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 — OpenEnergyMonitor 0.0.1 documentation

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?

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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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Thank you everyone for your answers, all very helpful.

Here are some more questions.

  1. I have come across the term ‘Flexible Heat Pump Technology’ which does something I has assumed ASHP did already as standard. Does anyone know what if any brands/products are using this already? See the following articles.

  1. Air Source heat Pumps have a reputation of not producing as much heat (or as quickly) as a traditional boiler. The recommended solution is to therefore either to use underfloor heating or bigger radiators, both of which result in a larger surface area to emit heat via and hence help with this. However I also see that for some makes/brands (see link below) there are choices available for an ASHP with low temperature heat pumps and high temperature heat pumps and possibly medium. My house is an old Victorian detached house with no cavity walls and even though I am planning to fit some internal wall insulation it will never be as good as a new build. So would I be better off getting a high temperature heat pump? Or do these operate less efficiently and would getting bigger radiators be better or both?
  1. Is any particular make/brand noticeably more efficient than all the rest? Or are they all fairly similar in performance?

The first step to upgrading to a ASHP is a full heat loss survey of your house. You’ll want to know how much heat is being lost from each room when it’s -3C outside, plus how much heat the radiators can emit at various flow tempatures. This will tell you either a) how hot the radiators need to be, or b) how big the radiators need to be. 45C is a common temperature that is designed for.

Note: a “bigger” radiator does not necessarily need to take up more wall space, as it can be thicker with more fins. A double panel radiator with double fins has twice the output of a single panel with no fins. If space is tight, there are some 3 panel radiators available.

Improving the insulation of a house is beneficial regardless of how it’s heated. I would suggest doing the insulation upgrades before getting a ASHP, as that might mean you can get a smaller unit that can modulate to lower power consumption.

I don’t know about high temperature heat pumps, but generally you get the best performance by running at the lowest possible temperature. This is achieved by fitting larger radiators, while insulation helps to reduce how much heat needs to be produced.

We’ve seen ASHP installations in solid-wall Victorian houses perform just as well as in more modern houses. You can see a couple of them at

The best performing heat pumps are the ones that are installed well; some brands may be better than others, but if overall system design is poorly done then performance will be equally bad. Look for a Heat Geek Assured installer.

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Everyone will have an opinion and it’s down to the individual to choose what fits their criteria. If you intend to use glycol in the system the a plate exchanger system is ok because it will separate the circuits from the heat pump so you could drain down the radiators and not loose any glycol. The downside is you do loose some efficiency both from the plate to plate and the use of glycol. Look at the mixergy cylinders they can be set for heating various amounts of water without waste. If you intend to use solar thermal you need a cylinder with large enough coils (surface area). Don’t get carried away with gimmicks and keep what you want the heat pump to do heat your home and water.

It probably should be the first step to installing any heating system else you’re just guessing in the dark, but boilers and energy (particularly gas) were cheap, so installing a gas or oil boiler with twice as many kW as needed, undervolume radiators (saving not much money), and running it inefficiently hot (preventing condensing) for short periods was cheaper than actually measuring things and designing.

ASHPs are currently so much more expensive than boilers and, more significantly, they only modulate down so far, so the survey is worth it not to buy a bigger one than needed, which would be more expensive to buy, and to run outside winter.

Do any of the articles actually say what the heck it means?

Well, some people report performance figures that I wish I had, but I suspect they’re doing unusual tricks (cold house, disused rooms, …) that I’d never get away with. I think installation quality makes the most difference, and some brands seem to attract a lot of complaints on forums, but I think the main thing is that different brands perform differently. We went with Mitsubishi Ecodan primarily because it’s quieter than some and can produce its nominal output well below zero (at reduced performance, of course), and accepted the limited-but-enough-for-monitoring/automation connectivity of melcloud and need to address the fairly high minimum output.

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I got the impression from the articles that what they do is use a small tank or coil to store heat already generated and re-circulate that rather than than using purely fresh heat. So some heat that could be generated during the day when it is hotter could be reused later when it is colder for example.

This is what I originally thought the big tank/cylinder was for but apparently that is purely for hot water like a traditional boiler.

