D_Hali - I did do as you suggested and used the link. However, I mentioned in another thread that the house is unusual in that it is double fronted, single backed, was part of one house now split in two etc. There is therefore an unusal layout etc. For that reason I done a room by room Heatloss calculation on the basis that it would be more accurate.
Sorry, I must have overlooked that post.
The BG guy came around and calculated the 13kw. I since done my own room by room calculation and got 13.5Kw. I am presuming a room by room calculation would be more accurate than the whole house calculation? Or is there something I am missing here?
A room by room calculation is obviously much better - provided your data is correct. I think 13-14kW is as good as you will get.
What I have not been able to clearly decipher is the length of time this might take place. My understanding is that when it goes below 12kw it will be getting close to the point where the room heat will be reaching the level set by the thermostat and boiler will then be knocked off in any case.
The heating requirement is directly proportional to the difference between internal and external temperatures. Assuming an internal temp of 20°C. you would get results similar to this:
[code:1]
Outside temp kw required
-1°C 14
2°C 12
5°C 10
8°C 8
11°C 6
14°C 4
17°C 2[/code:1]
There is data available which showing how the outside temperature varies throughout the year. But, speaking generally, the lower the temperature, the fewer the days. This means that, most of the year the boiler will be producing less than the design output - 14kW in the example.
If an 837 or 937 is installed it would only be able to use modulation of the outside temperature is below about 2°C. Any higher, and the only way it can produce a lower output is by cycling the boiler on and off.
In addition, I'm told the Vaillant will knock off for 5 minutes rather than continue to recycle.
But that's still cycling.
The Vaillant 831 has a lower setting of 7.7 if memory serves me correctly and the WB31 and 37 are in that range also.
The 831 is 8.7kW, WB37CDi is 9.4kW and the WB30CDi is 7.7kW However the hot water flow rate would not be adequate if the WB30CDi is used.
So from what I can ascertain, the amount of boiler cycling would be minimum if I got the 837. My priority is a good flow rate to the shower so on balance I am left with is obtaining the best flow rate versus the overhead of some unnecessary boiler cycling below 12kw.
The 837 and 937 would both be the worst choice for cycling.
You seem to be considering only Vaillant and WB products. I think you need to widen your net and include the
Broag Remeha range. They are well made and the firm has a very good reputation. All boilers will modulate down to 6kW (equivalent to an outside temperature of 11°C ), so the only thing to consider is the HW requirement. The 35C will give a 35°C rise at 14l/min and the 39C at 16l/min.