Poppy's Petulant Plumbing (again)

[Odana]

Last December we replaced our Shurflo pump under guarantee, and at the same time changed the PRV valve as it was banbging and leaking, and the pump was labouring before it dies.

 

Last week we replaced the pump again under guarantee. The PRV was leaking on joints, so we replaced that too. All was well... until today when the PRV started banging and dumping water into my airing cupboard (as well I assume as out the overflow pipe)

 

Can you help advise on what I need to check first, and what might be the issue here? Set up is as follows

 

• Shurflo Aquaking pump under front deck. set to factory 30psi. New last week.
• 5 litre accumulator tank next to pump, set to pressure just slightly less than pump cut out, as measured by Nicknorman's advice in last week's thread http://www.canalworld.net/forums/index.php?showtopic=61155&page=2
• 60-odd foot of pipe run to back of boat water tank cupboard
• large (domestic size - maybe 80 litre?) calorifier with immersion heater and also Aldi coil
• 3bar PRV on water tank, vented over the side
• new 8 liter expansion tank from Midland chandlers T-ed off the outlet pipe from hot water tank. set to just below pump cut out pressure. I can't remember if I set it just above or below accumulator pressure, but it was very close.
• no idea if I have a non-return valve somewhere

 

When we fitted new pump and did all the pressure set ups in the accumulator/expansion last week, all was good. The pump seemed to go on a while but we assumed that was due to now having, effectively, 2 accumulator tanks worth of water to fill. I have noticed the pump going on a bit longer during the week, but can't be sure it's not me now 'tuned in' to it.

 

The only thing I did differently today was run the Alde central heating which I think has a water coil (no idea how to switch that on/off) . However the water never got very hot from that as I used it all swiftly.

 

Could you suggest what the problem might be, and what the relative pressures of the accumulator/expansion tanks should be compared to pump and each other? What would be the simple thing to check first? The pum is a pig to get to under the front deck - needs removing completely to do any adjustments so fine tuning pressure switch on that is hard. Pump still under guarantee for another fortnight, if it may be that, but it seems unlikely

 

I hate my plumbing!! Thanks for advice

 

[bizzard]

3 bar is equal to about 43.1/2 psi so your PRV should cope with the 30psi pump, but the sudden pulses of a pump can pop it over that mark sometimes but in your case with two accumulators I doubt it. If on your calorifier especially if its a vertical one sometimes the PRV are not fitted to the top most outlet bush, instead the hot water outlet is. Well I've cured a couple of PRV's from blowing off with the Aquaking pumps by swapping the PRV with the hot water one and placing the PRV on the topmost bush fitting and the hot water outlet to the lower one and it fixed them permanently, maybe I think because there is usually always a little air that seems to find its way to the calorifiers crown and gets trapped there and so seems to cushion the valve from any sudden hydraulic shocks. These threaded bushes are usually the same sizes.

[smileypete]

Can you help advise on what I need to check first, and what might be the issue here? Set up is as follows

 

• Shurflo Aquaking pump under front deck. set to factory 30psi. New last week.
• 5 litre accumulator tank next to pump, set to pressure just slightly less than pump cut out, as measured by Nicknorman's advice in last week's thread http://www.canalworld.net/forums/index.php?showtopic=61155&page=2
• 60-odd foot of pipe run to back of boat water tank cupboard
• large (domestic size - maybe 80 litre?) calorifier with immersion heater and also Aldi coil
• 3bar PRV on water tank, vented over the side
• new 8 liter expansion tank from Midland chandlers T-ed off the outlet pipe from hot water tank. set to just below pump cut out pressure. I can't remember if I set it just above or below accumulator pressure, but it was very close.
• no idea if I have a non-return valve somewhere

 

Is the second bullet point a bit of a typo? The accumulator should be set to pressure just slightly below pump cut IN pressure.

 

When the tap is closed and the pump runs then cuts out, can you measure the pressure on the accumulator with a tyre gauge just to double check the cut out pressure is around the expected 30psi?

