Calorifier/Fresh Water Pump Question

[Slipperman]

Last week I posted a question about a leaky calorifier relief valve, and thanks to the helpful answers I think I have established that the valve is operating correctly ie it only lets out water when the engine is running and thereby heats the water in the tank. It has been suggested that it is good practice to leave the fresh water pump switched off unless it is required for drawing off water through the taps - my question is whether this would cause a problem, when the safety relief valve operates and no replacement water goes into the tank? Would the water in the tank keep getting hotter and hotter during a long run under engine, until something nasty happens?

 

[aluijten]

I'm not sure if I understand you correctly, but the water in the calorifier is very unlikely to become over 100 degrees because of the engine coolant. If it would, you engine itself would be overheating as well. Although the coolant can theoretically become hotter then 100 degrees this is an unlikely scenario.
As to the pump switched off, I never switch it off unless I leave the boat for longer periods at which point I also open one of the taps briefly to remove the pressure of the system. But that is more to preserve the Accumulator (pressure vessel).

Cheers,

Arno

 

[nedmin]

It depends on which type of water pump you have.If say you get a leak and it empties the water tank,if youve got a pump that cannot run dry you would burn the pump out and probably cause a serious fire.I usually leave mine on if we are living on board as it can run dry.When I leave the boat to come home I always do as the post above.See if you can find out the make of pump as it would be nice for you to know if it can run dry.

 

[Slipperman]

To clarify my question a bit: I have noticed that as the water in the calorifier heats up when the engine is started and has run for a few minutes, the pressure relief valve on the calorifier starts to weep, so i get a small amount of water in the bilge. Having watched it from engine start up, I can see/hear the fresh water pump work for a short burst to put cold water back in the tank. As I understand, all of this is normal. My question really was that, if I left the water pump switched off, would it reduce the leakage (because there would be no more cold water going into the tank to replace the water that has escaped through the PRV), and if so, would this cause a problem with overheating of the water in the tank if the engine runs for a lengthy period? Any thoughts welcome!

 

[Billjratt]

It would appear the water has nowhere to expand to - ie your accumulator is (A) not there, (B) needs drained.

Don't know if your conversant, but just in case it helps, the accumulator is the black cylinder on the bottom right of this sub panel.
The system is being tested for leaks by connecting to the garden hose.

 

[rogerthebodger]

To clarify my question a bit: I have noticed that as the water in the calorifier heats up when the engine is started and has run for a few minutes, the pressure relief valve on the calorifier starts to weep, so i get a small amount of water in the bilge. Having watched it from engine start up, I can see/hear the fresh water pump work for a short burst to put cold water back in the tank. As I understand, all of this is normal. My question really was that, if I left the water pump switched off, would it reduce the leakage (because there would be no more cold water going into the tank to replace the water that has escaped through the PRV), and if so, would this cause a problem with overheating of the water in the tank if the engine runs for a lengthy period? Any thoughts welcome!

PRV's have an opening pressure and a closing pressure which is lower than the opening pressure.

Fresh water pumps have a start up pressure and a stopping pressure, the stopping pressure higher that the starting.

Normally the both the opening and closing PRV pressures must be higher that both the pump pressures. If this is not the case i.e the pump stopping pressure is higher that the closing pressure of the PRV then you will get IMHO the problem you have. My PRV opening pressure is adjustable with a fixed difference so increasing the PRV opening pressure may solve your problem. If it does not may consider changing the PRV for one with a higher setting.

 

[PCUK]

Fit an accumulator tank to act as an expansion tank and all your problems are solved at a stroke!

 

[Slipperman]

Thanks for additional replies. A couple of points - my system has an accumulator immediately after the FW pump on the cold water system, but there is no expansion tank on the hot water system, and no real room to put one in the engine compartment, although I could possibly fit one some distance from the calorifier close to the sink in the saloon. And my PRV looks to be a very basic one with no adjustment (it is a Plastimo calorifier, with just a small nozzle and a small black spring loaded lever on the side). It does not have a red knurled knob which you can rotate like some of those I have seen on other calorifiers and on the ITT Cleghorn Waring website).

 

[Billjratt]

But the accumulator feeds both systems surely? The cold water is under pressure and accumulated, then some splits off and becomes hot and carries on to it's own tap.
Assume there's no nasty non-return valves anywhere?
Does the accumulator function? - ie do you get a litre or so of water "for free" before the pump starts - and does the pump run on after you have closed the tap?
When did you last open the screw and drop out water from it?

 

[duncan99210]

Your calorifier is fed directly from the cold water tank, so therfore is no need to fit a separate accumulator; one does the job of smoothing the presures for both systems. If you find that the PRV weeps a good deal and the pump switches on to top up the pressure, then the problem is that the PRV is set below the pressure that the water pump switches off. There are 2 work rounds on this: turn off the water pump when the taps are not in use (this will not damage the calorifier, provided the PRV closes and remains closed once the pump is off; only a small amount of water should be expelled as it heats up). The second way of stopping the PRV leaking is to turn down the pressure at which the water pump switches off: there should be a small screw in the pump that you can back off a little to lower the operating pressure a bit.
We had to do this when we fitted a new pump which came with a factory setting above that of the PRV on the calorifier. Simple job in principle but awkward to do 'cos of the location of the pump.

 

[Slipperman]

Thanks Billjratt. I admit to learning as I go here, but agree that the present accumulator acts to some extent on both systems, though I have seen some diagrams with an accumulator on the cold system and another 'expansion tank' (which I understand is the same as an accumulator?) on the hold. I believe the current accumulator is working properly, but I must admit I have not tried adjusting it at all.

