New Boiler sizing

[D_Hailsham]

If for example you have a 630 installed on a system that only requires (lets go extreme here) 18Kw and you don't range rate it then you will have a boiler that is producing 30Kw but only a heating system that can 'emit' 18Kw. Therefore you have 12 Kw of unnecessary energy being used.

This might be true of an unmodulating boiler. The boiler will be generating heat faster than the radiators can dissipate it , so the boiler will quickly reach max temperature, shut down, cool down, start up and continue cycling until the room stat is satisfied. There will not be any unnecessary energy used.

With a modulating boiler, the temperature will also rise very quickly but the boiler will then modulate down to balance input to output (as measured by flow and return temperatures). You will not have unnecessary heat being generated.

 

[Guzzles]

Just spoke to Vaillant Technical, and basically they believe that over-sizing (within reason) your boiler is not an issue and the system will modulate. The I stated my Radiator load (15Kw) and the cylinder load (20KW), and the 624 has come up as recommended. At the end of the day the 624 can modulate down to 9KW which is fairly low. Ranging the boiler will presumably reduce the percentage of modulation that the boiler can do as it won't be able to go lower than 9KW, which sounds like a disadvantage.

 

[Agile]

Just spoke to Vaillant Technical, and basically they believe that over-sizing (within reason) your boiler is not an issue and the system will modulate.

Well practically speaking they are not wrong but then they benefit by selling larger boilers dont they.

If you had attended an energy efficiency course ( which you should have done if fitting boilers ) and had questioned oversizing boilers you would have been very firmly told that its a no-no on account of the lower efficiency.

Your 15 kW of rads plus 2 kW for water equals 17 kW.

However the correct figure is to use the whole house heat loss figure as the rads may be oversized or may need to be augmented later if undersized.

Tony