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

The house has three separate roof areas, and luckily for this blog, three roof types. The main roof is has around 50mm mineral insulation between joists. The front of the house has a bay with its own small roof which has been modelled with no additional insulation installed.  The rear extension has a flat roof that was replaced in the mid 90s.

I've used our masterplan software to model the changes - I've set the base model to have a top efficiency boiler rather than the antiquated heating systems that are currently in place as I will definitely be changing this.

Bay Roof
The bay roof has been modelled with around a U value of 2.  There also is not much space to add additional insulation.  What are the options?  The void can be filled with insulation - some mineral wool or potentially something with a greater insulation value for smaller depth such as XPS or even more such as PIR or phenolic foam board.  This could be added from the outside but would require the tiles to be removed, or internally but would require the ceiling to be removed.  Another option is to insulate the inside face of the ceiling of the bay window.


Since I will be taking some polystyrene ceiling tiles off the ceiling of the room, and also I want to look at the timbers of the floor above I will be taking the internal ceiling of the bay down. I'll then insulate with foam board and then apply new plasterboard.

Estimated cost: £50 (DIY measure)
Saving: £3.30
Payback: 14.79 years

Flat Roof
The rear roof is a flat roof built in the mid 1990s and probably has a U value of around 0.35.  A new Building Regulations roof would need to have a U value of 0.18 or better.

If the roof was in good condition it could be insulated on the inside to keen the costs down.  In this case the roof requires some work to weatherproof it.  Strictly speaking this means it will need to be brought up to Building Regulations anyway. Scaffolding will probably be required to carry out weatherproofing works so I will be looking into quotes to bring it up to current Building Regulations as well as getting a solar thermal system installed on it at the same time.

Estimated cost: £1,000 (Professionally done)
Saving: £3.56
Payback: 281 years

Main Loft
The main loft is a standard pitched roof with around 50mm of insulation installed between the joists. This insulation is in a pretty badly installed.  Cheapest and easiest solution is to top up the insulation between and over the joists to 300mm.

Estimated cost: £250 (DIY measure)
Saving: £45.48
Payback: 5.5 years


Another option being considered to have a central area that has 100mm mineral wool between the joists and XPS board insulation and chipboard above the joists so that a solid platform remains for storage.  this will cost a little more but should result in similar savings.

Whist I've got the loft ladder out I'll be carrying out some work on the loft hatch:

Loft Hatch Insulation
Estimated cost: £10 (DIY measure)
Saving: £2.32
Payback: 4.3 years

Loft Hatch Draughtproofing
Estimated cost: £7.50 (DIY measure)
Saving: £2.09
Payback: 3.6 years

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.

As I've explained, the house is going to need some major work not least the addition of a central heating system.  This is a large change and will have a significant effect on the whole energy use of the house.  This is important for how I decided to model the property....


The first thing we do to analyse a property, once the survey has been carried out and the model calibrated, is to apply individual measures to test their individual effects. In this case since one of the measures (a new central heating system) is going have such a large effect on most of the others, and also since the new central heating system is definitely going ahead, it would see sensible to apply it to the base case and evaluate all the other measures against the property with a new central heating system.


I can still work out the benefit of the central heating system by either carrying out that analysis first or by putting in an 'if I don't change the central heating system' measure.  This latter one will just give some negative savings (I hope!) equal to the positive savings I would have got by doing it the other way round.


We often do this when modelling changes that we know will effect many other things or are definitely going ahead.

I've modelled the house with its current fabric and heating systems.  As the appliances will be brought with us I've modelled our current lights and appliances and our expected use of them.  The model is therefore a bit of a hybrid of our expected used in its current building fabric.

It's definitely at the boring stage for any readers! It's all about phoning mortgage companies and instructing solicitors..

I have carried out a survey of the property in order to model it up using Parity software. I'll publish the results of its current energy profile soon.  The only problem with modelling a new house is that we don't have historic energy bills in order to calibrate the model.  It's not the end of the world though as we are generally are within a few percent of the actual bills when we do have them to calibrate.