Slow heat release

New Test Method to Determine Efficiency and Emissions of Slow Heat Release Appliances Burning Solid Fuel... 

Slow heat release appliances are designed to supply heat after the bum cycle by applying mass heat storage. These appliances are characterized by the heat accumulation capacity in the constmction material. The heat is stored during the bum cycle where as heat is released with a time lag to the room over an extended time period. 

Typical examples of slow heat release appliances are soapstone stoves or tiled stoves (Kachelofen), Slow heat release appliances are widely used in the Alp regions, but also in Scandinavian countries and Finland. They are efficient, easy to operate and are appreciated by their users because of the continuous heat release. These appliances are suitable as the sole heating system for houses and also very appropriate for low energy houses. 

The existing type test standards are all adapted for continuous burning appliances. Due to the lack of adequate type tests for slow heat release appliances, they are temporary type tested with test standards for continuous burning appliances such as DIN 18891. Consequently the slow heat release appliances are overheated, due to the consecutive bum cycles as required by these standards. The corresponding test results do not properly represent the performance of the appliances operating under praxis conditions. 

The knowledge and the results of these research activities were an important basis for the evaluation of new type test standards for slow heat release appliances. 

The test procedure shall represent the real world operation of slow heat release appliances burning wood and take into account the transient parameters of a batch wise burning combustion process. It shall fulfill typical test laboratory requirements such as reproducibility of the results, laboratory independence, measuring accuracy and simple test procedures. The test procedure shall support the wood appliance industry for their high quality products and should have a good relation of price and performance. The test procedure shall refer as much as possible to existing international (ISO) and European (CEN, Austrian standards) test standards. 

The new test methods and procedures to determine heat output, efficiency and emissions of slow heat release appliances shall be the basis for new type test standards. The performance data puts manufacturers, architects, users and authorities in the position to compare and evaluate such appliances. 

The slow heat release appliances are operated in an insulated and air cooled calorimeter room (Fig. 3), where the heat output can be measured directly. In parallel, the efficiency is determined indirectly with measurement of flue gas temperature and concentrations of carbon dioxide (CO3) and carbon monoxide (CO) in the flue gas, in accordance with the CEN/prEN flue loss method. 

The test method has been evaluated on 3 typical slow heat release appliances with following objectives ... 

As typical representatives for slow heat release appliances a soapstone stove and a tiled stove were used for the tests. The third type of slow heat release appliance was equipped with an integrated heat exchanger to supply hot water for a central heating system. This appliance was chosen to check the feasibility of the new test method in view of the necessary recording of the heat split between heat released to the room and heat released to the central heating system. 

The measurerneiit of particle emissions was done over a entire bum cycle of the slow heat release appliance. The total suspended particles (TSP) were measured in the flue duct and in the dilution tunnel. The particle size distribution was just measured in the dilution tunnel. 

The efTiciencies of the slow heat release appliances are shown in Table 2. The results of the directly and indirectly determined efficiencies of the soapstone stove and the appliance with the boiler insert are very similar approx. 71 % 1.5% for the soapstone stove and approx. 88 % 0.6 % for the appliance with boiler insert. 

Further criteria for the justification of the TSP-measurement in the dilution tunnel are more of a practical nature. It turned out that some condensation problems arise on the filters at the flue duct because of strong formation of water vapor at the beginning of the bum cycle. These problems arise because of the characteristic bum cycle of a slow heat release appliance In the initial phase after igniting the fuel batch, the wood logs catch fire and release the fuel water over a short time period, thus generating water vapor while the stove is still cold. Even heating of the flat filter holder could not solve this problem entirely. 

The draft test method (see Appendix 1) has been presented to the Comite de Normalisation CEN TC 295/WG5 and is accepted as basis for a continuation of the work in the standard body. On a national level the test method has been proposed as a test standard for slow heat release appliances to be certified with the Swiss Quality Label for wood heat appliances. 

TYPE TEST FOR SLOW HEAT RELEASE APPLIANCES... 

Slow heat release appliances are classified in defined heating-interval-classes in accordance with the duration of heat they released beyond a minimum required heal output. 

To be qualified as a slow heat release appliance, the appliance has to achieve a minimum duration of heat release beyond a minimum required heat output. 

Fig. i Typical test cycle of a slow heat release appliance. The bum cycle is showed by the... 

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