 

[vyv_cox]

You only need a hot water accumulator if there is a non-return valve in the cold water supply to the calorifier. Otherwise the hot water expands backwards. This sometimes causes problems, dependent upon the layout, in that hot water issues from the cold tap.

As others have said, your problem appears to be caused by excessive pump discharge pressure relative to the calorifier relief valve setting. Turning the pump off would probably solve the problem but must be regarded as a temporary solution.

 

[PCUK]

Unless your system has been assembled wrongly the accumulator tank will also act as the expansion tank. You don't need one of each. You must have a non-return valve somewhere between the accumulator and the calorifier which is causing the problem. Pressurised systems are very simple but many installers make them overly complicated which is when problems occur.

 

[cdogg]

NO nothing nasty will happen, the water will only get to a little over 100 deg c in a very worst case (assuming you have a pressurise fresh water cooled engine) but by then the engine would be complaining. The calorifier is well within its operating spec a little over 100C, in fact you could remove all the water from the tank and nothing would happen, hot water from the engine would just circulate round the coil and heat the air in tank instead.

I think you may just have answered my intended post. My boat remains in the water over the winter and I want to fully drain the fresh water tank and drain the calorifier (somehow, not sure where from yet). However, I want to run the engine (fresh water cooled VP D1-20)up to operating temperature every 2-3 weeks over the winter and was concerned as to what would happen to the calorifier given it will be heated by the engine but have no cold water to heat.
What, for instance happens to the heated air within the calorifier, doesn't it need to escape when expanding?

Can you confirm that I have got it right?
Thanks

 

[Slipperman]

Thanks Vyv and others for additional thoughts. On further investigation this weekend I have found that, when tested with a tyre pressure gauge, I am getting no reading at all from the accumulator. I have not touched it for 5 years, so I guess the air must have gradually dispersed. The label on the accumulator says that its max pressure should be 20psi, whereas the fresh water pump label says its max pressure is 45psi. If there is no pressure in the accumulator, does that mean that the accumulator is not working at all, and that the cold water system pressure is being delivered at 45psi to the calorifier tank? And if I take a bicycle pump and put air in the accumulator, is this likely to make the situation better? To my thick brain if I pump in more pressure to the system it might make it worse!

My Plastimo calorifier relief valve does not seem to have any adjustment. Furthermore, unlike some PRVs that I have seen, the relief valve is built in to the cold water inlet connection - whereas most of the pictures I have found online show a relief valve plumbed into the tank separately, usually with a connection for a drain tube, which mine does not have. I guess being Plastimo is it a cheapo version, but then again it basically works OK, apart from this annoying leak!

 

[aluijten]

And if I take a bicycle pump and put air in the accumulator, is this likely to make the situation better?

This is perfectly alright.

The easiest way to progress:
- Turn off the fresh water pump.
- Open the sink tap (hot and cold water side).
- Connect bicycle pump to accumulators air valve
- Start pumping untill no additional water comes from the tap.
- Git it a few extra strokes. (just a little pressure build up)
- Close the tap
- Switch on the fresh water pump
All done.

If all goes well you will see your pressure water system runs much more smoothly.

The catch:

Inside the accumulator there is a bladder. On one side you have the air, on the other side you have the water (set under the pressure by the air)
If there is a rupture in the bladder (mostly because of age) this will be the end of it and you'll need to replace the accumulator (sometimes they can be repaired).

But first try to repressure the accumulator. You stand a good chance as the bladder is never completely 'air-proof'. It'll loose the air pressure ever so slowly hence the reason I take off the pressure from the system when I leave the boat. That way the pressure in the accumulator drops as well and the 'recharge' will last longer.

As to heating the air inside the calorifier, don't worry nothing will happen, at best just a slight build up of airpressure in the calorifier. If you open the hot water tap, it will be gone instantly. No risk on a meltdown unless you use the electric heater element inside the calorifier as well. You should never use that on an empty calorifier!!

Cheers,

Arno

Edit:
Just noticed that your accumulator is only specified to 20 psi. This may have caused a premature bladder burst, so be prepared to renew the accumulator with a type that can withstand the pressure delivered by the pump. But first try to 'pump it up'.

 

[vyv_cox]

Here's my method for pressurising the accumulator. Turn a tap on. If the accumulator is not pressurised the pump will run continuously. Start pumping with your bike pump. The water pump will run intermittently as the accumulator starts to work. As you continue pumping the period of water pump stopped will increase but then decrease as the accumulator pressure becomes higher. Bleed some air back out until the stopped period is maximised.

 

[aluijten]

Here's my method for pressurising the accumulator. Turn a tap on. If the accumulator is not pressurised the pump will run continuously. Start pumping with your bike pump. The water pump will run intermittently as the accumulator starts to work. As you continue pumping the period of water pump stopped will increase but then decrease as the accumulator pressure becomes higher. Bleed some air back out until the stopped period is maximised.

That will only work if your tap has a restricted flow compared to the freshwater pump.
On our boat it would not work.

 

[vyv_cox]

That will only work if your tap has a restricted flow compared to the freshwater pump.
On our boat it would not work.

Don't turn the tap full on.

 

[Slipperman]

Success! Yesterday pumped up the Accumulator to 17psi (max stated on label is 20 psi) and ran engine with trace of a leak! Today have had brilliant sail out south of IOW and returned to Portsmouth with dry bilges! As a backup I now also have a tube from the PRV into a plastic container, but clearly the lesson learned is to monitor the accumulator pressure. My sincere thanks to all who helped me solve this minor but irritating problem.