Hot water system problem

[Machpoint005]

Could I prevail upon the good nature of forum members to verify my logical(!) thought processes here?

(In other words, what do you think).

 

Domestic water system: new Shurflo 30psi pump fitted last year. There is an accumulator on a tee immediately downstream of the pump, and the pump has a local switch. The calorifier is vertical, and has a pressure relief valve on top piped to an overboard dump. The PRV claims to be rated at 2 bar. Calorifier and PRV may well be 15 - 20 years old. We're in a soft water area but quite a lot of crud has come through the system (rust particles) over the last several months.

 

Problem is that the pump, on demand, pumps water quite happily, but when you would expect it to cut out, ie when the taps are closed, it slows right down but doesn't switch off. The PRV then starts a 'dagadagadagadagadaga' sort of noise as if it is trying to stay shut but can't. Twisting the knob on it either has no effect, or dumps water overboard.

 

My diagnosis is that there may well be muck in the system, but I think the 2 bar PRV and 30psi pump are now mismatched, because the PRV should stay shut under normal system pressure. My best course of action is therefore to replace the PRV with a new one rated at 2.5 bar or even 3 bar. I don't see any reason why the calorifier couldn't withstand the increased pressure that might result.

 

Any thoughts, please?

 

 

[nicknorman]

2 bar is 30 psi near enough, so this could never be a good system. A tiny mis-calibration of the pump pressure or the PRV would give the results you describe. 2 bar does seem low for the PRV, but it may be thus for a good reason. If you fit a PRV with a higher pressure rating you will be exposing the calorifier to a higher pressure which it may or may not be designed for. First step would be to determine the manufacturer's pressure rating for the calorifier.

[OldGoat]

PRVs don't last forever. I'm on my third, 'co they get furred up and you can't (really) clean them. If the system has worked satisfactorily, then it's time to change the PRV.

 

AFAIK 'normal' systems have a 30 psi pump and a 3 bar (45psi) PRV. If bot are the same rating then the PRV is bound to cut in.

 

Newer boats seem to be fitted with 4Bar PRVs.

 

 

[smileypete]

Sounds like the pressure switch in the pump is bunged up with crud.

 

The pressure switch is sometimes in a 'cube' on the end of the pump with the wires coming out, and can be taken off/apart and the little 'bleed hole' cleared of crud, but best done while the chandlers are open!

 

If the accumulator is checked with a digital tyre gauge it should show how high the pump pressure is cutting off at.

 

cheers, Pete.

~smpt~

Edited April 13, 2014 by smileypete

[alan_fincher]

I agree with the diagnosis that a 30 psi cut-out pump and a 2 bar PRV are a poor combination.

 

Even if everything was calibrated spot on it could be prone to leaking, and if anything is out of spec it is almost inevitable.

 

It would be an unusual calorifier that is not good for 3 bar, but as Nick says, worth trying to see if you can find data for what you have.

[smileypete]

Has it always been like this?

 

The pressure switch is usually adjustable and can be unscrewed a bit to reduce the cut out pressure a little and give some 'wriggle room', but that won't solve a gunged up pressure switch.

 

cheers, Pete.

~smpt~

Edited April 13, 2014 by smileypete

[Machpoint005]

Problem developed intermittently after the new pump went in, but everything worked fine until very recently, and now I can't cure it. For the sake of less than a tenner for a new PRV I'm inclined down that route, but it may be worth tweaking down the pump pressure first?

[Iain_S]

I agree with what others have said, but it might be worth checking accumulator pressure as well, especially if the pump slows immediately on switching off the tap(s). If it isn't doing it's thing, I can see how the last stroke of the pump could cause pressure to rise enough to open the PRV, which releases enough water to lower the pressure to below pump cut in.

 

Iain

[ditchcrawler]

Problem developed intermittently after the new pump went in, but everything worked fine until very recently, and now I can't cure it. For the sake of less than a tenner for a new PRV I'm inclined down that route, but it may be worth tweaking down the pump pressure first?

That is a bit like saying because the fuse keeps blowing I will put a bigger one in because fuses are cheap.

[Machpoint005]

That is a bit like saying because the fuse keeps blowing I will put a bigger one in because fuses are cheap.

False analogy. I'm actually saying that the whole system pressure will be uprated slightly.

 

A 2.5 bar PRV seems to me the obvious solution, so that the system runs at a slightly higher pressure within the capabilities of the calorifier. The PRV isn't there to relieve system pressure, it is to stop the calorifier going bang when it is full of hot water.

I agree with what others have said, but it might be worth checking accumulator pressure as well, especially if the pump slows immediately on switching off the tap(s). If it isn't doing it's thing, I can see how the last stroke of the pump could cause pressure to rise enough to open the PRV, which releases enough water to lower the pressure to below pump cut in.

 

Iain

 

It doesn't -- it continues to run for a second or two (or always did so until the PRV problem occurred) in order to repressurise the accumulator.

[nicknorman]

A 2.5 bar PRV seems to me the obvious solution, so that the system runs at a slightly higher pressure within the capabilities of the calorifier. The PRV isn't there to relieve system pressure, it is to stop the calorifier going bang when it is full of hot water.

I'm just not quite sure that you have grasped the potential problem with this. If the calorifier is only designed for 2 bar, and you fit a 2.5 bar PRV, the pressure will get to 2.5 bar when the calorifier heats up and in fairly short order the calorifier will split, make a right mess, and have to be replaced. Of course it could be that the calorifier is designed for 2.5 bar or more, but then one wonders why someone would fit a 2 bar PRV.

[Machpoint005]

I'm just not quite sure that you have grasped the potential problem with this. If the calorifier is only designed for 2 bar, and you fit a 2.5 bar PRV, the pressure will get to 2.5 bar when the calorifier heats up and in fairly short order the calorifier will split, make a right mess, and have to be replaced. Of course it could be that the calorifier is designed for 2.5 bar or more, but then one wonders why someone would fit a 2 bar PRV.

 

Yes, I understand the basis of the PRV rating, and fully intend to check the pressure rating of the calorifier before taking this step. The system is, as far as I know, the original one, which would make it 19 years old, but the unknown is what margin of safety would have been built into a calorifier made in the 1990s. I suspect that a 2 bar copper cylinder would easily be capable of running to 2.5 bar, but less likely to 3 bar.

[Machpoint005]

I suspect a safety factor of 3 would not be unreasonable, but as I say, it's an old calorifier.

[David Mack]

Surely its better to turn down the pump cut out pressure, rather than increase the PRV rating. If the old pump didn't have this problem then it was almost certainly set to a lower pressure. So turn down the new one and you will be back where you were before.

[Machpoint005]

Thanks David -- that's still plan A.

[BruceinSanity]

AIUI, older calorifiers are more robust, as copper was cheaper in them olden days, so the cylinder walls are thicker.

[Machpoint005]

...but older calorifiers have been heated and cooled many many more times than new ones,so the seams might have weakened over time?

[Machpoint005]

Quick update -- pump cut-out pressure turned down with the central adjustment screw, problem solved.

[ditchcrawler]

Quick update -- pump cut-out pressure turned down with the central adjustment screw, problem solved.

Good. The right way to go IMO