Broken negative head pump?

Hopefully someone might help me...

We have a monsoon 46410 universal 3.0bar twin negative head pump supplying a wetroom. Has been working fine for 7 years.

There was a burst main down the road on Saturday morning, we didn't realise and both had showers, wondering why it was a bit hotter than normal!

Water came back on on sunday, and the rest of the house is fine. However the shower is playing up.

Once we turn it on we get about 20 seconds of normal flow, then it slows to a dribble. If we turn it right up to hot then the flow returns. But turn it to cold and it stops completely. Seems as if the hot is going back up (backflowing?) the cold as the pipe marked 'cold to shower' in the airing cupboard gets hot. I've been through the priming process a couple of times, but hasn't done anything. I can hear the pump is running ok and runs for about 20 seconds after turning the tap off to pressurise the tank as expected etc, it just doesn't seem to be letting any cold through (or is losing a fight against the hot maybe?).

I'm guessing we have broken it somehow by running it without a cold supply on Saturday morning, but hoping someone on here may have some magic fix before we resign ourselves to £££!!!

Cheers
Harvey in Bristol

 

Ah yeah - thought id forgotten something...

Hot is from gravity fed cylinder. Approx 1m run from cylinder to pump. Cold is from mains.

Hot and cold then run to wetroom via the loft. The pump and wetroom are in opposite corners of the house, approx. 20m I guess. Both hot and cold runs have air bleed valves in the loft which seem to be working fine.

Supply goes to a Hansgrohe ibox and splits between a ceiling rose and a usual shower head on a flexi pipe. Doesn't matter which shower head I turn on, neither work properly.

Cheers

 

Oh yeah - hot supply is via a connection that was added to the side of the cylinder (Essex flange?)

 

Just checked again, yeah your right, the cold comes from the tank in the loft which is full of water, so there's defineltey a good supply available to the pump. The cold tank is nearly directly above the pump in the loft.

Its a stuart turner monsoon U3.0 bar universal pump. As I understand it, once you turn the tap off, the pump runs on to pressurise itself - there's a small tank on the top of the pump. Then when the tap is opened again it recognises the drop in pressure and turns itself on.

I'm pretty good with DIY and plumbing etc.

Now that I've seen the supply tank in the loft and that there's no shortage of cold supply - the more I think there must be an issue with the pump itself...

Don't know if these pumps are repairable, or sealed.