What is a hybrid system?
A hybrid heating and hot water system is a combination of two or more technologies generating heat to provide heating and hot water to the home or building.
These are typically a gas or oil-fired boiler in combination with a heat pump, or a set of solar panels. For the purpose of this document, we are concentrating on a hybrid system comprising of a gas or oil-fi red boiler and an air source heat pump.
For a heat pump to work most efficiently the temperature of water being generated should be kept as low as possible, ideally around 40-45ºC maximum. Over 80% of UK homes were built before 1960 and are generally not that efficient with relatively poor fabric insulation and glazing. There are obvious exceptions to this with most homes now with double glazed windows however, not so many homes without cavity wall insulation have had it retrofitted and even fewer have had ﬂoor insulation improvements made.
A major advantage of a hybrid system is the reduced alterations needed to an existing heating and hot water system and the property itself.
How does the control of a hybrid system work?
There are quite a few different control methodologies that are available for a hybrid system, from a quite basic control that simply allows the heat pump to heat the system whilst the external air temperature is higher than 7ºC, to a control system that takes into consideration the cost of the fuels, the amount of over or under generation of grid electricity and numerous other factors. The major consideration of the designers and manufacturers of a hybrid system is to reduce the amount of complexity for a householder as this is when heat pumps have experienced issues with inefficiency and comfort issues. Ideally, the householder should be able to programme the heating system times and temperature requirements and the control systems will orchestrate what technology actually generates the heat required, not dissimilar to a hybrid car where the driver simply drives as normal without any knowledge as to whether it’s the combustion engine or the battery that is propelling the vehicle.
A modern hybrid control system ensures the most appropriate heat source is being used, whilst also allowing the heat pump and boiler to operate in parallel where the heat pump takes the lead while the boiler tops up the difference between what the heat pump is able to deliver and what the property demands. It is, therefore, cost effective and efficient to choose a heat pump with an output below the maximum heat demand of the building – this ensures that the heat pump is delivering its maximum potential.
A typical system will have an internal room temperature controller and an external weather sensor, or an internal room temperature controller that accesses the external temperature via the internet. This arrangement delivers the most efficient and carbon-reducing way of heating the system. The more complex systems also provide other inputs into the technology being used, by taking account of the fuel tariffs on offer, the amount of electricity available in the grid, the amount of solar PV being generated on the house, and whether an electric vehicle is charging or fully charged etc.
Most existing heating systems which are heated by a gas or oil-fired boiler, (circa 24m), have radiators installed that were sized on the basis that the water temperature within was around 75ºC and an external air temperature of -3ºC. They were also sized on the basis that the boiler would be operated intermittently, perhaps for one or two hours in the morning and 4 or 5 hours at night. Outside of these times, the heating system would be switched off with a small number of systems having controls that allowed the air temperature in the house to drop to a set-back temperature 4 or 5ºC lower than the comfort temperature (which is typically 21-22ºC). If the setback temperature is reached then the boiler will fire and keep the house at this set-back temperature until the next demand for the higher comfort temperature.
Consequently, there are many occasions where only putting air-source heat pump water temperatures, (40-45ºC) into radiators sized for temperatures more like 75ºC will result in lower room air temperatures being experienced and discomfort and perhaps secondary heating devices needing to be used with more carbon being emitted.
Conversely, there will be moments in the year when the outside air temperature is higher than the design temperature basis and the required room temperatures will be achieved by the lower ﬂ ow temperatures of a heat pump, with no noticeable drop in heat pump efficiency. Typically with anything over around 7ºC external air temperatures, it is likely that the heat pump will be suitable for the existing radiators to achieve the desired room temperatures and therefore with the lowering carbon intensity of electricity generation, a worthy consideration for carbon reduction measures.
A major advantage of a hybrid system is the reduced alterations needed to an existing heating and hot water system and the property itself. To substitute a gas or oil-fi red boiler totally with a heat pump is likely to require improvements to the fabric of the house to reduce the heat loss and also make the existing radiators and pipework suitably sized for the lower water temperatures. Also and perhaps more significantly there are around 17 million homes with combi or combination boilers installed and therefore no hot water storage cylinder. To remove a combi and replace it with a heat pump would require a hot water storage cylinder to be installed and a location for it found, and in most cases this will mean taking a portion of the landing or one of the bedrooms and constructional work to facilitate this. As you can imagine there will be very few people happy to accept this requirement.
Using a hybrid heat pump system would allow the heat pump to be sited externally and no or very few remedial changes are required to the heating and hot water system or the property. The boiler would heat the house during the colder periods and, when the weather isn’t quite so cold, yet heating is still required, a combi can provide hot water whilst the heat pump warms the property.
The cost of a hybrid heat pump installation will vary significantly on a number of factors;
- Is the installation both a new boiler and a new heat pump?
- If the existing boiler is retained and it’s just a heat pump that is the new addition
- The proximity between the heat pump and the boiler
- Is the hybrid system going to be with a Regular boiler system or a combi boiler system
- Is the heat pump a split or monoblock
Generally, a heat pump of around 6kW output is the size of the majority of hybrid installations and can therefore provide the heating requirements for around 70% of the time. The heat pump, excluding installation cost, would be around £4k and a boiler, such as a 30kW gas-fi red combi, would be around £1k. The controls and installation will again vary from site to site but the installation of the component parts onto an existing system, with labour and other materials, could be anything up to and around £8k-£10k.