"IMM" wrote in message ...
"John Aston" wrote in message
.. .

snip

Simpler Alternative:

1. Low Temp Circuit: One dedicated condensing boiler serving the UFH only
on a weather compensator. Simple, separate and sorted.

2. High temp circuit. Another boiler using a 3-way diverter valve serving
DHW and rads. You may want a weather compensator switching the boikler to
give the ideal temp for the rads.

Do you mean that the weather compensator changes the boiler setpoint
according to the outdoor temperature? If so, which (good quality)
boilers have this feature, please?

When DHW is called the boiler runs up to
max temp. This is similar to normal domestic setup.

3. A controller staging in the UFH and rads to give precise control of room
temps. UFH 1st stage.

4. Backup: Now you have heating backup if one boiler drops out.
Electrical backup for DHW.

5. Cold water storage tank instead of an accumulator with a booster pump.
Tank can be fitted anywhere.

6. The cylinder can be:

a) An unvented version,

b) A DHW only heat bank, such the DPS Pandora, which does requires an
overflow so can be fitted anywhere in the house.

You will find that two condensing boilers can be had for less than the price
of the Viessmann, and lots of change too.

For an inexpensive condensing boiler, if the resistance of the load is
greater than the boiler pump's residual head, can I simply put an
external pump in series with the flow output?