Stuart Turner Pump Strange Temperature Fluctuation

[Mrtrana]

Hi all,

I recently installed a 3bar twin universal stuart turner pump and I am having a strange issue with my cold water coming out hot.

The cold water from my taps come out warm then goes cool then goes back to warm and stays warm/hot. The inlet pipe to the pump is cold as normal but the outlet pipes are very warm. There is a 15mm pipe that connects the outlet of both hot and cold which is also as hot as the hot water pipes. I know this pipe is to allow the pressure vessel to keep both lines under pressure.

I spoke to stuart turner and they asked me to do a test to ensure both none return valves are working which it passed and now they are unsure why this is happening.

I find that my mixer taps are not balanced and opening the lever in the "middle position" gives more hot than cold. I have individual taps in my bath and the pressure on the hot is significantly more than the cold. It seems the hot water pressure is feeding into the cold outlet but I cannot figure out why as I would have thought the outlet pressure should be near equal on the pump.

My pump is fitted at the base of my hot water cylinder and is fed from the side of the tank and have a cold water tank in the loft.

Has anyone else experienced this? Any advice would be appreciated.

Thanks

 

[Johntheo5]

Which NRVs are they/you talking about?, is it the ones that are installed on the outlet of both the hot & cold discharge(s), I presume there are NRVs on the common 15mm line to the pressure switch so if so, how did you test them?, it looks like something wrong with the pump if the "cold" inlet is cold and its discharge is warm, but I suppose just possibly Xover at one of the mixer taps but as you said unlikely as both hot&cold pressures should be the same.
You might check and measure the flowrates from any mixer tap on both fully cold only and then on fully hot only.

Have you checked the pump suction strainers for debris, especially the cold since thats the one with the apparently poor pressure.

 

[Mrtrana]

Its the valves on the outlet of the pump. The test I was told to do was to close the inlet to hot and outlet to cold. This was to force water through the valve and out of the hot and then to do the reverse. They said a trickle/low flow is expected but if its high flow then the valves have failed. I would say the flow was more than a trickle but was not "high flow"

I have checked the strainers and they are all good. I have checked the flow rates on my mixer taps and the time to fill a jug is within 1sec of each other. The pressure on the cold is still very good but seems slightly less that the hot.

The pump is new so I can return it but didn't want to get another one and have the same issue if its something I have done that I can't see.

 

[Johntheo5]

If either of the pump NRVs are passing then the pump would be cutting in/out frequently with no demand for either hot or cold, like say at night, i can't visualize any NRVs installed on that common 15mm piping, (maybe there are) but if not and no restrictor then you should get a big flow on the tests you carried you, assumes that with the Pump Cold inlet shut and the Pump Hot outlet only shut that you are seeing this low flow rate with a Cold tap also opened? if so, can you actually measure this flowrate, then return the pump to normal and again measure the same Cold tap flowrate, with no Hot tap opened.
What is the pump design pressure?

 

[Mrtrana]

I took the 15mm pipe out to see if there was anything that could be blocked from the inlet sides but looked all good.

I get drips coming out of the taps when doing this test. I checked on a mixer tap and my single taps in the bath. I only get flow straight out the pump.

Head max is 3bar
Pmax is 6 bar

 

[Johntheo5]

Very strange, especially if the temperature of the cold water leaving the pump is the same as that coming out of the cold taps, you say that the cold water into the pump is "cold" so logically, one would suspect that 15mm connecting pipe especially as its hot, could you see right through it when you removed it, ie, no restrictor and did you check for restrictors at the pump, if no restrictors or NRVs then can only imagine that the pressure switch is a differential one, anyhow, if the flowrate from a "cold tap" is say 8.0LPM and its temperature is even only 30C, then it means that 2.3LPM at a assumed HW temperature of 60C is mixing with 5.7LPM of cold water at a present temp of ~ 18C to give 8.0LPM at 30C.
I would have said that the CWSC is at 30C if you hadn't said that the cold water into the pump is actually cold.

 

[Mrtrana]

That pipe is completely clear with nothing in it. When I took it apart I I checked the NRV's in the flow switch assembly and they had no debris in it and I blew into it to check it was sealing which it was, but it does allow air to flow through the 15mm and into the opposite outlet. I did this yesterday and surprisingly the cold doesn't seem to fluctuate hot as it did before which is strange. It still goes hot but seems to go back to cold quicker and stays at a cooler temperature than it was before. It's still not right but its better. I wonder if there actually is an issue with this pump. I have sent a video to stuart turner asking to confirm if the flow I got out in the test is classed as a fail just for my peace of mind.

