Original Article
February 16th, 2018


A FGHRS is a system for recovering heat from flue gases that would otherwise be wasted. They are only applicable to mains gas, LPG and oil condensing boilers and must be selected from the product database. Currently no oil compatible systems are listed. The schematic in Figure 1 shows an example of a Zenex Gas Saver and how the system works in conjunction with a combi boiler. In this case the energy saving for domestic hot water (DHW) is made when cold mains water is pre-heated by hot flue gases as it passes though the FGHRS, which is the ‘top box’ mounted above the combi boiler.

Figure 1

The system can also include a ‘close-coupled thermal store’ of either 25 or 50 litres. This allows higher instantaneous flow rates in larger multi-bathroom properties. When a tap is turned on pre-heated water from the FGHRS is fed to the combi via a heat exchanger coil in the close-coupled store, where it will be pre-heated. There may also be some heat input from PV or wind turbines.

The Ravenheat Combi Store operates on a slightly different principle as shown in Figure 2 below. When in space heating mode, a 14 litre un-vented DHW store is indirectly heated by flue gases. Thus when a hot tap is opened, this pre-heated water is sent to the combi boiler to be topped up to the final delivery temperature.

Figure 2

Having a small hot water store means the flue gasses during space heating can be recovered and stored until water demand is present. It is also important to note that this store does not re-classify a non storage system into a storage system.


Some more modern boilers have PFGHD (Passive flue gas heat recovery devices) built into them. Systems such as the Ideal Logic Code range. When selecting the boiler from the product database the FGHRS will automatically be applied.