Surely both makers give flow figures which can be used for comparison purposes?
Don't forget that high flow combis might well need an upgraded gas main as well as water.
I am in exactly your position but already have a good water main. The other thing that I noticed is on the 837 model you have to have a 28 mm gas main all the way to the boiler (I have 22mm). Even if you do it won't help if the gas supply in your area is not good enough. I know a lot of places Transco have fitted small bore plastic inside the old metal mains to save time and money but results in a supply that can't cope with the demands of such a boiler. In many areas the gas supply is not as good as it should be due to big expansions in housing tagging on to the old system. Transco cheat by increasing the pressure but that cant help if the pipes are to narrow. It is like using a hundred tones of pressure to force sand through an egg timer. It will only go so fast through a small hole
Anyway the boiler will still work but won't reach full potential. Also if Valliant see you don't have the 28 mm gas supply they won't support you if there is a problem
David
Look at the Worcester HighFlow or Vokera's similar model. Standard washing machine sized and will fit in a standard kitchen. High flows, doing two showers and not expensive.
Both systems will be at the mercy of your mains water supply. Both rely on mains pressure for their flow rates. The Megaflo instructions are just being a bit more realistic about it than the combi's, as they know that a 15mm main won't get the full benefit of the system.
However, if the 15mm pipework gives you sufficient flow rate at the mains fed taps now, then it won't be a problem. You may find it better to run the main feed to the cylinder in 22mm from the stop c*ck, though. Then you can upgrade the water main in the future.
Installing a second electric shower will still run into problems if the limiting factor is water main flow rate, rather than water heating rate.
Christian.
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