Is an Expansion Tank Necessary on a Fresh Hotwater System?

[sailorchic34]

From an engineering standpoint an accumulator is called an expansion tank. The design and function is the same. If an "accumulator" is installed on a pressure water system that includes a water heater/ clorifier, the "accumulator" also works as an expansion tank and will accept the expanding water from the heater.

In engineering speak there is no such thing as a accumulator, its always called an expansion tank. I call it a accumulator here as that is what many here call it.

By expansion tank, I mean a bladder style expansion tank. In olden days expansion tanks were just tanks with an air bubble in the top 1/2. But as Oliver L. indicated quite correctly the air is over time absorbed back into the water and the tank would get water logged and need to be drained.

While the manufacturers of VSD pumps are correct that you don't need a accumulator/expansion tank after the pump, the typical 6 gallon water heater will expand roughly 1/4 gallon (~.8 liter) of water. This is with engine heating. That water needs somewhere to go.

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.

Prudent design requires a small expansion tank somewhere in all pressurized water systems to allow for thermal expansion at the water heater / clorifier.

If you have an accumulator /expansion tank in your system because of the on/off style pump, then it also functions as a thermal expansion tank for the water heater.

Though the jabsco accumulator is not quite larger enough to handle the thermal expansion caused by engine heating. That is why I, with my 30+ years designing plumbing systems among many other things, recommend the small thermal expansion tanks at the big box stores. They are larger the smallest being about a gallon (4 liter) size and meet the plumbing code at least in the US. Because it does not have marine in the name it is also less expensive.

When adding an expansion tank to a system using a VSD pump, it is best to install a check valve after the pump. This will keep the VSD pump from hunting quite so much. I've done this on 1200 GPM booster pumps.

Myself I'm not quite a fan of the small VSD pumps now available for use on boats. They generally will use a bit more power then a standard on/off pump. This as the impeller is less efficient at lower speeds and due to parasitic electric use in the control circuit. On land side buildings it was quite common to use on/off pump on systems under 300 GPM as the power usage was overall lower.

I don't think the energy used in the small VSD pumps are that much more but it is more. Just a bit. Probably not enough to worry about unless your an engineer.

[Oliver L.]

Sailorchick,

thank god, a voice of reason! The only thing you've got to look up is the thermal expansion coefficient: If I heat water from 20 C (mid 60F) to 60 C (150 F), I am looking at just over 1% of expansion. Therefore 6 gal (25 l) should expand by about 0.25 l. I believe the thermostat in the Diesel won't let it get much hotter than that.

For any pump, the check valve needs to be between the pump and the tank, i.e. on the suction side - otherwise the pump would not stop cycling; typically they all have the pressure sensor built into the pump.

Smooth sailing, and warm showers...

Oliver

[ranger58sb]

Originally Posted by sailorchic34

If you have an accumulator /expansion tank in your system because of the on/off style pump, then it also functions as a thermal expansion tank for the water heater.

Though the jabsco accumulator is not quite larger enough to handle the thermal expansion caused by engine heating.
...

Hmmm... interesting point.

I didn't realize hot water could back-flow out of the HWH tank to effect pressure at the "accumulator" tank (so called, here, simply 'cause that's what Jabsco names it)... and I assume that in turn would effect operation of the pressure switch on the pump itself.

BTW, I simply quoted Atwood's manual (above) -- with no intention of defending their comments

-Chris

[sailorchic34]

Originally Posted by Oliver L.

Sailorchick,

thank god, a voice of reason! The only thing you've got to look up is the thermal expansion coefficient: If I heat water from 20 C (mid 60F) to 60 C (150 F), I am looking at just over 1% of expansion. Therefore 6 gal (25 l) should expand by about 0.25 l. I believe the thermostat in the Diesel won't let it get much hotter than that.

For any pump, the check valve needs to be between the pump and the tank, i.e. on the suction side - otherwise the pump would not stop cycling; typically they all have the pressure sensor built into the pump.

Smooth sailing, and warm showers...

Oliver

Most of the diesels I've seen are 180 degree F. Though 170-175 with heat exchanger approach temp is probably about right. I'm just conservative.

I was going off the top of my head as I didn't have the coefficients handy at that time, and was using 45 degree F (7C) to 180F(82C) or roughly 75c rise. I get about 2.5% (.15 gallon) expansion or a bit more then a pint. So I was a bit high before.

