What is a hybrid heat pump?
The general term hybrid means to have a combination of two things/systems. In terms of a heat hybrid heat pump or hybrid heating system, it is where there are two heat sources present ie a gas boiler and a heat pump. At it’s most basic level it is a duel fuel system. However as a boiler can do various combinations of heating and hot water, a hyrbid heat pump system allows various configurations in which the boiler or the heatpump can be doing either hot water, heating or both.
Following is an image of a hybrid heat pump system in which the combi boiler does both the hot water and heating, whereas the heat pump does just the heating.
How a hybrid heat pump system works?
As mentioned there are several configurations. Aside from being able to combinations of hot water and heating, when a boiler and heatpump are present, there are different modes of operation. Bivalent, Bivalent Parallel and Bivalent Series.
In a Bivalent Standard operation when the ambient air temperature gets too cold, it switches over to a backup heat source such as boiler, immersion etc.
Following is an example of this schematic. Please note, certain conditions are required to be met, ie bypass valve must only open if there is a restriction and also generally in this schematic there is no volumiser as your using an open heating circuit for defrost volume.
In a Bivalent Parallel operation, depending on the system schematic, the secondary heat sources ie boiler or immersion heater is able to switch on at the same time as the heat pump, and as it gets much colder it may stay on to assist or in the case of a boiler it will take over and the heat pump will switch off.
Lastly in a Bivalent Series configuration, the heat pump and the back up heat source are in series. In the case of a boiler being in series, this is usually a specialised packaged set which is designed to work with each other, the selected heat source is decided by which one is more cost effective to run ie using different outside temperatures or different rates of energy costs ie off peak electricity / gas etc.
Why go with a hybrid heat pump?
Economics : The HP has too low of a COP to run more economically than a gas boiler. This can happen when you have a high flow temp system, and the COP drops because the delta between outdoor temp, and flow temp is too high. Or if you have changes in gas or electricity prices. As an example current electric/gas ratio is 3.75 (30p/8p). Assuming a boiler is 90% efficient at best, you only need to achieve a COP above 3.38 in order to become more economical to run than a gas boiler.
Power output : The heat loss of the home is too high for the heat pump to provide it standalone, and therefore needs a top up. As an example, lets say your heat loss is 10kW, and you cannot install a larger heat pump due to planning permission, then you can install a smaller single fan heat pump such as the Vaillant Arotherm Plus 7KW in a hybrid setup. When the heat loss exceeds the capability of the heat pump, the gas boiler can do a top up for the heat pump, or replace it’s use during high heat loss.
Staged Journey : It may be that you are not familiar with heat pumps and there capabilities and so don’t want to ditch your boiler just yet. The hybrid way then allows you to have a greener home, by being able to determine what proportion of your energy needs will be served by the boiler and heat pump. (important to note, optimisation by your installer will determine the exact proportion mix)
Newly replaced / relatively new boiler: Similar to the above might be that you have recently replaced your boiler or is relatively new and does not make sense to ditch it prematurely, as that could go against being environmentally friendly. As a heat pump can last Upto 25 years, your boiler can be removed when it fails in the future and the heat pump can takeover dependent on upgrades you may have carried out over that time ie insulation.
Trivalent Mode: This is an extension of the bivalent mode which instead of simply deciding at a certain temperature for example 3 degrees celcius, below which the boiler / additional heat source kicks in, the trivalent allows you to enter gas price and electric rates, including off peak / peak rates in order to determine which appliance will be more efficient to run. This mode is found in a few heat pump systems and on standalone hybrid management systems.
Dependance on Foreign gas: If the last few years in the energy markets give any indication of the volatility of energy prices and dependance on foreign gas, a hybrid heat pump could allow you change the proportion served by your gas boiler and electric heat pump based on changes in prices in addition to variable rate electricity costs ie time of use tariffs including peak/off peak electricity rates.
Other reasons for a hybrid heat pump:
- redundancy / reliability
- standalone heat pump and associated upgrade costs are too high or not economically feasible
- for staged upgrades ie selecting a heat pump based on best case heat loss once all insulation and emitter improvements are made in the future.
Clarifications: Please note there are various terms such as hybrid, bivalent etc. Some manufacturers or sets of people may have come to associate the work hybrid with a packaged solution in which the heat pump is in series with the boiler. However according to the definition of hybrid, it simply means a thing made by combining two elements or systems.
|Payback and CO2 Savings||Work out the business case|
|Hybrid Calculator||Work out the share and bivalent point|
|Boiler COP vs HP COP||Work out the share and bivalent point|
Would you like to know more?
If you think a hybrid heat pump system might be the solution for your home instead of a stand alone heat pumps system with extensive upgrades, then visit our partner company Econic Energy, they specialise in hybrid heat pump systems using expertise and insights gained by working in a few european countries including Netherlands of which like the UK has many gas boilers. See their banner below and click to find out more.
Save up to £400 a year on your heating bill with a hybrid heat pump from Econic Energy