 

But it sounds like you'll need to first let the pressure in the accumulator down a little to say 20psi with the tap open and pump off.

 

If you haven't been able to measure the pump cut IN pressure then please say and we can post some tips on how to. But I think the first step is to check the pump cut OUT pressure and check it's not too high and in need of adjustment.

 

cheers, Pete.

~smpt~

Edited November 4, 2013 by smileypete

[nicknorman]

Yes, accumulator should be set to just below cut IN pressure, expansion vessel should be set to just below pump cut OUT pressure. However I don't think that would cause the problem since the key component here is the expansion vessel being set to around the cutout pressure, which you say it is.

 

As Pete says, maybe the pump cutout is higher than it should be, or maybe the PRV is operating at a lower pressure than it should do.

 

You say that the PRV was dumping water into the wardrobe as well as out of the overflow (which presumably exits overboard or into the pumped bilge) - in which case that sounds like something wrong with the new PRV - it shouldn't leak other than down the overflow pipe.

Edited November 4, 2013 by nicknorman

[Ark Right]

 

Is the second bullet point a bit of a typo? The accumulator should be set to pressure just slightly below pump cut IN pressure.

 

When the tap is closed and the pump runs then cuts out, can you measure the pressure on the accumulator with a tyre gauge just to double check the cut out pressure is around the expected 30psi?

 

But it sounds like you'll need to first let the pressure in the accumulator down a little to say 20psi with the tap open and pump off.

 

If you haven't been able to measure the pump cut IN pressure then please say and we can post some tips on how to. But I think the first step is to check the pump cut OUT pressure and check it's not too high and in need of adjustment.

 

cheers, Pete.

~smpt~

Hi Pete

Think the OP is talking about an expansion tank which you would usually use the cut out pressure not the cut in pressure of the pump to determine the correct pressure

Cheers

A

Edited November 4, 2013 by Ark Right

[nicknorman]

Blimey, someone likes to get the point across by repetition!

Edited November 4, 2013 by nicknorman

[Ark Right]

Of course I did it on purpose to stop Nick in his tracks and have the last word

[Odana]

Cheers all.

Accumulator AND expansion vessel are set to just below pump cut OUT pressure, as measured using nicknorman's advice. If Accumulator near pump needs to be below cut IN pressure, how do I do that?

 

Will purchase yet another PRV and see if that works. I had the one on Odana 8 years!! Poppy has already eaten 3 or 4!!

[nicknorman]

Firstly, I'm a bit puzzled that you mention the pump running for longer than usual, even though we suspect the accumulator pressure is too high (meaning it will take hardly any water)

 

Anyway, bear in mind that provided the accumulator is not completely full or empty of water, the air pressure and water pressure will be identical. There are probably various ways of doing it but this is what I would do:

 

Pump off, taps open. Adjust the accumulator air pressure to be lower than expected cut in pressure, say 10psi.

 

Taps closed, pump on. When pump cuts out, you might as well measure the air pressure to check that pump cut out is as you expect (30psi)

 

Turn a tap on to a modest flow. Measure the accumulator air pressure. It will be falling. At some point the pump will cut in, and the pressure indicated at that moment will be the cut in pressure.

 

Pump off, taps open. Adjust the accumulator air pressure to be a couple of psi below the measured cut-in pressure.

 

Taps closed, pump on. Finished

[MtB]

Best way to sort this out is to get a Morco, in my opinion....

 

 

MtB

[alan_fincher]

I'm trying to recall all previous debate on this, but am kind of surprised, (I think! ), that it is being suggested that the expansion vessel, (so I'm not talking about the accumulator), is pre-charged at less than the pump cut out pressure.

 

Unless I'm not thinking this through, that would mean that the pump always managed to pump at least some water into it before it cut out, effectively using it at least in part like an additional accumulator.

 

Surely the pre-charge needs to at least marginally higher than the pump cut out pressure? That would mean if you let the pump run on a cold system, it would not push any water into the EV. Then, as your water is heated, (and assuming none drawn off), and looking for somewhere to expand into, it has the maximum space available in the EV, before there is any danger of the PRV being used instead.

