I'm not a heating expert or indeed very good at technical stuff at all and would welcome some YACFers input! Thank you.
1. Boiler choice and size.
Our plumber has recommended the 832 on the basis of two people wanting powerful hot showers at a time, but he hadn't really considered the lowest heat output ('cos people always ask for more whoosh). I will look at other boiler options but to keep this simple, can we please just keep this to 'is the 825 an adequate option instead of an 832?'
My question about the Ecotec Plus 32w is whether the lowest heat output of 5.5kW means that, for almost all of the winter, the boiler is less efficient than if the lowest heat output was 3.8kW.
Might a 25kw (825, 3.8kW min heating output, 10.5LitresPerMinute hot water flow) be a better option for us, as outlined below, than the 32 kw (832 5.5kW min heating output, 13LPM)?
As I understand it our 3 storey end of terrace, 3 bed one bathroom plus en-suite shower room 1984 house is likely to have a maximum heat requirement of 7kW on a very cold day, as we have double glazing, cavity wall insulation and are about to be able to further improve our loft insulation to fabulous when the water tank goes. We are happy to heat the house in the 18-21 degree range.
I gather that the boiler thermostat should be at 65 degrees to get an effective convection return of 45 degrees, making sure to put it on in time to bring the radiators and hence rooms to the comfortable setting rather than turning up the thermostat for a quick blast.
While we want two showers operating from the boiler we don't need to have them working at a time (three people in the house, none needing to depart at the same time in the morning etc. and I tend to shower at the leisure centre. Not planning to move for at least five years), plus we have a cold input washing machine and dishwasher.
I'm assuming we'll have eco shower heads which reduce the water use by aerating it so it still feels reasonably strong, but less water needs to be heated.
I'm not sure what water flow is required at what temperature to have two showers running adequately at a time in this three storey house - Does the 832 allow two showers at a time (and does the 825 allow two feebler ones?). It seems that:
If we cater for both showers being used at the same time we require a 12-16 litre per minute hot water flow
If we cater for one shower being used at a time we require 9-12 litre per minute hot water flow
Vaillant Ecotec Plus 832, 5-10yr guarantee, Maximum DHW output kW 31.8, 13LPM
Nominal heat output range of CH at 50oC flow/30oC return kW 5.7 - 25.7
Nominal heat output range of CH at 80oC flow/60oC return kW 5.2 - 24.4
Vaillant Ecotec Plus 825, 5-10yr guarantee, Maximum DHW output kW 25.4, 10.5LPM
Nominal heat output range of CH at 50oC flow/30oC return kW 4.2 - 20.4
Nominal heat output range of CH at 80oC flow/60oC return kW 3.8 - 19.3
Bonus questions:
2. Central Heating Control system
Which one - the manufacturers (eg Vaillant vSMART) or a Nest OpenTherm or...?
3. Deaerator
Put one in?
https://www.jtmplumbing.co.uk/water-treatment-filters-c435/spirotech-domestic-range-c356/spirotech-spirovent-rv2-air-separator-brass-p20387/s24002