Domestic Heating s with Better Controls Martin O Hara Danfoss Randall Limited, Ampthill Road, Bedford, MK42 9ER, England Abstract: This paper presents results of system testing with 3 different domestic gas boilers (noncondensing, condensing and modulating-condensing) and 3 methods of room thermostatic controller (mechanical bellows, electronic on/off and electronic proportional-integral control). The results of measurements of energy consumption (gas and electricity), carbon emissions and comfort level are presented from tests in a controlled laboratory test house. The results suggest significant efficiency benefits can be obtained with the change of a controller from a simple mechanical type to a more sophisticated electronic product. In addition many of the efficiency benefits available from a high efficiency condensing boiler are lost if the controls are not updated to optimise the boiler performance. The energy consumption of a domestic heating system may be reduced by over 10%, with a similar reduction in carbon dioxide emissions, if the room controls are capable of operating the boiler in its most efficient mode. Introduction The basis of this work began with the switchover in the UK to condensing boilers as Part L of the UK Building Regulations in April 2005 for new and replacement domestic boilers (with a few exemptions). This left many home owners who had changed boilers to high-efficiency (HE) condensing types to complain that their fuel bills appeared to be barely changed, despite claims of up to 14% higher efficiencies available from these newer boilers. As part of a UK controls industry project, under the aegis of the UK trade association TACMA, Danfoss decided to perform tests on the impact that the room controls would have on the heating system efficiency to determine if the change of boiler alone was adequate to realise the efficiency savings that a HE condensing boiler should deliver. Laboratory Controlled Test House Danfoss Randall Ltd has a laboratory controlled test house. This consists of a 20 cubic metre single brick room with plastered internal lining (comfort house) surrounded by 50 cubic metres of controlled temperature air-flow (annular space), which represents the outside air temperature. The test house has a 1.3kW radiator to provide heat from the boiler to the room and a hydronic load dump allowing up to 25kW of additional load to be drawn from the boiler under room control (this load dump represents the rest of the house). The boiler is installed in a separate part of the laboratory hence does not contribute directly to the heating. Consequently, although operating on a single controlled room (as is typical in a UK dwelling), there is a whole house draw of energy from the heating system. The above system allows the effect of room control alone to be accurately monitored as we are able to set-up repeatable external temperature and system load conditions on the same boiler with the same radiator in the same test room (house). The only difference between each system test run is the room controller we use to determine the system activity. UK Winter Weekend Profile Figure 1: Comfort "Test House" at Danfoss Randall Ltd. The tests presented here all had the same profile on the system. The annular space (external temperature) is maintained at 10 C, the test room is allowed to cool to 15 C before commencing the test and the room control set at 20 C. The test is a single 12 hour operation representing a weekend dayprofile, turning the system on once and controlling at a fixed temperature; a single start-up and then constant control. The total system load is 13kW at start-up. afecor Seminar 2007 Page 1 25-26 th October 2007
Boiler Load Dump H H Heat Meters Annular Space (10 C) Test Room (20 C) Control Figure 2: Schematic Representation of Test House Boilers and Controllers Tested Four types of thermostatic room controller were used for these tests on three different boilers. Two of the boilers; a standard non-condensing and a HE condensing type, had the same three room thermostats; a mechanical gas-filled bellows thermostat, an electronic thermostat in on/off mode and an electronic thermostat with proportional-integral (PI) control operating in chrono-proportional mode (set to Chrono-6 for the tests shown here). The third boiler available is a modulating-condensing boiler with an OpenTherm digital interface, this boiler had the same mechanical and electronic on/off controllers used above, plus a modulating OpenTherm controller (Danfoss ORT-10) to determine the best control options. This paper is primarily interested in the impact of the control methods on each boiler type, hence no comparison between boiler types will be provided and due to slightly different boiler parameters (e.g. maximum load capacity and water temperature) it is in fact unfair to make comparisons between the boilers based on the data shown here. Explanation of Chrono-Proportional Control The chrono-proportional control algorithm adapts the on and off burn period to provide 6 cycles of the boiler per hour (chrono-6) for the given comfort demand as time progresses, hence initially during start-up some over and under shoot is to be expected. Other time-periods are possible, typically chrono-3 and chrono-6 are used for natural convection systems and chrono-9 and chrono-12 for forced convection systems. afecor Seminar 2007 Page 2 25-26 th October 2007
