williamwright wrote:
On 12/11/2020 11:01, Theo wrote:
williamwright wrote:
Anyone had any experience of this?

Currently planning an ASHP to replace an oil boiler, done a lot of
research. What did you want to know?

Theo

Snags!

Well, the first thing to say is that it needs a mindset change:
combustion-based heating is about making a small amount of very hot stuff
and then spreading that heat around. Heat pumps generate larger volumes of
cooler stuff. This means that some things that make sense for combustion
(eg microbore piping) don't for heat pumps.

Next up are a category of things which could be described as 'installer
error'. Unlike combustion, heat pumps have a smaller range of power outputs
they're happy with.

A lower heat output means a slower temperature ramp up - the unit runs for
longer, but that's mitigated by having insulation to keep the heat in the
house. Instead of turning on the heat for a quick burst when you get home,
you leave it ticking along when you're out and you only pay costs for losses
through the insulation. It's less good to leave windows open.

If you try and demand too much heat out of them, many have a backup
resistive heater. This is designed for occasional days in the depths of
winter when efficiency falls, but an undersized heat pump may rely on the
resistive heater too much - causing high bills. Our installer puts this on
a manual control, so you're at least aware of what's going on (and we'd just
burn wood on those days anyway). A number of horror stories have been
people moving into newbuilds with heat pumps installed and no guidance on
how to control the system (see also that mindset change). Some of those
newbuilds had funky ventilation systems that were installed wrongly,
trashing their insulation.

If the heat pump is oversized, it still runs but less efficiently, affecting
your COP.

As far as a retrofit goes, that cooler water means you may need a radiator
upgrade if you don't have UFH.

Therefore a key part of installation is the heat demand and loss
calculations, which require a more skilled installer than just sticking a
finger in the air and picking a size of boiler.

COP is something you don't really find out until you have the system
installed. I've heard various sources but think 2-3 is a sensible range.

Noise: the fans hum, but a lot less than our oil boiler firing

Servicing: there's fewer people who know what they're doing compared with
gas plumbers. This is improving. In other countries everyone has an
airconditioner, so there's nothing fundamentally new here.

Electricity v oil/gas prices: this is really hard to judge. Oil is cheap at
the moment, but I suspect that isn't going to last. With a solar+battery
setup you may be able to power some of the heatpump load in spring/autumn
(and hot water in summer too).


10 years ago they were very bleeding edge. Nowadays most of the kinks have
been worked out. They aren't mainstream, but they're now fairly mature.

Theo