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 normal house uses around £250 of hot water a year. A normal house could expect to save 50-70% of that with a £5,000 system. The maths isn't hard - 28 year payback at best. The key thing is that the proportion of our energy bills due to hot water is much less than most of us think.  It's also much easier and cheaper to make savings by reducing your demand (less baths, combi boilers, low flow showerheads etc) rather than trying to make you own. 

And once you do this the paybacks are even longer.

That's not to say solar thermal is always way down the list of things to do, and it can be a really good solution for properties off the gas network or where you have a rugby team staying in your home.

So back to the question at hand.  Why am I installing a solar thermal system? 5 reasons really...

1) I'm starting from a situation where there is no heating system (I took out the storage heaters and multi point gas boiler) so installing a solar thermal system in addition to the rest of the work will be a marginal extra cost.
2) I've got a young and growing family and there are likely be lots evening baths (especially when the rugby age arrives)
3) With a super insulated house the proportion of our energy bills that are due to hot water will be relatively large and so achieving our personal ambition to be as low impact as possible means starting to tackle some of those areas usually less worthwhile.  It'll be interesting how long we can survive on solar thermal and the wood burner before the gas boiler gets turned on for the winter.
4) I expect gas prices to rise
5) There is still a chance the Renewable Heat Incentive tariffs - rightly or wrongly - will be set at a level to make solar thermal systems financially worthwhile.

The Renewable Heat Incentive Premium Payment of £300 isn't one of our reasons as it doesn't make much impact.  We do however welcome the decision to gather more data before setting tariffs.

I'll blog a bit more about the details when it gets installed...

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.