An important point to bear in mind here is that many solar heating systems for DHW often require a) a new hot water cylinder b) solar controller and c) solar panels. With that in mind, I have seen prices as high as EUR9,000 for some of these systems. To put it in context, using solar for both DHW and space heating may only add an extra EUR2,500-EUR3,000 to the final cost, as the system is basically the same, albeit sized on a larger scale. That's why I am keen to explore this option!

Solar Heating Concept:

Solar collectors are placed on the roof, and the sunshine typically heats a water/glycol solution which passes through the collectors. A pump circulates this solution into a new dual coil tank, with the lower coil usually being fed by the heated water/glycol mix. This in turn heats the water in the tank. The other coil is then connected to the backup boiler, which could be an oil, biomass, gas or any other form of boiler.  

For DHW heating, the tank only needs to be the same size as a standard hot water cylinder; so usually the home owner can remove the old cylinder and replace it with the dual coil variety in its place. But more often than not, the installation company will recommend a larger tank size when replacing it, to take advantage of the larger volume of hot water which can be heated by the solar collectors.

That's all good news for DHW, but for space heating, having a new dual coil tank will not help towards the space heating element. So what is usually done is to replace the original cylinder with a buffer tank, which is basically a much larger tank with more inputs ands coils, e.g. 400litres and above. For example, if a 400 litre tank is split such that 150litres can hold the DHW, then the remaining 250litres is available for space heating. This 250litre section of the tank is connected to the heating distribution system, and the boiler is connected directly to the buffer tank. The solar feeds the coils in both tanks, in order to heat up the water, but the backup boiler only turns on when the temperature of the water returning to the buffer tank from the heating distribution system falls below the required setting. This is how the contribution towards space heating is made. 

Solar Roof Collector Considerations:

You should only use flat plate collectors if your budget is extremely tight or you are at preliminary roof design stage. These collectors look similar to the solar PV panels I use for electricity generation but are deeper, because within the frame the water/glycol mix circulates through a series of channels to allow it to absorb heat from the solar irradiation. They are not as efficient as evacuated tube collectors, so you will need more of these to give the equivalent return of evacuated tube collectors. But the benefit of the flat plate collectors is that you can integrate them very well into to roof structure, so that the sit almost flush to the roof tile or slate. They are also very durable and not as prone to breakage as evacuated tubes.

Evacuated tubes are more efficient, but also more costly. Most have a copper rod in the middle of the vacuum tube, which will heat up to a very high degree, and this heat is concentrated towards the tip of the tube. Here the water/glycol mix comes into contact with it and is heated and then circulated to the storage cylinder. They are also proven to be better at heat transfer during the dull overcast days and can still contribute to the space heating in these circumstances. However, they cannot be easily integrated into the roof fabric and are more susceptible to breakage than flat plate collectors. A well designed system should still operate even with broken tubes though, and in most systems any damaged tubes can be easily replaced. 

Roof Orientation:

The same considerations apply here is to solar PV panels- south facing would be ideal, but with my roof inclination at an East-South-East orientation that may require some extra collectors to be used to offset the lower gain. Also, when planning for solar space heating the number of collectors will be greater than if only being sized for DHW, so this needs to be factored in while assessing the roof space required. Another advantage of the evacuated tubes is that most can be rotated to a degree to compensate for poorer orientation or a non optimum roof angle.

Solar Buffer Tank Storage:

Unless you are building a new house with a buffer tank in mind, it is unlikely you will have space in the hot water cupboard for one of these beasts - they are very large with many standing 6-7 foot high and the full width of a door. Also, given the weight of the tank when full, most tanks will need to be sited on the ground floor; or on a reinforced concrete pad. In my case, even the smallest of these tanks would not fit in my boiler room, so I would need to use the connecting utility room for the tank, and move the washing machine and tumble dryer to a more suitable location. The tanks usually come with a special insulating jacket, that can be removed for installation and positioning. 

Which Solar System:

There are a number of manufacturers in the market, from well established heating companies through to standalone components which could be bought separately. For example, you could opt for the more expensive Thermomax branded evacuated tubes but then go for a non branded solar controller and tank, or you could go the other way and purchase unbranded tubes or flat plates from Navitron and build the system that way. Or perhaps you could source them all from a single company who manufacture all the components, such as Viessmann or Worchester-Bosch. On balance, I would want to ensure that the panels or tubes on the roof are of suitable quality, for these are the the primary heat source and I would not be willing to compromise on quality here.

In terms of costs, I would estimate that the solar space heating system will cost between EUR8,000 and EUR12,000. That takes into account the following: appx 6-9m² of evacuated tubes, a 750litre buffer tank, plus solar controller, expansion tank, and fitting costs.

There are grants available of up to EUR300 per m² of roof collector, so I would expect to offset the above costs by  between EUR1,800 and EUR2,700, depending on the tube aperture area. 

So lets review all the information we have and make the decision on the heating system changes...