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The result is interesting because it shows that with most pumps, which are set at about 30psi (gauge) cut-off, the PRV may still just blow if the water expands by its maximum. That's why I always turn my pump cut-off pressure down to about 27psi.

The pressure inside the vessel will therefore rise to 45 x 3.3 = P2 x 2.4 = 62 psia = 47 psig which will cause the PRV to open (assuming it's a normal 42psig type).

Of course. That's why I said above, viz: "using one tank to try to do both jobs also means that one cannot fit an NRV between the calorifier and the cold water supply"

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Only joking (half) but it does semm that Chris W was working out the pressure settings rather than the volume? I need to order one in the next couple of days as it one of my next jobs.

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So if we assume that there is 2 litres of expanded water and an 8 litre expansion vessel, the volume of air after expansion will be 6 litres.

Using one tank to try to do both jobs also means that one cannot fit an NRV between the calorifier and the cold water supply. This can lead to warm water coming out of the cold tap in the galley, as well as a possible issue with bacterial growth in the warm water. I know nothing technically about the bacteria but it does come up every time.

I'm not exactly clear what you mean......... water is incompressible, so having an extra length section won't accomodate any expansion.

The bottom line is that allowing for tolerances, your PRV is not guaranteed NOT to open even when heating from 20 to 60 degC. It probably will open. From 10 to 70 degC it will definitely open. A single vessel to do both jobs is a compromise and will never be perfect. To optimise it (it can never be perfect) requires careful measurements of all the pump's pressures and the real heating range and the adjusting of the pump's cut-out pressure. Even then, there is no guarantee the PRV will not open.

This is because (as the maths shows) the optimum set pressure for the 2 units is very different, ~15psi for the accumulator and ~30psi for the expansion tank.

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If you mean you have the extra length tee'd off the main line and this extra length is full of air (which gets compressed) then you will need a section of 15mm pipe that is 13 metres long to accomodate 2 litres of expansion!!!!

Furthermore, the air in the pipe will dissolve in the water over a month or so and thus the system would need some means of relenishing the air.

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Furthermore, the air in the pipe will dissolve in the water over a month or so and thus the system would need some means of relenishing the air.

Therefore the volume of water held in the accumulator will be 2 litres - 1.2 litres = 0.8 litres. If you have a typical 2 litre accumulator measure the water volume that comes out on turning a tap before the pump cuts-in. You will find that theory and practice agree very closely.

I was assuming we are discussing accumulators, tho expansion vessel was quoted so perhaps I have got the wrong end of stick (again) If we isolate one accumulator, pump off time is approx halved, with two it's handy in the night if one wants a drink/handwash

PS. I suspect the position of the accumulator might be a factor... does it help if it is downstream of a tap rather than upstream and beside the pump?

Just to be clear, an accumulator stores pressure to stop pump cycling, an expansion vessel prevents pressure relief valve weeping due to hydraulic locking (hot water expanding). If a non return valve is fitted then an 'expansion vessel' fits between NRV and PRV. If no NRV then accumulator acts as expansion vessel also.

Shouldn't they be pre charged to a pressure higher than your water pump can put out, so that the pump isn't capable of using them as another accumulator.

Chris has calculated this from a theoretical viewpoint, which I accept, although practical experience by several people seems to indicate you normally don't need as much as the physics and maths might imply.

This is a topic that I followed with interest when it first appeared. I followed all the recommendations and ordered direct from surecal. To allow for the expansion of my horizontal 65 litre tank to the formula of pythagerus squared x the inside leg measurement of grey squirrel the product arrived and I couldn't find any space in the forkin engine room!!!! Throw away all the calcs get an eight litre one as the calorifier has lasted twice as long as its predecessor so it must be doing the job within the limitations I have.

My cold feed into bottom of cal. is fed from a tee which also goes to cold taps, cal hot water expands back to tee and therefore taps, this cold feed is lengthened to accomodate enough hot water before it reaches the tee. Has been successful for our setup. Hope I have explained clearly.

The point I am trying to make (not very successfully) is that if an accumulator is big enough it can and does serve both purposes

Rearranging the equation and assuming that the PRV pressure is 42psi as measured on a gauge (but actually 57psi absolute pressure; ie: 42 + 1 atmosphere, where 1 atmosphere = 15psi)

If the water is below 10 degrees, the pump has cut off and no water drawn off, and the calorifier heated to 65 -70 degrees, I may get a bit of leakage from the PRV, but I can live with that.