While some heaters have a safety to prevent overheating from engine water, some don't have them. Plus some, possibly engineers and possibly blonde, tend to remove them, going for more thermal energy storage.

The small DC pumps have a build in check valve so no check valve is required before the pump.

Putting a check valve after the pump discharge, which is after the pressure sensor on a VSD pump, will help a little to prevent hunting by the speed controller built into the pump. It will work ok without the extra check valve, btw.

The VSD pump marketing guys were getting rid of the expansion tank/ accumulator to partly justify the more expensive VSD pump. Which is true. But that same tank also handled thermal expansion, and that part is still needed. Properly sized a standard on off pump with a tank works just about as well, costs less and also does double duty handling thermal expansion.

[sailorchic34]

Originally Posted by ranger42c

Hmmm... interesting point.

I didn't realize hot water could back-flow out of the HWH tank to effect pressure at the "accumulator" tank (so called, here, simply 'cause that's what Jabsco names it)... and I assume that in turn would effect operation of the pressure switch on the pump itself.

BTW, I simply quoted Atwood's manual (above) -- with no intention of defending their comments

-Chris

Yes there is a check valve in the water pump so everything downstream is one pressure zone. So when the water expands in the heater, it pressurizes the entire system. It does delay the pump switch, but only till the system pressure drops to the normal cut in point of the pressure switch.

If you added a pressure gauge you would see the pressure rise a bit as water fills the expansion tank(assuming no flow conditions).

What happens is the expanded water backs up into the accumulator / expansion tank. With the Jabsco that will just about fill the little tank and pressure will rise some from that.

Without an expansion tank the pressure will rise until the pressure relief setting on the water heater T&P relief is reached and then the T&P will pop off and burp some water out. Depending on the country the water heater was purchased in that pressure relief setting could be as high as 125-150 psig (in usa) before the relief pops.

[sailorchic34]

Originally Posted by JulianGullsway

I thought that it was essential to have an expansion tank if you required a reasonable pressure , I have just installed our hot/ cold pressurised system aboard, however I think I need a larger expansion tank on the hot water side and I am thinking of adding one to the cold water side as well. On the Hot water side I have a 35ltr Calorifier and a 5ltr expansion tank but the pressure drops off quickly, the cold pressure is good but only for a short period, any ideas!!!!

Unless there is a check valve at the water heater inlet, and normally there isn't, then you only need one expansion tank, located on the cold water side. If the pressure drops off quickly it could be that the expansion tank is water logged. Even bladder expansion tanks fail. Once the expansion tank gets water logged then the pressure would drop off very quickly.

If there is a valve (Schrader type fill valve) at the top of the expansion tank, Turn off the water pump, open a cold water faucet, connect a bicycle pump to the fill valve and pump for a bit. If the tire pump has a gauge see if you can pressurize the expansion tank bladder to 1.5 bar (about 20 psig).If it does not come up to pressure then either you have a tear in the bladder or you have an older type expansion tank without a bladder.

In either case remove the pump, close the faucet and turn the pump on. Once the pump comes up to pressure and shuts off, see if you have more pressure before the pump kicks on.

[JulianGullsway]

Thanks sailor chic I'll check that the weekend and report back 😊

[ranger58sb]

Originally Posted by sailorchic34

If there is a valve (Schrader type fill valve) at the top of the expansion tank, Turn off the water pump, open a cold water faucet, connect a bicycle pump to the fill valve and pump for a bit. If the tire pump has a gauge see if you can pressurize the expansion tank bladder to 1.5 bar (about 20 psig).If it does not come up to pressure then either you have a tear in the bladder or you have an older type expansion tank without a bladder.

In either case remove the pump, close the faucet and turn the pump on. Once the pump comes up to pressure and shuts off, see if you have more pressure before the pump kicks on.

It happens I was just messing with our "accumulator tank" last weekend, since our pump was cycling faster than I thought it should. Your description is almost word for word like the Jabsco Manual... although it also says best results should happen at 10 psi. My tire pressure gauge didn't show much of any air pressure at first, and it won't measure below about 20 psi anyway, so after adding air I've now got somewhere between 10-20 psi in there. While overall water pressure didn't significantly increase, cycle times evened out a bit better, and that's actually what I was working toward.

