"Andy Hall" wrote in message
...

All of this takes you to the point that it is most efficient to run
this type of boiler continuously at lower temperature than to force it
into a situation of reheating a cylinder in bursts.

Which is exactly what it will do when heat demand is les than the minimum
output of the boiler. Most of them it is around 8 kW, some go down to 5 kW.
You will get cycling.

If this type of boiler is connected directly
to the radiators and you have TRVs, as
the room temperature reaches the set point, you have
effectively balanced the building heat loss with
the radiator outputs.
THe TRVs will begin to close. If the boiler is directly connected to
them, it is able to sense the reduced heat
demand and modulate down accordingly.

But not enough and inefficient boiler cycling occurs.

If you put a store in the way, you
will be emptying it at a low
continuous rate of (e.g.) 8kW,
but then when most of the energy has
been used, replenishing it in bursts of
perhaps 25kW - all controlled
by a thermostat on the cylinder.

Putting a thermal store between the boiler and the rads with the temperature
controlled by an outside weather compensator will eliminate boiler cycling
and operate at very low return temperatures promoting efficiency.

When house only requires 1kW of heat, it can just take this from the thermal
store at the exact temperature dictated by an outside weather compensator.
The boiler stays off until the store water drops below what the outside
weather compensator dictates. Then it comes in to reheat to the required
temperature in one long low temperature efficient burn.

THe two exceptions that I made were a) if it's a conventional or
non-modulating boiler - there you would make an improvement by having
it recharge the cylinder once as opposed to driving the radiators
directly and cycling to match effective power output to load;

Most modulating boiler modulate to maintain a flow temperature. These are
perfectly suitable for a thermal store. Even those modulating on load
compensation operate quite well too. Best to buy:

a) A condensing non-modulating boiler
b) A condensing modulating to maintain a flow temperature.

These generally are cheaper and less sophisticated and less to go wrong.
Keep it simple.



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