Hot feed from twin pump stalls when cold feed starts in bath mixer tap

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foxotron

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Hello All,

Hoping for a bit of advice/info in this one:

My Setup
Small system in apartment with cold tank above hot tank in the hotpress, so fairly low head pressure. Installed a pump about 15 years ago and worked fine until last week (original pump was an Stuart Turner ST55 replaced after approx 10 years with an ST20 2 bar).

The pump feeds 2 basins (bathroom hot & cold, kitchen sink hot only) as well as the shower/bath tap. Its extremely rare (or never) that the pump would be feeding more than one tap at a time.

I'm aware that I should probably have a negative head pump, but I've always been able to start the shower by lowering the shower head until flow is established, no problem, so the huge cost difference didn't seem worth it.

The Problem
In the last week only - I'm finding that when running the bath/shower tap, I can run hot water no problem but as soon as I introduce a little bit of cold water, the hot cuts out and the pump seems to stall on the hot side. This happens regardless of whether I'm filling the bath or directing the water to the shower.

I should say that both hot and cold run absolutely fine when run at the same time through separate taps, this problem only happens on the bath tap when mixing hot and cold.

What I've tried
I've opened up the bath tap by unscrewing the redirection knob (the thing that you pull up to change from bath to shower) to check for debris - nothing. I've also taken out the hot water cartridge and it looks fine. No blockages in the shower head or line feeding it either.

I figured it must be the ST20 pump, so I bought a replacement Stuart Turner showermate 1.5 bar positive head regenerative twin model, confident that this would sort the problem.

The problem still happens with the new pump.

My Questions
Anyone got any idea whats happening?
Is the problem likely to be the bath tap?
If I replace it, would I be better buying a more expensive model with a mixer cartridge built in? (this is in a hard water area so I want to avoid maintenance of cartridge if I can)

Any and all help greatly appreciated!
 
You shouldn’t just feed the kitchen tap with hot only as it’s a dual pump you need both supplies or non at all as it will effect the pump not having one side flowing eg cold side
 
ShaunCorbs said:
You shouldn’t just feed the kitchen tap with hot only as it’s a dual pump you need both supplies or non at all as it will effect the pump not having one side flowing eg cold side
Click to expand...
Thanks for the tip, I didn't know that.

Would this cause the problem with the bath mixer when the kitchen tap is off? What I don't get is why this has started happening recently after many years, and it still happens with a new pump 🤔
 
Does the basin on cold and hot work ?
 
Is the bath tap temp controlled ?
 
Could you buy/obtain one of those shower hoses that you just shove onto the hot and cold taps and connect it to the basin taps and see does it mix when you open up the taps.
another test, open the hot and cold basin taps fully, then keep throttling in the hot tap until you get a very small flow, is it still hot?.

Also check hot water temperature not > 60C.
 
Last edited:
John.g said:
Could you buy/obtain one of those shower hoses that you just shove onto the hot and cold taps and connect it to the basin taps and see does it mix when you open up the taps.
another test, open the hot and cold basin taps fully, then keep throttling in the hot tap until you get a very small flow, is it still hot?.
Click to expand...
I could but I'm not sure it would tell me anything else. The problem happens without engaging the shower. I can try the other test tomorrow but from doing it today I think the hot starts to stutter as soon as the cold is on....
 
Your problem may be further back at the combination cylinder, can you post a photo of the hot and cold draw off points.
Sounds like a cavitation problem once there is any back pressure on the pump, you could also try reducing the HW temperature temporarily, unlikely but also a possible restriction in the HW supply to the pump.
 
Last edited:
John.g said:
Your problem may be further back at the combination cylinder, can you post a photo of the hot and cold draw off points.
Sounds like a cavitation problem once there is any back pressure on the pump, you could also try reducing the HW temperature temporarily, unlikely but also a possible restriction in the HW supply to the pump.
Click to expand...
 
Thanks!

Here's a photo of the pipe work - the cold supply comes from the tank above and hot from the top of the cylinder
 

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