Self install inverter heat pump---what to buy?

My existing system which I converted from an oil boiler 11 years ago uses a 22kw air source heat pump running through a buffer store and consists of radiators with some ground floor under floor heating. The heat loss according to the heat geek easy calculator is around 8kw. DHW is via solar PV and inline water heaters + input from the ASHP if needed. The house has most rooms as zones with individual thermostats. The normal system running temp is 42-45C.

I would like to change the ASHP for a modern inverter type with weather compensation and run direct without the buffer over the summer if it ever arrives.

I previously bought a full monitor kit as I would like see how the system performs. I was originally going to monitor the existing ASHP but I think it would rather change the existing ASHP and modify the system before installing.

Question is what heat pump to choose as most seem very similar on specs but not on prices!

I’ve installed all my own plumbing and electrics (most of which requires notification now) and I’ve been looking into this myself. I like to engineer things correctly and install everything safely, so take everything that I say with the appropriate amount of salt.

Vaillant’s Arotherm range of heat pumps seem the easiest for DIYers to buy and install. There’s a 9kW version. See “Heat Geeks” and “Urban Plumbers” on YouTube for useful information.

Choose a fairly large unvented hot water cylinder for hot water efficiency (Heat Geek recently did a few videos on this). Ideally, all of your primary pipework needs to be 22mm (perhaps some at 28mm) and you’ll want to remove as many bends as possible for efficiency. I’m guessing that your heat emitters are probably already large enough (I had to change almost all of my radiators). All the manufacturers recommend buffers, low loss headers, etc but I’d plumb everything direct, use the pump in the monoblock, and see how that works before trying those things. If there are performance problems, I’d suggest upgrading the pipework and straightening it before adding buffers and low loss headers. Simpler systems are better.

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I have a 12kW Gen6 Samsung, just coming up to a year old, so can’t comment on long term reliability, but it has been fine since installation

  • It does not have an embedded pump or pressure vessel. This has the advantage that you can use/easily change industry standard items rather than manufacturer specific items (cheaper, easier to source)
  • It is one of the few heat pumps that does not require a pressurised system, so will work quite happily on old style vented systems (maybe not so important these days)
  • Samsung will sell you a Modbus interface that fits in the outside unit. No requirement for Github hacks to access heat-pump data. I have it connected to my HEMs and have complete control over it, so not restricted to Samsungs’ interpretation of what good control should be (their built in weather compensation is a bit crude, IMHO)
  • I used to work for Samsung long ago, and was quite impressed with their attitude to quality.

On my short list when starting out were Daikin and Mitsubishi (been in the HVAC business a long time, kit should be reliable), Vaillant (it uses eBus which I hacked 20 years ago for boiler control, so could easily connect into HEMs), Hitachi (good reputation but not so easily available) and Samsung (reasons as above).

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Thanks for the info—all useful stuff. I see that the Samsung is now G7, assuming that its minor improvements from G6 and good point about the external pump which I already have.

A lot of the data sheets dont give the min output for the unit, which I find surprising…unless I’ve missed it somewhere. Also I’m pretty clueless about the various interfaces like Modbus, at the moment I dont see why I need anything like that apart from the controller…perhaps I will as I learn more!

Is mainly a heat pump change as I aready have the unvented cyl installed and hot water system working.

The changes have been prompted by looking at how things seem to have progressed since I intalled mine but the biggest influences to date are the postings by urban plumber and heatgeek.

The chinese ASHP I’m using now has Hitachi scroll compressors and has been faily reliable apart from changing all the power capacitors in the motor circuits, its kept us warm for 9 years. The oil boiler is still in place and I’ve fitted motorised ball valves so that I can change the entire system over to the oil boiler at the flick of a switch in the event that the ASHP cannot cope but Ive never really used it so in the next upgrade they will go along with the buffer.

Other things I forgot to mention in my list of Samsung advantages

  • It has a boiler enable output, so you can use it directly in a hybrid system with oil/gas boiler without extra controls.
  • Samsung will sell you an interface box that allows you to update firmware and examine much more working data than is available through Modbus, useful if your installer has the usual “average” abilities.

Gen 6 uses R32 as the working fluid, Gen 7 uses R290 (propane), a more environmentally friendly refrigerant. No doubt R32 will get more expensive with the move to propane and CO2. The compressor and motor are in a hermetically sealed canister, like a fridge, so assuming no bad pipework, it shouldn’t need a recharge in its’ working life. By the time it does, then replacement rather than repair will be the better alternative. I see in the first Gen 7 devices they incorporated the pump and stuff inside the heat-pump (G7 integrated), but they seem to now also make a version without, like the G6, probably by popular demand.

