Vented Cylinder - Pump Keeps Cutting Out

[screwfixsparky]

Greetings Everyone,

Long time spectator, first time poster. Generally I've managed to find the answer to any issues I've come across on here, but after a lengthy search online, on here and across other forums, I still haven't been able to figure this out. I'm an electrician by trade, but have done some basic DIY plumbing in the past, including radiators, hot and cold feeds and waste.

I've got a setup (I've included a basic diagram I made up in paint below), which includes a vented hot water cylinder (direct), feeding a kitchen sink, bathroom basin and a bath shower mixer tap (not quite sure if that's the exact term for it), with a single shower pump for the hot water (I've also included pictures of this below).

The issue I have is only actually present at the bath tap/mixer, and specifically only when the showerhead is activated. All the taps work fine (even the bath tap, as long as the showerhead is not activated).

If only the hot water is turned on, the pump works as it should, pumping hot water at pressure out of the showerhead, however once the cold tap is opened even slightly, the hot water pump cuts out. I have not experienced this issue at any other point, and as I've mentioned, even the bath tap works fine when the hot and cold are adjusted together.

I can upload a video (if someone guides me on where to upload it to or how to attach it). The lady from whom I purchased the home said upon moving in, a plumber diagnosed the problem as a broken pump, however this does not make any sense to me, and before I go and spend £150+ on a new pump, I wanted to know what you guys thought.

I was also suggested that my hot water cylinder may be set too high, and the sheer heat of the water is causing a problem (again, not a plumber, but sounds far fetched).

Unless there is a problem with my setup (one pump feeding too many points maybe? or perhaps the sizing of the feeds?), I would've thought there was an issue due to imbalance of pressure.

The hot water has been ran in 22mm and the cold in 15mm (again no idea if this makes a difference or not).

Any tips or suggestions would be great. The diagram (although awfully drawn) is fairly accurate in terms of the height of the cylinder relative to the taps (the cylinder is above a washing machine, on the same floor level as all the taps in the system).

 

[Madrab]

Sound like you are getting crossflow in the mixer tap and the unbalanced cold mains might be restricting the pumped supply when it comes to mixing into the narrower hose, in effect stopping the flow through the pump. Try a single check valve on the HW supply before condemning the pump.

It's can be a problem when the supplies are unbalanced like a pumped hot and mains cold. The ideal would be anything pumped would have both hot and cold pumped, that way the supplies would be balanced.

 

[screwfixsparky]

Good Evening Madrab!

Thanks for the reply! I did think that a dual pump could be a potential solution, effectively equalising both hot and cold (regardless of whether cold needed it), albeit a more complex one than trying a single check valve.

Just to confirm, the single check valve would be placed in line, after the pump (again I've attached the diagram).

 

[Madrab]

If you are only seeing the issue @ the bath mixer then I'd be tempted to try it just before that tap. Depending on the head of water available though, sometime with a gravity systems a single check valve can slow the flow to the hot, whereby the pump doesn't see a suitable flow and fire up. That's really down to how high the cold water cistern is above the head of the shower and how sensitive the pump switch is.

 

[screwfixsparky]

If you are only seeing the issue @ the bath mixer then I'd be tempted to try it just before that tap. Depending on the head of water available though, sometime with a gravity systems a single check valve can slow the flow to the hot, whereby the pump doesn't see a suitable flow and fire up. That's really down to how high the cold water cistern is above the head of the shower and how sensitive the pump switch is.

Thanks, I've done a bit of reading and it seems your right, it should go as close to the mixer as possible. Luckily, I installed an access panel when the bath was being tiled over and I can reach the pipework right underneath the taps (just about), I'm going to pick one up tomorrow and give it a go, I'll let you know how it goes.

 

[Johntheo5]

If the cold water pressure (mains??) is greater than the pump discharge pressure then the pump flow switch will shut it down, have a look at the pump label which will give the discharge pressure (post any details), a PRV (pressure reducing valve) on the cold supply set slightly lower than the pump discharge should, IMO, solve your problem, once you have the pump details easy then to select the PRV setting.
The pump should have its own non return valve.

 

[Madrab]

The pump should have its own non return valve.

You do get them on universal/negative head pumps, for obvious reasons, but they don't come as standard on most positive head pumps.