Trials of Vexo inhibitor in domestic central heating systems by Rayner Mayer d25.03.2013
Executive summary A new type of inhibitor has been formulated by VEXO International to improve the efficiency of domestic central heating systems. The claims for X-PO10 inhibitor include prevents scale build-up throughout the system dissolves existing scale back into solution inhibits corrosion and formulation of hydrogen gas To examine these claims metering has been installed in two existing residential dwellings and measurements have been made of the heating system by running a back to back trial before and after the inhibitor was added to these systems. In this report, the various measurement methods are described and the results analysed over a 10 week measurement period. The following conclusions were reached about the Vexo inhibitor the thermal conductance of the radiators has increased as the whole radiator area is now more effective in transmitting heat the thermal efficiency of the boiler has increased the life of the heat exchanger will be extended if the acidic level of the central heating water can be maintained close to neutral level, due to the prevention of corrosion of the heat exchanger plates The advantages to the consumer will be lower heating bills and reduced maintenance costs, whilst the advantage to society will be reduced greenhouse gas emissions and more efficient use of resources. It is recommended that a larger trial with more dwellings should be initiated to quantify the benefits in more depth; also that the central heating system should receive an annual dose (250 ml) of inhibitor to maintain the corrosion protection with the annual maintenance of the boiler. Signed Rayner Mayer Director Sciotech Projects Sciotech Office Engineering Building University of Reading Reading RG6 6AY 1
Contents Executive summary... 1 1. Background... 3 2. Trial dwellings... 4 3. Metering... 4 4. Methodology... 5 5. Adding inhibitor... 5 6. Results... 7 7 Discussions and conclusions... 11 8 Recommendations... 12 9. References... 12 2
1. Background To limit the production of greenhouse gases (GHG), resulting from the combustion of fossil fuels and the impact of global warming, it is essential to use energy as efficiently as possible. Some 40% of domestic energy is used to heat homes in the UK so even a small increase in efficiency of the production and distribution of heat will significantly reduce GHG emissions. Carbon Trust has identified one limiting factor namely that of scale build up on the inside of central heating radiators and heat exchangers [1]. Vexo International has developed an inhibitor which independent laboratory tests have shown will take this scale back into solution [2]. Current installation practice for new boilers When fitting a new boiler, boiler manufacturers recommend that an inhibitor be added to the water when refilling the radiators to limit corrosion [3]. The function of this inhibitor is primarily to maintain the water at an acidity level (ph) of 7.0 that is neither acidic nor alkaline to protect the heat exchanger against corrosion. Manufacturers also recommend that the radiators be power flushed to get rid of any solid material and scale by pumping water around the radiator system which contains a flushing agent [3]. This adds typically 300 to 400 to the cost of replacing the boiler and is only effective for two pipe radiator systems. However for one pipe systems, it is not possible to power flush because the pipe work layout is such that the flushing water only washes out the pipe work and bypasses the radiator. Manufacturers recommend servicing the boiler annually which includes cleaning the heat exchanger, clearing the condensate trap and checking the carbon monoxide levels in the exhaust gas stream. As these boilers are micro-processor controlled, burner adjustments are neither required nor possible. During this service operation, it is not common practice to check the inhibitor level or to add more inhibitor unless specifically requested by the home owner to do so. As the vast majority of home owner s do not know of inhibitors, inhibitors are very seldom added to the heating system. Boiler efficiency Tests in the laboratory indicate that the efficiency of modern central heating boilers of the condensing gas type can achieve an efficiency in excess of 90% (SEDBUK A rating) [4]. However when these boilers are installed in new and older domestic central heating systems, the overall efficiency is decreased and some measurements are described in section 6. This reduction in efficiency is partly due to the boiler being operated much of the time on part rather than full load and partly because the thermal conductance of the radiator and heat exchanger inside the boiler can be reduced by the build-up of mineral materials (scale) coming out of solution in the water and settling on the walls of the heat exchanger and radiator [1]. There is little that can be done about improving part load efficiency other than to size the boiler to the heat loss of the dwelling. However the build-up of scale should be a reversible process if an inhibitor solution can be devised which can take the scale back into solution and keep it in solution. This is what VEXO claim that their inhibitor is designed to do [2] and this set of trials has been designed to check the ability of their inhibitor to undertake this task. 3
