Which type of boiler ?

[Nickfromwales]

Excerpt from a previous thread. A member asked.....

"Starting our extension and renovation fairly soon so I am now looking at the type of heating system to go for.

The house is roughly 100m² footprint and 450m³ volume.
Insulation levels are expected to be better than building reg standard but not upto PH standard.
Plan for heating and DHW in the house is as follows:
UFH throughout with no rads, two towels warmers to be used though.
One bath, two showers and four sinks (not decided whether both showers will be DHW or one electric yet but say both DHW for this example). Average usage of one shower and one bath per day
From jsharris spreadsheet I figure that my worst month at minimum OAT heat input would be 1398kWh based on daily heat loss power of 1878W.

Ideally we would like the boiler hidden from view and this could be house in either the garage or loft as both have ample space.
We have both gas and electricity to the house so we are open to a wide range of possiblilities.

Anyone have any thoughts on which type of boilers I should be considering?
As far as manufacturers go I am willing to invest a little more for better quality (brands I have investigated so far are Valliant, Baxi and Ideal). 

[Nickfromwales]

The first reply was.....

The biggest problem is getting one that can go down low enough. There is a huge dead zone between 'build regs' standard and passivhaus standard that is simply not served by the heating industry. 

The problem is that you need to have a boiler with a fair bit of power so that you aren't waiting days for a shower, but you then need it to modulate down very low to handle the highly insulated house. And it can't do that. 

So you end up with a boiler that is constantly cycling and on pump overrun, which isn't good for the boiler and isn't good for fuel economy. 

If you get to passivhaus standard then you can warm the house in the depths of winter with a small electric fan heater. So then you just go for a small boiler to warm the water up when you're not getting enough divert off your PV into your tank. 

But going all electric quickly gets very expensive if your power requirements are much above passive house standard. 

You seem to have to have a maximum demand of about 1000W or the next step up is 10000W (bearing in mind that most of the year you'll be well below the maximum power requirement). 

The Ideal Vogue 18 system boiler I'm looking at can deliver up to 19kW of power, but can only modulate down to 3.6kW. The 15 can only get down to 3.0kW