Water Systems

[stagedamager]

Evening all!

On Oak, we have one water pump to service both the hot and cold systems, as i have always thought it should be. However on Ash, the tank feed splits into two, and both the hot and cold legs have their own individual pumps. Is this normal? I can only assume that it is to help with the showers, as the majority of the accomodation was there when they ran as hotel boats. However, the installed pumps were of different ratings and when showering this causes the pumps to cycle at different times, and therefore the water temp in the showers is up and down like a yo yo! In the box of spares is a large 38 gpm 45psi pump. My question is would i be better re plumbing both systems onto this one big pump or buying two smaller, but matched pumps??

Thanks in advance

 

Dan

[Speedwheel]

It's not uncommon. It's very helpful if you have a gas water heater and you want to fill the kettle while someone is in the shower without the boiler going out.

[stagedamager]

but no gas water heater. Just an Eberspacher and 2 showers.

[Speedwheel]

To prevent losing shower pressure when using kitchen cold tap?

[stagedamager]

no kitchen, it's all cabins.

there are wash hand basins however.

But then why not do a twin system on Oak, which had 3 hand basins, a sink and large shower. I'm very confused!!

[Neil Smith]

You could leave as is and put in a thermostatic shower head.

 

Neil.

[MtB]

It's a dopey system, twin pumps. Just leads to unnecessary complication and poor shower performance. I suggest you revert to one pump.

 

MtB

[smileypete]

The 45psi pump may well be tooo much pressure for a calorifer if it can't be adjusted down.

 

You'd then need to add a pressure reducing valve, £15 from the likes of Ebay.

 

cheers, Pete.

~smpt~

Edited February 21, 2015 by smileypete

[nicknorman]

Two pumps means they cycle out of phase thus constantly changing the temperature of warm (ie hot and cold mixed) water. Even with a thermostatic mixer I think there's likely to be some temperature fluctuations. Best to revert to 1 pump and keep the other for a spare. You could even plumb itin with isolators so you could rapidly switch to the other pump, or even keep both in parallel, both feeding hot and cold.

[dor]

I installed twin pumps when I fitted out my boat, with a link with a full flow ball valve between the hot and cold. My thinking was that it would help with shower performance, and the thermostatic mixer should keep the temperature constant. The ball valve was to link the two systems if one pump failed.

 

In practice, the pumps cycled at different times and the thermostatic mixer couldn't react fast enough. After some fiddling I gave up on the idea and now use one pump with the ball valve permanently open. The pump (Par Max 2.9) is more than adequate, even if a tap is turned on in the galley while someone is having a shower, and I have a reserve pump which can be switched to by the flick of a switch.

 

So no, don't bother with twin pumps unless you want an easily switched backup.

 

 

Edited to make sense of the mashup caused when it was posted.

Edited February 21, 2015 by dor

[smileypete]

A decent size accumulator and pressure reducing valve, set to say 1 bar, can give a perfectly stable flow

 

But provided the pumps cut out higher than 1 bar and can keep up with demand.

 

cheers, Pete.

~smpt~

Edited February 21, 2015 by smileypete

[nicknorman]

A decent size accumulator and pressure reducing valve, set to say 1 bar, can give a perfectly stable flow

 

But provided the pumps cut out higher than 1 bar and can keep up with demand.

 

cheers, Pete.

~smpt~

It will give a stable flow, but at 1 bar a pretty slow flow. I find our hot flow is significantly less than the cold, and I put this down to the NRV at the inlet to the calorifier. I wonder what its opening pressure is? I suspect, close to 1 bar.

[MtB]

It will give a stable flow, but at 1 bar a pretty slow flow. I find our hot flow is significantly less than the cold, and I put this down to the NRV at the inlet to the calorifier. I wonder what its opening pressure is? I suspect, close to 1 bar.

 

Usually kept closed by a hairspring so a lot less than one bar. One can fairly easily blow through a spring-closed NRV.

 

My lungs can only generate up to about half a bar (IIRC, when I tested), and that was with me blue in the face with effort. Surprisingly low!

 

I'd guess a cauliflower NRV opens at about 0.1 bar.

 

 

MtB

[nicknorman]

 

Usually kept closed by a hairspring so a lot less than one bar. One can fairly easily blow through a spring-closed NRV.

 

My lungs can only generate up to about half a bar (IIRC, when I tested), and that was with me blue in the face with effort. Surprisingly low!

 

I'd guess a cauliflower NRV opens at about 0.1 bar.

