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

[foxotron]

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!

 

[foxotron]

If you just run with a hot and cold tap fully open and you then keep closing the hot tap, does it still keep flowing hot water right down to a trickle until almost fully closed?.

One other thing, there may have been a special type of dip tube installed in the top of the cylinder 15 years ago when pump installed, this (if installed) may be partially blocked so something else to check out some time.

I'll check on the first thing.

The initial installation 15 years ago was done by a cowboy as I later found out. I doubt he opened the cylinder if I'm honest. When the original pump died and was replaced by a proper plumber, he may have installed the dip tube but I wouldn't count on it.

I guess we are saying that there may be a partial air block in the pipework above the cylinder and the normal best practice ways to prevent this might not have been followed? If so it seems I need to call in a pro!

 

[John.g]

I suspect there isn't a vent. That fitting looks to me to be a non return valve, have a closer look (maybe photo) you should see a arrow on it but doesn't matter, that shouldn't be fitted either but again it worked for years with it installed so might be partly seized, mind you its surprising that the proper plumber didn't remove it, maybe he installed it!.
Not having a dip tube doesn't necessarily mean it won't work.

Getting a pro in is a very good idea because with (if) no vent and if the isolating valve on the cold feed to the cylinder is shut then a very dangerous situation when cylinder starts heating.

 

[foxotron]

I suspect there isn't a vent. That fitting looks to me to be a non return valve, have a closer look (maybe photo) you should see a arrow on it but doesn't matter, that shouldn't be fitted either but again it worked for years with it installed so might be partly seized, mind you its surprising that the proper plumber didn't remove it, maybe he installed it!.
Not having a dip tube doesn't necessarily mean it won't work.

Getting a pro in is a very good idea because with (if) no vent and if the isolating valve on the cold feed to the cylinder is shut then a very dangerous situation when cylinder starts heating.

Thank you so much. I am now officially out of my depth and will call a plumber tomorrow first thing!