Hybrid heat pumps – the way forward for our aged housing stock

Some background

Many people will have heard of heat pumps and certainly AC – air conditioning units? Firstly a heat pump IS an air conditioning units and IS a refrigeration unit.

The function of a heat pump (the generic name for all 3) is to extract heat from one source and use it in another source. With a fridge, it’s job is to take heat from the warm air in the fridge and deposit it outside the fridge, leaving the fridge cooler. The heat pump does the same but takes heat from outside a building and sends it into a building to heat the living space.

We call this process a reversible refrigeration cycle because it uses no fuel such as oil or gas and all the energy it creates it reuses.

The process requires an electric pump (compressor) to circulate a fluid (gas and liquid) round the system from indoors to outdoors, it is this pump that requires energy in the form of electricity.

A heat pump is very efficient because it is using free heat from outside (as long as it’s above 7°c) and often has an efficiency of 500%. We call this efficiency a COP or coefficient of performance, which in essence means that for every KWh of energy consumed (in electricity) the heat pump produces 5KWhs of heat energy. Unfortunately the great drawback of heat pumps is two fold. 1) they run on electricity which is about 40p/KWh at current (June 2022) prices and 2) they are much less efficient on cold days so reducing the benefit.

So how can we use that efficiency in heating our homes? One approach is to us a gas boiler and a heat pump in tandem. The heat pump works during the cooler, spring / autumn months (6-8 months) and then the gas boiler takes over during deep winter (December and January). This is believed to reduce the gas consumption by over 80% which of course means a reducing in greenhouse gases.

This technology can be further advanced by installing a system where the electricity used is obtained for free*. If a property is capable of supporting solar panels (PV) then a good proportion of the heat pumps annual electricity usage can be supplied by a modest PV array.

The solar panels can also be linked to a battery pack that will store the electricity until it is needed, say in the early morning when the sun is still in bed! The battery pack would then charged up during the sun light hours when the heating system is switched off as the occupants would be out of the house at work, in many cases.

I am starting a project at home to link all of these technologies and will be updating my blog with progress. So far I have partly installed the buffer tank.

First progress 6th June 2022

Heat pump progress so far, 25th June 2022
Progress today (27/6), taken the flow and return outside and pressure tested the system so far.

More progress this week. Heat pump out door unit brackets fabricated and the heat pump is due to be delivered and fitted this Tuesday (2nd Aug), very excited

Another little milestone this week (4/8/22) as I manage to get the heat pump lifted and mounted on the wall outside, not easy given it’s 4m up and weighs 200kgs.

Hiab does the trick no problem
Happily mounted on the external wall waiting for the pipework and electrics to be connected up.

Pipe work now installed outside but insulation and trace heating still to do

Pipework in place ready to be tested and insulated

We had electricians install the cables and wire up the charger / inverter and the indoor and out door unit so that it’s now ready to be switched on when the pipework is complete.

With the insulation phase now completed I’m ready to connect the heat pump into my existing central heating system and wire up the electrical controls so that heat is pumped at the right time and to the right place.

Pipes and cylinder now fully insulated to prevent heat loss
External flow
and return pipes insulated

Anti-freeze valves fully insulated to minimised heat loss

Electronic control (Nov 2022).

Unfortunately the Viessmann 150a does not have integrated software to allow inter-heat source control; ie. It won’t talk to my Viessmann 200W system boiler and work out the best way to heat the house and hot water. I’m told by Viessmann that the software is being developed and so waiting to do what they come up with in that area. Meanwhile…..

I’ve had to design and configure my own external hybrid switching circuits to allow one unit to work at certain times when it’s inefficient/ expensive (£) for the other unit to work.

In principle, above 5°c the heat pump should work 100% of the time, save for the hot water demand (come back to that later). When the heat pump is working we don’t want the gas boiler coming on and so I have designed some relay circuitry to switch the boiler off.

Heat Pump Hybrid Wiring Diagram

The above circuit diagram deals with the switching off of the boiler but not switching off of the heat pump, that’s is still under development.

The relays and wiring is now in place and ready to test

One more step close to the battery installation. The batteries are charged up on the Octopus Go night time rate of 12p/KWh and then the Victron Multiplus does the sharing of energy from the batteries to the house loads (heat pump) until the batteries are exhausted or the loads are reduced.

Hybrid Heatpump with battery power supply

With the hardware installed it’s now a question of setting up the systems to optimise the energy available. The 48v battery pack is converted into 230v via a Victron inverter which also charges the batteries with off-peak electricity at 12p/KWh.

Octopus Go low cost electricity deal

The cheap electricity is designed for people to charge their cars with lithium-ion batteries but there is no reason why you can’t charge a battery pack that powers your home heating system.

It’s early days but you can see how my gas usage has reduced from 620kwh in September to only 221kwh in November so far (1 week to go) and a much cooler month. This is massively reducing my CO2 output.

The real time data from the Victron VRM system can be seen here:

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