Is an Expansion Tank Necessary on a Fresh Hotwater System?

[Deep6]

Hello everyone, I've replumbed my entire boat now using PEX and sharkbite fittings. I have fitted a new isotemp basic 30 hotwater tank, and a new whale ic pump. Looking at the diagram on the pump box it doesn't show an expansion tank, and yet the manual calls for one on the hotwater side. I am also looking at other pages around plumbing and they're saying that a hotwater tank can in of itself act as an expansion tank by having an air pocket at the top. This has left me rather confused, as to whether I do, or do not need one.

Can anyone clear up this mystery?

Thanks,
Jayson

[Lancerbye]

I have a 10 Gal Isotemp and a 6 gal as a spare, neither have ever had expansion tanks. Expansion tanks are not normally found in hot water systems, but are used in boiler or heater applications where water has to be air free, moving and runs at a much higher temp. Similar to the cooling system in your car. You do need an expansion tank for the heating water coming from your engine that goes through the heating coil inside the hot water tank.

[sailorchic34]

Actually for the last, oh, 15 years, thermo-expansion tanks have been code required on all land side water heaters in the USA. While not required for marine systems, it's a good idea to give the expanding water some place to go. A air bubble in a hot water tank is not going to work for long.

VSD pumps generally don't need a expansion tank. But if adding one to the system, I would install a check valve after the pump and then tee in the expansion tank.

The check valve will help keep the expansion tank from effecting the VSD on the water pump.

BTW the small expansion tanks at the Big box stores are cheaper then the marine accumulator (Fancy name for expansion / bladder tank)

[DavefromNZ]

I fitted a new hot water tank and considered an expansion tank unnecessary as I had one upstream at the cold water pump.

A loud bang and the end of the hot water tank bulged out. I had to replace the new tank and fit an expansion tank.

[Jon Neely]

Sorry for not knowing more how expansion tanks work but I'm guessing the tank can flex more than the SS tank in our hot water heaters? all the accumulator tank does is fill with water correct? Having never installed one, not knowing the potential issues that my new $700 Quick water heater might have without one. Now that I think about it, I do think it mentions one in the manual.... Ill have to check on that. Sounds like it must be on the hot end, downstream of the heater. Do we not hear about these being used maybe do to lots of boat (mine included) use braided PVC hose that flexes?

[Oliver L.]

ok, a bit of reason in here...

First, the function: Your water pump is controlled by a pressure switch with a certain hysteresis. For example, if the pressure dips below 1 bar it switches on and switches back off when reaching 1.5 bar (or on at 15 psi and off at 22). Water is an imcompressible medium. This means that if you don't have an expansion tank, your pump will come on instantly when you open a tap, and will cycle in very short bursts, which is not good for the pump and not good for peace and quiet on board.

The expansion tank is in itself also incompressible, i.e. it does not "expand", as it's made of steel. But it contains a bladder filled with nitrogen gas, which can be compressed, and thereby it can act as a "pressure buffer". Two things you have to watch for: Do NOT use an expansion tank for heating systems - they are less expensive, since they are just made of steel, but they will rust and you will have rust in your coffee. Second, the bladder inside is filled with a certain pressure, which in most cases is too high for boating systems. City water pressure can be as high as 6 bar or 85 psi, while boats usually operate in the 1.5 bar region (22 psi). If your bladder is pre-pressured at 50 psi, it will be as incompressible as steel at the lower pressures. The solution: There is an air valve at one end of the tank (just like on a car tire) where you can adjust the pre pressure. Switch off the pump, open a faucet to drain all pressure and then use a bicycle pump with a gauge to set the pressure in the bladder to the minimum pressure at which your pump will switch on. This will give you the best compensation volume.

Now to "exploding tanks": First, no water tank is built to use air as "expansion volume"; this can't work since air can get dissolved in water and there would be no way to guarantee that there is a defined amount of air in there. Also, the tanks are made for hot water, and if you fill a part with air, you lose that volume of hot water. But all hot water systems need to have a safety valve. On boats this is typically set at 2.5 bar (35 psi) and combined with a high temperature limit. If your water tank bursts before the valve pops and floods your bilge with hot water, the tank was seriously corroded to begin with.

Lastly VSD pumps: Yes, in theory and according to the manufacturers they can operate without an expansion tank. But it does not hurt them either - they will just run a little less hectic.

Have fun with the plumbing - it was one of the first jobs I did on my boat to replace leaking safety valves and shitty expansion tanks...

Oliver

[Jon Neely]

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


Jon

[ranger58sb]

Out Atwood EHM11 hot water heater has an onboard pressure relief valve, and also incorporates an internal air gap. This from the manual:

A Pressure Temperature Relief Valve, dripping while the water heater is
running, DOES NOT mean it is defective. During normal expansion of
water, as it is heated in the closed water system of a recreation
vehicle, the Pressure Temperature Relief Valve will sometimes drip. The
Atwood water heater tank is designed with an internal air gap at the
top of the tank to reduce the possibility of dripping. In time, the

expanding water will absorb this air and it must be restored.

That's followed by instruction on how to "replace the air gap."

All that is completely separate from the accumulator tank in the freshwater line, just after the freshwater pump.

-Chris

[goboatingnow]

I think there is some confusion of terms. Most are talking about accumulator tanks as opposed to expansion tanks.

