JG - thanks for the information (and the heads-up on the Aerona) although I would have to say I had to read it several times to try and understand!
Whether it is either / or for the heat source, I hadn't really considered. My initial thought was that as the CoP dropped off, the oil boiler would take over, but if they can be used in conjunction to get the most efficient solution, happy days.
Although you have now got me thinking, could you have a floating bivalent point? As the price of oil & electricity fluctuates, so will the preference as to which would be the less expensive heating option.
I think that you are saying that the ASHP needs to maintain a constant flow rate and that zones etc can upset the balance. As we have 18 rads over 2 zones, it strikes me we would be on a hiding to nothing as the volume is going to be variable. As an aside, the existing system is currently open vent, does this mater? If it is an issue, could the ASHP be integrated via an PHE? That way the systems are split, the ASHP can be pressurised as required and the volume of fluid in the ASHP circuit will be constant...
As for the radiator sizing issue, I was figuring that if the external temperature was high enough for the CoP of the ASHP to be acceptable, the heat demand would not be overly high and a slower rate of heating would be acceptable. Or is this naive?
I have to admit, I don't quite understand what you mean by
Daikin's need a 5 degree delta T which is difficult to hit on rads, Mitsubishi's work to a 10 degree delta...
It was my intention to leave the DHW running off the solar / WBS / oil boiler as ASHP's struggle to heat to 60'.
From what you appear to be saying about the Aerona, what I save on the unit, I will loose out on performance.
Thanks for taking the time to reply, and hopefully I have understood what you are saying.
