Newbie to ASHP

I have been researching various topics in preparation for a major home renovation project and one aspect I am currently planning to include is to replace a traditional gas boiler with an ASHP.

My house is about 140 years old, detached with solid brick walls. I therefore plan to also fit internal wall insulation and in order to minimise the thickness I am planning to use combined plasterboard/aerogel as the solution. Whilst this will not provide as much impact as either external wall insulation or traditional thicker rigid insulation panels it will provide the best solution for the thinnest approach although with a commensurate higher cost of course. I will be sticking with radiators although I will replace the current ones with newer, bigger ones.

I have also found some smart air bricks and I am considering fitting these as this would help retain heat under the ground floor from the pipes which feed the radiators.

I do however still have areas about ASHP I do not yet feel I know enough about yet.

  • Am I right in thinking that the outdoor unit heats water which is stored in a water tank which is then circulated around either radiators or of course underfloor heating, rather than directly circulating hot water? (A traditional boiler circulates hot water direct to the radiators.)
  • Am I also right in thinking the same hot water tank is used for supplying hot water to taps and showers?
  • As ASHP have a reputation of taking a long time to heat up a house would this then also mean a problem in supplying sufficiently hot water to a shower?
  • What if any options are their for smart home heating controls and ASHP? Is there any point to this? (There is for traditional boilers.)
  • What ASHP brands are friendliest for smart home implementations?
  • Do the hot water tanks sold by ASHP brands all support also linking to STP - Solar Thermal Panels i.e. hot water from the roof?
  • Would this boost the performance of the system and be worthwhile?

Hi John! I can give some answers to these, based on my own experience so far…

The water does actually circulate around the house to heat the radiators directly, and the cylinder is only used to store domestic hot water. Some installations may have an additional “buffer tank” to increase the volume in circulation, though this is generally to be avoided.

The hot water tank only stores “potable” water, for taps and showers. There’s either a coil (like traditional cylinders) or an external heat exchange (favoured by some manufacturers) that keeps the circulating water (often containing glycol) separate from the water in the tank.

An ASHP can heat up an entire tank of water in maybe an hour or so, hot enough for several showers. One would typically have the tank heated up once or twice a day, depending on size and demand. Can be topped up with solar PV or thermal.

“Smart” heating controls for ASHP are about as “good” as they are for traditional boilers, depending what you want out of them. Given the lower heat output of an ASHP, many owners leave them “on” all day rather than trying to be smart with them. Good heating controls can have a significant impact in overall system performance.


Don’t know the answer to this. I think a lot of manufacturers have proprietary systems, and many ASHP owners have installed their own hardware to monitor and control their systems.
See Heat Pump Monitoring - Guide | OpenEnergyMonitor

I don’t know about all brands, but I have seen some that sell models with solar thermal support.

I feel that solar PV is more versatile, especially paired with a battery, unless you happen to have solar thermal installed already.

Be sure to get a thorough heat loss survey done of each room of the house, so that you can get the correct sized radiators and heat pump unit.

You may find this video useful, where @glyn.hudson shows off his self-installed heat pump:


All great answers. I’d like to add something to this one:

I’m not sure about best. Maybe Daikin? You can get some idea what’s out there by searching source code sharing sites for terms like ASHP, HVAC and heat pump, like Search results · PyPI or heat-pump · GitHub Topics · GitHub

In theory, the Mitsubishi Ecodan is quite good, as people have figured out how to connect a DIY controller to the cn105 socket and Mitsubishi haven’t prevented it (possibly either they can’t, or they are confident that good enough protections are between that socket and the heat pump, possibly to protect the hardware from their own controller if it got damaged or hijacked — it should be the second of those!).

In practice, if you were on the RHI+MMSP grant schemes, that socket is hogged by the MELCloud device for the duration of the MMSP grant payback. The NZ equivalent can be controlled with the echonetlite protocol but the UK (EU?) MELCloud device only seems to expose HTTP(s?) to the local network and I’ve not yet found the protocol documented. I think all UK DIYers have ended up controlling the local ecodan over the Internet via the MELCloud web app, which is disappointing.

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Additional note. Ideally the water is directly circulated from the ASHP to rads via existing pipework.
However, there is a school of thought that a plate heat exchange to separate the water in ASHP and the water in the heating circuit may be “a good idea” for older systems as it keeps the muck/dirt/corrosion out of the ASHP. Plate heat exchanger are very cheap.

Keeps stuff out of the ASHP but costs a couple of degrees across the heat exchanger, further reducing efficiency. Ladies and gentlemen, place your bets! Efficiency now or a pump failure at some unknown later date?

Samsung are pretty good, they have a fully documented Modbus module that allows you to get lots of data and control all the functions. We’ve implemented support for this in Emoncms:

The nice thing about Samsung is this modbus interface is officially supported and fully documented by Samsung, unlike the Ecodan interface which has been reverse engineered. I don’t have experience of other heat pumps, I believe Vailant units can be controlled via eBus protocol which is documented by the hardware required to use it is not very available.

As long as the system is properly flushed and a good quality magnetic filter and strainer is used, there shouldn’t be too much dirt circulating. I would avoid using a heat exchanger, minimising heat loss is key to making a heatpump work well. Worst case scenario is a pump needs replacing, pumps are relative cheap and easy to replace.

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