Accumulator tanks are filled with air ( from a Schraeder valve ) and may have a rubber bladder. Their purpose is to even out the pulses from a diaphragm pump. It's a nice to have feature. ( the system will work fine without it , even though the pump will cycle a bit more )

Expansion tanks are to allow an incompressible medium room to expand as it heats up.

Neither are necessary in marine hot water leisure systems. There is enough give in the plastic piping and the calorifier has a pressure release anyway. ( and water temps are lower then heating systems. )

VSD pumps in my experience ( having fitted three ) don't benefit from having accumulators the whole point of VSD ( which I really like by the way ) is that the pump runs continuously under variable demand

Dave

[Oliver L.]

Chris,

that's my point. Without an expansion tank the valve will drip; with one at proper pressure it should simply not happen.

Dave,

I disagree with your statement that in the marine leisure industry you don't need them. Yes, the systems will work, but the pump will cycle a lot and the bilge will receive a fresh influx of precious potable water every time the heater works. In my mind that makes a less-than-optimal system...

As to the plastic hoses, you can forget them. PEX pipes which are the most widely used have virtually no give under pressure. Otherwise the safety valve would not drip...

Oliver

[Dennis.G]

Originally Posted by goboatingnow

... and water temps are lower then heating systems.

Dave
Agree with most all said, but know that in my system when heating from engine, the water gets hotter than normal domestic HW system. In fact a serious scald hazard. I make sure to warm newbies before they use shower.

I don't have any expansion tank btw. Or tempering valves.

( if you were referring to hydronic heating system, agree they are hotter than domestic water systems)

[ranger58sb]

Originally Posted by Oliver L.

Chris,

that's my point. Without an expansion tank the valve will drip; with one at proper pressure it should simply not happen.

Dave,

I disagree with your statement that in the marine leisure industry you don't need them. Yes, the systems will work, but the pump will cycle a lot and the bilge will receive a fresh influx of precious potable water every time the heater works. In my mind that makes a less-than-optimal system...

As to the plastic hoses, you can forget them. PEX pipes which are the most widely used have virtually no give under pressure. Otherwise the safety valve would not drip...

Oliver

And from earlier: First, the function: Your water pump is controlled by a pressure switch with a certain hysteresis. For example, if the pressure dips below 1 bar it switches on and switches back off when reaching 1.5 bar (or on at 15 psi and off at 22). Water is an imcompressible medium. This means that if you don't have an expansion tank, your pump will come on instantly when you open a tap, and will cycle in very short bursts, which is not good for the pump and not good for peace and quiet on board.

Yes, ours dripped once, last year sometime.

I mean it happens only very rarely.

Our accumulator tank keeps the pump from cycling on/off quickly.

Perhaps the "air gap" Atwood mentions (along with the pressure relief valve) is at play, too... in essence, acting much the same as an expansion tank would, until the "gap" gets filled up with water over time and needs to be replenished with air.

-Chris

[Bash]

Originally Posted by Dennis.G

I make sure to warm newbies before they use shower.

I'm trying to visualize how you warm your newbies. Or was that one of the most fabulous CF typos of all time? Please tell me you don't merely warn your newbies.

[redsky49]

Hmm... Some misinformation being given out here.

First, for (at least) US built boats, there should be a T&P valve (temperature and pressure relief valve) on the water heater that will open in the event of excessive internal pressure. This generally occurs, not from excessive water pressure from the supply pump, but from either an internal fault with the electric heating element, the engine coolant temperature or from trying to heat a mostly empty vessel, thus creating steam. This valve should occasionally be exercised to ensure that it functions properly when needed, and should be located so as to not create problems in the event of a discharge.

As to the expansion of water, the degree of expansion can be calculated based on the delta T, but in most cases any expansion will be accommodated by the water "expanding" back into the cold water storage tanks. The exception to this is if there is a check valve or similar acting device located in the system which prevents this. This is where the water supply pump comes into the equation. [Note: It was the enhanced requirements for backflow protection on domestic water systems in the US which have made the expansion tank prevalent in many homes and commercial installations. This varies per locality and Plumbing Codes.]

If the pump supplier calls for backflow protection, or the installation of an expansion tank, then such an item should be installed, and installed correctly. Hot water heaters do not have a "pocket of air" (other than the initial fill or following drainage or some other occurrence) to allow for expansion.

An accumulator tank is to address pumping issues. An expansion tank is to handle the, well... expansion of water. Two completely different issues, and two completely different devices.

As for piping and/or tubing expanding when heated, it certainly does, the data is published and easily accessible, though for this discussion it is mostly irrelevant.

An "air gap" was mentioned above, as well as "dripping" of the relief valve. As a rule, there should not be any dripping of this valve as a normal operation. There should be an indicator on the valve to show its opening set point. This may be anywhere from 75 psi to 175 psi for typical devices, and should be appropriate for your particular installation. Higher setpoints may eliminate dripping, but expose the piping system to pressures it cannot handle as well as the possibility that the storage tank may exceed its pressure capacities. Then things will indeed go "BOOM".

My sense is that pressure levels of marine water systems should be held as low as possible, 20-25 psi or less. This helps to conserve water and protects the system against excessive pressure. Domestic water systems at home have differing requirements and would likely have higher pressure requirements. The 85 psi domestic water pressure mentioned above would be a likely candidate for a pressure regulating/reducing valve.

The best advice: Read the directions!

[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!!!!