Flue Gas Recovery Systems FGRS

Flue gas recovery systems (FGRS), or more accurately also known as flue gas heat recovery systems (FGHRS) capture and reuse waste heat from exhaust flue gases, these are used to preheat water or other fluids, improving energy efficiency by reducing demand and thus reducing emissions.

The UK government document "Review of the methodology for FGHRS in SAP. Final report" describes the systems thus:

"Flue gas heat recovery units are devices that can be used with combi boilers to improve the efficiency of domestic hot water (DHW) production. They are designed such that incoming mains water is preheated before entering the boiler by exposing it (via a heat exchanger) to the exiting flue gases and extracting the otherwise wasted heat. The amount of available energy for recovery is limited by the flue gas temperature (FGT), the water vapour content and state, and this in turn is determined by the type of boiler and central heating temperature.

"The effect of pre-heating the incoming mains water means that less heat is required to be input from the boiler to raise the temperature of the DHW to that required for delivery. This can lead to savings in terms of gas used by the boiler and, as water at the right temperature can be delivered more quickly, there can be additional savings from reducing the amount of lukewarm water that is rejected by the user before it is hot enough."

FGRSs can be classified by the type of heat exchanger being used (e.g., air-to-air, air-to-water), the method of heat recovery (e.g., regenerative, recuperative), or the specific application and equipment, such as economisers, air pre-heaters, waste heat boilers, or heat recovery steam generators (HRSGs). Other systems also include heat pumps and dedicated flue gas heat recovery systems (FGHRS) for boilers and Storage Flue Gas Heat Recovery Systems (SFGHRS)

SFGHRS are described in the same document thus "in such devices the store of water is heated by the flue gases via the heat exchanger. This volume of water may be contained in the same unit around the flue, or in a cylinder next to the primary FGHRS unit. The store may be insulated, depending on the store location and the manufacturer’s design."

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