Fluctuating shower pressure

[mk2phil]

My shower and bath taps are both fed by a Hansgrohe 2 bar pump. The Hot Water is fed from the tank and the cold feed comes directly from the Cold water tank in the loft (I think, not sure it its mains or not).

When the shower is running the pressure seems to slowly drop off quite badly. I've found that if I turn the shower off and then turn on the HW from the Bath tap (which is also fed from the same pump) it seems to sort the problem out. When I turn the bath tap on there is sometimes a cough and a splutter then it blasts out. BUT, if you leave it for a few seconds the pressure will slowly die down, then a few seconds more and it picks up again, and continues this cycle. If I then turn the tap off and turn the shower back on the pressure is back at the shower?

There is a small air vent type pod with a small black cap that tightens and unscrews on the pipe that exits the top of the cylinder through the surrey/essex flange. The cap is loose to allow air to escape.

The pump doesn't sound too quiet when its running either, but it was fitted when I moved in and therefore I've assumed its a normal level of sound it makes. Although a retired plumber friend said it was too loud when he heard it running, and suggested that I strip and rebuild it if possible.

Any ideas chaps?

 

[Tennooge]

Your domestic hot water header tank cannot fill fast enough for your demand, therefore drawing air into the pipe. The pump is probably noisy because the barrings are ****ed, due to being run almost dry. Check your mains water flow rate. Could be throttled down on the stop tap in the road.

 

[mk2phil]

The header tank is MASSIVE. The previous owner plumbed it all in and he also increased the size of it. I'm sure there'd be enough water in there to supply the tank with for ages. And it doesn't explain why when I run the bath tap for a short while and then the shower the pressure is restored, albeit for around a few minutes.

Could it be that the HW tank isn't filling as quickly as its being emptied? The HW tank is in our bedroom airing cupboard. If the head of water is measured from the outlet pipe on the tank in the loft to the entry point in the HW cylinder I'd say it was about 2 metres. Is this sufficient?

 

[Tennooge]

It shouldn't matter what the head of pressure you've got from the tanks if you've got a booster pump. If you run the shower and the tank in the loft is filling fast enough to keep up with the flow then it could be your pluming somewhere. i.e. a restriction in the pipework or just a faulty booster pump. Hope you can get to the bottom of it mate. I'll keep you posted if i can think of anything.