 

Consider the more extreme case - if the EV pre-charge was low enough that the pump alone was able to nearly fill it, it would be little use as an EV. I'm not suggesting you are in this extreme situation, but if there is a rationale for setting the pre-charge on an EV less than the pump cut-out, I'm struggling to remember why that might be.

 

What have I got wrong, please?

 

EDIT:

 

Found this.

 

In this very old thread Chris W, (remember him!), uses a lot of complex maths and physics to reach the conclusion I would reach using almost no maths and very much less complex physics!

 

I still think the EV pressure needs to be a few psi higher than the pump cut out pressure, not lower than it, otherwise it is in part acting as an accumulator, not an EV, as intended.

Edited November 4, 2013 by alan_fincher

[MtB]

I'm not sure about boat cauliflowers but household mains pressure unvented hot water cylinders often have non-return valves incorporated into the cold water inlet port. If this applies to boat caloriflowers then having an accumulator will afford no protection against excess pressure in it leading to the PRV discharging.

 

If there is no NRV at the inlet, as far as I can see the accumulator will act as an expansion vessel, but the symptoms listed by the OP suggest to me there IS one.

 

MtB

[nicknorman]

Alan, the optimal pressure for the EV is the pump cutout pressure. However, bearing in mind the vagaries of measuring the pressure accurately, setting the ai pressure equally accurately, that the empty air pressure will vary with temperature, and the pump cutout pressure may not be perfectly repeatable, it's also worth looking at the consequences of a slight under or over pressure.

 

For an over pressure, the vessel remains empty at pump cut out. As the water heats, the pressure has to rise a few psi before the EV starts to do anything. Thus the extra air pressure translates directly to an unnecessarily higher pressure with the water heated.

 

For an under pressure, a small amount of water will enter the vessel thus slightly reducing the effective volume. However, as soon as the pressure starts to rise more water can flow in thus minimising the increase in pressure due to thermal expansion, but with slightly reduced efficacy due to its slightly lower effective volume.

 

So which of these 2 scenarios is better? I don't think there is an answer, it depends on the relative volumes of the calorifier vs EV, how much the temperature rises and probably some more things, however my gut feeling is that a slightly lower pressure is less debilitating than a slightly high pressure in a typical scenario. In truth, it probably makes very little difference. Perhaps it would be better to just say to set to cutout pressure and accept a small error either way.

[alan_fincher]

I'm not sure about boat cauliflowers but household mains pressure unvented hot water cylinders often have non-return valves incorporated into the cold water inlet port. If this applies to boat caloriflowers then having an accumulator will afford no protection against excess pressure in it leading to the PRV discharging.

 

If there is no NRV at the inlet, as far as I can see the accumulator will act as an expansion vessel, but the symptoms listed by the OP suggest to me there IS one.

 

MtB

 

I assume, though, that Odana is not expecting the accumulator to act as an EV, now, because they have intentionally added a separate EV on the hot water outlet, so the correct side of any inbuilt NRV to stop a PRV venting.

 

I know that the pre-plumbed Surejust/Surecal units do incorporate a less than obvious NRV buried in the plumbing of the cold water inlet. It is for this reason its manufacturers say an EV should always be fitted on the hot water side.

 

Rereading her original posts, I'm convinced the pressures in both vessels are incorrectly set.

 

The accumulator should be less than pump cut-in pressure. that's typically about half the cut out pressure, as I understand it, so should be less than 15 psi I believe.

 

The EV should be pre-charged higher than the pump cut out pressure of 30 psi.

 

From memory our set up with a similar pump has the accumulator pre-charged to only about 13 psi, whereas the EV is at something like 33 psi.

 

No way should both be close to the same number, so that's the first thing to sort out IMO.

[nicknorman]

I'm not sure about boat cauliflowers but household mains pressure unvented hot water cylinders often have non-return valves incorporated into the cold water inlet port. If this applies to boat caloriflowers then having an accumulator will afford no protection against excess pressure in it leading to the PRV discharging.

