Poor pumped shower pressure - advice needed!

[-scrumpy-]

We have a shower pump (sited at the foot of the hot water tank), and powers our shower, and has done so with no problems. Last year we had a Solar Hot Water system fitted, and since then we've had several plumbers attempt to fix the problems with cavitation/poor pressure in the shower.

I'll describe the current setup (bearing in mind I'm not a plumber!). There is a 22mm vent pipe coming out of the top of the hot water tank. There is a flange on the side of the tank about 300mm from the top. This feeds directly into a thermostatic mixer valve. The mixer valve cold supply is 22mm and fed from a tee on the 22mm downpipe from the cold water tanks to the bottom of the hot water cylinder. The mixer valve is required as the hot water in the tank can get very hot with solar heating in the summer.

The flow out of the mixer valve then travels around the back of the tank, at the same level as the mixer valve. The first tee downwards feeds the upstairs hot water taps, the next tee downwards feeds the hot supply for the shower pump, the last elbow then feeds the downstairs hot water supply.

If I turn the pump off, and turn the shower on, then eventually the air will be expelled, so that when the pump is turned on, the pressue is good. However, the next time we come to use the shower, air will have been built up in the supply again (confirmed by the fact that I can "bleed" the system again to remove the air).

We've had two plumbers look at it now, but no luck in solving the problem. I've turned the thermostatic mixer valve down so that it's near the minimum temperature. Some people have suggested that we should have a separate supply from the cold tank directly to the mixer valve - does this sound a valid suggestion? Could air be getting in from other places? (e.g. where the flexible pipes join to the pump?

Thanks in advance for any help, and if I've not been clear, please ask and I'll do what I can to clarify.

Many thanks,

Nick

 

[Moss5]

You said
"The mixer valve cold supply is 22mm and fed from a tee on the 22mm downpipe from the cold water tanks to the bottom of the hot water cylinder".

The shower pump is stealing cold-feed from the hot tank.
You need a dedicated cold oulet from the cold tank, direct to shower pump inlet.

Cut a new hole in the cold tank. Cut and blank the branch from the Tee (above) and insert a new piece of pipe up to the new tank outlet.

 

[Nige F]

Also.....why is the mixer necessary if the shower is a thermostatic mixer itself? I would think it struggles to draw water thru` the mixer that regulates the other hot draw offs and the shower feed too...Understood the safety mixers were to be fitted @ point of use..not one for all?...see what others think

 

[-scrumpy-]

"The shower pump is stealing cold-feed from the hot tank."

Thanks for the advice - would you mind expanding more on how this "stealing" works? Is it because it's easier for the water to be drawn out of the hot tank, than it is to get the water from the existing downpipe?

thanks again,

Nick

 

[-scrumpy-]

Also.....why is the mixer necessary if the shower is a thermostatic mixer itself? I would think it struggles to draw water thru` the mixer that regulates the other hot draw offs and the shower feed too...Understood the safety mixers were to be fitted @ point of use..not one for all?...see what others think

I think the mixer is necessary as the hot water in summer can be very hot, and so become too hot for the shower pump. In the summer the water at the bottom of the tank can be well over 75 degrees - so that water at the top must be hotter than that!

cheers,

Nick

 

[Nige F]

75@ bottom so nearly boiling@ top didn`t realise it was so effective..........makes me think..I`ve got a nice S,facing roof @ home

 

[Moss5]

Thanks for the advice - would you mind expanding more on how this "stealing" works?

Imagine you are a lump of water. You have just left the cold tank inside a 22mm tube. You come to a fork in the route - straight on to the base of the hot tank, or turn right to a mixer on the suction-side of a pump. Being a lazy lump, most of you chooses the soft-option.

The hot water only leaves the tank under the driving force from the cold water delivery. You system is dividing that driving force.

All I am advocating is creating two outlets from the cold tank and utilising the existing pipe run. Total cost about £10.

I am unable to comment on the correctness of your mixer. Ignore my comment about sending cold direct to the inlet of the pump. Leave the cold run as-is, except for connecting it to a second outlet from the cold tank.

 

[-scrumpy-]

Thanks, thats a good clear explanation of how it works - I will certainly give this a try.

cheers,

Nick