2. Trial dwellings Two typical dwellings were selected, a two pipe and a one pipe radiator system. a terraced house of about 90 m2 at 70 Tudor Drive, Yateley a detached house of about 100 m2 at 167 Branksome Hill Road, Owlsmoor These are identified in the report as 70TD and 167 BHR respectively. Details of the dwellings and their central heating systems are given in Table 1. 70 TD 167 BHR Size (m2) 90 100 Age 1969 1963 Boiler type Condensing gas, Condensing gas combination Year of boiler installation 2010 2013 Radiator pipe system 2 pipe 1 pipe Power flushed Prior to installation Not possible Number of radiators 13 14 Age of radiators 3 10 years Up to 50 years Pressurised system yes yes Inhibitor Added in 2010 when new boiler installed None prior to adding VEXO Table 1: Descriptions of the trial central heating systems 3. Metering As the existing gas meter measured the supply of gas to the boiler as well as the cooker, a second gas meter was installed on the feed supply to each boiler (Figure 1). A heat meter of the ultrasonic type was installed adjacent to the boiler flow and return pipes (Figure 2). One thermocouple was installed by fitting a T joint in the flow (supply) pipe and the second thermocouple in a T joint of the return pipe together with an ultrasonic velocity sensor. The sensors were then coupled to the heat meter which calculated the instantaneous heat output by multiplying the difference in the water temperature between the flow and return pipes and the volume of water in a specific time period. The technical details of these meters and sensors are given in Table 2. Meter Manufacturer Model number Gas BES secondary meter G4 FXN GKT Heat Sharky hydrometer S/N 44668901 Table 2: Gas and heat meters Figure 1: Meter for monitoring gas into boiler Figure 2: Heat meter 4
4. Methodology For 70 TD, the central heating system was fully drained to get rid of any residual inhibitor before refilling with clean water from the supply pipe. For 167 BHR, the central heating system and all the radiators were fully drained prior to installing the new boiler but the radiators could not be power flushed as it is a one pipe system as previously described. Because heat loss and comfort levels differ from house to house, the same houses and occupants were used as the controls. This is the same strategy that was used recently for trials of the hot fill washing appliances for OFGEM [5]. After fitting the metering, each central heating system was run without any inhibitor for a period of time of 3 to 4 weeks before any inhibitor was added. In order to measure the efficiency of the boiler, the gas entering the boiler and the heat output of the boiler as measured by the heat meter were recorded at regular time periods. These measurements together with other monitoring methods were used to characterise the benefits (Table 3). Sensor Location Units Gas meter Input to boiler kwh (gas) Heat meter Output of boiler kwh (heat) Infra-red thermometer Record temperature of Degree 0 C radiator at specific points Infra-red camera Record heat distribution Degree 0 C Acidity level of water in CH ph Degree of acidity or alkalinity system Presence of Vexo inhibitor Radiators Concentration of molybdate Table 3: sensors and location The presence of inhibitor was detected by measuring the concentration of molybdate in the central heating (CH) water, which is present in the inhibitor supplied by Vexo. 5. Adding inhibitor Vexo inhibitor X-PO10 is an inhibitor [2] designed to inhibit scale build up on the inside of radiators and the heat exchanger limit corrosion and build-up of hydrogen gassing reduce boiler noise A single dose of 250 ml of inhibitor is recommended for hot water volumes up to 70 litres and an additional dose if the volume is greater or there are 12 or more radiators. The global aim of these trials was to add Vexo - PO10 to two existing domestic central heating systems and quantify the benefits to the occupants. 5
5.1 Vexo valve and inhibitor The VEXO system comprises a 250ml pouch, a valve and a valve sealing cap as is illustrated in Figure 3. The valve is designed to replace the blank plug in a radiator and is of the nonreturn type so the inhibitor can be added, but liquid cannot flow outward. In these trials the valve was located in the pipework adjacent to the boilers, which were located in the garage (Figure 4).. Figure 3: Vexo system 5.2 Adding the inhibitor Figure 4: Inhibitor pouch attached to Vexo non-return valve situated adjacent to boiler After a measurement period of three to four weeks without inhibitor, the boiler was switched off. The procedure for adding the inhibitor as recommended by VEXO was adopted that is the pressure in the system was dropped by bleeding some water from the system the sealing cap was then taken off the valve the pouch was then secured into the valve which procedure also ruptured the seal of the pouch (Figure 4) the contents of the pouch was then added to the central heating water supply by squeezing the pouch (Figure 5) the sealing cap was then screwed back into the valve (Figure 3) system pressure restored by adding some more water from the domestic supply 6