 

If there is no NRV at the inlet, as far as I can see the accumulator will act as an expansion vessel, but the symptoms listed by the OP suggest to me there IS one.

 

MtB

Certainly our boat has an NRV at calorifier inlet. I think its fairly common, to prevent the possibility of very hot (80 deg) water from coming out of a cold tap near the calorifier. Even without one, because the accumulator will be mostly full at pump cutout, it's not very good at absorbing increased water volume due to temperature rise. An EV, pressurised to remain virtually empty at cutout, will have a much better ability to absorb expansion.

[alan_fincher]

So which of these 2 scenarios is better? I don't think there is an answer, it depends on the relative volumes of the calorifier vs EV, how much the temperature rises and probably some more things, however my gut feeling is that a slightly lower pressure is less debilitating than a slightly high pressure in a typical scenario. In truth, it probably makes very little difference. Perhaps it would be better to just say to set to cutout pressure and accept a small error either way.

 

I do remember that it is (very) surprising if you calculate how big an EV theoretically needs to be to cope with the very largest range of expansion that you could get between a tankful of cold water and a tankful of hot water.

 

If Odana's numbers are right, their 8 litre EV is servicing a calorifier of probably at least 80 litres.

 

Now in my head allowing for 10% expansion sounds ample, but I'm sure I have seen calculations that suggest that theoretically you need more than that.

 

If that were the case, then wasting any of that 8 litre capacityby setting the pre-charge to low, means you have less available space for actual expansion before the PRV might open.

 

As many calorifiers now come with 4 bar PRVs, which is around double the pump cut out pressure, my assumption is that most calorifiers are good for 4 bar. So I really can't see what issue you would face if the pressure had to rise marginally over 30 psi before anything flowed into the EV. You still have a huge safety margin.

 

My choice would be slightly over, rather than slightly under, (what we have done), but as long as it is close, it probably doesn't make a huge difference, I accept.

 

Too late to go and remind myself why such large EV's are recommended, but I might tomorrw if this is still running!

 

The accumulator pre-charge however is clearly set to high, (to serve its intended purpose) - however I can't see any way that fact would cause water to get discharged from the PRV - in fact, if anything, I'd actually expect it reduced the possibility, provided there is no NRV present, as it could be further increasing the expansion space available.

[nicknorman]

So its wasted space vs pressure offset up by the over pressure. There's only one way to find out.....F - no, that ones getting a bit old hat!

 

I certainly agree with your last para.

 

Our installation has an NRV and no EV. We accept that a cupful of water or two may discharge harmlessly via the PRV. So what, it seems much simpler than all this EV lark! That said, we have a horizontal calorifier which I suspect traps air at the top thus giving a degree of EV function.

[bottle]

Odana, are you still with us.

 

A revision of all above.

 

you have in order,

 

pump (30 psi)

 

an accumulator, set it to 10 psi, (tap open pump off),

 

Calorifier

 

PRV. (45 psi approx),

 

expansion vessel, set it to 30 psi. (calorifier cold but system up to pressure.)

[smileypete]

80L calorifier and 8L expansion vessel looks like it's cutting it a bit fine, not a lost cause with the current setup but might need the pump pressure tweaking down a little.

 

Normal worst case heating the calorifier from 4°C to 85°C would cause 2.6L of expansion, causing the pressure in the expansion vessel to rise from 30psi to 40psi, still just within the rated lift pressure of a 3 bar PRV. (3 bar is 48psi).

 

First thing to do might be to measure the pump cut out pressure, if its too high there's no point in setting the expansion and accumulator pressures til it's adjusted.

 

To do this, with pump off and cold tap opened, let the accumulator pressure down to say 10psi, then close tap, switch on pump, after it's cycled off read the new pressure in the accumulator, it should now read the pump cut off pressure.

 

ETA Something that can make things easier to check and diagnose is a PRV with built in gauge:

 

http://www.ebay.co.uk/itm/3-Bar-Pressure-Relief-Valve-With-Gauge-/280923665374

 

 

cheers, Pete.

