No PRV?

[DavidExpert]

How is it possible that my boat (built by a reputable builder circa 2000) does not appear to have a pressure relief valve on the waterworks? It's clearly survived all this time without one but I thought they were an intrinsic part of the hot system. Any ideas?

[Paul C]

Does it have an expansion tank of some kind instead?

 

If not, then there must be some kind of pressure relief, after all water expands (lowers its density) when hot and this applies a significant force, enough to blow connections on pipework etc. You'd hope the pressure relief is through a yet-to-be-found PRV however...

[Flyboy]

How is it possible that my boat (built by a reputable builder circa 2000) does not appear to have a pressure relief valve on the waterworks? It's clearly survived all this time without one but I thought they were an intrinsic part of the hot system. Any ideas?

It might have an expansion vessel (like an accumalator) on the calorifier.

 

 

https://www.bes.co.uk/products/109.asp?kw=11352&ref=googlebase&gclid=CjwKEAiAmY-3BRDh7pjvg46p1iYSJADQ78gNXO8hyMyhBM0VJdkG_lTL89t7_6zgYPpgCMDh6-v2QBoCJbjw_wcB#11352

Edited March 12, 2016 by Flyboy

[DavidExpert]

There is a normal accumulator but nothing that I can see of such a thing on the calorifier itself which just looks like a normal oval shaped small tank with pipes running in and out.

[Paul C]

The PRV doesn't need to be located near the calorifier. It could be located anywhere on the hot water pipework. Indeed, it is sensible for it to be located together with other pipework eg ours is located in the bathroom and drains into the shower sump.

[DavidExpert]

Understood. I've had a good look round though, under the bed where the tanks are, in the engine room, where the water pumps are under the bathroom sink etc. Cannot for the life of me find one. No signs of any piping to take the excess water elsewhere or overboard either. Doing my head in!

[b0atman]

is it not at the base of the calorifier maybe hidden round the back

[by'eck]

Unless there is a non-return valve in the cold feed inlet to the calorifier, excess pressure up to a point, would be absorbed by the normal accumulator usually sited close to the water pump.

[blackrose]

Unless there is a non-return valve in the cold feed inlet to the calorifier, excess pressure up to a point, would be absorbed by the normal accumulator usually sited close to the water pump.

 

Yes, assuming the accumulator wasn't already at peak pressure when the hot water expansion needed to occur?

 

Plastic pipework can expand a little bit too, but you should have a PRV on the calorifier even if you find a hot water expansion vessel somewhere in the system. They're not difficult to fit. Don't forget this... I use gas PTFE for water too.

 

[NMEA]

If you have a pretty large accumulator and no non return it should be fine, don't forget that the pressure charge in an accumulator is around the cut in pressure of your pump which is well below the usual 3 bar charge in a hot water expansion vessel and allows plenty of room for expansion, if correctly sized to the whole system. Boiling events aside of course!! Though with a thermostat and a fail-safe cut out that should not happen. Really high pressures would probably get past the water pump and vent through the potable tank anyway.

Edited March 12, 2016 by NMEA

[Tony Brooks]

A warning.Accumulators have been known on more than one occasion to puncture their diaphragm or otherwise leak air pressure. If the accumulator on the system we are talking about de-pressurises itself then I think there is a fair chance the calorifier will eventually split unless a pipe joint is forced open.

[blackrose]

If you have a pretty large accumulator and no non return it should be fine, don't forget that the pressure charge in an accumulator is around the cut in pressure of your pump which is well below the usual 3 bar charge in a hot water expansion vessel and allows plenty of room for expansion, if correctly sized to the whole system. Boiling events aside of course!! Though with a thermostat and a fail-safe cut out that should not happen. Really high pressures would probably get past the water pump and vent through the potable tank anyway.

 

But why not just fit a a PRV and eliminate the "should" and "probably"?

 

It's not like they're expensive or anything.

 

http://www.ebay.co.uk/itm/Boiler-Safety-Pressure-Relief-Valve-1-2-BSP-3-Bar-FXF-FREE-POSTAGE-/201244769841?var=&hash=item2edb1f7e31:m:m5znM5lllfoQVhO6g19NnJw

Edited March 12, 2016 by blackrose

[NMEA]

No reason not to at all.

[bizzard]

And in the event of the pumps pressure switch failing ''ON' 'quite common'' and there is no PRV to relieve the pressure, the pump can pump the system up to a pretty high pressure, high enough to bust things, and the load may not blow the pumps fuse before this happens.

[DavidExpert]

I am taking all of this onboard, so to speak, and being a mechanical numpty have asked someone who knows about this stuff to fit one for me.

 

Thank you all very much.

A warning.Accumulators have been known on more than one occasion to puncture their diaphragm or otherwise leak air pressure.

If the diaphragm splits, does the water leak into the air side or the air leak into the water side? Would that explain why my accumulator had too much air pressure so I had to let some out?

[ditchcrawler]

 

 

If the diaphragm splits, does the water leak into the air side or the air leak into the water side? Would that explain why my accumulator had too much air pressure so I had to let some out?

No, what made you decide there was too much air in it, or was it a new one ?

[DavidExpert]

No, what made you decide there was too much air in it, or was it a new one ?

Not a new one, quite the opposite, probably the original. I've been having an intermittent "pump running of its own accord" issue and it was suggested to me that the pressure was too low but when a gauge was put on it, there was too much (I was bowing to the advice of the engineer bloke at the marina here at this point)

 

I am aware this usually means a leak but I can't find any leaks in the pipework, no taps are dripping. Until yesterday it had always been the case that the pump ran immediately if I put a cold tap on but would take 10 seconds before kicking in if I used the hot. Also, if I used the hot but not long enough for the pump to run, it would do so about half an hour later, just the once.

