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Central Heating

What is the correct mains water rate required for greensta 38cdi to work effectively?

Had asked a VHL (a local CH Company) and British Gas to visit and advise about most effective and best value solution to replace our old (20yrs old) Potterton Prima 60F. We currently have a very small capacity copper immersion cylinder which means that when we have visitors, we have to wait more water to heat up or switch on immersion as well as the Potterton to get more hot water.
We cannot replace this small cylinder without having to undo new bathroom which was tiled 4 years ago!

The mains water pipe was replaced about 4 years ago from stop-cock to under the stairs in plastic blue pipe and from there it goes down to 15mm copper pipe work to the kitchen and loft storage tank.

I am getting conflicting info from local CH guy and British Gas==both have suggested changing to a Worcester-Bosch Greenstar Combi either 38CDi or 42CDI for 10 rads, 3 bed rooms, 1 x bathroom and downstairs showerroom.

The local guy simply turned the mains tap in the kitchen on & off and said the 42CDI would do the job.
However, the British Gas guy measured the flow rate and said it was 12 lpm and this would not be enough unless we also extend the 22mm pipework to the kitchen as well!?

Both confirmed that we would need new 22mm gas pipes for the Combi Boiler as our existing pipe work is 15mm diameter.

(1) Could someone that has had experience of installing Greenstar 38 or 42CDI advise what is the required flow rate to get hot water flow rates from the Worcester-Bosch combi?
(2) Who is correct and what else should I considering?

Many thanks,
Sunil J

2 Answers from MyBuilder Heating Engineers

Best Answer

Once again this problems crops up. You can only get out what comes in ,The mimum pressure the water companys can give is 1bar pressure thus a flow rate around 9litres Although many are above this pressure as for extending the pipework if the pressure is low coming into the house then increasing pipesize in my opinion a waste of time.You must always remember when changing the system to all mains you could have up to 10 to 12 outlets on mains water.The main water can not cope with all outlets
open and you can lose water at many out lets also don't forget water pressure vary. In my opinion just change the boiler and forget about the combi and if possible change the cylinder although I would have thought you would have a 30 gallon cylinder .Would also check the cylinder stat needs to be as low as possible on the cylinder. Also is the immersion heater the correct length for the cylinder. hope this assist you Dave WARD

2015-07-16T09:55:02+01:00

Answered 16th Jul 2015

Need more information. Do you use a lot of hot water?
Please reply as this information is crucial!

2015-07-16T09:55:02+01:00

Answered 16th Jul 2015

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