Figure 5: Pouch after inhibitor added Two pouches of 250ml X-PO10 inhibitor were added to each central heating system: for 70 TD because the number of radiators was above 11 and for 167 BHR because it had not been possible to power flush the system due to it being a one pipe radiator system. After six weeks, the inhibitor level in 167 BHR was checked and found to be minimal (Molybdate tracer had decreased from 200 ppm to 25 ppm). Another pouch of inhibitor was then added and its presence and impact will continue to be monitored. The empty pouch was unscrewed and the valve cap added. The system pressure was the restored by adding water to the central heating system. The occupants knew that the inhibitor had been added and they were asked to continue to use their heating system in the normal way. 6. Results 6.1 ph level This checks the acidity level of the water in the central heating system and is listed in Table 4. 70 TD 167 BHR Prior to fitting gas and heat 6.83 6.89 meters 3 to 4 weeks after fitting meters 6.85 6.85 but no inhibitor 3 weeks after inhibitor added 6.98 nm 7 weeks after inhibitor added 6.81 6.79 Table 4: Acidity level of water 70 TD The inhibitor level remains high but the ph level has decreased slightly, but is still within the acceptable range of neutral ph7. 167 BHR As the ph level was slightly lower, but is still within the acceptable range of neutral ph7. The molybdate level was checked and was found to be very low and presumably it has been used up dissolving the sludge in the bottom of this single pipe system as it could not be power flushed because it is a single pipe system. A further dose of inhibitor (250 ml) was added (24 March 2013). For this type of system, it would be recommended a de-sludging agent be added for 24 hours before the system is then drained, re-filled and inhibitor added. 7
6.2 Temperature distribution in radiators 70TD The temperature distribution across three radiators was measured four weeks after the system was drained and refilled (31/1/13); the inhibitor was then added and the temperature distribution was then re-measured on 21/2/13 using an infra-red laser beam thermometer (Table5). Inhibitor presence Top right Bottom right T Top left Bottom left T Radiator location Pre 43 34 9 41 33 8 Dining Post 58 57 1 45 44 1 room Pre 37 30 7 36 30 6 Post 49 45 4 42 41 1 Hall Pre 42 35 7 39 33 6 Post 55 53 2 41 43 2 Lounge Table 5: Temperature distribution in radiators pre and post adding inhibitor 70 TD It can be observed that the differential temperature distribution from top to bottom of each radiator is very much reduced temperature drop from the right to left is much higher This indicates that the radiator has a more uniform distribution post pre-inhibitor. The thermal image of the hall radiator after the inhibitor was added (Figure 6) shows how uniform is the temperature distribution. Figure 6: Thermal image hall radiator post inhibitor 8
6.3 Temperature distribution in radiators 167 BHR This was also measured using an infra-red laser beam thermometer. The temperature distribution was measured 08/02/2013, some three weeks after the new boiler was fitted; inhibitor was then added and a further set of measurements was made on 23/02/2013 (Table 6). Inhibitor presence Top right Bottom right T Top left Bottom left T Radiator location Pre 59 42 17 61 38 23 Post 57 50 7 59 49 10 Hall Pre 51 43 8 50 41 9 Dining Post 56 55 1 53 54 1 room Table 6: Temperature distribution in radiators pre and post adding inhibitor 167 BHR The result of adding inhibitor is that the temperature distribution of both radiators is much more uniform. This will result in the radiators being more effective at radiating heat and keeping the room warm. The thermal image of the radiator in the dining room after adding the inhibitor is shown in Figure 7. Figure 7: Thermal image dining room radiator after the inhibitor was added 9
6.4 Boiler efficiency The boiler efficiency was determined by dividing the heat output by the heat content of the gas input. This is tabulated in Tables 7 and 8 for the two dwellings together with the inhibitor content. Heat meter (kwh) Gas meter (KWh) Efficiency (%) Inhibitor level ppm Pre inhibitor 12.01.2013 to 30.01.2013 1834 2119 86.5 0 Post inhibitor 06.02.2013 to 21.02.2013 1571 1797 87.4 200 Post inhibitor 13.02.2013 to 24.03.2013 4417 5047 87.5 200 Table 7: Boiler efficiency pre and post inhibitor 70 TD The highest efficiency recorded was 89.0% from 21/28 February; the average value is lower at 87.4% and since the ph is slightly acidic (section 6.1), it might be desirable to add an extra dose of inhibitor. Heat meter (kwh) Gas meter (KWh) Efficiency (%) Inhibitor level ppm Pre inhibitor 15.01.2013 to 07.02.2013 3685 4362 84.4 0 Post inhibitor 08.02.2013 to 17.02.2013 1341 1571 85.4 200 Post inhibitor 17.02.2013 to 23.02.2013 959 1097 87.4 nm Post inhibitor 17.02.2013 to 23.03.2013 4731 5520 85.7 25 Table 8: Boiler efficiency pre and post inhibitor 167 BHR The highest efficiency recorded was 87.5% over period 17/23 February. It is not surprising that this efficiency then decreased again as the inhibitor came out of solution (as detailed previously), so by 23 March there was minimal inhibitor left in solution. As this is a one-pipe system, the only remedy was to add a de-sludging agent (as detailed previously), followed by another dose of inhibitor. 10
