AN INVESTIGATION OF THE PERFORMANCE OF VARIOUS HEATING SYSTEMS IN A MATCHED PAIR OF HOUSES
The objective of this project was to reduce energy consumption in the domestic sector while maintaining or improving comfort conditions. The approach used was to evaluate a group of conventional heating and control systems under practical usage conditions in terms of their efficiency and overall performances. Having thus identified the deficiencies for each system, a number of innovations were introduced in an attempt to improve the performance of the systems and they were then re-tested. Four basic systems were evaluated in the initial test. These were oil, gas and solid fuel independent boilers and a solid fuel back boiler on an open fire. A common low pressure hot water distribution and radiator system was used with all boilers. The initial, basic control scheme used was on/off operation of the pump under the control of a room thermostat monitoring air temperature. Detailed measurement of all temperature and energy flow parameters was carried out using a computer based monitoring system which also controlled the real time occupancy simulation. The measurement technique used was to compare the energy consumption of the test house with that of a control house, operating under similar conditions but using direct electric heating. This was termed the "twin houses technique". The tests showed that the oil boiler was capable of meeting the heating demand but that the system efficiency varied widely, ranging from 41% to 70% over the 28 test days. It was clear that the boiler was unable to operate satisfactorily at the low loads imposed on it. The load ranged from 8.4% to 23.6% of full rated load. A similar problem was encountered with the independent solid fuel boiler. In this case, the efficiencies obtained ranged from 38% to 53% but the heat input to the house was significantly above that necessary to meet the specified temperature regime and resulted in overheating. The boiler was unable to operate at the low level of output required by the imposed heating load and the output was determined more by the external wind conditions than by the load on the boiler. The solid fuel back boiler system achieved efficiencies of between 30% and 40% in the initial tests and proved capable of meeting the load imposed. However, the start-up period, between the initial lighting-up and the beginning of pumped output was typically two hours long, and the system was slow to respond to changes in demand. The system also suffered from the inability to control the flow of combustion air which limited the efficiency obtainable. The tests on the gas boiler showed that the unit, a wall mounted, low capacity, balanced flue boiler, was well matched to the imposed load operating at an overall efficiency of between 70% and 82%. However, due to the nature of the control system, the specified temperatures were achieved only in the hall, where the thermostat was located. The temperatures in the other heated areas varied with external conditions and also with time. The lower efficiencies occurred at low load levels and were due to rapid cycling of the boiler. The permanent pilot light was another source of loss of efficiency. From the results of the initial tests it was decided that the gas boiler and back boiler presented the best opportunities for improvement as both the oil boiler and the independent solid fuel boiler would require design changes outside the scope of the project. In the case of the gas boiler, the change related not to the boiler itself but to the control system associated with it. The back boiler was improved by fitting a firefront to control the combustion air flow and by adding a water store to act as a buffer between the boiler and the distribution system. An improved temperature control system, involving the use of thermostatic radiator valves (TRVs) and a water flow sensor, was used in conjunction with the store. This control system was similar to that used to improve the gas boiler system. The addition of the firefront to the back boiler resulted in an increase in the average overall efficiency from 34% to almost 47% and also improved the response of the system to changes in demand. The addition of the store to the system did not result in any efficiency improvement but the overall performance, measured in terms of availability and controllability, was significantly improved. The modified control system used with the gas boiler resulted in a marked improvement in performance, with the actual achieved temperatures closely matching the desired temperatures in heated areas. The conclusion of the report is that the gas fired system is, in general, the most cost effective choice of heating system. However, where a chimney, flue and fireplace are being provided as standard anyway, then a coal fired back boiler is competitive. In such a case, the addition of a firefront is also cost effective. The water store buffer is not, however, economic in spite of the improved performance of the system. The provision of TRVs appears to be cost effective but further data is required to confirm this. Where TRVs are in use the addition of a flow sensor is economic. From the results obtained in this project, it would appear that further work in a number of areas could yield worthwhile results. Given the current trend towards low energy houses, the development of an oil boiler capable of operating efficiently at outputs in the range 4 KW to 8 KW seems necessary. The use of a redesigned control system, possibly associated with a small thermal store, to reducing cycling, and smaller nozzles with oil pre- heating, should improve the performance of oil boilers in modern houses. The firefront could be redesigned to improve its performance by reducing the direct radiant output as the tests showed a tendency to overheat the room in which the fire was located. The development of an effective and smaller store, operating at a higher temperature differential, would be a significant advance. The use of phase change materials may be the direction in which work in this area should move. Work on control systems which combine the use of TRVs on some radiators with a room thermostat for other areas could produce an optimum design. Designs which include a flow sensing system would require an improved adjustment method and the interaction between such systems and the boiler must be studied, particularly where, as was the case during tests on the back boiler, there is not direct, positive control link between the system and the boiler.
Bibliographic Reference: EUR 10450 EN (1986) MF, 136 P., BFR 300, BLOW-UP COPY BFR 700, EUROFFICE, LUXEMBOURG, POB 1003
Availability: Can be ordered online
Record Number: 1989124106100 / Last updated on: 1987-01-01
Available languages: en