pumped shower & small cold water tank

12 Jul 2005
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United Kingdom

I have a conventional hot water system with a 25 gallon cold tank in the loft, and a 125 litre hot water cylinder in the airing cupboard. My bath cold tap is fed from the rising main.

I'd like to fit a pumped shower. The manufacturers instructions state that the cold tank must be a minimum of 50 gallons. Mine is only 25.

I'm doubtful that the floor in the loft is strong enough to take a 50 gallon tank, never mind the difficulty of actually getting one in.

Obviously, to fit the shower i need to take a cold feed from the tank to the shower so that the hot/cold supplies are similar pressures. How much of a problem is a small tank going to be?

Is the problem solely based around running out of water? If so, could I get away with improving the fill rate of the tank (by adding more ball valves ( the water supply to the tank is in 22mm except for the last metre or so)?

Since I have only 125 litres of hot, assuming a 50:50 mix rate, then i'll run out of hot water at a similar time to the tank emptying (ignoring any extra water from the simultaneous fill), so is there therefore no problem.

Am I missing anything? what would you pro plumbers do?


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Time the fill you have. You can enlarge the hole a bit in the valve, makes a huge diffrerence. Or add a second/different valve. or a second 25g cistern.

Since I have only 125 litres of hot, assuming a 50:50 mix rate,
Makes no difference, you have to take it all from the CW cistern, unless you use a venturi shpwer... Ie you can't just mix tank & mains.
ChrisR said:
Since I have only 125 litres of hot, assuming a 50:50 mix rate,
Makes no difference, you have to take it all from the CW cistern, unless you use a venturi shpwer... Ie you can't just mix tank & mains.

I realise it all comes from the cold tank; i just reason that once the hot water runs out you'll more than likely turn the shower off!!!
i would fit the shower to the tank you have. If you do find it to be in sufficient then you could add another tank.
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I'm not a pro...but have faced this exact issue.

We have a 25 gallon tank and standard 900*450 cylinder - as you do. They are pumped by a 2.25 bar S&T twin impeller.

There's only two key variables, as others have suggested: inflow to cold tank from mains (ours is 14l/min - we didn't try making the valve bigger, though), and outflow from shower head. You should get a good idea of the latter from the pump manual charts, which plot head vs flow rate. Our is 18l/min.

This gives us a net outflow from cold tank of 4l/min - which gives 25 minutes before the cold tank drains.

Of course, you'll run out of hot water by then anyway. Our Aqualisa shower manual suggested a 2:1 hot:cold ratio (which will vary with the temp of the mains in summer/winter) if temps are 65 and 10 degrees respectively.

A shower head flow rate, and this ratio, will tell you how much hot and cold water a (say) 10 min shower would use. Once the volume of hot water used (which would be 66% of the total water used in a 2:1 ratio) exceeds 125l, you have run out of time!

In practice - at 18l/min (get a decent pump, such as a Stuart & Turner), you will be fine even on a 25 gallon tank. In the year we have had our shower/pump in place, we haven't run out of hot while showering; our theoretical maximum shower time is 11 minutes.

And the cold tank will refill quickly (5 minutes at most), even if the hot isn't replenished as quickly...

In summary: yeah, the manuals will tell you to have a 50 gallon tank. My experience is that you'll be fine - the hot will run out sooner than you'll drain the cold tank.

If you want a spready template to work this through, PM me (email in profile).

You could fit a 3/4" ballvalve to the tank, which will refill it quicker, but take the good advice offered and try what you have first. If a problem arises, deal with it then.

Just out of interest did you know that the Water Regs require you to notify the Water Authority before you install a "fitting" which takes more than 12 litre/minute from the mains :!: Does this apply to combis? Bath taps fed from combis or Megaflows? 3/4" ballcocks?
Thanks for the useful ideas. I think i'll just install it and see how well it works. It's an aqualisa axis- so i've not been able to find any flow rate figures for it. I will just have to experiment.

I think i'll make sure that the (new) cold feed from the tank is a bit lower than the feed to the HW cylinder, so that if i do run out of water, i should know because the shower will go really cold, rather than the other way round.
You may not be a pro, joncox but that`s a bloody good answer
Interesting reading all of the comments and advise about fitting a mixer to a small cw tank. I have an additional problem whereby my tank is galvanised and the tank outlet feed to the cylinder comes straight out of the bottom. I suppose this was the way to do it in the 60's maybe before domestic mixers came out. I have had good look at the tank and it seems solid with no signs of rotting. I understand that the cold water feed to the mixer from the tank should be below the main outlet but of course this is not possible due to the main outlet coming out of the bottom. Could you please advise many thanks.

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