Hybrid Heat Pump

Hybrid Heat pump by Original Twist

Heat pumps of all sorts (ground source or air source) are going to save the World. The debate over ground or air rages on but is largely irrelevant when a hybrid design could cherry pick the best bits from both. Both suck heat out their surroundings and pump it to a higher level.  You get 3 or 4 times more energy out than is put in, a seemingly impossible engineering miracle which still, for many, defies belief. The in-out energy ratio is the coefficient of performance or COP.

First lets join the debate about ASHP vs GSHP by comparing the COPs of both across the season.

The ASHP operates in a wide range between the red line (day time running) and the blue line (night time running). Cold night time temperatures look pretty disastrous but warmer day time running produces consistently high COPs. On the black line the GSHP does without the wild swings and gently declines in performance as it sucks the heat out of the ever cooling ground.

showing the COP difference between ASHP and GSHP

Day COP vs. Night COP

While the GSHP (black line) gradually loses performance over the winter the ASHP can always do better on warmer days (red line) making the two systems closely matched during the daytime, up until Christmas anyway. Soon after that the daytime COP on the air source starts to improve while the ground source continues to deteriorate.  The only redeeming feature of the ground source is that it is miles better than the air source blue line at night and cold days.

Even though the GSHP rules the night an ASHP system geared towards more daytime running would be a match for the much more expensive GSHP which, let’s face it, doesn’t always need to run at night at all.

However, some new electricity deals for electric car charging (5p/kW.hr at night) make a GSHP, with more night time running, quite compelling. It might be thermally less efficient but the running cost is only 25% that of daytime rates. There’s more bounty though; the Renewable Heat Incentive which is heavily biased towards GSHP at 20.89p/kW.hr versus half that for ASHP.

In my view the Government have not thought it through properly; the rates compensate for the cost difference  between the systems and not for the performance gap if indeed there even is one. But who cares, there is a lot of money on offer and we can have some.

If only there was a system that could cherry pick the best COP line on the chart for any particular moment but still keep the high RHI payment of the GSHP. It would need to extract heat from the air on most days – red line – and extract heat from the ground at night – black line. The air side would not run at night so the blue line can be ignored and the system would cherry pick between the red and black lines. The average COP over the season would be around 4 which is better than either system on its own, so best in the world then. Luckily designing such a system is easy really and not particularly expensive either.

 Original Twist Hybrid heat pump.

GSHP unit

For the core of the system we start with a GSHP unit.  As the name implies we need to feed it with some warmed water from a ground loop or slinky and more on that later.

Air source module

There is already water going in and out of the GSHP unit so warming it with air is simple. Just connect a parallel circuit incorporating, say, a couple of car radiators (surprisingly cheap) and a fan or two to make the equivalent of an ASHP for about £200. 12v fans for cars are waterproof and easily available. A simple solar controller brings in the AS module whenever it can contribute.

Hybrid heat pump

Buffer tanks

Two buffer tanks are used to handle the cold water coming out of the heat pump and the warmed water going in. The tanks allow the air side to store daytime energy.

Warm air is a great resource so we decouple the air source module and let it run whenever there is a benefit, whether the heat pump is on or not. The cold tank will nearly always be available to charge and sometimes the hot tank too.  Like the solar stripper circuit on the heating system the solar controller turns on the fans and the circulation pump and also optimally switches tanks. When the colder tank is warmed up the flow back into the slinky is highly beneficial by raising the COP line slightly and delaying that end of season fall off in performance. The straight black line on the COP chart will bend upwards into better COP territory as shown in the new chart.  The new system always selects the best source for the best COP  and  is  always  superior  to  conventional  systems.



Slinky coil

Normally the ground starts the winter at around 12C then the GSHP and cold weather gradually take that down to around 0C. Permafrost around slinkies is not unusual. Normal ground loops are designed to perform at the worst end of this so they have to be huge. Not so with ours which can be very much smaller because:-

  • The ground starts the winter overcharged. We dump heat into it in the summer.
  • Frequent recharging takes place.
  • The cold return is usually pre-warmed by the air side before going back to the ground.
  • Day time running of the GSHP is less frequent so the ground temperature can recover better.
  • Towards the end of winter, as the air warms up, the Air Source Module takes on practically all the load. The end of season performance stress on the slinky is avoided so the design is less critical and it can be considerably smaller.

I’ve done a job where the slinky was trucked down from Switzerland along with a man in a white coat and a bill for thousands; a bit over the top when polyethylene tube from the local builders merchant is just as good and costs under £300.

Air conditioning

The RHI rules forbid a combined chiller unit in the GSHP unit. No worries there mate. In the summer the cold tank can be left to get really cold and a coil can send the cold to the main heating system and onwards to the fan coil units.  Aircon sorted for almost no cost!

BTW – the heat pump  module will be making heat when the aircon is running so you can heat the pool! Think of the pool heat exchanger as just another radiator on the domestic circuit, it couldn’t be simpler or cheaper.


So that’s it. Better COPs than a GSHP or an ASHP, full RHI payments, aircon, ground loop recharge, smaller ground loops and a better match to PV panels (more day time running).  The smaller ground loop might just swing this for owners of regular sized gardens.

Thought for the day

The RHI pays for heat made but not the electricity used to make it. So on a COP of 4 they pay for an effective COP of 3.  So if we ran a 3kW GSHP for 4 hours at night on 5p/kW.hr (electric car rate) it would cost 60p. But we would be paid for the 36kW.hrs produced at 20.89p      £7.52  NICE!

So, your PV panels run the heat pump for free during the day and you get paid for night time use – makes you think eh?

(Actually the RHI pays you based on your EPC certificate total heat demand with the rate at 20.89p/kW.hr, and for 7 years too)

ultimate eco heating

Ultimate Eco-heating system

If you are looking at heat pumps then I’ll assume you will have seen this heating system which particularly favours daytime running of Air Source Heat Pumps, a feature in keeping with the free power from PV panels.

In the light of my comments in Heating News this system has been revised and simplified. Revision 2

overclock and tilt PV panels

PV overclock and tilt

Using big PV to go off-grid – or nearly off-grid – makes sense especially when driving a heat pump. You’ll need every trick to make a limited energy supply go further so make sure you check out the 6kW overclock and tilt concept.