~smpt~

Edited November 5, 2013 by smileypete

[ditchcrawler]

I'm not sure about boat cauliflowers but household mains pressure unvented hot water cylinders often have non-return valves incorporated into the cold water inlet port. If this applies to boat caloriflowers then having an accumulator will afford no protection against excess pressure in it leading to the PRV discharging.

 

 

MtB

You could well have a point there.

[dmr]

How do you know that your pump really is 30psi????

 

if you fit a pressure guage like Pete suggests you might find its a fair bit more than 30psi, you might even find that the pressure oscillates a bit whilst the pump is building up the pressure. You could try turning the pressure down using the adjusting screw on the bottom of the pump but this is not always satisfactory as it adjusts both the cut in and cut out pressures which can lead to an "unsatisfactory shower experience".

 

Fitting a Jabsco pump will likely solve this problem.

 

I had a similar problem and fitting a pressure guage really helped to work out what was going on.

 

..........Dave

[Odana]

Still here, reading avidly between classes and marking, and processing the To Do list. Thanks all

 

I checked the cut out pressure when we fitted pump last week - it was about 29 psi if I recall which is pretty much what it should be. I discovered last year that turning it down below that was not good for the consistency of water flow - the 'unsatisfactory shower experience' dmr talks of! It may of course have leaped up in the week due to it being a crap shurflo (new) - we exchanged it under guarantee because the previous pump was reading 40 psi and refusing to adjust down - that was 30 in its early life too.

 

Will do all pressure checking of pump/accumulator/EV on Saturday again, to see nothing has moved. In the meantime I like the PRV with gauge - ebay here I come!

[bizzard]

Still here, reading avidly between classes and marking, and processing the To Do list. Thanks all

 

I checked the cut out pressure when we fitted pump last week - it was about 29 psi if I recall which is pretty much what it should be. I discovered last year that turning it down below that was not good for the consistency of water flow - the 'unsatisfactory shower experience' dmr talks of! It may of course have leaped up in the week due to it being a crap shurflo (new) - we exchanged it under guarantee because the previous pump was reading 40 psi and refusing to adjust down - that was 30 in its early life too.

 

Will do all pressure checking of pump/accumulator/EV on Saturday again, to see nothing has moved. In the meantime I like the PRV with gauge - ebay here I come!

Its worth trying the PRV high up on the calorifier or higher even perhaps by using an extension stalk with possibly an elbow if the thing is situated on the cals side to take advantage of the cushioning effect of the bit of air that is usually trapped up there. Those pumps are a bit unstable and can give powerful spasmodic pulses. I feel sometimes an odd pulse can be well over their supposed 30psi cut out pressure. Once a valve is knocked off its seat say at 3bar and discharges they don't generally reseat fully and tightly until the pressure has dropped a little below 3 bar, often quite a bit below.

[Odana]

Update. New PRVS with built in pressure gauge arrived and fitted today. This tells us that the pump cut OUT pressure is 2.9 bar, or about 40psi, very different from the 30 psi it was cutting out at when fitted a fortnight ago. This is the reading when cold so any heat may up the pressure in tank and trip the PRVS. The pump cut IN pressure seems to be somewhere about half to 3/4 bar.

 

Stage next is to dismantle front deck to re-adjust the pump pressure but this seems a very wide range to me and I suspect turning pump down will cause spluttering. We also have to adjust pressures in the accumulator and EV but I don't think this is related to the cut out pressure?

 

John insists that it can't be yet another faulty pump. I think it can. Any views before we hike back up the M6 to midland chandlers to exchange it again? If we DO have to change pump the replacement will only be a spare and we will also buy a jabsco - I am sick of this.

[nicknorman]

Depending on the design of the specific pump, you may be able to fit an external pressure switch. You would need to be able to extricate the wires to the internal pressure switch, although I suppose if you wound the pressure up to its max value, the internal switch would always stay on, and you could then control the pump using an external pressure switch just in the normal wiring to the pump. This makes it easier to replace just the switch if it fails again.