 

[Johntheo5]

I wasn't thinking properly yesterday re your tests, even with that straight through piping then there should be little or no flow through it, as you showed, however, even with a perfect pump if both operating at different flowrates, I would expect a fair bit/alot of bypassing, if the Hot end is pumping say 10LPM then its head is 2.4bar and if the cold end is pumping say 5LPM then its head is 2.7bar, a differential head of 0.3bar which would bypass a lot of water through that very short length of 15mm pipe, diluting the hot flow, however if so, one would think that that would have have noticed by lots of others.
Can you check your supply arrangements from the CWSC, normally, the cold supply is piped directly to the HW cylinder and the hot cyl outlet then supplies the pump hot end inlet, the cold supply from the CWSC is taken from a different cistern outlet and piped to the pump cold end cold inlet, just check this out, if all OK then suggest getting a new pump.

 

[Mrtrana]

Just got off the phone to stuart turner and they looked at the video and said the test is a pass and the pump is fine. They said it is something after my pump but it feeds 1 mixer tap, 2 individual taps in my bath and hot water to my kitchen tap. He didn't know what else to say as the only time this happens is if the valve test fails but mine is a good and he consulted someone who said the same.

I've checked all the supply piping and everything is as it should be. I have created a bypass on this so when I swap over so the water just runs under normal gravity, I have no issues. It's when the pump runs.

I'm thinking in just getting a positive head pump which does not have this 15mm pipe connecting the 2 outlets and that will stop this issue. May not solve a bigger problem I may have but can't see what that would be.

 

[Johntheo5]

You might consider getting back on to ST and ask them about that pressure switch arrangement, ask them if that is a completely unrestriced piece of 15mm piping with the pressure switch just teed off it with no NRVs in that system as you seem to see it, and if they confirm it then ask them does that not imply that the two pumps are running in parallel with consequent dilution of one side or the other.

You might try running with a number of Hot taps only open and check if that connecting piece runs cold, then repeat the test with Cold taps open only and see does that piece of pipe then run hot, (might have suggested this previously)

Edit: You can also do a Mixing valve Xover test, just shut the cold side discharge valve and put each mixing tap to fully cold (and open), also the same with the shower, no water should come out.

 

[Mrtrana]

I do suspect it is running in parallel which does allow flow to dilute either side. I've attached an exploded view of the pump. Nothing stands out to me.

I did try something like this and when running just the hot, the 15mm pipe was very hot and the cold outlet was hot
Doing the opposite kept the 15mm pipe hot and the cold outlet was warm.

 

[Johntheo5]

It does seem to be a straight through bit of pipe which doesn't seem very clever to me anyhow, there is a 4.1MM O ring on the T off to the PG but nothing like that on the straight pipe.

You might just do that test I suggested in "edit", post #10.

 

[Mrtrana]

So I did the xover test and no water came out. When I run just cold water, the 15mm pipe gets hot and I can feel the hot water pullung through and up the cold outlet. When I run the hot water I can feel the cold running through the pipe and cooling the hot outlet. It seems that its made to do this which is strange. I managed to get my hands on a positive head pump for testing which I connected last night and works with no issues at all. I am going to return my pump this morning and look to go with positive head.

 

[Johntheo5]

Its more than strange, its almost unbelievable that its done it this manner, it makes some sense if a say a 1mm/1.5mm orifice was fitted in the pipe to give a very small flow of ~ 0.2LPM, brings in either hot or cold + a very small flow from the unused end, to stop that side overheating and burning the seals out, say washing a car with cold only.
I think originally, they had two pressure switches, one on each outlet, with the EV/pressure vessel on the hot end, they then had just one switch + the EV/pressure vessel on the Hot end only, assumes that hot is normally requred as well as cold.

 

[Mrtrana]

Yes I agree. I gave the tech team my feedback but he was adamant that was was not the pump but its clearly setup to allow flow to travel between the 2 sides. The positive head one is also significantly quieter so this is certainly the way to go for me.

Thank you for your advice. You've been very helpful