The next stage in Adrian’s Wood Lane retrofit decarbonisation journey was to look at heating and hot water. This followed a process of insulating and draft proofing the house. Adrian looked in detail at the carbon savings using his free Gas\/electricity SMART meter data, free Weather data, free Heat Pump Datasheet data, a spreadsheet, and a set of scales for both Ground Source and Air Source Heat pumps.<\/p>\n
Heat pumps are one of the UK government’s preferred options for decarbonising household heating and hot water. Around 24 million households in the UK have some form of gas central heating, often in the form of a combi-boiler that provides domestic heating and hot water. This has been helped by the relatively low cost of gas per kWh compared with electricity. At the time Adrian was creating the comparison gas was 7.5p\/kWh and Electricity around 35p\/kWh.<\/p>\n
Gas condensing combi boilers are advertised as being circa 90% efficient. But is that the case?<\/p>\n
Adrian says-<\/p>\n
Gas condensing combi boilers are now circa 90% efficient\u201d \u2026 that\u2019s what I thought, but not so<\/i>! Gas boilers have a SCOP just like Heat Pumps do, because they too have a varying efficiency based on flow temperature, flow rate, DCH flow return temperature, servicing and demand usage characteristics. The [general headline] 90% \u0220 is the headline \u201ctop level\u201d achievable efficiency. As well as CO2<\/sub>, they also emit CH4<\/sub> and NOx; my calculations did not include realistic combi- efficiency for my boiler, nor CH4<\/sub>\/NOx<\/sub> emissions, but I did include CH4\u00a0 <\/sub>for my Grid Electricity (but not NOx<\/sub>). Overall, the point is that combi-boilers are not<\/i> 90% efficient in the winter when you use them most.<\/p><\/blockquote>\n
Heat pumps however are between 200% and 600% efficient! The reason for this efficiency gain is the electricity powering the pump is being used to transfer heat from one source to another, rather than direct electric heating. This efficiency puts heat pumps on a par with gas combi boilers in terms of operational costs. With the addition of smart tariffs, time of use tariffs, and solar PV, heat pumps can be less expensive to run. Over time as the grid is decarbonised it is expected that electricity costs will reduce in comparison to gas and the greater efficiency will mean heat pumps become more cost effective to run than gas.<\/p>\n
Air Source Heat Pumps<\/h2>\n
Air source heat pumps (ASHP) typically have a large fan to pass ambient air over a heat sink. They will work at temperatures as low as -20 degrees C. They are between 2x efficient in the winter to 5x efficient in the summer, though even in winter 3x efficiency can be achieved.<\/p>\n
Ground Source Heat Pump<\/h2>\n
Ground Source Heat pumps (GSHP) transfer heat from underground, either through a borehole, or though shallower pipes laid underground across a larger area. With the borehole method the boreholes\u00a0is lined with a steel 8\u201d pipe down which two plastic pipes are dropped (with a \u201cU\u201d bend at the bottom); these connect to the GSHP unit \u2013 forward flow and return \u2013 and are filled with anti-freeze and meticulously filtered as this same fluid is what goes through the vapourisation nozzle in the heat pump to do the heat transfer.<\/p>\n
Of the alternative shallow pipe laid underground method across a large area Adrian says…<\/p>\n
…digging up soil is a really<\/i> bad idea because i) it emits CO2<\/sub> and ii) it destroys the eco-system that was living in the \u201csoil\u201d before we dug it up. This excellent video: Dr Matthew Shepherd: A fascinating introduction to life below ground\u00a0 <\/u>[best of around 15 webinars I\u2019ve watched on soil so far]<\/blockquote>\n