I've been beginning to think that maybe air pressure in that tank could become a Spring maintenance issue, after winter temps have affected previous pressure levels. Or else maybe I'm beginning to see the effects of a slow leak in the bladder

-Chris

[sailorchic34]

Originally Posted by ranger42c

It happens I was just messing with our "accumulator tank" last weekend, since our pump was cycling faster than I thought it should. Your description is almost word for word like the Jabsco Manual... although it also says best results should happen at 10 psi. My tire pressure gauge won't measure below about 20 psi, so I've got somewhere between 10-20 psi in there now, and while pressure didn't significantly increase, cycle times evened out a bit better, and that's actually what I was working toward.

I've been beginning to think that maybe air pressure in that tank could become a Spring maintenance issue, after winter temps have affected previous pressure levels. Or else maybe I'm beginning to see the effects of a slow leak in the bladder

-Chris

After you recharge the expansion tank, if it starts short cycling after a few weeks, there is a tear in the bladder. Happened to me, twice.

I like to set the bladder tanks to about 40% of system pressure. So if static pressure is 40 psig then about 16-18 psig will work most of the time. If you have a 30 psig system then 10-15 psi might be good. 60 psig set it a little higher. Luckily it does not need to be exact and close is good enough.

[goboatingnow]

In european marine hot water systems , I have yet to see a specific expansion tank , but accumalator tanks are common so they are providing both functions, Ive also seen as quite common , system where burping of teh pressure relief effectively provided the pressure relief.

I wasnt suggesting best practice, merely an observation

heres a good guide http://www.jabscoshop.com/itemimages...on%20Guide.pdf

including expansion tank computations

dave

[sailorchic34]

Originally Posted by goboatingnow

In european marine hot water systems , I have yet to see a specific expansion tank , but accumalator tanks are common so they are providing both functions, Ive also seen as quite common , system where burping of teh pressure relief effectively provided the pressure relief.

I wasnt suggesting best practice, merely an observation

heres a good guide http://www.jabscoshop.com/itemimages...on%20Guide.pdf

including expansion tank computations

dave

Oh, I love the copper tank of the C-warm heater. That should last a long time. There you have a separate expansion tank for the heater. This is because the heater is only rated a 44 psig and the relief pops at 44 psig (3 bar).

Where there is a pressure reducing valve or check valve on the inlet of the C-warm heater then you will of course need a separate expansion tank for the heater. If you only have a pump that generates 2.5 bar pressure then the check valve and pressure reducing valve (PRV) is not needed.

The check valve is there to prevent hot water from mixing with the cold side. But its not normally that big a problem on boats. Plus the check valves after some time tend to stick open a little.

BTW I'm aware that the EU and England have rather different standards then the US. Specially where water heaters/ clorifiers are concerned. A biggy is the tanks are designed to different pressure standards. If your cruising the US and need to repair a EU or English water heater / clorifier, be advised that the T&P relief valves available in North America, will most likely have a higher pressure rating then your tank is rated for. Just FYI.

[goboatingnow]

Primary because in Europe domestic system do not exceed 3 bar pressure, and has carried over to marine systems

[goboatingnow]

Oh, I love the copper tank of the C-warm heater. That should last a long time.

Again a carry over from domestic practice. A 'copper cylinder' in every home

dave

[Dennis.G]

SailorChic you rock.

If you have an "accumulator" tank, and no check valve between cold and hot systems, that tank acts as your expansion tank for hot water expansion. That's all I have in my system and works great.

Photo below might help in understanding expansion (aka accumulator) tank (cutaway tank on right). The little fitting on top is a Schrader valve where you set pressure with a tire pump.

[ranger58sb]

Originally Posted by sailorchic34

After you recharge the expansion tank, if it starts short cycling after a few weeks, there is a tear in the bladder. Happened to me, twice.

I like to set the bladder tanks to about 40% of system pressure. So if static pressure is 40 psig then about 16-18 psig will work most of the time. If you have a 30 psig system then 10-15 psi might be good. 60 psig set it a little higher. Luckily it does not need to be exact and close is good enough.

Thanks for the tip; I'll watch for that.

The water pump cut-on is 20 psi and cut-off is 40 psi, so I had thought setting the accumulator air pressure to about 35 psi would be nifty... NOT. The pump cycled very quickly! Once I reduced accumulator air pressure to something less -- enough to move the gauge, but less than 20 psi, so I'm guessing between 10-20 somewhere -- the pump cycle calmed down.

And in the meantime, the Jabso material says 10 psi is likely optimum... even though they make scant mention of pump cut-in/cut-off...

At that point, I guessed the accumulator air pressure should be lower than the pump cut-in pressure. If so, how much less, I haven't a clue.

-Chris