Results: Comfort Control All tests were done with the thermostats set to 20 C, however, some variation on this set-point is to be expected and the mean value was allowed to be within ±0.5 C of this target temperature. The set point was determined as the mean value of the room temperature cycle, not the minimum value as is often used. Non-Condensing Boiler The non-condensing boiler is the stock type of boiler fitted in the UK up to 2005. There are probably over 20 million or so installations in domestic dwellings of this boiler type still in regular use and until they fail beyond economic repair, this will remain the case for several years to come. Figure 3: Non-Condensing Boiler Room Comfort Control The room comfort levels achieved in this test show that the electronic on/off and the chrono-proportional (PI) controllers provide significantly better overall comfort levels that the mechanical thermostat which has a peak-to-peak temperature swing of over 1.5 C compared to 0.8 C for electronic on/off and 0.2 C for the chrono-controller. It is observed on both the electronic on/off and chrono-6 comfort controls that the boiler hit maximum water temperature just as the room entered the control band (shown by the slight dip in room temperature as the boiler is switched off internally). In the case of the basic on/off control this quickly is recovered but this caused a slight delay in the PI-controller, as there is no boiler feedback the algorithm drops out of control and takes some time to regain proper room control again. This is an accident of the position of the maximum water set point and the load demand, when run with a room demand of 21 C or a higher maximum temperature this effect was not observed. High Efficiency Condensing Boiler The HE condensing boiler is a SEDBUK Grade-A stock type fitted in the UK since April 2005 when the Part-L building regulations came into force. There have been close to 3 million installations of this type of boiler in the UK and this is growing at the expense of the non-condensing type above. afecor Seminar 2007 Page 3 25-26 th October 2007
Figure 4: HE Condensing Boiler Room Comfort Control The room comfort levels achieved in this test (figure 4) again show that the electronic on/off and the chrono-proportional (PI) controllers provide significantly better overall comfort levels that the mechanical thermostat that in this case has a peak-to-peak temperature swing of 2 C compared to 1.2 C for electronic on/off and 0.3 C for the chrono-controller (once in the control band). With this more modern and faster-reacting boiler the chrono-6 control gives very fast precise control and again significantly improved comfort levels compared to the other two control methods. Modulating-Condensing Boiler The OpenTherm enabled modulating-condensing boiler, is also a SEDBUK Grade-A type. These are not yet a common product on the UK market but have seen success in Holland and the Benelux regions. Many boiler manufacturers do sell these products with proprietary controls throughout the EU, but as they are bundling controls and boiler there is little explanation of the technology. The boiler will work with simple on/off control signals (hence operation with mechanical and electronic on/off room controllers), but is designed to operate with a room controller than can send a digital signal to control the boilers flame modulation level to satisfy the room demand. Hence this control-boiler combination can offer digital modulating of room comfort from a modulating flame boiler, this enables low temperature water heating once the control band is entered and assists with low demand level optimisation. As can be observed from the room comfort plot (figure 5), the modulating control offers exceptionally well controlled room temperature control with less than 0.2 C fluctuation over the majority of the test cycle once in the control band. The mechanical and electronic on/off controllers on the modulating-condensing boiler provide no better levels of control than they do on the HE condensing boiler, suggesting that unless the controls are optimised and a digital modulating control is used with this type of modulating boiler, the householder is not benefiting from the ability of the boiler to modulate the flame. afecor Seminar 2007 Page 4 25-26 th October 2007
Figure 5: Modulating-Condensing Boiler Room Comfort Control Results: Efficiency The tests above were run in the test room and at the end of each test the total energy consumption was recorded, both gas and electricity. This has been used to determine the total cost and carbon footprint of the resulting tests to determine which control schemes offer the best efficiency for the equivalent comfort level. Although flue gasses for the gas boiler are capable of being directly measured, the electricity contribution to carbon emissions is not, hence for the purposes of this report and to maintain consistency, the reported carbon emissions are calculated from the fuel consumption rather for both gas and electricity (see Appendix A for data on emissions calculations). Non-Condensing Boiler Control Cost ( ) Emissions (kg CO 2 ) Mechanical On/Off 4.19-16.20 - Electronic On/Off 3.69 11.94 14.25 12.02 Electronic Chrono-Proportional 3.63 13.39 14.00 13.60 Table 1: Non-Condensing Boiler Cost and Emissions The results for the non-condensing boiler show that the better level of comfort control offers significant savings in energy and hence reduced emissions. Even changing from a mechanical on/off control to a narrower band electronic on/off control can save 12% on the fuel and carbon, using the tighter control of the chrono-proportional control gains a further 2% on the heating cycle used here. afecor Seminar 2007 Page 5 25-26 th October 2007
