Low pressure in shower

[BertHarris]

Hi all,
I have a gravity fed system but am getting low pressure in the shower. Taking the shower mixer off and then letting the water run shows excellent pressure going into the mixer. Assuming the mixer was faulty I purchased the same one and installed it only to have weak pressure again. I’m perplexed as to what the issue might be. I had a plumber round who initially helped to take off the original mixer to reveal a lot of debris and rust - after cleaning the debris and rust and not eliminating the problem it was concluded that the issue must be with the shower mixer - but alas, that wasn’t the issue either.
Any thoughts would be most welcome, cheers

 

[Ben-gee]

Did you replace hose and head too? - these could be creating a back pressure if blocked or hose herniated.

Has the shower ever been good?You need a shower specifically designed for low pressure.

 

[Shower Engineer]

Hi all,
I have a gravity fed system but am getting low pressure in the shower. Taking the shower mixer off and then letting the water run shows excellent pressure going into the mixer. Assuming the mixer was faulty I purchased the same one and installed it only to have weak pressure again. I’m perplexed as to what the issue might be. I had a plumber round who initially helped to take off the original mixer to reveal a lot of debris and rust - after cleaning the debris and rust and not eliminating the problem it was concluded that the issue must be with the shower mixer - but alas, that wasn’t the issue either.
Any thoughts would be most welcome, cheers

Is the shower you installed suitable for low pressure gravity supplies?

 

[Ric2013]

Excellent pressure? You may have a good flow with no mixer connected, but if it's gravity-fed the pressure will be low - typically 0.1 to 0.2 bar - and most mixers will reduce your flow very greatly unless designed with gravity pressure operation in mind.

At best (e.g. some of the best Mira models) you'll be looking at 6 litres per minute from a gravity pressure shower mixer. Many shower mixers really aren't designed to run at pressures below half a bar and you'll need to pump them to get a decent flow.

If you know the cistern water level height in metres above the shower handset and divide that by 10, that's (approximately) your pressure in bar. You could consult the shower manufacturer and ask what flow they would expect to see at the sort of pressure you have.

 

[michcosg]

Check also to see if your new shower has non-return valves inserted usually in the water supply arms. These are not necessary with gravity tanks having what I think is referred to as an AB air gap ie an overflow warning pipe. But will reduce the pressure!

 

[Ric2013]

Check also to see if your new shower has non-return valves inserted usually in the water supply arms. These are not necessary with gravity tanks having what I think is referred to as an AB air gap ie an overflow warning pipe. But will reduce the pressure!

An AB air gap would not usually be used in a domestic cistern - I suspect you mean an AC air gap, but it doesn't really matter as the principle is much the same.

Unfortunately UK legislation requires you to protect anything else that might feed from that pipework, cistern, and cylinder, and not just the mains (otherwise you could, say, contaminate your washing-up water) so it would not necessarily be in order to remove non-return or check valves.

That said, check valves are normally there to reduce backflow issues which is not a risk if both hot and cold inlets to the shower are at the same low pressure.

If the shower hose is long enough to allow the handset to fall below the shower tray overflow level, then there is a possible (though unlikely) back-siphonage risk hence the need for check valves. However, particularly if the shower has dedicated feeds, I can see that a gravity-fed shower installation MIGHT not be at risk from back-siphonage even in fault conditions. If this is the case, then I would be inclined to agree that check valves would not be needed. (In technical terms you'd be thinking of Type B upstands but I'm struggling to understand the finer points of the relevant legislation).

Seeing as the OP probably won't have the technical knowledge to determine whether check valves are required or not, and on the Chesterton's Fence principle, I would advise him to contact one of the local plumbers who he has previously used before removing any check valves.