can i use an unvented cylinder

12 Feb 2014
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West Midlands
United Kingdom
Hi i have a 4 bed house with a mixer shower in 3 of the rooms. I do not have adequate mains pressure so I plan to put a 50 gallon water tank in the loft supplying a 300l indirect unvented cylinder . The cylinder coil will be supplied with a 30 kw combi
My plan is to fit a 3bar pump on the cold feed between the 50 gallon water tank and the cylinder, also t-off the cold pipe from the tank just after the pump for a clod supply to the mixer showers, this should split the 3 bar into 1.5 for cold and 1.5 for hot.
i believe this will keep the hot and cold running to the mixer showers at the same pressure.
Can anyone tell me if this is doable or not, If not say why and possibly give an alternative.
I would replace the mains but i have had a new drive laid. Any help appreciated.

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Probably possible but why?
The usual arrangement would be cold storage in the loft, vented cylinder with twin pump after it to supply showers etc.
A 50 gallon tank is too small to supply a 300 litre cylinder.
And why would a combination boiler be required with a 300 litre hot water cylinder?

I do not have adequate mains pressure
What is the pressure? And do you really mean flow?
Sounds OK to me, but as above you'll want a bigger loft tank than that

I'd use the cold connection on the cylinder control valve to supply your cold rather than teeing off before it. Bear in mind that all this work must be carried out by a G3-registered installer - you can't do it yourself

As for the pump, not all pumps are created equal so 3 bar isn't always 3 bar. You need to understand more about them to spec one. What is your static mains pressure?
If the static water mains pressure is over 2bar an accumulator would be an infinitely better solution.
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If you are planning to do as you say you definately dont need a 30KW combi speak to a decent heating engineer you are way on the wrong path. you need a system boiler and an unvented cylinder at the very least, you definately dont need a combi
Thanks for your replies. Can i ask when you say a system boiler do you mean a heat only boiler. I did have a gas engineer come round and he had suggested a combi, he said the hw side could supply hot water to the bathroom and kitchen sinks and the other side for the heating and the coil in the cylinder. The engineer measure the flow and it was 12 ltrs/m at ground floor. I was planning to put the boiler on the top floor and the cylinder in the loft with the storage tank. The gas engineer has suggested that the existing 22mm gas will have to be replaced with a 28mm pipe hes say gas pressure will be to low.. I am bit uncertain about the pressure created for the showers, if i do have a pressure of 3bars and 1 shower is turned on is 3bars to powerful or ok. Is an accumulator like a large storage tank.
Seems to be a gas engineer with not much of an aptitude for explaining all these things that you are asking on here!


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