You may well be happy with the controls provided, but it is always useful to have a known way out if there is some show stopper you find after installation. I believe HA (Home Assistant) has a Modbus interface so that could provide you with alternative control with minimal software skills needed.

Not straightforward to answer - some vendors (Samsung included) fit the same compressor to a range of heat pump sizes, and since the onset of compressor surge will be one of the factors that sets unit turndown, any vendor-supplied minimum output must be interpreted accordingly.

I have an 8kW Samsung HTQ (call it Gen 6.5 if you like) but this has the same compressor as the 12kW and 14kW versions. Samsumg UK Tech Helpline told me that the minimum heat output for my unit is “about 4kW” or 50% of nameplate.
I’ve verified this on several test runs - when the inverter frequency drops below 20Hz (in response to weather compensator or roomstat algorithm demands), the controller stops the compressor. This corresponds to a heat duty of a little below 4kW. (The nameplate 8kW is a bit conservative except at really cold source conditions, and 9.5kW is generally achievable, so 20/50*9.5 = 3.8kW which is what I see when the compressor stops.)

But this limiting heat duty will also apply to the larger units. So turndown should indeed be expressed in terms of heat duty (in kW) rather than as a fraction or percentage.


Edit: BTW I am very happy with my Samsung (1 year old today) and would strongly recommend…

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Many thanks for the replies—thought I had missed the min figure somewhere also perhaps is not so important as its working into a buffer rather than direct to rads as per heatgeek and urban plumber utube video’s.

The Samsung Gen 7 12kw seems to be fine for what I want and is around the size of the existing 22kw so an easy swap as pipes and elec already exist. Looked at more utube and doesnt look too bad to set up.

Planning a start later in the “summer” now that the solar PV is coping easily with the DHW

Sprsun/Cool Energy. A very capable machine, built on a complete Carel refrigeration controller platform.

The 9kW will run down to 4kW at the bottom end. Very good weather comp, fully configurable PID controller, built in PWM pump and flow sensor.

DIYd my install last September and have been very pleased with the performance this past winter.

I’ve got it running open loop (no buffer, llh or zoning) on pure weather comp. Although I’ve recently fitted a thermostat to act as a hi-limit stop now we’re into warmer weather.

EDIT: They do three versions - this one is the 9kW and uses R410a, it’s not in stock at the moment but the almost identical version that uses R32 and has EVI, and gives 10kW is in stock. They also do an R290 unit that’s 9kW, although it is pricey compared to the competition.

Thanks for your post—where did you get the HP from?

Cool Energy in Grimsby, can’t say a bad thing about Chris and his operation, great product, great support.

I particularly like the non-linear weather comp option, and the fact it has 2x 240v valve outputs (heating and DHW) so you can use 2 port valves if you want. Published modbus register list too…

EDIT: SInce then we’ve settled on 45.5 @ 0 and 35.5 @ 15 - we’re on radiators, with UFH in the extension running at 28 flow into 5" thick slab.

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Yes its an interesting option and the spec looks good and under consideration. Price is good but also found a Samsung Gen7 12kw for not much more at £4300. We are similar with rads upstairs and UF down + rads. Our UF is from 25 years ago and it’s rubber pipe, still intact but not very good for conductivity and originally ran at 65-70C so added some rads downstairs and big improvement overall at lower temp.

Think I’ve seen the none linear waether described as altering the curve on some videos but not sure.

Have you got full monitoring as well?

Isn’t the 12kW samsung the one with really bad turndown as it’s a 16 underneath?

No monitoring here, can’t justify it to myself. All I know is ‘it’s a bit cheaper than gas’

EDIT: Found a 12kW R32 unit on the dashboard:

Not bad performance.

+1 for the CoolEnergy unit - we have one.

I was thinking of putting a flow sensor into the system, so we could use the openenergy heat pump software but if there’s a flow senor I can access, maybe I won’t need to break into the system.

Do you know how you access the flow sensor data?


PS Can you point me to the Modbus info - thx

The flow sensor is a simple on/off sensor, it doesn’t give flow rate.

I’ll upload the modbus register table when I’m in front of my PC.

Is yours the old 9kW r410 unit, same as mine?


inverTech Modbus Agreement.xlsx (33.3 KB)

"Generally we would use the spare rs485 terminal that’s normally used for wifi for modbus control.

Then if wifi is also required we would add the bms/rs485 expansion card and connect the wifi module to this, see attached picture."

Thanks Hugh.


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Glad to help. If you need any more assistance, just ask. I’ve been playing with mine for most of the winter and have a few pointers if you need to run it open loop (tweaking the PID and runtime values)