 

 

MtB

OK So I'm not quite sure why there is such a big difference in the flow rates then. Yes the hot has to go slightly further, but not much in % terms to eg the bathroom basin. When the hot is on that valve does make quite a noise, it certainly sounds as though the water is struggling to get past. Maybe there are NRVs with a higher opening pressure? (and we have one of those?)

[smileypete]

It will give a stable flow, but at 1 bar a pretty slow flow. I find our hot flow is significantly less than the cold, and I put this down to the NRV at the inlet to the calorifier. I wonder what its opening pressure is? I suspect, close to 1 bar.

 

1 bar is 10 metres head, which should give pretty good flow, but would need a fair size pump on a boat.

 

Sounds like your NRV is a somewhat restrictive double check valve, there's a difference between the 'cracking pressure' (usually pretty low) and the size of the flow path through the valve.

 

cheers, Pete.

~smpt~

Edited February 21, 2015 by smileypete

[FadeToScarlet]

I'd be inclined to keep both pumps wired and plumbed, but make one the main one and, with a T port valve, the other a backup- because it's guaranteed to conk out at the most inconvenient time, such as when SWMBO is in the shower getting ready for an important night out, and turning a valve and flicking a switch to bring the backup on line can avert inconvenience and domestic disharmony straight away!

[dccruiser]

As has been said i would revert to one pump and a decent accumalator, but i do everything according to the K.I.S.S. principal .... if its tried and tested, dont change it!

 

Rick

[blackrose]

It's a dopey system, twin pumps. Just leads to unnecessary complication and poor shower performance. I suggest you revert to one pump.

 

MtB

Yes, I agree. If a pump is selected based on its capability of running a certain number of outlets at the same time, then there's no need for two.

[smileypete]

OK So I'm not quite sure why there is such a big difference in the flow rates then. Yes the hot has to go slightly further, but not much in % terms to eg the bathroom basin. When the hot is on that valve does make quite a noise, it certainly sounds as though the water is struggling to get past. Maybe there are NRVs with a higher opening pressure? (and we have one of those?)

 

Dug out a pic of a standard check valve/NRV, showing how restrictive it is:

 

 

Looks to me the inner diameter of the orifice is half the pipe diameter, so 25% of the area which is rather restrictive.

 

There will be plastic on the other side which restricts things further, plus a double check valve will have two of these in series.

 

A trawl on Ebay should turn up a 'full flow' single check valve which has a much fatter body:

 

 

Or use a 22mm one with a couple of adapters.

 

cheers, Pete.

~smpt~

Edited February 22, 2015 by smileypete

[MtB]

OK So I'm not quite sure why there is such a big difference in the flow rates then. Yes the hot has to go slightly further, but not much in % terms to eg the bathroom basin. When the hot is on that valve does make quite a noise, it certainly sounds as though the water is struggling to get past. Maybe there are NRVs with a higher opening pressure? (and we have one of those?)

 

 

I'd say it is due to the cross sectional area of the open NRV being quite small, along with the hydraulic resistance of the extra length of pipe especially if it is 15mm HEP20.

 

HEP20 is only 11mm in diameter internally whilst copper is 13.6mm. A reduction of 35% in CSA, so turbulent flow will therefore kick in at a far lower flow rate with HEP20 than copper. As I suspect (and you will know) turbulent flow soaks up a lot more energy than laminar.

 

Piping up with 22mm tube will probably eliminate the difference.

[nicknorman]

Maybe it's a double check valve? Dunno, but this is what it looks like:

 

Check valve by nick.norman, on Flickr

[smileypete]

Looks like a double, swapping it for a full flow single should help get the flow close to what comes from the cold side.

 

The doubles are often used to meet water quality regulations on mains supplies, where a bit of restriction won't make a lot of odds for something like a garden tap.

 

Seeing as it's push fit, could try replacing with a length of plain pipe to see how performance may improve.

 

cheers, Pete.

~smpt~

Edited February 22, 2015 by smileypete

[MtB]

Yep, a 15mm double check valve. Very restrictive with a tiny, tiny water path through it.

 

I was expecting to see a 22mm single built into the cauliflower inlet. I'd suggest fitting a 22mm check valve like this if the low flow rate bothers you:

 

 

 

http://www.ebay.co.uk/itm/22MM-FULL-BORE-NON-RETURN-VALVE-/251638131812?_trksid=p2054897.l4275

 

Beware many check valves with 22mm connections, they have the same tiny 15mm plastic component mounted inside as your current fitting.

 

 

MtB

[nicknorman]

Ok thanks guys, i guess that explains it. At pump cutout pressure (30 psi) the flow rate is fine, but at cut in pressure (20 psi) the hot flow is quite weak compared to the cold. Now I know why!