I recently visited a London based Daikin installer and was able to see and discuss the Daikin product. This was helpful. One aspect I discussed was the possibility of combining heating and cooling and as I expected they confirmed that the ‘normal’ ASHP configuration of an ASHP with either underfloor heating or radiators was not suitable for cooling as well even though brochures imply it can run in reverse for cooling. Even ignoring the greater inefficiency of trying to cool a room by running cold water through underfloor heating or radiators this could lead to condensation. Therefore as I had been already considering I moved on to thinking about a separate Daikin AC external unit to support a couple of internal wall mounted systems.

Really you need to have an Air to Air system and not an Air to Water system as most ASHP setups would be.

An area I did not at the time get answered completely was which Daikin Altherma models might include in its units the buffer and/or volumizer items. I believe I would be right in assuming the valve to direct the circulating water either to the hot water tank or heating would be a separate item. Is the pump in the Daikin internal unit?

Coincidentally just after the above Daikin installer meeting, I discovered a new(er) Samsung Mono HT ASHP. This seems to be intended to be a similar solution to the Daikin Altherma HT with the HT meaning it can produce a higher temperature up to 70c and hence be a better match for less well insulated houses with radiators i.e. like mine.

Samsung as discussed in other threads here are more friendly in terms of smart home integration and monitoring so this was of interest.

I did then also find mention of the Samsung TDM Plus with Climate Hub and the fact that this can be configured to drive not only underfloor heating or radiators and a hot water tank but also drive standard wall mounted AC units. This would all be from the single external unit to drive both air to air for the AC and air to water for the heating.

If anyone is familiar with the Samsung TDM Plus system with Climate Hub I have the following questions.

  1. It would seem logical to use the ASHP to extract heat via the internal wall mounted AC units - when in cooling mode to then re-use that heat to heat the hot water tank. It is not clear this happens and if not the heat would be wasted.
  2. The Climate Hub supports a wired remote which can be used to control the hot water tank heating, configure 1 or 2 room heating zones, monitor energy usage and (possibly) configure use of solar power, but it is not clear if it also can control the AC via the same wired remote. It maybe a separate very similar wired remote has to be wired to the individual wall mounted AC unit.
  3. I cannot tell if the TDM Plus is able to deliver the same ‘high’ temperature that the Samsung Mono HT and Daikin Altherma HT can. Anyone know?
  4. Does the Climate Hub contain the buffer tank/volumizer and pump and valve? (I am trying to get an understanding of what space I would need to provide for the equipment and locations.)
  5. Is anyone able to confirm as I suspect that the Samsung Mono HT even if used with the Climate Hub cannot drive both heat and AC unlike the TDM Plus? I am fairly sure this is the case. It’s a shame as it looks nicer than the TDM Plus external unit and might be both more efficient and quieter as a newer product.

Apart from possible answers to the above is anyone familiar the Samsung TDM Plus and able to provide any additional comments?

Also does anyone have any recommendations for a London installer/supplier of either the Dakin Altherma HT or the Samsung TDM Plus?

I’m afraid I can’t answer all your questions. But I do have a Vaillant Arotherm 7kW ASHP with the cooling module installed. We have Dunham-Bush fan coil units installed in the loft to cool the upstairs bedrooms with water cooled via the ASHP.

The system was designed and installed by Elite Renewables who are based in outer London and hence may be suitable for your needs. I have no hesitancy in recommending them.

In response to your question 1. the system is unable to transfer the heat extracted from the rooms to the hot water tank.

Q1 - There are many many ways of doing this, my conclusion after nearly a year of educating myself was that the most efficient [lowest running cost for heating] will be a simple single circuit without heat exchangers and without low loss headers [of course it should be appropriately sized for the heat loss otherwise you will be at the inefficient extremes of operation of the unit] [edit - I suspect if you can’t achieve the optimal flow rate with this setup - it may not be the best option]

Q2 - a separate, and heat pump specific tank should be used for supplying hot water to taps and showers - for maximum efficiency the heat pump should be configured to vary the water temperature to your radiators based on heat loss (which changes with outside temperature) - not what you want in your shower (cold[er] showers on warm[er] days) :slight_smile:

