It's Christmas so I thought I'd spoil myself and do some additional fun
analysis on Air Source Heat Pumps and different Coefficients of Performance. (N.B. as with other analysis this is only valid for the house I am looking to buy.) For those that don't know a Coefficient of Performance (COP) is the % heat energy that you get out of a heat pump compared to energy in. An example is a COP of 200 means you get 200% energy out for energy in or twice as much energy out as energy in. I know it sounds like some basic laws of physics are being broken but they aren't. The energy in is actually 'work' and the energy out is the conversion of low grade heat (in the outside air) to high grade heat (inside). The COP therefore doesn't take into account the amount of energy in the low grade heat as it is essentially free.
The graph above shows the results of my efforts. The yellow line is the basic savings that can be expected compared to the current heating system in the house. You can see that a COP of 225 is required for there to be benefit. The blue line is the savings including a rough figure for the Renewable Heat Incentive payments - in this case a COP of around 120 is required. For these calculations I've using a one tariff electricity rate. Anyone want to share their thoughts?
So the results of the initial
heating system analysis are in...for this analysis I haven't carried out any behavioural or zoning analysis or changes to the shower heads. The model therefore has the whole house heated to 21 degrees - 2 heating times during the week and one longer one on weekend days. For each system I have modelled putting in modern controls appropriate for the system i.e. thermostats, radiator valves, boiler interlock
To begin with I looked at 9 different major changes to the current heating system (electric storage heaters supplemented by gas room heaters on the ground floor):
- A top specification condensing combination boiler for a radiator heating system and hot water
- As 1. but with the addition of a weather compensator
- A top specification condensing boiler with a modern fully lagged 200litre cylinder and lagged pipework for a radiator heating system
- As 3. but with the addition of a weather compensator
- As 4. but underfloor heating
- An air source heat pump system for the heating and hot water (coefficient of performance used - 2.5)
- As 6. above but using the average coefficent of performance found during recent Energy Saving Trust fieldtrials - 1.94)
- As 7. above but with the heat pump for heating only. The hot water being provided by an electric immersion heater.
- As 7. above but with an estimate of the Renewable Heat Incentive payments based on original consultation figures.
Here I have compared the different predicted total CO2 emissions for each type of system. Again it is worth stressing that the rest of the building remains unchange - i.e. superUNinsulated! What this shows us is that the gas boiler systems are predicted to roughly half the emissions (or over half when you take into account that some of the emissions are due to lighting and appliances). The air source heat pumps are predicted to reduce the emissions by about about 2/5 unless the hot water is provided by an immersion heater.
Another way of showing this is to concentrate on the CO2 savings - these are expected to range from between just over 6 to around 8 tonnes (excluding the immersion heater for hot water no.8). This really highlights why the strategy of working out the heating system and then including in the base before evaluating other insulation measures is a good idea.
The boilers obviously use gas and the air source heat pumps uses electricity. The system being replaced uses gas and electricity for heating and gas for hot water. This chart shows the expected savings from the different options. The higher coefficients of performance of the air source heat pumps are offset by the much higher cost of electricity compared to gas. Option 9. stands out as an outlier and is very high as it includes a large annual payment that might be possible under the Renewable Heat Incentive. This payment will come from the 'energy companies' who will have to pass it on to all consumers. We'd be interested in your thoughts on these payments taking into account the large capital costs that only some people will be able to afford.
I've taken some rough estimates for the different heating and hot water system options. These are obviously rough estimates as I haven't yet been able to really investigate the property and work out exactly what is required. The gas boiler with radiators is the cheapest and the air source heat pump with underfloor heating is much more expensive.
Here is the results of dividing the estimated cost by the estimated savings. The gas boilers with radiators have a payback range of between about 13 and 15 years. The air source paybacks are positive when the manufacturers COP is used but negative when the average COP from the EST field trials are used - and that is compared to the current heating system. The potential Renewable Heat Pump payments skew 9 considerably.
So what does all this mean for our heating system decisions? Personally I'd like to see a bit more independent data on the performance of air source heat pumps in many different scenarios. Even without this the paybacks don't seem to be as good as for the gas boilers. I'd also like to see more information about the long term performance and life expectancies of air source heat pumps. Finally I'm not sure I'd be comfortable with such large payments which would essentially come from people who either did not have the means to install an air source heat pump or other RHI plant.
All this means is that although no decisions are being finalised at present, I'd going to model the property with a gas boiler. As it is a fairly large house and will have two bathrooms I'll also model it with a hot water cylinder. I'm going to have as the base case a radiator system but will evaluate underfloor as an option.
There are other considerations that will come into play with regards to the radiator vs underfloor decisions such as the floors being taken up for rewiring, plumbing, insulation (and potentially damp remediation). Finally as the walls will probably be insulated over a period of time, it will be best if the central heating system was installed throughout first and did not need moving over time - radiators would unless I was to have them standing over 120mm away from the walls to start with.
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