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Hi

I have a calorifier with a PRV fitted which expels water into a water bottle. This works fine in a Heath Robinson kind of way but it's a bit of a pain emptying the bottle every few days or even more frequently if I've been using the hot water more than usual. Also, if I forget to empty it I end up with a lovely wet patch which then needs drying out.

I would love to find an alternative solution to the overflow bottle. I have a couple of ideas which I'm hoping will generate useful feedback:

 

1. There is an unused outlet hole in the engine bay (to the left of the cupboard where the calorifier is). Is there any reason I couldn't extend the pipe from the PRV directly to an outlet?

 

2. I've been looking at expansion tanks. I don't understand fully how they work - the PRV sends water to the expansion tank but then would the tank need emptying or does the water somehow end up back in the calorifier?

 

What I'm after is a system that deals with overflow in a more elegant and self sufficient way than the current plastic bottle. All suggestions are welcome!

 

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Edited by Yeletah
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It's not an overflow, it's for releasing pressure when it gets too high so it's for safety so in day to day operation it's not meant to let out water. The expansion tank allows the hot water to expand so keeping the pressure at normal levels and therefore the PRV won't overflow.

 

So yes an expansion tank is good, plumbed in the hot water outlet of calorfier (as with the taps). And yes you can redirect the PRV outlet overboard to a nearby fitting.

 

The expansion tank basically works by been full of air, as you can compress air and not water and water expands when it gets hotter you compress the air.

Edited by Robbo
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Fit a new PRV?

 

Could you expand on this? Just wondering how replacing the PRV would solve it. My understanding of a PRV is that it allows water through when the pressure meets a certain threshold which seems to be what it is happening. My technical boat knowledge is a bit lacking in the plumbing department so apologies if I'm missing something obvious!

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I don't think your PRV should be operating to that extent. Mine certainly didn't. Normally the pipe from the PRV would empty overboard via a skin fitting, which is what mine did, or into the engine bilge, the bit where your stern tube drips into, to be got rid of periodically by the bilge pump.

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There is no reason for you not to pipe the PRV outlet into the engine bilge or even overboard by using a skin fitting. As you link the amount of water you get in the bottle to the amount of hot water you draw I very much doubt you need a new PRV - the "leakage" is just the expanding cool ater in the calorifier as it is heated.

 

Question 2 shows a fair degree of misunderstanding.

 

A expansion tank is simply Td off a convenient hot water pipe and you pressurise it to around the pump cut out pressure. If you fit one I doubt the PRV will vent. Now as the water expands it will flow into the expansion vessel compressing the air but NOT raising the water pressure to any great extent so the PRV is not forced open.

 

 

 

 

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There is no reason for you not to pipe the PRV outlet into the engine bilge or even overboard by using a skin fitting. As you link the amount of water you get in the bottle to the amount of hot water you draw I very much doubt you need a new PRV - the "leakage" is just the expanding cool ater in the calorifier as it is heated.

 

Question 2 shows a fair degree of misunderstanding.

 

A expansion tank is simply Td off a convenient hot water pipe and you pressurise it to around the pump cut out pressure. If you fit one I doubt the PRV will vent. Now as the water expands it will flow into the expansion vessel compressing the air but NOT raising the water pressure to any great extent so the PRV is not forced open.

 

 

 

 

 

Makes a bit more sense. So an expansion tank is to do with regulating pressure rather than providing an overflow. Gotcha

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Could you expand on this? Just wondering how replacing the PRV would solve it. My understanding of a PRV is that it allows water through when the pressure meets a certain threshold which seems to be what it is happening. My technical boat knowledge is a bit lacking in the plumbing department so apologies if I'm missing something obvious!

 

Fitting a new PRV will only help if the old one is malfunctioning. If you have a non-return valve (NRV) in the cold water feed pipe just before your calorifier (to prevent the calorifier heating the cold pipe), then your calorifier is effectively a pressure vessel with no means of accommodating the expansion when it heats the water. So the PRV will operate and let a bit of hot water out to prevent something cracking - like a joint or even your tank. But what you have is an inefficient system because you're bleeding hot water.

 

As Robbo says, you can fit a hot water expansion tank between the NRV and the calorifier (from memory it should be minimum 8% of the volume of the calorifier but someone can correct me if I'm wrong about that). This will accommodate the expansion and should prevent the PRV from activating. So your PRV then becomes the last line of defence and you're not relying on it to protect your tank and plumbing (Boats with a NRV and no expansion tank rely on the PRV and they do get stuck shut).

 

The other thing you should do is connect the PRV to a skin fitting in the hull. If there's an existing one that's unused so much the better.

