Sensible ASHP quote?

I've got a Daikin 11kw split ASHP but with an overlay u/f system which we've had since September 2010. It's in a 140sq.m bungalow of 1986 vintage with original cavity wall insulation, 300mm+ in the loft and 1999 double glazing.

Ours has a flow rate of between 30-40degrees depending on the outside temperature. I personally wouldn't like to run a system at 50degrees as I'm guessing that it's running a bit close to having the back-up/boost heater.

It's reckoned that every degree above about 35degrees flow temp will impact efficiency by around 2.5%, so running it at 50 dgrees will reduce your efficiency by nearly 40%.

I'd strongly advise that you look at your radiator sizing because one that's rated at 2kw with a flow of around 65 degrees will be giving a lot less, probably 50% less at 35 degrees so you'll be cold.

I would never ever recommend just sticking an ASHP onto an existing system. As JeepJunkie says the pipework need to be adequate for the flow and the emitters should be capable of heating the the room adequately.

I'm not an expert but I'd guess that an 8.5kwh ASHP wont be big enough and will have to use the back-up heater to maintain it's output when it very cold.

I'e do my own calcs if I were you because if the system isn't spec right you'll be cold, it will cost you and you may not get the RHI unless it conforms to their latest specs.

It's a big investment and need to be carefully considered. Do it right and they are excellent, do it wrong and you'll be very disappointed.

I'm not convinced that a solar array will help all that much. We heat our house in the winter when the sun doesn't shine much, it's at the wrong angle and we use the majority of our energy in the early mornings, late afternoons and evening when it doesn't shine at all.