Reducing the Carbon Footprint of Existing Domestic Heating: A Non-Disruptive Approach

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1 EEDAL June 2009 Reducing the Footprint of Existing Domestic Heating: A Non-Disruptive Approach Martin O Hara Danfoss Randall Limited Abstract There is insufficient time between today and 2020 for renewable energy technologies to greatly impact the carbon footprint of the existing domestic housing stocks heating systems, hence the 20% savings the EU has committed to has to come primarily from other approaches. Of the homes we will be living in by 2020, over 90% are those that we live in today. This paper looks at what can be applied to existing domestic boiler space heating installations to produce savings in system energy efficiency, and subsequent carbon reduction, by better use of heating control technologies alone. By improving the space heating controls, while retaining the same user interface as today s existing technologies, it can be shown that up to 10% reduction in energy consumption and a similar carbon emission reduction can be achieved without the need for plant change or changing the user interaction with the heating system. Test results on gas boilers without condensing capability (still a large installed base), as well as more modern high-efficiency condensing boilers and modulating flame boilers, running a standard domestic space heat load of 13kW, have proven that controls alone can greatly impact the efficiency and hence carbon emission of a domestic heating system. Introduction Like most EU governments, the UK is committed to trying to achieve the 20% reduction in carbon emissions by There are many steps to achieve this goal, but in 2005 the UK put in place a buildings regulation change it hoped would improve the efficiency of UK domestic heating by up to 14% when a change of heat plant (primarily domestic gas boilers) is required. The UK building regulations (known as Part L) changed to effectively allow only high efficiency (HE) condensing boilers to be installed in domestic heating systems (with a few exceptions)[1]. This regulation change brought about an almost overnight switch from older non-condensing boilers in new installations, with the above stated expected increase in domestic heating of 14% seasonal efficiency benefit. Unfortunately the resulting changes of plant alone failed to provide the expected improvement and as well as a relatively low carbon saving, home owners complained of a lack of saving in their energy costs when they had replaced what is a relatively high cost part of their heating system[2]. Changing the boiler is a disruptive process to a household, as well as an expensive one, particularly where flue issues and condensate traps are difficult to fit retrospectively to some existing installations. As a consequence many installers report an increase in the requests for repair of existing boiler plant rather than change to a HE boiler, further defeating the carbon saving goals of government. As an industry, the controls association in the UK; TACMA, and Danfoss Randall Ltd in particular, started a project to examine how the change of the system thermostatic control alone (a nondisruptive approach) could impact the efficiency of not only newer HE boilers, but also older noncondensing boilers and more advanced modulating-flame boilers more commonly found in the Benelux regions. The project, known as the Advanced Controls Project, looked to see how a change in the operation of a relatively simple room thermostat, and in particular how advanced control algorithms in electronic controllers could impact existing installed domestic heating system efficiency. Although work had been conducted by the UK government on improving the modeling of UK domestic heating systems by computer based simulation[3], it was decided to use real-system testing that could simulate the impact in a genuine installation, using available domestic boilers in the TACMA Advanced Controls Project. Latterly the possibility of using computer based simulation to extend the test scenarios has been performed [4], but the result of these simulations is outside the scope of this paper. Workshop: Heating 1

2 Laboratory Controlled Test House Danfoss Randall Ltd has a laboratory controlled test house (figure 1). 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 system space 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. Boiler Load Dump H H Heat Meters Annular Space (10 C) Test Room (20 C) Control Figure 1: Schematic Representation of Test House 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. The heat transfer into the room is by natural convection from the radiator, as it would occur in a real home, hence the speed of reaction of the control (placed on the wall opposite the radiator) and the delivery of heated water to the radiator is consistent with how these system components react in a real home. The main artificial concept is the fixed external temperature, in reality this would vary and although the annular space temperature is programmable it was decided to maintain a fixed temperature to enable repeatability tests to be easily conducted and to remove additional variability controlling this parameter may introduce. UK Spring/Autumn Weekend Profile The tests presented here all had the same time-temperature profile on the system. The annular space (external temperature) is maintained at a constant 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 run of June 2009 Berlin 2

3 hour operation representing a weekend profile, 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. Boilers and Controllers Tested Four types of dial-set thermostatic room controller were used for these tests (figure 2) 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 that operated in on/off mode and an electronic thermostat with proportional-integral (PI) control operating in chrono-proportional (time-proportional) mode set to Chrono-6 (6-cycles per hour) for the tests shown here. Physically the electronic on/off and Chrono-proportional thermostat was the same device; the operating mode is an installer selected option. 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 the three specific boilers 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. Figure 2: Simple dial-set thermostats used for tests (from left; mechanical thermostat, electronic on/off and PI Chrono-proportional and OpenTherm modulating control). Results 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. 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 using National Foundation contribution factors[5]. Non-Condensing Boiler The non-condensing boiler is the stock type of boiler fitted in the UK up to There are known to be over 15 million installations in domestic dwellings of this boiler type in the UK alone still in regular use and until they fail beyond economic repair this will remain the case for years to come June 2009 Berlin 3

4 Comfort Control The room comfort levels achieved in this test (figure 3) show that the electronic on/off and the chronoproportional (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-proportional 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 electronic on/off control this quickly is recovered, but this maximum temperature limit 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 water feed temperature this effect was not observed. Figure 3: Non-Condensing Boiler Room Comfort Control Consumption The results for the non-condensing boiler show that the better level of comfort control offers significant savings in energy and reduced emissions (table 1). Even changing from a mechanical 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% over the heating cycle used here. Table 1: Non-Condensing Boiler Cost and Emissions Control Cost ( ) Emissions (kg CO 2 ) Mechanical On/Off Electronic On/Off Electronic Chrono-Proportional June 2009 Berlin 4