Q3 - for the highest efficiency “they say” you should leave the heating on all the time. I don’t quite understand the maths on the efficiency/heat loss side and have not proven it to myself [yet] mathematically, but have tried to explain it to myself in the following way… think driving for 3 x 1 hour slots over a 6 hour period at 100 miles per hour, vs 6 hours at 50 miles per hour. (one is less efficient than the other due to cube law of air resistance but both get you there in 6 hours - £60 vs £40 petrol)

Q4 - something like the homely system backed by some degree of AI in the cloud (need compatible ASHP for that) - I have not tried it though

Q5 - don’t know - I went for a Samsung Gen6, seems to cater for all my smart home needs with a reasonably well documented modbus interface - my fall back will be to install homely if I give up on my planned diy “smart controller”

Q6 - Not sure - I converted my vented hot water tank to be [more] heat pump friendly while retaining my solar thermal coil and back boiler- early days though - there are two heat exchangers between my heat pump and the water [eventually] going into my shower, which is not ideal but I have put in a waste water heat recovery system that compensates somewhat for having an indirect tank (64% of heat from shower drain goes back into the shower) - will report back at some point on the success [or not]

Q7 - If by “this” you mean Solar Thermal - my [unproven] theory - if my heat pump gets a break every summer and works less in autumn and spring it should last longer

That depends what you mean by “better match”. You won’t have to replace any radiators but it will almost always cost more to heat water hotter, so you will pay more to use it. I suspect HT is best used on systems where the pipes are narrow and the owner will not or cannot replumb.

You will start to go round in circles and get confused and misled with all this information and opinions. so first like i said before the most important thing you can do is get the correct heat loss calculation for your property and see what size heat pump you require and what size emitters are needed in each room to satisfy your needs. secondly size your hot water cylinder based on occupancy and usage 45ltrs per day per person (guide only but industry standard). Thirdly keep it as simple as you can, avoid buffer tanks, low loss headers keep your zones as open as you can to maintain volume and flow rate. Remember that increasing your flow tempreature increases your running costs. Your hot watyer cylinder only needs to go to 60degrees once a week for a legionella cycle so you could use an emerson heater on a timed program so there is no need for a heat pump that can achieve 75 degrees flow rate. Most heatpumps will do reverse operation if you think about the defrost cycle but you cannot have it with UFH because it will cause damp and condesation build up and you can only use it with emitters if you use fan colis to blow the cool air into the room.


Depending on the type of tank. If the supply water passes through a coil to be heated as used, there is no need for this cycle.

Hi Brian
We may be at odds or just crossed wires, do you mean supply water as in what comes out of your tap or supply water as in what circulates around the coil to heat the stored water in the cylinder. If we are trying to be as efficent as possible you would not be heating your stored water to 60 degrees nor running your heat pump everyday to 60 degrees. You want your water to be between 48-50 degrees from your tap so why add cold to mix it down. Personally I would not bother with legionella cycle as there has never been a case of legionella bacteria associated with an unvented cylinder due to no air being present in stored water. There’s more chance of it developing in all the dead legs you see on water circuits.

Supply → from the mains, through a coil in the hot tank tank heating DHW as used. i.e. you never store the water you actually use.

Vented or unvented, makes no difference. If the water used is not the water stored, no need for the cycle.

It depends on the tank size and the volume you need. Also if you are also heating from another source, the stored tank temperature can easily be above the DHW outlet temperature.

Also depends on length of runs. Unfortunately, my house has long runs as the design of the house was heavily influenced on location and the planners. Longer runs, hotter water required leaving the tank (even when heavily insulated).

Also depends how hot you like your showers :wink:.

Finally, if you heat the whole tank to say 55°, your DT as it approaches full drops and so does your efficiency (I’m still on gas BTW, but the principle stands). If you heat until the DT starts to drop, you don’t end up with a full tank at the desired temperature. Heating part of the tank to 55 (rather than 50) could well be more efficient than filling the whole tank to 50.

So you mean like a combi and because now I know you are on gas yes through a plate to plate and heat the water you use, this would be impossible with an ashp. So do you have a secondary return on your hot water from your cylinder because this would be more efficent and reduce the amount of water run before reaching temp at the tap. Mixergy cylinders now have the ability to heat varying amounts of wate4 depending on demand.