Shower pump - hot only, unpumped cold tank supply

Hi all.
I've just signed up and found this forum useful, but having searched exhaustively, I could not find this info.

Q. Can I pump just hot water (single impeller pump) and not pump the cold water from the cold water storage tank? Or use two separate pumps?

I ask because most of the water needed is hot. Adjusting the thermostatic valve to full cold leads to pump shut down. Full hot allows continual activation of pump (before it broke) Also, my folks had just hot pumped to their shower before their pump failed.

Only info on this Q involves cold mains supply not cold water tank supply.

Another question - why is the hot water flow lower than cold , despite the cold water tank pressurizing the hot water?

Background info.
Detached 2 storey house with vented hot tank 120l on 1st floor, 220l cwt in loft. Shower is on 1st floor close to hot tank, minimal pipe run. 3m? Up , across then down.

Problem - low hot water pressure to shower with new thermostatic valve. Last one was not great. New one rated to 0.1 bar, including shower head. Cold water gives 3 to 4 times more flow than hot.

Plumber fitted twin pump salamander 2 bar (hot and cold). This has 22mm pipe run supply from hot tank to pump with flange at top of hot tank, but goes to 15mm pipe bore a metre after 22mm pipes exiting pump to shower.

This worked well for 3.5 yrs, used every 3 days. The pump one day did not work. Checked pump plug fuse, inlet filter to hot supply filter had a little fibreglass insulation which did affect flow. I guess this knackered it.

I didn't want to spend £300-500 every few years on new pump. I've considered unvented system , but happy with hot water pressure elsewhere in home. Also, the need for overflow pipe would mean lots of ripping up floors with current location of vented tank over stairs.

Considered unvented 50 or so litre tank in loft next to cwt dedicated to just shower using immersion heater elements to heat water. I could strengthen platform, but don't think 4 "or 3"x2" joists gang nailed are up to extra weight. It would not cost much to have put 4 decent joists under cwatwhen property was build. (Persimmon). The tank is sat on beefier joists 6" x 2" timber above these . Truss roof supports connect to 4" or 3" x 2" joists.

Another problem would be using the nearby 15mm cold supply that Tees off to cwt. When cold water is demanded for shower or bath would this affect the unvented 50l hot tank water pressure?. Putting a dedicated 22mm or 15mm pipe run to loft would be messy (nice plasterboard walls to rip out)

Long post, but your input is appreciated.

Yes you can just pump the hot side. Trouble you may have is balancing the cold pressure to the hot so the shower works correctly. It's usually done where some of the cold is mains pressure and the hot is pumped to bring it into line but that's a hit or miss game.

The hot water flow could be lower due to pipework supplying it, more bends in the pipework or the outlets that are being used etc

Unvented is a good idea but if siting it isn't practical ..... cold water supply would need to be tested too to ensure it was suitable.

Could tee'ing from the supply to the cold water storage cistern (CWSC) affect the supply to the unvented, yes it could, it all depends on the mains supply. The idea of a small direct unvented just for the shower is an option but it costs way more to heat water by electricity, so the economics aren't ideal.

My solution was a bespoke vented hot water cylinder with a second coil in the top of the cylinder. This coil is fed with mains pressure water to be heated to feed the shower. Cold feed to the shower is also main pressure.


The cost of this bespoke cylinder in 2011 was under £200

Thanks Bernard.
Any links to product. Is it basically an unvented cylinder in a vented cylinder. Will this set up need an expansion vessel? What volume is the hot cylinders? Thanks

The green/orange in the diagram is a coil inside the tank. No requirement for expansion tank as the voulme in the coil is less then 15 ltr.

The nominal capacity of the cylinder is 117 ltr It is a standard 900 X 450 with the second coil added to order.

Rereading. It is a coil heating element near to a mains cold supply water pipe passing through hot water cylinder. I'm surprised the water will heat up enough once it flows to shower.

bernardgreen said:

My solution was a bespoke vented hot water cylinder with a second coil in the top of the cylinder. This coil is fed with mains pressure water to be heated to feed the shower. Cold feed to the shower is also main pressure.

View attachment 172469
The cost of this bespoke cylinder in 2011 was under £200

Click to expand...

Can you test the hot supply with the shower removed? I'd expect a slight imbalance in the supplies, but the level you describe suggests there may be a restriction on the hot supply, be interesting to see what kind of flow you get with the shower removed.

Hugh Jaleak said:

Can you test the hot supply with the shower removed? I'd expect a slight imbalance in the supplies, but the level you describe suggests there may be a restriction on the hot supply, be interesting to see what kind of flow you get with the shower removed.

Click to expand...

The thermostatic valve has been removed x2since the pump was fitted to check for solder flux and lime scale on the inline filters. The lever valves were opened full and the cold consistently had more flow x3than the hot.
I wondered if raising the cwt 40cm would be worthwhile. So, holding the shower head 40cm lower resulted with a decent flow from shower head. But ,this was just cold only set on thermostatic valve. Moving the water tank would mean it sits further to right on its platform , nearer the corridor void rather than the concrete 2 storey supporting wall. 0.4m is only 0.04 bar so not really much gain.

With a 2bar pump the flow was very good. I am guessing the flow for hot is reduced due to extra resistance to flow through tank and slightly extra pipework.


I'm interested in this coil set up. WonderING if the cold mains pipe 15mm? Goes above the heating coil?

Thanks for useful replies

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