 

But then suddenly yesterday the pump started grrrr'ing for a second every two minutes. As part of investigating it, the aforementioned overpressure was discovered.

 

Since reducing the pressure, both taps now run without the pump kicking in (for about 20 seconds) then the pump runs for about 30 seconds. Then about 4 hours later it might have another little run eg this morning it went off at 3am and 5:30am but not since.

[BruceinSanity]

Not a new one, quite the opposite, probably the original. I've been having an intermittent "pump running of its own accord" issue and it was suggested to me that the pressure was too low but when a gauge was put on it, there was too much (I was bowing to the advice of the engineer bloke at the marina here at this point)

I am aware this usually means a leak but I can't find any leaks in the pipework, no taps are dripping. Until yesterday it had always been the case that the pump ran immediately if I put a cold tap on but would take 10 seconds before kicking in if I used the hot. Also, if I used the hot but not long enough for the pump to run, it would do so about half an hour later, just the once.

But then suddenly yesterday the pump started grrrr'ing for a second every two minutes. As part of investigating it, the aforementioned overpressure was discovered.

Since reducing the pressure, both taps now run without the pump kicking in (for about 20 seconds) then the pump runs for about 30 seconds. Then about 4 hours later it might have another little run eg this morning it went off at 3am and 5:30am but not since.

Sounds like the non-return valve in the pump maybe allowing the pressure to reduce by blowing back into the tank just a tad.

 

Having reduced the pressure in the accumulator, it is now working properly and storing a bit of water. They are usually supplied a little over pressure, so that the fitter can bleed a bit off to get it down to suit whatever pump has been fitted.

 

The pump may also run overnight because the cooling water in the calorifier contracts and draws a bit more from the inlet, eventually using up all that was in the accumulator and tripping the pump into running. If there's an expansion vessel on the calorifier, this supplies a bit more water to compensate as well.

[MtB]

Not a new one, quite the opposite, probably the original. I've been having an intermittent "pump running of its own accord" issue and it was suggested to me that the pressure was too low but when a gauge was put on it, there was too much (I was bowing to the advice of the engineer bloke at the marina here at this point)

 

I am aware this usually means a leak but I can't find any leaks in the pipework, no taps are dripping. Until yesterday it had always been the case that the pump ran immediately if I put a cold tap on but would take 10 seconds before kicking in if I used the hot. Also, if I used the hot but not long enough for the pump to run, it would do so about half an hour later, just the once.

 

But then suddenly yesterday the pump started grrrr'ing for a second every two minutes. As part of investigating it, the aforementioned overpressure was discovered.

 

Since reducing the pressure, both taps now run without the pump kicking in (for about 20 seconds) then the pump runs for about 30 seconds. Then about 4 hours later it might have another little run eg this morning it went off at 3am and 5:30am but not since.

 

 

My I enquire what the pressure was, that you say was 'overpressure'?

 

It's hard to imagine how the gas pressure in an accumulator can ever spontaneously rise above pump switch pressure when previously it was set correctly.

[DavidExpert]

My I enquire what the pressure was, that you say was 'overpressure'?

 

It's hard to imagine how the gas pressure in an accumulator can ever spontaneously rise above pump switch pressure when previously it was set correctly.

Unfortunately I can't tell you because, being rather ham fisted, I let loads of it out as I was attaching my air pump to the little nozzle thing, before the marina guy came over with a pump with a gauge on it and said "it's a big high, I'll let some out" and let even more out.

 

So, yes, all a bit odd but the marina chap says it's now at the right level (and he knows his stuff, I believe) and it certainly works the taps much better.

 

Apart from the two runs overnight last night though, it doesn't seem to have run spontaneously all day.

Sounds like the non-return valve in the pump maybe allowing the pressure to reduce by blowing back into the tank just a tad.

Hmm, so the pump may still need replacing? Or can you get spare "bits"? It's a Shurflo pump.

[Iain_S]

The PRV doesn't need to be located near the calorifier. It could be located anywhere on the hot water pipework. Indeed, it is sensible for it to be located together with other pipework eg ours is located in the bathroom and drains into the shower sump.

 

It could also be on the cold water side, provided it it isn't between the pump and a non return valve.

[BruceinSanity]

Unfortunately I can't tell you because, being rather ham fisted, I let loads of it out as I was attaching my air pump to the little nozzle thing, before the marina guy came over with a pump with a gauge on it and said "it's a big high, I'll let some out" and let even more out.

So, yes, all a bit odd but the marina chap says it's now at the right level (and he knows his stuff, I believe) and it certainly works the taps much better.

Apart from the two runs overnight last night though, it doesn't seem to have run spontaneously all day.

 

Hmm, so the pump may still need replacing? Or can you get spare "bits"? It's a Shurflo pump.

Depends on your irritation threshold. The pump on SA runs occasionally in that unexpected way, but not often enough to be a nuisance so we just leave it be. (We've checked the system for leaks and so on.) Edited March 12, 2016 by BruceinSanity

[Paul C]

We had a failing pump (ie it didn't hold pressure and didn't leak - so suspected it was leaking pressure back into the water tank) and soon developed the habit of turning it off in the evening, so it didn't activate at random times through the night. In theory you could put a non-return valve before the pump, which would cost far less than a brand new pump, but in practice I suspect you're buying only a little time and the pump would need replacing sooner rather than later.

[ditchcrawler]

I will never try repairing one again

[Tony Brooks]

I will never try repairing one again

 

 

Me too, also the Cummins engineer who had his boat next to us. We both got leaks we could not cure.

 

Re a NRV before the pump. In the 70s Godwin pumps had one of these and they lasted longer than the scooter inner tube they used as an "accumulator" in the pressure chamber. I always fit one at the first sign of pump trouble.

Edited March 13, 2016 by Tony Brooks