Shower Pump Dry Run Protection Activates When Water Gets Hot

[totallychris]

I've got a negative head pump fitted to my shower which isn't working properly. The system is as follows:

• Hot water tank with integrated cold storage (combination tank/Fortic?)
• Pipes from tank into loft, then down to pump
• Negative head pump (Salamander Pumps 2.2bar)
• Thermostatic mixer shower valve (Deva Lincoln)

When I turn the shower on, the pump activates as intended, but then begins to struggle and turns off as the hot water reaches the pump. (I can tell this is when it's happening, because it's the exact point when the pump's hot inlet pipe heats up). The pump then turns off, with the "dry run protection" LED flashing.

If I hold the shower head at the bottom of the shower enclosure while this is happening, the pump will start back up. If the shower head is left at normal height, it will sometimes turn back on, but sometimes it will stop entirely and I'll need to turn the pump off and on again.

The pump also "hunts" sometimes, not often but maybe three or four times every 24hrs.

I've tried pumping the pressure vessel up to the required 14PSI, but this hasn't fixed it. I've also removed the non-return valves from the shower valve, but again, no dice.

My current working theory is that there's some air in the hot water tank, and this bubble is reaching the pump at the same time as the hot water, which is why these issues are coinciding. But I have no idea how to confirm that diagnosis, or how to solve that if it is the problem!

For what it's worth - I'm aware that a pump shouldn't be used with a Fortic tank! I didn't do the installation and wouldn't have had it done this way, but we are where we are. If I run the shower without the pump, I get essentially zero flow, so that's sadly not an option. A new system (or maybe just a new tank) is inevitable eventually but I'd love to delay that cost as long as possible.

Would really appreciate any advice - thanks in advance! And let me know if there are any details missing.

 

[Exedon]

The open vent on Fortic cylinders is really short ( nature of the beast) .
Can't see how you can possibly stop air being drawn down it when pump running.
As you already know you can't pump Fortic cylinders despite the fact someone has had a go.
Pressured cylinder probably way forward if water supply adequate.

 

[totallychris]

The open vent on Fortic cylinders is really short ( nature of the beast) .
Can't see how you can possibly stop air being drawn down it when pump running.
As you already know you can't pump Fortic cylinders despite the fact someone has had a go.
Pressured cylinder probably way forward if water supply adequate.

Thanks, that's what I feared. It does confuse me that the pump only has issues when it first starts up though, do you have any idea why that might be?
I reckon an unvented cylinder is what I'll go for when the time comes, although the flat does have a (mostly unused) gas connection so I'd also be tempted to switch over to a combi boiler - not sure that the cost would actually be that different and it gives me the option of installing radiators too rather than using electric heaters.