Following a slightly leading question from my colleague David, I've done some
quantitative analysis on the potential savings now and under a hypothetical Renewable Heat Incentive regime.  The RHI will now require metered amounts for which I've used the modelled kWh savings from a solar  thermal system over a non solar thermal system.  In reality these may be below the actual meter amounts as these will include any storage losses.

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I've modelled:
   -  1) an A rated combi boiler  (although the house and number of bathrooms is too large for a combi)
    - 2) an A rated combi boiler with low flow showerhead
    - 3) an A rated boiler with a solar ready 250litre cylinder
    - 4) an A rated boiler with a solar ready 250litre cylinder and ultra low flow showerheads
    - 5) an A rated boiler, solar thermal system and 250litre cylinder
    - 6) an A rated boiler, solar thermal system and 250litre cylinder and RHI payments
    - 7) an A rated boiler, solar thermal system and 250litre cylinder and low flow showerhead
    - 8) an A rated boiler, solar thermal system and 250litre cylinder and low flow showerhead and RHI payments

Not done yet....I've also looked at the paybacks with fuel prices and RHI prices remaining static as well as with a 10% annual increase in fuel prices and a 2.5% increase in RHI payments.

I've applied costs that are either marginal (i.e. for solar thermal) or that apportioned to the hot water (i.e. in the case of a boiler).

And here are the results:
       Annual Saving        Install Cost             Payback                        Payback                        20 year savings
            (year 1)                                        (static prices)        (with prices increases)            (less capital outlay)
1)          £125                    £200                    <2 years                        <2 years                              £6,959
2)          £143                    £250                    <2 years                        <2 years                              £7,940
3)          £107                  £1,000                   <9 years                        <7 years                              £5,071
4)          £146                  £1,050                   <8 years                        <6 years                              £6,853    
5)          £171                  £3,500                  <20 years                      <12 years                             £6,638
6)          £309                  £3,500                  <11 years                        <9 years                            £10,163
7)          £195                  £3,550                  <18 years                      <11 years                             £7,676
8)          £310                  £3,550                  <11 years                        <9 years                            £10,614

A little thought experiment....
1 - put aside any subsidies for the time being
2 - put aside any differences between the amount you pay for energy and the amount a manufacturer pays for energy
3 - put aside different carbon intensities of energy saved and energy of manufacture/tranpsort
4 - the following diagram therefore must be true when the product has paid itself off:

Therefore in a simple situation (without subsidies and with no difference in energy prices) when a product has paid for itself in energy savings it has also necessarily paid for itself in embodied energy. 

So the only question is how much to 1, 2 and 3 change things? My view is not very much. What are your thoughts - anything major missing here?  I'd love to have your thoughts.

Because of this for my project, I have decided to concentrate on paybacks and CO2 savings rather than embodied carbon as it will come out in the wash so to speak...

The house has a range of different types of window ranging from bay windows at the front, a single glazed top light above the front door, to 15 year old uPVC windows in most of the rest of the property.

For this exercise I've ignored three windows - the dining room window and side kitchen window as they will be altered as part of a potential extension, and the tiny toilet window as I haven't decided what to do with it yet.

When we model changes to windows as part of our Masterplan, changes to solar gains are included in the calculations as we take into account the amount of light that comes through different window types as well as their orientation and the geographical location of the house.

I've presented the results in a different format to highlight other useful ways of looking at savings other than just absolute financial or CO2 savings.

The peach rectangles represent the £ expenditure per annual £ saved by the measure - i.e. the smaller the better and also they are essentially equivalent to paybacks - £25/£ saved ~ 25 year payback.

The orange cylinders show the £ spend for each annual kg CO2 saved.  Again the smaller the better.  ~~Both of these represent cost effectiveness of measures either in saving money or reducing CO2 and are useful for setting thresholds. In the graphs I have set a £25/£ saved threshold as indicated by the red vertical line - only measures that don't exceed the line will be considered from an eco perspective.

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):

1. A top specification condensing combination boiler for a radiator heating system and hot water
2. As 1. but with the addition of a weather compensator
3. A top specification condensing boiler with a modern fully lagged 200litre cylinder and lagged pipework for a radiator heating system
4. As 3. but with the addition of a weather compensator
5. As 4. but underfloor heating
6. An air source heat pump system for the heating and hot water (coefficient of performance used - 2.5)
7. As 6. above but using the average coefficent of performance found during recent Energy Saving Trust fieldtrials - 1.94)
8. As 7. above but with the heat pump for heating only.  The hot water being provided by an electric immersion heater.
9. As 7. above but with an estimate of the Renewable Heat Incentive payments based on original consultation figures.

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.