O molecules WILL expand, that is the molecules will each increase in size just a bit as the electrons take a slightly higher orbit and the molecules will move apart from each other.

So in a 6 gallon container, say for example a water heater, heated from say about 45 degrees to 180 degrees F, you will then have about 6.18 gallons of water in a 6 gallon container. If there is no where for the extra .18 gallons of water to expand to and the T&P relief did not open, then a seam on the water heater will fail or a pipe will burst. In other words somewhere, where ever the weakest part of the system is will spring a leak. Water is not compressible under normal pressures so that .18 gallons will find somewhere to go.

In the old days in buildings the expanding water would just back up into the cold water main and push a little water back to the street. Then in the 80's everybody started installing backflow preventers on the water service entrance which prevented the water from backing up into the street. That then required the use of thermal expansion tanks on water heaters as there was no where for the water to expand too. The same would apply if there is a pressure reducing valve on the service entrance or say on the water heater cold water supply

On a boat with a pressure pump the water would try to push back through the pump into the tank. The pump by design has check valved internal to the pump to prevent that. So the expanded water would then dribble from the T&P relief into the bilge. This will over time cause wire drawing of the T&P valve seat and it will start dripping all the time. Then it needs to be replaced.

Adding a thermal expansion tank prevent the dripping and wire drawing from happening..

[chala]

Originally Posted by sailorchic34

Well luckily most of the defective T&P reliefs just start dripping from corroded/wire drawn seats. There have been a few cases where the T&P relief did not open, which was somewhat annoying.

I suppose the annoyance will be no more than

Originally Posted by sailorchic34

If there is no where for the extra .18 gallons of water to expand to and the T&P relief did not open, then a seam on the water heater will fail or a pipe will burst. In other words somewhere, where ever the weakest part of the system is will spring a leak.

Damage the plumbing installation and at the same time may spray someone with hot water.

Originally Posted by sailorchic34

Its actually recommended that the T&P relief be tested every 6 months or so, by pulling the lever for a second and seeing if it resets. That is almost never done BTW.

More important the test is to see if the valve will open. In fact the T&P relief valve are to some extend self-testing as per your example.

Originally Posted by sailorchic34

If there is no where for the extra .18 gallons of water to expand to

Then the valve should operate and be tested.

Originally Posted by sailorchic34

So in a 6 gallon container, say for example a water heater, heated from say about 45 degrees to 180 degrees F, you will then have about 6.18 gallons of water in a 6 gallon container.

45 degrees (7.22) to 180 degrees F (82.22) is a little extreme. It would be silly to draw water at 45 degrees from a hot water faucet and also 180 degrees F is unnecessary unless you which to heat up the space in which the water heater reside. So if the water heater is properly sized for its use it is unlikely that the valve will see much discharge of water.

Originally Posted by sailorchic34

In the old days in buildings the expanding water would just back up into the cold water main and push a little water back to the street. Then in the 80's everybody started installing backflow preventers on the water service entrance which prevented the water from backing up into the street. That then required the use of thermal expansion tanks on water heaters as there was no where for the water to expand too.

I have seen large electrical hot water systems build in the 1920 that where equipped with non-return valve and steel expansion tank with pressure relief valve. This was more to prevent the unit from emptying into a break in the main water supply and to protect the units from running dry and so burning the heating resistances. I would say that T&P relief valves did exist for smaller units as earlier than 1930. Normally the water freely expands `drain` to waste.

Originally Posted by sailorchic34

On a boat with a pressure pump the water would try to push back through the pump into the tank. The pump by design has check valved internal to the pump to prevent that. So the expanded water would then dribble from the T&P relief into the bilge. This will over time cause wire drawing of the T&P valve seat and it will start dripping all the time. Then it needs to be replaced.

I have seen T&P relief valves as old than 40 years and still working fine in contrast accumulators are fare more troublesome. Also a boat pressure pump rating should match the T&P valve rating or the pump will become the weakest part of the system.
In a boat where water is precious the discharged water can be collected and not sent to the bilge.

Originally Posted by sailorchic34

Adding a thermal expansion tank prevent the dripping and wire drawing from happening.

