How to size an expansion tank?

[chris w]

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.

But your pipes are already full of water, in the tee'd section, in the lengths up to the pipe, everywhere............. where is the space into which the hot water can expand????

 

As I already calculated, even if you had an EMPTY length of 15mm pipe (which you don't), it would need to be 13 METRES long to accomodate the expansion!!!!!

 

Chris

Edited May 13, 2009 by chris w

[nb Innisfree]

But your pipes are already full of water, in the tee'd section, in the lengths up to the pipe, everywhere............. where is the space into which the hot water can expand????

 

As I already calculated, even if you had an EMPTY length of 15mm pipe (which you don't), it would need to be 13 METRES long to accomodate the expansion!!!!!

 

Chris

It expands into the calorifier/exp vessel further up the line. I have got approx 4 metres of 22mm, I decided on this length for practical reasons, expecting it to be insufficient and to see how it worked, so far no hot makes it back to the cold taps etc. and it has been up to engine temp (80c)

[chris w]

It expands into the calorifier/exp vessel further up the line. I have got approx 4 metres of 22mm, I decided on this length for practical reasons, expecting it to be insufficient and to see how it worked, so far no hot makes it back to the cold taps etc. and it has been up to engine temp (80c)

You seem to be missing the point that there is NOWHERE for the water to expand into. Even your 4 metres of 22mm piping would not hold 2 litres of water EVEN IF IT WERE EMPTY (which it's not). You stated earlier that you don't have an expansion vessel - now you mention the "calorifier/expansionvessel further up the line". Which is it?

 

Where is this mythical empty pipework you have?

 

Chris

[bottle]

[/quote

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.

 

 

Your hot water does not expand into that length of pipe (as explained by Chrisw) what may/does happen is that the heat from the tank is transferred to the water along the pipe, (conduction ?) the amount heat transferred along the pipe may indeed stop before the tee but if left long enough without any water being used the heat (not expansion) would probably go beyond the tee.

[nb Innisfree]

You seem to be missing the point that there is NOWHERE for the water to expand into. Even your 4 metres of 22mm piping would not hold 2 litres of water EVEN IF IT WERE EMPTY (which it's not). You stated earlier that you don't have an expansion vessel - now you mention the "calorifier/expansionvessel further up the line". Which is it?

 

Where is this mythical empty pipework you have?

 

Chris

I never said I didn't have an accumulator, what I did say was that I had two accumulators. The 4m of 22mm pipe holds, I think, approx 1.25 lts, not enough in theory to accomodate 50lt calorifier expansion. I could, with a bit of work, extend this to, say, 8 mts which would in theory be long enough. But why bother with theory if it works in practice? Remember, no hot water from cold outlets.

[chris w]

I never said I didn't have an accumulator, what I did say was that I had two accumulators. The 4m of 22mm pipe holds, I think, approx 1.25 lts, not enough in theory to accomodate 50lt calorifier expansion. I could, with a bit of work, extend this to, say, 8 mts which would in theory be long enough. But why bother with theory if it works in practice? Remember, no hot water from cold outlets.

But you DON'T have any expansion room. The pipe is already full of water even when it's COLD. Do you not understand that you can't compress water. You have no room available for any expansion. Do try to understand please.

 

you stated earlier:

I have installed an extra length of cold water inlet pipe to accomodate expansion, works well, simpler and more reliable than NRV."

 

An extra length of cold water pipe will NOT act as an expansion vessel. Secondly, you say it is "more reliable than an NRV". An NRV is not there to accomodate expansion - it has a different function. Bottle was on the nail when he said that all you are experiencing is the heat transfer along the pipe not the actual expansion because that's IMPOSSIBLE. Your PRV (or your opening of a hot tap) is relieving the expansion pressure. Trust me!!!

 

Chris

Edited May 13, 2009 by chris w

[Charlie Breakfast]

It strikes me that Allan, Dor, Innisfree and myself are all saying roughly the same thing.

 

I may be wrong but looks like We all run calorilfiers without a NRV . We have accumulators in the cold feed ( between tap and calorilifier?) to stop the pump pulsing and at the same time to take care of the expansion cased by heating the water in the calorilifier. I think we are all saying that with this set up the prv does not leak on our boats , that hot water does not come out of the taps and that to date bacteria is not a problem. In short it works...tried and tested.

 

Innisfree I understand your point....I too think that the longer the pipe between a cold tap and the calorilfier the less chance warm water will reach the cold tap....interesting that we both are quoting somewhere in the region of 14 feet.

 

I suspect if I was heating water to 80 degrees in a horizontal calorilifier with a cold tap 2 feet from the calorilifier I might be using a non return valve to keep the hot water where i want it....but i'm not

 

Cheers

Chas.

[chris w]

It strikes me that Allan, Dor, Innisfree and myself are all saying roughly the same thing.

 

I may be wrong but looks like We all run calorilfiers without a NRV . Chas.

Innisfree stated that he didn't have any expansion vessel. Is he now saying that he has an accumulator? He said that his additional 4 metres of pipe is absorbing the expansion. That's total crap. How can a pipe that is already full of water absorb more water? It's nonsense. The heat will get transferred along it but not the expanded water.

