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Are passive flue gas heat recovery devices savers?

Passive Flue Gas Heat Recovery Devices enable you to reuse heat from your boiler that would otherwise be wasted. They are retrofit devices for older boilers that capture waste heat from flue gases and reuse it, to improve the efficiency of hot water production and save fuel and money.

Passive Flue Gas Heat Recovery Devices

Inside a passive flue gas heat recovery device

Inside a passive flue gas heat recovery device

Normally, flue gases escaping to the atmosphere can exceed 60°C. Passive Flue Gas Heat Recovery Devices reclaim much of this wasted heat, letting the flue gases then vent to the atmosphere much cooler.

A heat exchanger is fitted around the flue coming from the boiler to heat up mains cold water that is fed into it.

The heated water is then either stored for use later in a tank, or used as pre-heated incoming water for the boiler so it has less work to do.

How much can they save?

Combiplus passive flue gas heat recovery device on top of a combi boiler

Combiplus heat recovery device on top of a combi boiler

Passive Flue Gas Heat Recovery Devices can work on boilers whatever their fuel (depending on the model and flue size). But not all boiler types are suitable. Exact savings depend on the type of boiler to which they are fitted:

  • If attached to the flue outlet of the best band ‘A’ condensing boiler, it reduces the amount of gas used for hot water by an extra 7%
  • If attached to a non-condensing boiler, up to 52% can be saved
  • On an older combi-boiler, up to 37% savings on water heating expenses are possible.

Chris Farrell, MD of Zenex Technologies, and inventor of GasSaver, the model used by Baxi and others, believes these devices can provide help reduce fuel poverty by substantially reducing fuel bills.

Benefits of Passive Flue Gas Heat Recovery Devices

  • Easily installed by a gas safe registered plumber
  • Cuts fuel bills
  • They are an eligible measure under the Green Deal Initiative with a cashback amount of £90
  • They are particularly effective during the winter when energy demands are highest
  • Can cut water consumption by 6%
  • Reduce carbon dioxide emissions by around half a tonne a year.
  • Can be linked to weather compensation control technology to further improve efficiency
  • Zero maintenance
  • Require no auxiliary energy (e.g. mains electricity) for operation.

Passive Flue Gas Heat Recovery Devices are listed under SAP appendix Q with an independently assessed, additional energy saving rating.

Finding a suitable device

Multifit GasSaver flue gas heat recovery device

Multifit GasSaver flue gas heat recovery device

Besides GasSaver, other makes are: Vaillant’s recoFLUE and Energycatcher Combiplus.

The difference between the GasSaver and the Energycatcher is that with the latter, the reclaimed heat can be stored in a tank and used when required, in much the same way as a solar thermal system. With the former, it pre-heats the water used by the boiler.

Not all devices fit all boilers. To find out which is the one for yours, or if yours is able to have a retrofit, first make a note of the type and model number of your boiler, then visit the suppliers’ or manufacturers’ websites where there is a list of which models of boiler their devices are suitable for.

Fitting instructions are different for each device. Follow the instructions in the technical manual.

How much do they cost?

The Vailliant recoFLUE - one of the passive flue gas heat recovery devices suitable for older non-condensing boilers

The Vailliant recoFLUE is suitable for older non-condensing boilers

Costs start at around £1,200. Payback time depends on the type of boiler and usage level, and whether you fit it yourself or get in a plumber.

You’ll save more money if you have a water meter, since less water is used.

Check your boiler

Before buying one, check what type of boiler you have, to see what savings are possible. Since 2005 virtually all gas boilers that have been fitted in the UK have been more efficient, condensing boilers.

Seasonal demand for hot water graph

There is significant seasonal variation in demand for hot water

Condensing boilers have bigger heat exchangers that already recover much heat from the burning gas, making them more efficient than non-condensing ones.

Combi boilers heat water on demand and don’t need a tank to store hot water.

How to tell if your boiler is a condensing boiler:

  • If the flue is made of plastic, it’s a condensing boiler.
  • If it is made of metal it probably isn’t.
  • If it has a plastic pipe coming out of the bottom, through the wall and into a drain, then it’s a condensing boiler.
  • If it runs on gas and was installed after 2005, or runs on oil and was installed after 2007, then it’s very likely a condensing boiler.

© David Thorpe Jan 2013. David is author of Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency

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Also See:
Biomass boilers, combi boilers, condensing boilers at free eco open house events in September
Green boilers