Quality T&P relief valves should not drip and are essential but definitely adding a thermal expansion tank will make the plumber richer.

[sailorchic34]

Originally Posted by chala

More important the test is to see if the valve will open. In fact the T&P relief valve are to some extend self-testing as per your example.

Then the valve should operate and be tested.

Yes the valve will open, but like any valve that is just barely lifted off it seat time and again the seat will erode / wire draw till the valve will no longer seal when closed and drip. I just replaced an 18 year old T&P relief valve that was doing a bit more then dripping.

As to testing the T&P valve, yes it should be tested, but I know of no one that actually does that.

Originally Posted by chala

45 degrees (7.22) to 180 degrees F (82.22) is a little extreme.

Oh agreed it high, but on a boat with engine heating its very common. Least wise in the winter. Last few boats I was on were set up that way. With fresh water cooled diesel or gas engines with thermostats fully open at 180 degrees F, it is very possible to have 180 degree hot water stored. Even if you have a temperature regulator on the water heater to shut off the engine water, the regulator can fail.

So its not at all uncommon. But even if the water only expands just a little, its enough to cause damage.

Originally Posted by chala

I have seen large electrical hot water systems build in the 1920 that where equipped with non-return valve and steel expansion tank with pressure relief valve.

Oh it was quite common to install a check valve / non-return valve in hot water sytems, well commercial and larger systems, before oh 1990 even in the US. I've designed thousands of systems from 40 gallons to large systems producing 200 gallons a minute, with check valves on the inlet. Still common where you have a hot water circulator to maintain loop temp. And like you indicated the 1920 system had a expansion tank too. Good engineering that.

But on the typical CF boat a check valve on the hot water inlet is not required (unless its a British systems or similar). If there is a check valve then there should be a expansion tank installed too.

Originally Posted by chala

Also a boat pressure pump rating should match the T&P valve rating or the pump will become the weakest part of the system.
In a boat where water is precious the discharged water can be collected and not sent to the bilge.

Well the pump body test pressure may meet the T&P relief pressure setting, but the rubber diaphragm in the typical 12V pressure pump probably will have issues with the higher pressure.

Thing is plumbing devices, faucets, pressure switches, ballcock valves, etc. using in plumbing systems are only designed for about 80 psig, in the US anyway. Oh the pipe and fittings are designed for a much higher burst pressure.

But the cheap plastic valve faucets seen in the faucet displays at the big bog stores are only rated to 80 psig, for example. However the standard T&P relief pressure in the US is 150 psig. I think the British have a far more sensible T&P pressure rating then the US.

Installing a $40 thermal expansion tank in a boat is not going to make a plumber rich. It will however prevent water being wasted via the T&P relief.

[jeremiason]

Ranger

I would be interested if any of the smarter people in here have done studies on how much water you lose to a pressure relief valve, but my guess is it is minimal.

In my opinion, the pressure relief valves don't drip enough water to worry about it or spend the money/time on an expansion tank.

ABYC Standards require a pressure relief valve on all Hot Water Heaters and that it be discharged over board, not to the bilge.

Mine is factory set up with a hose running from the pressure release valve to a above the water line thru hull, which is located just below the height of the water heater. It has worked great for almost seven years.

[sailorchic34]

Originally Posted by jeremiason

Ranger

I would be interested if any of the smarter people in here have done studies on how much water you lose to a pressure relief valve, but my guess is it is minimal.

Nice one...

It depends on the system. At 120 degree F, the typical 6 gallons in a marine water heater will expand about 4 ounces or ~0.06 gallons.

If your heating with a fresh water cooled engine and have a thermostatic valve on the hydronic side of the water heater, the water will expand, based on 140 degrees F, about 6 ounces (0.09 gallons)

If you have engine water directly connected to the water heater then the expansion, based on 180 degrees F could be 9 ounces (0.15 gallons)
So it depends quite a lot on the storage temperature.

The T&P relief generally will not open up at all if the boats pressure water system has an accumulator /expansion tank in the system, as was required with the older on /off pressure water pump.

The issue with the OP was his new system which includes a variable speed pump, where the accumulator is no longer required.