 

If you are able to follow my maths above then you will understand that just using an accumulator to do both jobs (ie: accumulator and expansion vessel) is a real compromise unless your accumulator is even bigger (~10 litres) than a normal expansion vessel. It'll partially work but the PRV will still open. What is probably happening is that someone uses a hot tap before the water is fully expanded and that relieves the expansion pressure before the PRV opens.

 

Chris

Edited May 13, 2009 by chris w

[Keeping Up]

Exactly right Chas.

 

{pedantic mode on } Strictly speaking I do have an NRV. It's positioned AFTER the hot tank. I'm not sure it ever does anything useful. In theory it's there to prevent the hot water from syphoning back to the cold system if you open both a high-up hot tap and a low-down cold tap at the same time when the pump is off; in practice the only time I'd ever do that is when I actually WANT to try and empty the tank for the winter and it's a nuisance then and I had to put a special tap in at the top of the tank to let the air in when I want to drain it.

 

{pedantic mode still on} It's quite possible that my PRV does weep sometimes. My accumulator is quite small. But it weeps safely away down a pipe to the outside, and the PRV hasn't yet worn out after 18 years, so I don't know and don't care. In fact I reckon it does away for the need to perform the occasional maintenance activity of clicking the valve to free it of accumulated scale.

 

{pedantic mode off }

 

Still agree with you Chas. There's no expansion tank, no hot water in the cold taps, and no bacteria problem (yet)

[chris w]

......................and no bacteria problem (yet)

But how do you know for sure that the bacteria are not affecting your reasoning?

[casper ghost]

My calorifier has worked fine for 8 years now and I don't have an expansion tank, never thought it was required. It's only a 10 gallon tank and is only heated by the engine but does produce nice hot water. Are people really sure they need this expansion tank, not just another gizmo is it..

[chris w]

My calorifier has worked fine for 8 years now and I don't have an expansion tank, never thought it was required. It's only a 10 gallon tank and is only heated by the engine but does produce nice hot water. Are people really sure they need this expansion tank, not just another gizmo is it..

No one is saying that it won't work.... BUT you are relying on the correct and continued functioning of your PRV to prevent your calorifier from exploding! Further you are running (the expanded) hot water over the side of your boat which is just a waste. You won't know that your PRV has failed until your boat floods one day.

 

Chris

[casper ghost]

No one is saying that it won't work.... BUT you are relying on the correct and continued functioning of your PRV to prevent your calorifier from exploding! Further you are running (the expanded) hot water over the side of your boat which is just a waste. You won't know that your PRV has failed until your boat floods one day.

 

Chris

There's no pipes to the outside of the boat?

There's a presure release valve on the top but no water ever comes out.

[chris w]

There's no pipes to the outside of the boat?

There's a presure release valve on the top but no water ever comes out.

If you have no water pipe to the outside of the boat from the PRV then any expanded water ejected through the PRV is still somewhere inside your boat!!

 

If the PRV is not opening then someone (luckily) is always opening a tap before the expanded water pressure builds up. If that is not the case, where do you think that a couple of litres of expanded water is going? Maybe it's a magic boat

 

Chris

[Keeping Up]

But how do you know for sure that the bacteria are not affecting your reasoning?

I asked them and they assured me they weren't

[chris w]

I asked them and they assured me they weren't

Gotcha !!

[Colin Smith]

The maffs for your set-up is as follows:

 

Pressures used in the equation are in psi atmospheric as needed by Boyle's law.

 

so P1 x V1 = P2 x V2

 

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)

 

Thus, 30 x 5 = 45 x V2

 

Therefore V2 = 3.3 litres (of AIR)

 

So the volume of water = 5 - 3.3 = 1.7 litres of water

 

The pump will cut in at around 20psig (35psia)

 

At that pressure the equations reveal that 45 x 3.3 = 35 x V2

 

therefore V2 = 4.2 litres of air.

 

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

 

This latter figure corresponds to what you are seeing in reality.

 

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.

 

Therefore the volume of air inside the vessel will decrease by this amount from 3.3 litres to 2.4 litres

 

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).

 

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

 

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.

 

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.

 

Chris

 

Hi Chris,

 

Thank you for posting all of this.

 

I'm in the middle of putting together my water system and would be grateful if you could advise: My pump is a Flojet R4300143A - cuts in at 2.1bar (30psi) - cuts out at 3.2bar (45psi).

These pressures are quoted in the docs that came with the pump and on the Flojet website. My question is…are these figures likely to be psig or psia? I’ve looked at a lot of manufacturer sites but they only quote “psi” or bar.

 

I intend to fit a calorifier of around 85lts with a suitable expansion vessel, NRV and a suitable accumulator etc. but just wanted to be sure I'm putting the right numbers into the equation!

 

Thanks,

 

Colin

[nb Innisfree]

But you DON'T have any expansion room. The pipe is already full of water even when it's COLD. Do you not understand that you can't compress water. You have no room available for any expansion. Do try to understand please.

