The majority of schools use gas boilers for their space heating with radiators, and also for the heating needs for hot water, usually for showers and washbasins, the latter often referred to as ‘domestic hot water’ (DHW). One area of energy efficiency receiving increasing attention is the economical recovery of heat from boiler flue gases. It has recently been shown that these flue gases contain much more recoverable heat energy than had been previously recognised. In actual boiler operation over the year there is significant heat loss of some 20% to 25% in the flue gases of the most efficient boiler and most of this can be recovered.
It is perhaps not generally known that “condensing” boilers, now mandatory in domestic use, are not required legally to condense any minimum percentage of the water produced as steam in the combustion process. In fact around 2% is condensed and thus 98% of the latent heat of condensation is wasted to atmosphere via the flue. Surely this is an Alice in Wonderland situation.
A recently patented device to recover this heat energy has been developed by Zenex Technologies of Plymouth as the Zenex GasSaver. It has been tested by the Building Research Establishment (BRE) at Watford, defined as a Passive Flue Gas Heat Recovery Device (PFGHRD) and marks a step forward in energy saving and efficiency. The BRE has added the device to their list of energy saving devices in SAP (Standard Assessment Procedure). This device offers substantial energy saving for schools and can be used with other features of Zenex heating system design technology.
The GasSaver consists of a water store in a six litre container placed in the flue above the boiler together with a heat exchanger coil through which the incoming cold main for the domestic hot water supply, (DHW), passes. . The principle is perhaps best illustrated in the GasSaver cross-section shown in Fig 1 and the schematic of Fig 2. The flue gases are directed under the container and then pass through a central cylinder in the container and out into the flue. The water is heated and some condensate falls into the water as the gases pass out from the unit through the perforated lid of the water store. Much of the latent heat of condensation is captured in the stored water and, together with the heat from the condensate, is available for transfer to the incoming cold main. The cold main thus effectively passes through three heat exchangers namely, the tube above the water level in the store around which the cooled flue gas is passing to the final part of the flue and atmosphere, the coil of corrugated tube in the water and finally through the tube with a conical base in the hottest part of the flue gas. This arrangement reduces the exit temperature of the flue gas to around 35C, compared with the usual 80-85C of a SEDBUK A rated boiler. The DHW leaving the store reaches a temperature of around 80C after which it is mixed with cold water from the main, in a temperature controlled mixing valve, to reduce the temperature to 30C before it is fed into the boiler.
Tests by certified independent bodies, including Gastec and TNO in The Netherlands showed significant energy savings. Annual saving of the gas required to produce DHW is some 40%, resulting in overall savings of 7% to 14% of the annual gas heating bill and, of course, similar reductions in the emissions of carbon dioxide. An additional advantage is the fact that the device produces hotter water faster and, as a result, less lukewarm water is drawn from the hot tap and wasted before the water is hot enough for the user’s needs. Other independent tests have verified a saving of some 5% in the annual use of DHW.
Chris Farrell and his team at Zenex have developed the Zenex Blade, an economic and attractive design for the use of the GasSaver in larger domestic, institutional and commercial applications. This uses a modular approach to the supply of heating and hot water systems. The boiler and GasSaver are fitted into a cabinet together with 50 or 100 litre storage cylinders and fittings so as to give a complete heating system ready for installation. Several of these modular units can be connected in parallel to give a flexible easily, maintained, highly efficient configuration. The standard construction and sizing assists with the design of plant rooms and reduces on site heating system assembly costs. A typical installation is shown in Fig 3
An interesting practical application of the concept is in the Plymouth leisure centre, the Plymouth Pavilions. Here three 30kW Blades replaced 600kW of inefficient boilers and the hot water storage volume was reduced from 10,000 litres to 5,000 litres. As a result the problems in the former hot water shortfall were solved, the overall gas consumption was reduced by over 30% and the cost of the installation was recovered in less than 18 months.
Many schools are now specifying the Zenex Blade concept for their boiler replacement. The first system in England was installed at a primary school near Slough and came into use at the start of term in September 2007.
