Thanks. Looking online there only appears to be about £90 difference between the two so I might as well go for the 18Kw just in case and have that set to 15kw. I suppose the extra cost would be for a bigger burner/heat exchanger or whatever it’s called inside - is that right? If so, I suppose having an 18Kw turned down to 15kw will put less stress on it or don’t they work like that?
Hello Steve57,
Please excuse me for not looking at the Vaillant Boilers Installation Instructions before writing this but I defer to
Member Riley for His knowledge / comment about these particular Boilers being able to be `Range Rated` in terms of being able to adjust their Input / Output.
A lot of Condensing Boilers have pre-set Gas Valves and cannot be `Adjusted` for a different Input / Output.
However - If You have a 18 Kw Boiler running on a system that `normally` requires no more than 15 Kw`s
and which has a Room Thermostat it will just heat up the System / Property a bit quicker than a 15 Kw Boiler would and be turned off by the Room Thermostat when the set temperature is reached.
You would not use any [perceptible amount] more Gas than with a 15 Kw Boiler because when the Room Thermostat is satisfied the 18 Kw Boiler will be shut down until the Room Thermostat requires heat again.
The 18 Kw Boiler would be using slightly more Gas when the burner is running - BUT for shorter periods to Heat the property / to satisfy the Room Thermostat.
The 18Kw Boiler will heat up the System / Property a bit quicker and IF the Property Heat requirement has been calculated as close to 15 Kw`s you would have an `extra Heat Producing Capacity` for any period of extremely Cold temperatures
as Member King of Pipes has mentioned.
If the Property Heat Loss Calculations have been done correctly and the figure is close to 15Kw`s I would definitely go for the 18 Kw Boiler because as Member King of Pipes has eluded to - Heat Loss Calculations are basically formed around `the coldest expected outside temperature of minus 2 degrees` [my wording] and various Room Temperatures of 18 - 20 - 21 degrees.
If your Property`s Heat Loss is
close to the 15 Kw figure using that `coldest expected outside temperature of minus 2 degrees` / `Standard Room Temperatures` and there is an outside temperature drop down to perhaps `Minus 6 or 8 degrees` the 18 Kw Boiler is going to have the additional Heat producing capacity to combat the extra / quicker Heat Loss from the property that a much lower outside temperature causes.
This message is taking for granted that your Property`s Radiators are correctly sized and are currently capable of Heating the various Rooms ?
Otherwise the Heating System will not correctly heat the Property irrespective of the Boiler Input / Output.
I hope that this is helpful.
Chris