I'd have thought the only thing that should be forcing water into them is expansion as the hot water heats. Other than when you first open a tap, and this gets pushed out, I'd then expect them to stay full of air, and that to only be replaced by water if you got into another heating, and hence expansion cycle.

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Why would an expansion tank have anything to do with the capacity of colorifier? Unless as other have stated you are talking about an accumulator.

I took the question to be about adding an expansion vessel to the domestic hot water from the calorifier, to avoid water being ejected through the PRV as the system is heated.

So a 15 mm pipe run from the water tank ...off this came the galley cold ..then the shower cold ...then accumulator... then bathroom basin cold and then the vertical calorilifier.

An accumulator must be pressure set BELOW the pump's cut-in pressure (this is typically around 15psi). If not, one is not maximising the volume of water that can be held in the accumulator before the pump cuts-in. For space reasons below a sink, a typical accumulator is usually around 2 litres in size. The typical pump's cut-out pressure is around 30psi.

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The precise "maffs" is as follows and utilises Boyle's Law of gases where P1 x V1 = P2 x V2 where P1 and P2 are the expansion vessel pressure before and after water expansion and V1 & V2 are the volumes of air inside the expansion vessel before and after water expansion. Pressures must be input as absolute pressures not gauge pressures (absolute pressure = gauge pressure + 1 atmosphere where 1 atmosphere = 15psi). So 12psig = 27psia and 30psig = 45psia

Once a hot tap is opened of course, the excess expansion pressure is released and the expansion vessel has no role to play unless the water is reheated and expands further.

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The point I am trying to make (not very successfully) is that if an accumulator is big enough it can and does serve both purposes

Rearranging the equation and assuming that the PRV pressure is 42psi as measured on a gauge (but actually 57psi absolute pressure; ie: 42 + 1 atmosphere, where 1 atmosphere = 15psi)

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PS: Note that in the next post I show that an expansion vessel needs to be set ABOVE the pump's cut-out pressure, so an accumulator and an expansion vessel use widely differing set pressures. Trying to use one vessel to do both is a very poor compromise.

With the pump cut-off, if the water is heated from 20degC to 60degC, the water will expand by 1.44% (from water expansion tables) or 0.9 litres.

This caused me some worry when I fitted out our new boat.... in the end I copied what was on the last one as it seemed to work.

Of course that's true but, if you are able to follow the maths, you will see that the required size of a combined unit would be very large (>10 litres) in order to do an effective job as both expansion tank and accumulator. This precludes its being fitted under a sink in the galley in most cases.

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I know Chris argued strongly that the maths and physics show that what I have was not sufficient, (5 litre EV with 55 litre calorifier), but in practice I've not seen anything come through the PRV yet. Several others claimed the same for EVs that Chris would say are not large enough.

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If it's an expansion vessel (hot water) rather than an accumulator (cold water) being talked about, then I'm not convinced they should be having a massive effect on pump cycling times, in the normal state of things.......

Of course. That's why I said above, viz: "using one tank to try to do both jobs also means that one cannot fit an NRV between the calorifier and the cold water supply"

It comes up every time, but always in the context of "I don't know but I've heard it can happen". You don't get a problem in the hot pipes, which constantly heat up and cool down, nor at the warm interface between hot and cold, wherever it might be. I most strongly suspect it's another urban muyth - but I'm sure it will keep on coming up, time and time again.

My gut feel is that even if the science says otherwise, an EV that's at least 10% of your calorifier(s) will generally be enough. So with 80 litres total, I think you'll get away with an 8 litre EV. They are getting quite big, by 8 litres.

Distance from accumulator to calorilifier about 10 feet, distance from cold galley tap T off to accumulator about 4 feet. Calorilifier 55 litre, accumulator can't remember but normal blue job from midland. Did not fit a NRV.

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So the volume of water delivered from your accumulator before the pump cuts in will be the difference in the air volume = 4.2 - 3.3 = 0.9 litres

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Think about it - water will expand by around 3% over the range of temperatures which we find on a boat. The larger the calorifier the larger the volume therefore of expanded water.

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Innisfree has confirmed his remarks actually applied to accumulator(s) fitted on the cold side, which is not what I understand OP to be talking about.