5 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 approximately 4.5 million installations of this type of boiler in the UK since 2005 (source: HHIC) and this number is growing at the expense of the non-condensing type above. 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. Figure 4: HE Condensing Boiler Room Comfort Control Consumption The savings benefit of the better electronic on/off control over the mechanical thermostat is only small with the HE condensing boiler 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 (table 2), the reason for this can be easily observed by looking at the return water temperature profiles for the system (figure 5). Table 2: HE Condensing Boiler Cost and Emissions Control Cost ( ) Emissions (kg CO 2 ) Mechanical On/Off Electronic On/Off Electronic Chrono-Proportional June 2009 Berlin 5

6 Figure 5: 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 approximately the first hour of the test run (the start-up phase, see figure 4). 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 (figure 5). The return water temperature using the mechanical or electronic room controller is never in full condensing mode during the on cycle due to 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 at the boiler. 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. 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 controlboiler 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-level demand optimisation. Comfort Control As can be observed from the room comfort plot (figure 6), the modulating control offers exceptionally well controlled room temperature 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 modulatingcondensing 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 fully benefiting from the ability of the boiler to modulate the flame June 2009 Berlin 6

7 Figure 6: Modulating-Condensing Boiler Room Comfort Control Consumption Here the electronic on/off controller is giving significant savings of up to 10% over the mechanical thermostat (table 3), 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. Table 3: Modulating-Condensing Boiler Cost and Emissions Control Cost ( ) Emissions (kg CO 2 ) Mechanical On/Off Electronic On/Off Modulating (OpenTherm) Cost-Benefit and Impact Analysis Clearly a room thermostat is significantly lower cost than a boiler, hence the savings that these results suggest can be achieved simply by replacing mechanical room thermostats with electronic equivalents and can be done at less than 5% of the cost of replacing the boiler. The payback time is equally small, with typical space heat fuel savings representing in the region of 80 per annum suggesting a payback of under 1-year for replacing the control alone, compared with over 10 years payback for a change from a non-condensing to HE-condensing boiler or modulating flame boiler (and assuming the boiler efficiency benefits can be realised). Even with the projected savings for changing the boiler itself, it is the disruptive effect of a boiler change that will discourage many home owners as much as the cost. Unless there is a suitable flue June 2009 Berlin 7

8 for the boiler and appropriate drain-off for the condensate traps the total installed cost may prove prohibitive for some systems. There will also be many home owners that simply will not want the decorative mess that is associated with such a change of heating plant as a boiler replacement. It had been assumed that non-condensing boiler efficiency could not be improved and that only a change of this boiler type to a HE-condensing or modulating flame type could improve the system efficiency. This is shown here not to be the case, even older systems with non-condensing boilers can have their efficiency improved by over 10% simply by the replacement of the room control from an electromechanical type to an electronic on/off or Chrono-proportional type (see table 1). Given that electromechanical room thermostats are the most common type installed and possibly present in over 70% or more of existing domestic installations combined with single home boilers, in the UK alone this could represent over 16 million homes. The energy savings here would suggest every home owner could save over 80 per annum with the change of thermostat alone and the UK as a whole save in the region of 6.4 MT CO 2 [6, 7]. Where boiler plant has already been replaced with HE-condensing type, further savings can be achieved by improvement of the control. It is the lack of replacing the control at the same time as the boiler that is probably responsible for the poor system performance that installed HE-condensing boilers have achieved and lack of owner satisfaction that is reported in the press[1, 2]. Similar energy cost savings can be obtained by the home owner ( 60 per annum), due to the lower installed base of around 2.6 million in the UK annual savings would represent 0.36MT CO 2, but this could be ongoing for all new installations. Combined CO 2 reductions of the control changes given here represent some 4.4% saving on the total domestic carbon emissions for UK homes[7]. It is believed similar results could be obtained for other northern and central EU countries where boiler to radiator central heating is the dominant space heating system. Note: Total consumption figures based on 200 heating days per annum. Conclusion Modern electronic controls are capable of providing high levels of room comfort in domestic heating systems that utilise gas or oil 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 over 10% in a non-disruptive manner in most home systems by simply replacing any mechanical thermostatic 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 are possible by optimising the room control and using a digital modulating controller. Table 3: Optimum Boiler-Control combinations and predicted savings over base-line electromechanical controlled system. Boiler Control Non-Condensing Chrono-proportional HE-Condensing Chrono-proportional Modulating (OpenTherm) Modulating A low cost, non-disruptive, change of thermostatic control can provide improved system efficiency without compromise on comfort. In fact improved comfort and improved system efficiency can be provided simultaneously without having to change any of the heating system plant or installation June 2009 Berlin 8

9 References [1] The new boiler that's causing a heated row, The Guardian, 2 April 2005, [2] Are Condensing Boilers more efficient in real life? [3] Cockroft, J., Samuel, A., Tuohy, P Development of a Methodology for the Evaluation of Domestic Heating Controls, Market Transformation Programme Report RPDH19-2, Department for Environment, Food and Rural Affairs, UK. [4] Cockroft, J., Kennedy, D., O'Hara, M., Samuel, A., Strachan, P., Tuohy, P Development and validation of detailed building, plant and controller modeling to demonstrate interactive behaviour of system components, Building Simulation Conference 2009, July 2009, Glasgow, UK. [5] [6] Consumption in the United Kingdom, Department of Trade and Industry, July 2008, URN 02/1049, [7] Our Challenge, securing clean, long term affordable energy for the long term, Department of Trade and Industry, January 2006, URN 06/ June 2009 Berlin 9