power showers and the size of water tanks

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I've been told by a plumber that if I install a pump-assisted shower over bath, the pump will make water run so fast that the water tank may empty very quickly. Then he suggested that possibly turning down the pump would help. They did look at the water tank and didn't say that it was the smallest - but he still was asking whether filling a bath is a problem. It's not though the water flow slows down after filling half or maybe 2/3 of the bath.

So I'm not sure whether he knows the power shower wouldn't work beyond a short period, but still wouldn't want to refuse the job, or he is being over cautious?

If it does turn out that shower runs out of water in five mins, how expensive would it be to put a bigger tank in - and is it at all possible? And would turning down a pump work without defeating the purpose of having a decent shower (I don't need it to be extra powerful, just better than a drip I'm getting with non-combi boiler).

look up the specification of the pump. For example, if it delivers 20litres per minute (which is quite a forceful shower), and you have a typical hot water cylinder that holds about 100 litres, it will run out of hot after about 5 minutes, if all hot, or 7 minutes if 70% hot.

You also have a cold water tank. If it is a big black plastic one in the loft, it may hold 100 litres or possibly only 50litres. If it is a small one on top of the cylinder, it might only be 20litres so will run out after one minute. As soon as the cold tank is empty, flow will stop, even if the cylinder still has hot water in it (cylinders never empty).

I just looked at some sample shower pumps, and the ones I looked at delivered 30 to 40 litres per minute, so would run out even faster. I have not seen pumps that can be turned down, perhaps they exist.

You will get some extra time because the cold tank will be (slowly) filling up while you are emptying it. If you put a bucket or measuring jug under the ballcock you can see how many litres per minute it delivers.

Your bath holds about 100 litres, so if you can't fill it without the flow dropping, that suggests that your existing cold tank is less than 100litres. A bigger one will be heavy, so needs carpentry in the loft to make a strong platform.

Yes what your plumber says is correct, a pumped shower I.e a shower with an internal pump or a separate shower pump will require adequate water supply to prevent the pump running dry. You will need 100 g water storage to provide adequate water for the shower pump. You may already have a 50g tank, in which case a another 50gal can be linked onto it, as long as the plumber uses a bye law 30 kit.

Turning down the pump although will help with the flow of water sort of defeats the object as the shower will not have the same force as it should if it were running at full flow and you might as well not have bothered having it installed in the first place.

If it were me and I wanted a pumped shower I would go the hole hog and have the water storage upgraded as well, rather than worrying and running the risk of running the pump dry - which may then need replacing.

thanks for the replies, v.helpful!
How can I tell what size is my cold water tank without doing the bucket test? Would this info be printed on it? it's in the loft, yes.
He hasn't suggested upgrading the tank storage - sounds like he should have (even though he metioned that it can be an issue). I wonder if he didn't want to scare me off the job due to the cost being very high - how much could it cost to put an additional tank, What?
PJ so what do most people do if they have power showers? is it standard to already have bigger tanks, or do people normally upgrade?

if it is a black plastic one, there will be numbers embossed on the side.

There may be two, e.g. 70/100 where 100 is the nominal "up to the brim" size and 70 is actual usable volume.

It may be in gallons or litres according to age.

Otherwise tape-measure it and we will try to guess.