Size of CW storage tank

I can't find what I'm looking for and hoping someone can help please. What capacity cold water storage tank do I need? The house has three bedrooms (no ensuites but one bedroom has a vanity unit), 1 bathroom with shower, downstairs loo, kitchen & utility room. At present there are only three of us here, but I guess the size should be big enough for a family of 4 or 5. I've just enlarged the HW cylinder up to 165L (48 x 18"). Mains pressure isn't all that great.
I plan on raising the CW tank in the loft to give a bit more head and take the chance to put in a bigger tank if necessary. The current CW tank is 40 galls - is this big enough?
Cheers Peter
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In an earlier contribution to this discussion,

Is the header tank just for the hot water system - or are some of the cold taps also supplied from it? What sort of shower is it - is it electrically heated, taking a mains cold water feed - or does it use stored hot and cold water?
If the header is just for hot water, and the shower is either electric or will never be used while running a bath, you might just get away with your existing tank - but even so, it is marginal.
You really need to have some water left in the cold tank when you have expelled all the hot water out of the hot tank and while - if appropriate - running other cold taps at the same time, without relying on the mains refilling the cold tank. I would have thought that something like 60 gallons would be adequate. There may well be some rules of thumb - I don't know - others may!
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wrote:

Sorry - I should have explained this. It's a traditional installation. CW storage tank supplies low pressure cold to all outlets except the kitchen and Utlity sink, plus the HW cylinder.
The shower is a Grohe Automatic 2000 thermostatic gravity-fed low pressure, with 3/4" supplies and outlet pipe. It has an 8" dia fixed shower head on a long arm. Pressure is only 1m head, but the flow rate makes up for it. It could be better though, hence the idea of raising the tank.

The shower is over the shallow end of the bath, plus there is a handset and shower mixer in the bath. There's not really any point is running the bath and the shower together unless you like waterfalls.

I was Googling for a rule of thumb, but couldn't find anything.

Thanks Peter
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On Wed, 31 Dec 2003 17:24:09 -0000, "Peter Taylor"

This all depends on what you want.
I think that the main criterion is to make sure that the tank is not going to run out of water under worst case conditions of use.
For this purpose, you can pretty much ignore the loos, kitchen, bedroom and everything except the bath and shower because they use relatively small amounts of water and generally infrequently.
Since you say that the shower is in the bathroom, I assume you don't envisage a situation of both being used concurrently.
I think that the main scenarios to consider are:
a) Somebody runs a bath to generous depth, uses it fairly quickly and empties it. Then the next person comes in and does the same. Is the supply going to keep up with that?
b) Somebody uses the shower. How long will it run before the tank empties for the first person doing this? How long does the tank take to fill?
Given that, I would do the following:
Run off a fair amount of cold water from the tank so that it is filling. Measure the flow rate into the tank from the ball valve by timing the filling of a vessel of known size.
Measure the flow rate at the bath taps with them full on - adding the figures to get the total usage rate.
Do the same for the shower.
With the tank full, turn off the mains supply and run the bath taps until the tank empties. From this you can deduce the actual volume of water in the tank.
By putting these numbers together - i.e. by taking the actual emptying rate and subtracting the filling rate, you will know the net emptying rate. For the bath, you will almost certainly have a net emptying rate, for the shower, possibly not if it isn't pumped.
This will tell you whether the supply is adequate for your requirements and you can increase the storage if not.
If you are going to go for a new tank, don't forget to consider the weight and loading on the ceiling joists. Generally, tanks are placed over load bearing walls if possible and on a thick board on additional cross timbers to spread the load as much as possible.
Another technique that you can use is to have a second tank. I used this approach very effectively in a house where the loft space had limited height. This was in the days before the so called coffin tanks from Polytank were available. With two tanks you arrange the cold feed into one and all the exits from the other. Then put a length of 28mm tube between the two.
.andy
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wrote:

downstairs
I
enlarged
the
galls -

I'm not sure what my son gets up to in the Bathroom :o) but when he comes out, all the hot water has disappeared. There have been rare occasions when the cold tap (LP) coughs and splutters due to no water in the tank, but I can't say this is a masjor problem.

That's right.

Got it! It's like one of those maths questions about filling up a bucket with a hole in it.

New tank or old, when I reposition it will be in much better position over a loadbearing wall, with far less stress ion the timbers than existing.

This sounds a good idea. Both at the same level, obviously. I had toyed with the idea of increasing the head by having a small header tank (like a F/E tank) at high level with a sealed bulk water tank below it, but I figured you don't gain much as the water level will quickly lower itself.
Cheers Andy, many thanks. Peter
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On Wed, 31 Dec 2003 18:52:28 -0000, "Peter Taylor"

Tell me about it. Mine's worse for vanity than the women of the family.

Basically, yes. The other thing is to do the filling rate test at different times because the pressure does vary.

Yes they have to be. The other possible benefits of the Polytank are that they are easy to get through loft hatches. I replaced my old original tank a couple of years ago and I know that the original was put into the loft before the ceilings went in.
Also if you go for the long low ones you effectively spread the weight load a bit more. I also quite like their construction of a small hatch type of lid rather than a large floppy thing that doesn't sit properly on the tank.
www.polytank.co.uk
Screwfix do a couple of the sizes but there are more available and orderable from the merchants.

Exactly. You'd end up with the original head very fast.

You're very welcome.

.andy
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Peter Taylor wrote:

Don't bother raising the tank in the loft unless your loft has incredible headroom... Some years ago I raised the CW tank in my loft as much as I could, about 1.5M, and it made sod all difference to the water pressure of the 1st floor shower. When I thought about it I realised it would have had to be raised 10M for a useful 1bar pressure increase....
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