7 Discussions and conclusions The effect of adding inhibitors to central heating systems in two dwellings are summarised in Table 9. Item Before After Comment Radiators Non uniform heat distribution Much more uniform heat distribution Entire area of radiator effective in delivering heat ph level Slightly acidic Neutral Longer life of heat Boiler setting in cold weather High Can be lowered by 5 o C giving a saving in gas consumption exchanger Lower setting possible because radiators are more effective in transmitting heat Boiler efficiency 84% 87.5% Increase in efficiency is incremental and this measurement period is too short to determine ultimate efficiency Gas bills High Likely to be 5 to 10% lower for similar winter temperatures Greenhouse gas emissions High Table 9: The effects of adding inhibitor Likely to be 5 to 10% lower for similar winter temperatures Effect will be incremental as the scale will be taken back into solution over a time period longer than this measurement period Effect will be incremental as the scale will be taken back into solution over a time period longer than this measurement period These results are valid for one as well as two pipe heating systems and also for radiators whose age is up to 50 years. Boilers may be either instantaneous water heaters (combination boilers) or those with a storage tank (cylinder type). For these two dwellings, the boilers are relatively new and a greater improvement might be obtained with older boilers which would have to be the subject of a further trial. From these results we can conclude that the VEXO inhibitor can Increase the thermal efficiency (conductance) of radiators Increase the thermal efficiency of the boiler Decrease gas bills Reduce greenhouse gas emissions Extend life of heat exchanger if water can be maintained at neutral level The actual savings are difficult to quantify over such a limited trial period and number of dwellings. The available evidence suggests that savings in the gas bill between 5 and 10% are possible whilst the benefits of extending the life of the heat exchanger could amount to at least 1000 over the lifespan of the boiler. At a pouch cost of ca 16.50, the payback time would only be a few months. Maintaining inhibitor levels The inhibitor is effective for both one and two pipe radiator systems; however the results are reversible in that the efficiency decreases again if the inhibitor is taken out of solution in dissolving the radiator and heat exchanger scale and sludge. For one pipe systems where it is not possible to power flush the radiators in situ, it would be recommended a de-sludging agent be added for 24 hours before the system is then drained, re-filled and inhibitor added. 11
8 Recommendations The initial results are very promising and suggest that a larger trial for a longer time period would be very desirable because this technology could help the UK to meet its carbon reduction targets. To attract further funding, copies of this report should be sent to organisations like the Energy Savings Trust, OFGEM and DECC. The initial dose level of 250 ml of inhibitor may not be sufficient for central heating systems even those with less than 12 radiators. It is conditional upon the volume size of the system, age and the size of the existing mineral deposits within the system. It is therefore desirable to check the inhibitor and ph level of the central heating water after a period of one to two months and if necessary add an extra dose of inhibitor. Because the efficiency improvement is reversible if the inhibitor is removed from solution because of the inhibitor dissolving large amounts of existing scale and sludge particularly on one pipe systems, it is very desirable to check the inhibitor level annually when the boiler is serviced and to add a 250ml dose of inhibitor at the same time as recommended by the Carbon Trust [1] and VEXO to maintain the radiator and boiler efficiency. 9. References [1] Low temperature hot water boilers, introducing energy savings opportunities for business. Carbon Trust CTV 008 [2] Vexo X-PO10 Inhibitor leaflet http://www.vexoint.com/ [3] Installation instructions for any central heating boiler e.g. Worcester - Bosch [4] SEDBUK efficiency bands http://www.boilers.org.uk [5] Carbon savings with hot fill appliances: analysis of demonstration trials by R Mayer, D Saker and P Lewis. Sciotech Report #0112 02/01/2013 supplied to Ofgem 12