Edited by blackrose
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Fitting a new PRV will only help if the old one is malfunctioning. If you have a non-return valve (NRV) in the cold water feed pipe just before your calorifier (to prevent the calorifier heating the cold pipe), then your calorifier is effectively a pressure vessel with no means of accommodating the expansion when it heats the water. So the PRV will operate and let a bit of hot water out to prevent something cracking - like a joint or even your tank. But what you have is an inefficient system because you're bleeding hot water.

 

As Robbo says, you can fit a hot water expansion tank between the NRV and the calorifier (from memory it should be minimum 8% of the volume of the calorifier but someone can correct me if I'm wrong about that). This will accommodate the expansion and should prevent the PRV from activating.

 

The other thing you should do is connect the PRV to a skin fitting in the hull. If there's an existing one that's unused so much the better.

Brilliant advice. Thanks everyone

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You do need to be clear what you mean by "expansion tank", though.

 

My original thought (perhaps wrong!) from your description was you were thinking about piping the overflowing PRV outlet into an open, non sealed, tank, such as often used on central heating system. This would not help - over time it would overflow, just as your existing bottle does.

 

What I think is actually needed is what I would call an Expansion Vessel (EV) which is a sealed unit like an accumulator, with an air reservoir in it. (Often it is the same physical item as an accumulator, but charged to a much higher "pre-charge" pressure.)

 

This should be fitted on a tee anywhere on the hot water outlet from your calorifier, (as Tony states), and needs its pre-charge pressure set higher than the cut off pressure of your pump, but less than the PRV should open at. (An example Pump Cut Out 30 psi, Pre charge pressure on EV 37 psi, opening rating of PRV 45 psi - but needs to be set up to match your actaul values).

Set up this way the expanding water will go into the EV, not out the PRV, but as soon as you open a hot tap, the water will be expelled from the EV, leaving it ready again to have expanding water go into it.

Edited by alan_fincher
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My calorifier is connected to the skin fitting with car radiator hose, I think you need something fairly heat resistant - not because of the occasional hot water inside it but because it's the PRV does get quite hot and will be constantly in contact with the hose. Or you may find that ordinary nylon braided hose can cope?


You do need to be clear what you mean by "expansion tank", though.

 

My original thought (perhaps wrong!) from your description was you were thinking about piping the overflowing PRV outlet into an open, non sealed, tank, such as often used on central heating system. This would not help - over time it would overflow, just as your existing bottle does.

 

What I think is actually needed is what I would call an Expansion Vessel (EV) which is a sealed unit like an accumulator, with an air reservoir in it. (Often it is the same physical item as an accumulator, but charged to a much higher "pre-charge" pressure.)

 

Yes, and make sure it's stainless (or epoxy coated?) on the inside. I made the mistake of fitting a cheaper expansion vessel for a central heating system which was supposed to have coolant/inhibitor inside to prevent corrosion, not fresh water. After a few years it rusted through from the inside and I had to replace it.

Edited by blackrose
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Nobody has mentioned the fact that a PRV gets calcified 'over time' and needs an occasional tweak of the knob on top to clean and reseat it.

So to my mind - rather than just discharging overboard when it does vent, it's a good idea to be able to monitor it. I have a pipe to a gallon container in the engine bay bilge (not engine bilge) - so that I can see if its regularly passing water - any excess gets pumped overboard as with comes out of the prop shaft.

 

Some years ago the PRV failed while we were boating and the whole contents of the main tank ended up in the bilge. We were without any water for two days....

Now I check it regularly and carry a spare.....

  • Greenie 1
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Can I also suggest putting the expansion vessel on the cold water inlet betweeen a check valve and the calorifier? The bladders seem to last longer if not in contact with hot water....and they will still do the same job. Also make sure it's somewhere you can check the pressure in the expansion vessel quite easily and add some air as required.

 

Gareth.

  • Greenie 1
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Can I also suggest putting the expansion vessel on the cold water inlet betweeen a check valve and the calorifier? The bladders seem to last longer if not in contact with hot water....and they will still do the same job. Also make sure it's somewhere you can check the pressure in the expansion vessel quite easily and add some air as required.

 

Gareth.

If it's a Surecal then I believe they're fitted with an NRV in the inlet connection which would make this impossible as supplied.

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If it's a Surecal then I believe they're fitted with an NRV in the inlet connection which would make this impossible as supplied.

Ahhhh. Fair enough. Mine is a C-warm from Cleghorn Waring and they are supplied without NRV so you can fit in system as required

 

One interesting point from that diagram is the Flow/return from the engine. I was always told to put the flow from the heat source in the top of the coil and have the return back to the heat source at the bottom of the coil. Might not make much difference to an engine but on my Alde with a smaller pump it seems to work well.

 

Edited to add

I've just checked the instructions for the C-Warm range and they do show the flow from the engine going into the top of the coil and exiting the bottom of the coil.

 

Gareth

Edited by frangar
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