High Efficiency Condensing Boiler Control Cost ( ) Emissions (kg CO 2 ) Mechanical On/Off 3.04-11.76 - Electronic On/Off 2.98 2.13 11.51 2.18 Electronic Chrono-Proportional 2.74 9.91 10.54 10.37 Table 1: HE Condensing Boiler Cost and Emissions The savings benefit of the better electronic control is only small with the HE condensing boiler over the mechanical thermostatic controller at just over 2%. Since this is a higher efficiency boiler in the first place the benefits of the tighter thermal regulation alone clearly does not have such a large impact as it did with the non-condensing boiler. However, very large savings can be obtained, close to 10%, by using the chrono-proportional control algorithm for control of the HE condensing boiler compared to mechanical on/off control, the reason for this can be easily observed by looking at the return water temperature profiles for the system (figure 6). Figure 6: HE Condensing Boiler Return Temperatures for the Three Control Types During the initial heating phase, all the controllers will operate the HE condensing boiler in its condensing mode since they are all on for the first hour of the test run. However, by zooming in on the boiler return water temperatures during the central period of the test cycle, when all three thermostats are in their control band, significant differences can be observed. The return water temperature using the mechanical or electronic room controller is never in full condensing mode during the on cycle due the relatively slow response of the control, meaning that there is too much heat lost during the off period from the water in the system to maintain the condensing temperature. The chrono-proportional control forces a faster operation and the return temperature is observed to be almost permanently below the condensing temperature and hence the boiler is running at optimum efficiency all the time. afecor Seminar 2007 Page 6 25-26 th October 2007
Modulating-Condensing Boiler Control Cost ( ) Emissions (kg CO 2 ) Mechanical On/Off 3.92-15.18 - Electronic On/Off 3.50 10.4 13.58 10.5 Modulating (OpenTherm) 3.35 14.3 12.91 15.0 Here the electronic on/off controller is giving significant savings of up to 10% over the mechanical thermostat, this is primarily due to the faster reaction speed rather than the tighter control. The modulating-condensing boiler has internal control electronics that is able to provide some downmodulation of the flame with faster on/off signals and this is enabling the more economical running of the boiler using the electronic controller (this can again be observed in the return water temperature, not shown here). When using the optimised digital controller, savings of over 14% are possible over the mechanical room controller and the boiler is capable of being run at very high efficiencies even when operating in the low load of the control band. Summary Modern electronic controls are capable of providing high levels of room comfort in domestic heating systems that utilise gas boiler to wet radiators. The higher comfort levels also come with significant energy savings, even when coupled with older traditional non-condensing boilers. Homeowners can reduce their fuel bills and carbon emissions by 10% in most home systems by replacing any mechanical room controller with an electronic thermostatic controller capable of operating in a chrono-proportional mode. Where a boiler is capable of modulating its flame further savings still are possible by optimising the room control and using a digital modulating controller. Note: The percentage differences observed between energy costs and carbon savings are not identical due to the mix of gas and electricity consumption for the different controllers. Generally the electricity cost is very small in comparison to the consumed gas cost, but the faster controllers do run the pump more frequently, hence consume more electricity but less gas. Conclusion Improved system efficiency can be obtained without compromise on comfort, in fact improved comfort and improved system efficiency can be provided simultaneously if advanced controllers running chronoproportional or digital modulation control are used. The improvements in lower energy cost and lower carbon emissions demonstrated here can be obtained for an existing domestic heating installations with the change of the controller alone, significant energy savings are possible without having to resort to a change in the boiler plant. Danfoss Randall Ltd Ampthill Road, Bedford, MK42 9ER Tel: 01234 364621 Fax: 01234 219705 email: danfossrandall@danfoss.com website: www.danfoss-randall.co.uk afecor Seminar 2007 Page 7 25-26 th October 2007
Appendix A: Calculation of Costs and CO2 Emissions The figures in this paper for energy costs are calculated using the prices from the British Gas website on 1 st March 2006 for both gas; 3.325p/kWh and electricity; 9.444 p/kwh (pence per kilowatt-hour, GBP:Euro conversion rate of 0.68 was used). Although flue gas is monitored in these tests, to include the CO2 factors for the electrical consumption only calculated CO2 data is provided based on production factors of 0.19 kg per kwh for gas and 0.43 kg per kwh for electricity (as per National Foundation; www.nef.org.uk). afecor Seminar 2007 Page 8 25-26 th October 2007