Is I have mains supply of 18lpm should I get a 40kwh combi

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Hi I am in need of a new combi. We don’t want vented cylinder for various factors including space. We currently have a old valiant. 30kW. If we shower and do dishes it’s still usable but we would like a bit more pressure between the 2

min looking at a baxi. Given 40kw does 16.1lpm isn’t it best we just get that? Given our water pressure is 18lpm?
 
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I’m no heating expert but changing the boiler won’t make any difference to the water pressure, will it?
 
Given our water pressure is 18lpm?

You're mixing your terms.

Pressure is measured in bar (1 bar equivalent to a column of water 10m high) and is quoted as a static figure ie. with the no water flowing (so you are effectively measuring the pressure in the street main) and the dynamic pressure (where you are measuring the pressure at various flowrates).

The flowrate is the water flowing per time ie. litres/sec or litres/min etc.

So a good shower would perhaps require 10 litres/min at 1 bar pressure etc.
 
1. As Gasguru says, pressure and flow rates are different things.
2. Your current flow rate of 18 lpm would be shared between hot and cold, so unlikely that you will be able to get 16 lpm unless its hot water only.
3. You also need to look at the modulation range of the boiler to make sure it isn't over the top for the central heating side of things.
 
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Ok . Thanks.
My terminology is a bit wrong. My water flow measuring jig said 18lpm.

we currently have a 30w valiant from 2000z we want to change it: what size combi would be good. We have good pressure and lpm (it actually could be more then 18lpm) . At the moment with our old boiler. The most we do is 2 showers at once. Or a shower and a kitchen tap at once. At present it’s a bit weak when they both run. Would a 35kw baxi help?
 
As above, you need to take into account the size of your heating system. An oversized boiler will cost a lot to run. How many radiators do you have?
 
As above, you need to take into account the size of your heating system. An oversized boiler will cost a lot to run. How many radiators do you have?
11 Rad’s. We also have a tado. So we want the boiler to support open them. We would want one with 10 year warrantee
 
Baxi don't really support OpenTherm properly, and their modulation is poor. Look at the Intergas Xclusive 36 if you're happy with 36kW - 10:1 modulation, fully OpenTherm compatible, and a 12 year warranty when installed with the excellent value Intergas system filter. If you want to go up to a 40kW then the Navien NCB 40kW is an excellent choice - again 10:1 modulation, and fully OpenTherm compatible, and I think it's a 10 year warranty although would have to check.
 
I asked my boiler man if his intergas approved. He replied why not try 938. Thoughts?
Baxi don't really support OpenTherm properly, and their modulation is poor. Look at the Intergas Xclusive 36 if you're happy with 36kW - 10:1 modulation, fully OpenTherm compatible, and a 12 year warranty when installed with the excellent value Intergas system filter. If you want to go up to a 40kW then the Navien NCB 40kW is an excellent choice - again 10:1 modulation, and fully OpenTherm compatible, and I think it's a 10 year warranty although would have to check.
 

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