Now silly me with my 30+ years of mechanical engineering in building systems mind you, my PE and my certification in plumbing engineering from ASPE, that I got way back in 1983 which was the first year they had that test, I somehow believe that with the new variable speed pump that it might be a good thing to install a expansion tank. Lordy I must be a fool to believe that..

But its OK. I have credibility issues for sure.. I've even had other engineers tell me in owner meetings that I did not know what I was taking about and their system would work fine. As it turned out, I WAS exactly right then and the facilities director lost his job over that one.

Anyway, I believe that the ABYC standard predates the introduction of variable speed pressure pumps and so it is a little bit out of date with new equipment. Be interesting to see if the ABYC updates their standards in the next few years.

So I stand by my believe that if your installing a new system with a variable speed pressure pump that does not require an accumulator / expansion tank, that a thermal expansion tank should still be install anyway. Why waste water?

[Delfin]

Originally Posted by Jon Neely

thank you for the detailed response, answered all my questions.


Jon

Yes, he answered your questions, but some of the answers are highly debatable. An expansion tank on a hot water system is desirable because hot water expands, which is why they are so commonly installed. No, the tanks don't rot out because they are usually lined with polyethylene or equivalent. Yes, the pressure relief valve will relieve the pressure, but it is a nuisance to have hot water dribbling into the bilge, on your deck or wherever it goes. A cheap and simple to install expansion tank eliminates that problem. No, you don't need a separate expansion tank on the cold water side if you use VSD pumps. Mine are 5 years old and there is no surging, just a steady stream of water at the rated flow. I put two on to increase that flow and the system surges a lot less than the PAR diaphragm pump with expansion tank on a prior boat. Besides, the expansion tank downstream from the hot water provides an expansion buffer for the cold water anyway, because the hot and cold are all connected upstream from the water heater, heat exchanger, etc.

[Delfin]

Originally Posted by jeremiason

Ranger



Mine is factory set up with a hose running from the pressure release valve to a above the water line thru hull, which is located just below the height of the water heater. It has worked great for almost seven years.

You have that thru hull because it is possible for the relief valve to get stuck open, so builders have to route all that water someplace. If you leave your boat with the water pump on that amount of water can equal all ya got, so its not a trivial problem.

That still leaves open the question of whether it makes sense to have the relief valve dribbling hot water over the side every time it heats up or whether adding a $100.00 expansion tank that eliminates the issue is appropriate.

[goboatingnow]

Mins dribbles , yet I have a cold water accumulator on this boat !

Check valves on the hot water side are not common in British systems , thermostatic valves on the engine feed are as are the mixing valves on the cylinders.


My PRV empties into the bilge ! ( mind you so does my shower !)
Dave

[Delfin]

Originally Posted by sailorchic34

Then in the 80's everybody started installing backflow preventers on the water service entrance which prevented the water from backing up into the street. That then required the use of thermal expansion tanks on water heaters as there was no where for the water to expand too.

Agree with everything you wrote, except this. Back flow prevention devices simply dump the water on the ground if back pressure occurs. So they don't prevent backflow, except into the city water system, instead diverting the backflow onto the ground. I've had to install and maintain quite a few of them over the years because of downstream fertilizer injection systems....

[ranger58sb]

Originally Posted by jeremiason

Ranger

I would be interested if any of the smarter people in here have done studies on how much water you lose to a pressure relief valve, but my guess is it is minimal.

In my opinion, the pressure relief valves don't drip enough water to worry about it or spend the money/time on an expansion tank.

ABYC Standards require a pressure relief valve on all Hot Water Heaters and that it be discharged over board, not to the bilge.

Mine is factory set up with a hose running from the pressure release valve to a above the water line thru hull, which is located just below the height of the water heater. It has worked great for almost seven years.

Heh... I certainly haven't. As I said earlier, I can see evidence that ours has dripped at least once and that's not ringing my chimes in the worry department.

I happened to have checked our PR valve the other day, and it seems to be working. It's not plumbed overboard, in a boat that otherwise seems to be built to applicable standards...

In any case, a drip here or there doesn't worry me too much

-Chris