Homemade 'plumbing unit'

[jamse]

Hi,

I've posted on here before - thanks to all who have helped me shape my thoughts so far.

For reasons of space and difficulty in running pipes, we are hoping not to have a cold water tank in the loft of the place we are renovating. One option we have seen on the web is so-called "plumbing units", which include a cold water tank immediately above a hot water cylinder - this PDF has a pretty good picture of the sort of thing:

http://www.hartons.co.uk/pdfs/Plumbingsystem4.pdf

I wanted to know if it's possible to make the equivalent ourselves. Is there any "special sauce" involved in these pieces of hit, or is it just a cold water tank and a hot water cylinder plumbed in a vented arrangement as if the CW tank were in the loft, then with a pump to make up for the fact that you've got approximately zero head to provide pressure?

Thanks in advance,
Jamse

 

[muggles]

I suppose you could do that, and DIY too, it's not a very nice way of doing things though and I think you'll find that it's a compromised 'solution' - the cold water tank may be undersized for the cylinder in order to get it all in the cupboard, bearing in mind that both hot and cold draw-offs are used and the system is pumped. Also I'd be interested to see how you get at the ball valve for maintenance purposes.

If you're renovating the property, why not look at getting either a combi boiler or an unvented hot water cylinder, both of which would remove the need for loft tank(s) and avoid this kind of setup

 

[jamse]

Thanks muggles.

Re combi boiler - There's no gas in the property at the moment, so trying to avoid that route.

Re unvented cylinder - We are considering that as an option (and are aware that it can't be DIYed), although I'm not sure whether our water pressure and flow rate are up to it. The rising main into the property is only 15mm and I can stop the flow on the outside tap with my thumb - I've bought a pressure meter to get a more scientific reading!

What would you consider to be a suitable size CW tank in proportion to a HW cyclinder to avoid it being undersized? About the same capacity? Significantly larger? Say we get a 150L HW cylinder - what size tank would you recommend?

Thanks,
Jamse

 

[muggles]

I think you are asking the wrong question here, the correct question is 'what size cylinder do I need for my property', this needs to be established as well before the cold water cylinder can be sized. For this more info is needed - how many hot/cold outlets are going to be expected to run off the tank, and what are they?

 

[jamse]

I appreciate all the help with this - thanks for getting me along the right lines!

To answer your question - it's a small 2 bed cottage, with one bathroom and a kitchen. The bathroom has a large bath with a shower over it, and a wash-hand basin. The kitchen will have a sink. I've not thought through the kitchen details yet, but I'd vaguely assumed that we'd get a cold fill washing machine, as that's what we've got at the moment, and that I'd plumb it off the mains - but I'm not tied to that idea.

Thanks again for all your help with this.
Jamse

 

[Agile]

The hot water cylinder is properly sized on the number of people living there!

A typical size is 50li plus 50 li per person. That can be reduced a bit for more than 5 people as a 300li cylinder will serve 6-7 if they are not too demanding.

Tony

 

[jamse]

OK, thanks Agile - that sounds like my initial estimate of 150L is about right - it's just my wife and me at the moment.

So, based on a 150L HW cylinder and the outlets I've mentioned, what sort of size CW tank should we be banking on? Is there a rough formula or way of working it out?

Thanks,
Jamse

 

[Onetap]

You could, but why would you want to (unless you've got a really pathetic flow rate from the mains)?

Combination cylinders were used pre-1987 (?) when the UK Water Regulations only allowed a cold water storage tank and the kitchen tap to be connected directly to the mains supply. The rules were changed to conform with the EEC, you can now connect almost everything direct to the mains and use an unvented cylinder. If the pressure's too high (it probably is for the existing taps), then fit a pressure reducing valve.

So what is the point now of a combination cylinder?

 

[Agile]

A combination cylinder was designed primarily for a 1-2 person use and the volume of stored water is pretty irrelevant by current standards.

The size of storage depends on the expected useage. The general rule of thumb would be to store about 25% more than the dynamic storage volume of the tank.

Tony

 

[jamse]

Thanks Agile and Onetap.

Yes, Onetap, one of the main reasons we're thinking about this instead of an unvented cylinder is doubts over our water pressure and flow rate. Do you have a general rule of thumb for what would constitute an acceptable pressure / flow rate?

Agile, we had pretty much ruled out conventional combination cylinders on the grounds that they all seem to have pitiful amounts of CW storage. But if we can put together an arrangement with, say, a 150L HW cylinder and around 190L CW tank it sounds like we should be OK. Am I understanding OK?

Thanks,
Jamse

 

[Agile]

You have to work out the dynamic storage volume that takes into account the inlet flow rate at the ball valve.

If the flow rate is say 7 li/min which might be typical then you can take 7 li/min all the time.

Or you can have a 100 li tank and take 17 li/min for 10 minutes.

Ball valves have a very small outlet orifice and fitting a larger one, say 1" instead of the standard 1/2" will considerable increase the flow rate. Thats all depending on the mains flow rate into the property and sometimes thats poor because it was the reason a storage system was chosen.

Generally a dynamic storage volume calculated on the maximum flow rate to be taken from the cylinder could be used. Its typical to assume a maximum flow rate of 20 li/min.

There is little point in having a dynamic storage much higher that the volume of the cylinder with a small allowance for the reheat time.

An actual storage volume equal to the cylinder or a little more would usually suffice as long as the tank is not supplying large volume cold water outlets. Usually its only the cold taps in the bathroom and toilet ciusterns and can usually be ignored.

Also consider the house! A six bed house needs a larger cylinder as a current two people would likely be more if the house was sold.

Tony