 

you stated earlier:

 

 

An extra length of cold water pipe will NOT act as an expansion vessel. Secondly, you say it is "more reliable than an NRV". An NRV is not there to accomodate expansion - it has a different function. Bottle was on the nail when he said that all you are experiencing is the heat transfer along the pipe not the actual expansion because that's IMPOSSIBLE. Your PRV (or your opening of a hot tap) is relieving the expansion pressure. Trust me!!!

 

Chris

I didn't say cold water pipe was acting as expansion vessel, my accumulator is doing that, cold water pipe passes expanded hot water along to teeoff and then to accumulator

Cold water pipe is long enough not to let hot water reach teeoff and therefore doesn't enter cold flow to taps. Of course NRV doesn't accomodate expansion, it's there to stop hot water expanding back into cold tap supply, that's what the extra length of pipe does. Got it yet?

[chris w]

I didn't say cold water pipe was acting as expansion vessel............

But you did.....................

 

"Instead of NRV I have installed an extra length of cold water inlet pipe to accomodate expansion"

 

Remember?

[chris w]

Hi Chris,

 

Thank you for posting all of this.

 

I'm in the middle of putting together my water system and would be grateful if you could advise: My pump is a Flojet R4300143A - cuts in at 2.1bar (30psi) - cuts out at 3.2bar (45psi).

These pressures are quoted in the docs that came with the pump and on the Flojet website. My question is…are these figures likely to be psig or psia? I’ve looked at a lot of manufacturer sites but they only quote “psi” or bar.

 

I intend to fit a calorifier of around 85lts with a suitable expansion vessel, NRV and a suitable accumulator etc. but just wanted to be sure I'm putting the right numbers into the equation!

 

Thanks,

 

Colin

The figures will be psig.

 

The immediate problem I see is that your pump cut-off pressure is far too high. At 45psi it's going to blow the PRV everytime it switches on because the PRV will be set at about 42psi.

 

A friend of mine fitted the same Flojet pump and the PRV started opening and closing at rat-a-tat speed everytime the pump activated. I diagnosed the problem for him and he had to change it for a Jabsco 30psi cut-out version (20psi cut-in) as the Flojet does not have an adjustable pressure switch setting. If someone sold you a 45psi pump (rather than your specifying it) then take it back and swap it. They are wrong to have specified it knowing that PRV's are invariably 42psi (to protect the calorifier).

 

If you follow the maths, you will see that actually you want to turn the pump down to about 28psi to be absolutely sure (with a reasonable safety margin) that the PRV will not open owing to expansion. A 10 litre expansion vessel and a 2 litre accumulator will be appropriate in terms of size.

 

Chris

[nb Innisfree]

But you did.....................

 

 

 

Remember?

Ok I think we're quits on misquotations, what I should have said was "to accomodate the PASSAGE of hot water"

Are we reading from the hymn sheet now? Very difficult to explain all this in words, picture and paintings and all that jazz, must try to get to grips with posting a drawing.

[casper ghost]

If you have no water pipe to the outside of the boat from the PRV then any expanded water ejected through the PRV is still somewhere inside your boat!!

 

If the PRV is not opening then someone (luckily) is always opening a tap before the expanded water pressure builds up. If that is not the case, where do you think that a couple of litres of expanded water is going? Maybe it's a magic boat

 

Chris

 

I cruise single handed so no one is turning the hot tap on, unless it's the ghost .

The calorifier is in my engine room and it does get water in it at the point but i expect that's from the stern tube, never checked if it's hot water, anyway i can see the prv from where i steer and never seen any released water. I suspect the pressure isn't enough to trip the prv.

[chris w]

......... i can see the prv from where i steer and never seen any released water. I suspect the pressure isn't enough to trip the prv.

It's a matter of physics fact that water expands when heated. Over the range that the engine (or a diesel heater) will heat the water this will amount to about 3% of the size of your calorifier or around 2 litres for a 60 litre calorifier.

 

This has to go somewhere or your calorifier will split with catastrophic results. As you say, you have never checked the water in the calorifier area and you are just assuming it's due to the stern tube.

 

You won't see its gushing out of the PRV. As the pressure builds the PRV will weep and then close again as the pressure is released. Then the pressure will build up again and the PRV will weep some more.... and so on.

 

Chris

[RLWP]

It's a matter of physics fact that water expands when heated. Over the range that the engine (or a diesel heater) will heat the water this will amount to about 3% of the size of your calorifier or around 2 litres for a 60 litre calorifier.

 

Chris

 

If I have occasion to move our calorifier I can hear the water slopping about in it. I concluded that it must therefore have quite a bit of air in it as well as water, probably more than 2 litres.

 

Ours is a horizontal one.

 

Richard

[chris w]

If I have occasion to move our calorifier I can hear the water slopping about in it. I concluded that it must therefore have quite a bit of air in it as well as water, probably more than 2 litres.

 

Ours is a horizontal one.

 

Richard

For a horizontal one that will be true if the take-off is on one end a little down from the top surface. A vertical one will be full (assuming the take-off is from the top as per normal).

 

Chris

Edited May 14, 2009 by chris w