Kayak are you asking about the dometic water (hot water which comes out of your tap) or the heating water (water which is heated by boiler/engine/fire and runs through the coil in the colorifier.

The bigger the calorifier(s), the bigger that potential expansion is, and hence the larger an EV needs to be to guarantee being able to contain it all.

But if NRV is fitted then exp vessel is needed between NRV and PRV to prevent hydraulic locking which would open PRV whenever hot water expands slightly. Instead of NRV I have installed an extra length of cold water inlet pipe to accomodate expansion, works well, simpler and more reliable than NRV. I find that two 5lt accumulators set at same pressure (2x50lt calorifiers) has worked very well for 4 years without any pressure check or adjustment

If the water is heated instead from 10degC to 70degC, the expansion is 2.2% or 1.3 litres. Therefore the air volume will decrease from 3.3 litres to 2 litres

Only joking (half) but it does semm that Chris W was working out the pressure settings rather than the volume? I need to order one in the next couple of days as it one of my next jobs.

The pressure also needs to be set BELOW the PRV pressure (which is usually around 42psi) otherwise the PRV will still blow as the water expands. Therefore the expansion vessel pressure needs to be set somewhere in the range 30 - 42 psi.

The vessel will be initially empty with a pressure of 15psig (30psia) in it. After the pump cuts out the inside pressure will be 30psig (45psia)

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Once a hot tap is opened of course, the excess expansion pressure is released and the expansion vessel has no role to play unless the water is reheated and expands further.

Therefore at pump cutout pressure, 30 psi, the accumulator will contain about 2.5 litres of water. This seems to allow about a litre to be drawn off before the cut-in pressure of the pump is reached and the pump restarts.

Is there anyone out there who can tell me how I should size an expansion tank on my hot water system. I have 2 twin coil calorifiers at 40 lt each connected (or will be!) in series. Any help is gratefully received as always.

I guess we got lucky....and a few factors may have helped here...... A longish distance from galley tap to calorilifier (14 feet), a vertical calorilifier, only heating water to 60 degrees f (old engine and setting thermostat low) , and a low pressure shureflow (approx 20 lbs per sq inch not the 30 lbs sq inch one). We also dose the integral water tank regularly with purifier.

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The calorifer can then heat from about 20 degrees say up to bout 60 degrees with the expansion being taken up by the accumulator and the PRV doesn't release. Of course drawing off a little water takes out some of the expansion and allows more expansion to occur if the water heats further.

Chris will tell you what the maths indicates though, I feel sure - probably more than 10 litres, I'm guessing, (if you can fit it in !).

Shouldn't they be pre charged to a pressure higher than your water pump can put out, so that the pump isn't capable of using them as another accumulator.

You could of course fit two smaller ones, we have 2 x 5lt with 2 x 50lt calorifiers and works very well, ages before pump cuts in (and ages before it cuts out!)

The pressure inside the expansion vessel needs to be ABOVE the pump's cut-off pressure (typically ~30psi) otherwise the pump itself will pump up the expansion vessel's diaphragm and thereby waste some of the effective volume which could have beem used for hot water expansion.

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Approximately 8 litres per 60 litres of calorifier, so about a 10 litre expansion tank for 2 x 40 litre calorifiers combined.

This is because (as the maths shows) the optimum set pressure for the 2 units is very different, ~15psi for the accumulator and ~30psi for the expansion tank.

In use we are not getting warm water in cold galley tap , calorilifier(vertical) stays hot /warm for 18 hours, no signs of bacteria , pump comes on /off with about 5 second delay and prv does not release.

So the water expands up into the EV from the domestic side without being refreshed. Surely this creates the effects of a dead leg.

Re-running the equation, the pressure inside the vessel will increase to 45 x 3.3 = P2 x 2.0 = 74 psia = 59 psig which will cause the PRV to open widely.

So if we assume that there is 2 litres of expanded water and an 8 litre expansion vessel, the volume of air after expansion will be 6 litres.

I'm not exactly clear what you mean......... water is incompressible, so having an extra length section won't accomodate any expansion.

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I'm equally sure that you can't get hot water coming out of your cold taps (as has been suggested in the past) although clearly it would be possible for the expanding water to drive some warmth back down the cold feed pipe if you were to open a cold tap that was very near to the hot tank. However the amount would almost always be negligible and usually zero; we've certainly never had any warm water from any of our cold taps.

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