Stuart Turner Negative Head 3bar Twin

[John Carroll]

Has anyone noticed any problems with this type of pump where the cold outlet can and does get warm/hot and the hot outlet goes cool depending on whether more hot or cold water is required, this is because the hot and cold outlet are directly connected with a short piece of 15mm (~ 13mm ID) piping on which the pressure vessel & pressure switch are mounted, this, in effect, means you have two pumps running in parallel, with no control over the hot/cold mixing, this is a brand new pump, the owner took the discharge ends apart to confirm that there was no orifice plate in this piece of pipe, despite this ST would not admit that there is a apparent problem with this design, luckily, he found that a positive ST pump is quite adequate for his requirements and returned the above pump. I know this sounds almost unbelievable that no orifice of say 1.0/1.5mm is installed in this bit of pipe which would still allow the pressure switch to operate properly but only allow say a "bypass" of ~ 0.2LPM which wouldn't cause too much dilution even though not ideal especially from a renowned outfit like Stuart Turner.

If you want to test the above, just open a cold tap only for a few minutes and see does it then run warm/hot and then open a hot tap only and see does this then run cool, the connecting bit of pipe should run either very hot or just cold during these tests.

Stuart Turner Schematic.docx

[Alan Ambrose]

That’s curious - I have a couple of those pumps and never noticed that problem. One is 30 years old and one 20-25 ish. The pressure vessels used to fail within, say, 5 years, and the kits from ST got a bit silly in price. So, I replaced those with 3rd party ones maybe 10 years ago and they’re good.

 

I wonder whether you got an example with a fault on it.

[John Carroll]

Yes, Alan, that was my immediate thought, it would seem utterly bizzare if the pump is constructed as shown. ST asked the owner to check that both NRVs were OK, which they were, ST then said the problem could be due to Xover in a mixing tap, tests were carried out, there is/was no X over. That piece of 15mm short pipe, say ~ 500mm long will only have a friction loss of well less than 0.1M at a flow rate of 7LPM, so, in the context of the pump flowrates, almost a full by pass, if unrestriced or controlled in some manner.

 

Do your pumps have this single piece of pipe with the PG & pressure vessel mounted on top?, I have seen some types of negative head stuart turners with two pressure switches and two pressure vessels and also I think some with just one pressure switch and one pressure vessel mounted on just one side, presumably expecting that hot and cold water are both required.

 

Could you please post a photo of one of these pumps showing the pressure vessel/pressure gauge etc. and maybe carry out the hot/cold only tests sometime.

Edited Saturday at 06:58 by John Carroll

[Alan Ambrose]

Sure will do, not at that location at the moment though. From memory, it looks pretty much like the picture you posted, but with my pressure vessel mod. I’ve never really thought much about how these work - I guess when only hot or cold is used, one pump just thrashes. It looks like there are a couple more sensors additional to the pressure switch - not sure what those are for.

Edited Saturday at 07:40 by Alan Ambrose

[Nickfromwales]

1 hour ago, Alan Ambrose said:

Sure will do, not at that location at the moment though. From memory, it looks pretty much like the picture you posted, but with my pressure vessel mod. I’ve never really thought much about how these work - I guess when only hot or cold is used, one pump just thrashes. It looks like there are a couple more sensors additional to the pressure switch - not sure what those are for.

They have flow and pressure switches. 
 

[John Carroll]

15 minutes ago, Nickfromwales said:

They have flow and pressure switches. 
 

 

There will always be two flowswitches, the one pressure switch may be on the system as described above or there maybe just the pressure vessel+pressure gauge taken off one outlet end, in which case the pumps are completely and properly isolated from each other. Have you got a link to that schematic above that shows a outline of the pump associated with it.

[John Carroll]

Just spoke to someone in Dublin who overhauls/reconditions these (ones with the 15mm connecting pipe) and he confirmed that the pipe has no reduced ID O rings etc, the only restrictor is the 4.1mm ID O ring  where the pressure vessel is mounted on the T piece, the schematic shows 15MM ID O rings at either end so certainly looks like a unrestricted 15mm piece of pipe between the two outlets, extraordinary, and IMO can only result in dilution, varying from very little if both sides puming similar volumes of water to pretty hefty dilution is only hot or cold water required from one end.

 

[Nickfromwales]

This is what I just scan read through. 
 

https://www.stuart-turner.co.uk/product/monsoon-u3-0-bar-twin-46410

[Nickfromwales]

6 minutes ago, John Carroll said:

Just spoke to someone in Dublin who overhauls/reconditions these (ones with the 15mm connecting pipe) and he confirmed that the pipe has no reduced ID O rings etc, the only restrictor is the 4.1mm ID O ring  where the pressure vessel is mounted on the T piece, the schematic shows 15MM ID O rings at either end so certainly looks like a unrestricted 15mm piece of pipe between the two outlets, extraordinary, and IMO can only result in dilution, varying from very little if both sides puming similar volumes of water to pretty hefty dilution is only hot or cold water required from one end.

 

I’m assuming this is to balance the chambers and possibly the output supply?

 

If it doesn’t affect the shower, then I guess a design that allows it to work is not a flaw? 

[Nickfromwales]

I’ve fitted loads of these, always ST, and never a moments grief; other than them clapping out after 100k miles, with a full military funeral. 

[John Carroll]

4 hours ago, Nickfromwales said:

I’m assuming this is to balance the chambers and possibly the output supply?

 

If it doesn’t affect the shower, then I guess a design that allows it to work is not a flaw? 

 

I doubt if theres any requirement for balancing, the positive head pumps don't have any connecting pipe I think.

 

I wouldn't have any big worries with a shower where the HW is say 60C and the cold 10C, you need 9LPM at 60C mixing with 6LPM at 10C to give 15LPM at 40C, the hot might be reduced to say 50/55C by dilution but no problem for a thermostatic shower, however if say running the HW cylinder at even 50C where you the require 11.25LPM of hot mixing with 3.75LPM of cold to give 15LPM at 40C, this might well result in significant dilution to then cause shower temperature control. People with ASHPs and vented HW cylinders probably often run with a cylinder temperature of 45C or even lower, this might result in a HW temp as low as 35C, not too pleasant for either showering or normal kitchen use.

Anyhow, all it requires is for someone with one of the hundreds and hundreds of these pumps  to run a few tests at different flow rates and temps, that will soon put all this to bed, hopefully not in the way  I fear it might.

Edited Saturday at 14:45 by John Carroll

[Nickfromwales]

1 minute ago, John Carroll said:

People with ASHPs and vented HW cylinders probably often run with a cylinder temperature of 45C or even lower, this might result in a HW temp as low as 35C, not too pleasant for either showering or normal kitchen use.

They will likely have unvented hot water cylinders, so the point is moot?

 

[John Carroll]

I'll just leave it at that. 😀