Shower problem; vacuum forming in hot water pump inlet

[Rich40w]

Unusual one that to date has confused several plumbers;
I have a Salamander pump that is powering a shower. The issue I occasionally get is the shower just goes to a trickle particularly if I go away for a weekend. The problem is fixed by isolating the pump feeds and then sucking the hot water feed through the pump hot water chamber to break an apparent vacuum lock in the hot water entry from my hot water cylinder. If I turn off the hot water when I go away for a few days, it normally works and let’s the shower work fine but over the weekend it went to a dribble even though the shower was used on both days.
I am thinking perhaps there is an air leak that as the hot water in the cylinder cools, it perhaps sucks air in to cause air pockets but would love to hear if anyone has a killer idea that may solve this long running issue.
Thanks

[chrrris]

What are the relative heights of the cold water cistern, the pump and the shower head, and what specific model of pump is it?

The reason I'm asking is that if a positive head pump is used in a negative head (or marginal) scenario, then the problem you're describing is one of the potential issues I believe. But even if that isn't the problem, the above info may help someone work out what's going on. TBF, I'd have expected the plumbers who've already looked at it to have considered the above, but you never know...

[Bob225]

What size is the hot feed to the pump, It sounds like the hot side of the pump maybe getting the water pulled back by running other hot taps or the seal/impeller on the pump is starting to fail

what has been done so far by the plumbers ? what model of pump is it

[Rich40w]

Ok so the cold water cistern is about 10 ft above the pump in the loft, pump sits in the airing cupboard (450 litre cylinder) about 30 cm from the floor. Shower head is about six foot higher with pipe work going up from the pump, into the loft, across by about 10ft and then down. Pump is a Salamander ESP 50 CPV with 25mm pipework entry/exit.
Work to date has been to fitting the pump, fitting a u-bend on the hot water entry, supposedly replacing the S-valve, fitting bleed nipples in the loft.....that’s what I remember!
Any thoughts?

[chrrris]

Hmmm... it sounds like the pump is fitted within spec. So I'm afraid that I'm all out of ideas. Maybe someone with more experience will have a suggestion or two, but I reckon it's going to be a tricky one to solve remotely as it'll need a bit of hands-on fiddling around. Has it behaved like this since it was installed, or is this a recent development? If it's a recent development, and it's not a new install, then I'd try a mechanical service kit (new seals, O rings and filter washers) first (basically, what Bob225 said).

Some people don't seem that keen on Salamander, but I've never had any probs with them and tend to favour them on the few occasions I've fitted pumps. Pretty sure I've fitted that exact model of pump in the past without any dramas, and it's seems like a decent, solid, unit -- plastic impellers I think, but metal housing and body. Although I have a feeling they don't actually make the "ESP" range anymore.

[Bob225]

Have you tried a non return valve on the hot output of the pump ?

http://shop.salamanderpumps.co.uk/non-return-valve.html

[Rich40w]

Thanks Bob225, yes there is a NRV on the hot water output line....

[Rich40w]

Just to answer your questions Chrrris, the issue has always been evident in various guises, even as a new install. One point I have reached out to Salamander on is the pressure canister. I have found this sometimes gathers water so I was wondering if I empty it of water, what pressure should I pump it back up to......any ideas?

[Bob225]

If that's the case replace the pump rather than throwing parts at it

Edit. It may be worth putting a double check valve on the incoming hot pipe work (before the tails)

[wine~o]

Outlet from the HW tank is ???

Pics please.