Plumbing advice. Water tank

[Lethagized]

I've purchased an immersion heater cylinder tank which has a seperate smaller tank on top to hold the cold water. I'm planning on fitting it up this weekend, but i've been told i need an extra cold water tank above this tank to increase the pressure otherwise the cold mains water from the mixer taps will push the hot water back up meaning no hot water at the taps. Is this true? Do i need an extra large cold water tank, and does it have to be mounted above the small cold water tank attached to the immersion tank?

 

[cactuscake]

hmm, depends where all of this is situated in your house (in relation to where the hot taps are)

You get one bar of pressure for every 10 meters the water is raised above the outlet, so my guess is that the tank needs to be at least 10 meters raised per bar of mains pressure... though I don't know how you can check what the mains pressure is.

However, if the hot water tank is fed directly from the mains without using a cold tank in the loft then it should already have enough pressure behind it to stop any backflow from happening... I think. (This is how the hot water tanks work at my workplace anyway)

Edit - Shouldn't mixer taps have check valves in them to stop backflow anyway?

 

[5punk3monk3y]

depends on the taps you have and also are you replacing an existing direct combi cylinder?

It mainly happens with valved showers not so much with taps as most taps have an inner and outer feed on them easy way to spot this is turn just the cold on on the tap if it appears to be coming from the centre or outer ring of the outlet then do the same with just the hot if it does this then no worries about the tap

If you do have a valved shower and mains differential pushes the hot water back up the cold then I would suggest investing in a single impeller pump (these can be had for around £150 quid) to pump the hot water from the cylinder. DO NOT FIT A PRESSURE REDUCING VALVE to the cold feed if someone suggests it to you because this will seriously reduce the water flow to the shower basiacally you would be better off using a watering can.

Also a check valve in this situation is useless as the cold will stop the hot travelling past it due to the different pressure in other words it will never open

also don't be fooled by anyone suggesting an isolation valve set partially open as mains pressure is a constant(which is the problem) all you will do is reduce the flow

hope that helps

 

[Lethagized]

Thanks for the replies so far. I forgot to mention that this will be a fresh installation concerning the cylinder. The pipes are already installed but never connected up. We don't have central heating or a combi boiler.

The combination cylinder will only be supplying...
The bathroom beday which has mixer taps and the same thing with the bath.

The combination cylinder is in the loft, and the bottom of the cylinder is about 3 meters above the level of the bathroom floor.

Another question is, if you had 2 tanks in the loft. One tank holds 50litres of water, the other tank holds 1 litre of water. If they were both mounted at the same height, would the pressure of the water coming out the pipe be equal? If yes then my original question is already answered.

I've also found out that i will need to connect the mixer taps to a gravity fed cold supply because my taps mix the hot&cold in the block instead of the head of the spout. I found this out some time ago when i connected them up to the mains and water was backfed up the hot pipe. I need equal pressure at the mixer taps. Damn crappy B&Q taps. Cost me over £160 for them as well!!

 

[cheets64]

Dont see why you need an extra tank, your main one will be feeding from the main and means piping more to another tank just to feed the your main one,

This is something you should be looking at.