Correct Size (Kw) Boiler ?

[Dan Robinson]

Pressure is not the only thing to consider.

Flow rate through the mains is also important.

I still don't think you need to worry about the boiler just yet.

the 37CDi is a combi and will struggle with two outlets simultaneously.

 

[ALEC1]

whole house boiler is right in this case...its a question of looking at both...and making a judgement call...

as you have two figures the existing boiler 15kws, and the whole house 16kws, I'd stick with that...

I see so many oversized boilers that the rad multiple system usually works for me...certainly too bigger radiators for the heat loss of the house will work better than the other way round...

 

[D_Hailsham]

1) Upgrade current boiler to 18-20kw System which is the most efficient boiler for my home or invest in a larger boiler and have it down rated to allow for future expansion.

That depends on the probability of expansion, how big it will be and the time scale.

You also need to make sure that (a) the boiler can be down-rated, and (b) that the minimum output of the boiler is well below current requirements (say 50%).

2) Put in a large 210-300l Unvented Cylinder. Larger the better to avoid hot water shortage especially in the mornings.

You should be able to calculate the maximum simultaneous demand, e.g. two showers at 10 litres/min for 10 minutes = 200 litres. But don't forget that this will be a mix of hot (60C) and cold (15C).

3) Try to avoid heating the water at the same time as CH, which will increase recovery times.

If you install weather compensation controls, this happens automatically. The reason is that weather compensation varies the water temperature according to the outside temperature, so it can be anything from 30C to 75C. Heating the water in the HW tank requires the boiler water to be at max temperature.

Have a look at the Remeha Avant System Boilers. They meet the requirements for items 1 and 3.