You can download the document for “An Introduction to Renewable Heating” here. Or view all documents available for downloads here.

So far we have discussed how to size a heating system and how traditional heating systems work. Now we will begin to look at how we can integrate a renewable heating system into an existing system. The first thing to realize is that your Hot Water Tank is going to need replacing (unless you already have a hot water tank that can accept multiple heating inputs). Traditional Hot Water tanks can only accept one source of heating (unless you have a dual coil hot water tank), whereas with a renewable heating system you are going to need two (or more) sources of heating (as the renewable heating system will not provide 100% of your heating needs).  You could, of course, use the renewable heating source as your only form of heating and your back-up heating source coming from an immersion heater, however this is not advisable unless you have a high grade heating system and a very well insulated property. This sort of heating design tends to be installed on new build properties (such as the Cernunnos Eco Homes).


The tank used for renewable heating systems is called a “Thermal Store” (also known as a “Heat Bank”). These are slightly taller than traditional hot water tanks, and come in all sorts of sizes. The primary advantage of Thermal Stores is that they can accept multiple heating sources.


Thermal store cylinders work on the process of stratification whereby hotter water rises to the top of the cylinder and cooler water goes to the bottom. Each level of the cylinder therefore has a different temperature. It is for this reason that thermal stores are taller than conventional water tanks. Under-floor heating, which require lower water temperatures and a more constant flow of heat would be connected towards the bottom of the cylinder, whilst the domestic hot water needs would come from the top. The domestic hot water needs are not stored in the tank itself, but is fed from the mains system and heated through a heat exchanger. The heat exchanger draws the hot water from the top of the thermal store, and passes it through a heat plate. As the mains water passes around the heat plate, it absorbs the heat from the thermal store water, thereby heating the mains water. The water from the thermal store that has passed through the heat plate is finally pushed back into the thermal store at the bottom for it to be heated again. This way the thermal store is the closed loop system for central heating, whilst the mains water feed is passed through to heat the mains water. The thermal store provides mains pressured domestic hot water needs at up to 45 litres per minute. Also, this allows the thermal store to be a vented system (i.e. non-pressurised) and thus it is a lot safer and easier to maintain. Quite often these heat plates are stored within the thermal store, but for the purpose of analysis we have shown it externally below.

Thermal store cylinders also allow multiple heat sources to contribute to the heating of the water in the tank. It can have more than one heating coil (also known as heat exchanger or heat plat exchanger), and therefore solar thermal systems or heat pumps could heat the water in the tank. Also, in a conventional Hot Water Tank the heat exchanger is connected directly to the boiler, which pumps hot water through the coil that heats the water in the tank before being fed through the central heating system (radiators etc). The DHW needs are run from the tank, being replaced by cold water from the cistern, which is then heated by the coil. This coil has to transfer all the boiler heat and this is the weak point, resulting in long warm up times and long recovery periods. Thermal Stores differ in that the boiler (or multiple heat sources) is (are) connected directly to the hot water tank, circulating the whole tanks contents. This direct transfer of heat means faster response and warm up time for the water in the tank. The transfer coils are also much larger in surface area and much longer (typically 10 meters long). There are also 2 transfer coils, one at the bottom, feeding the heating system (which requires lower temperatures) typically to 60 degrees, and one at the top to feed the DHW needs, typically heating the water to 75 degrees. The bottom heat source tends to be the renewable heating system, which provides heat up to 50 degrees Celsius. The higher heating source tends to be the more traditional system which can be used as a “top-up” system that brings the water temperature up to 60 or 70 degrees Celsius. This can be your existing oil/gas system or an immersion heating system.


As the Thermal Store is much bigger than a traditional water tank, it can run for a long efficient burn rather than repeated smaller cycles that are seen in conventional tanks. This makes them ideal for under-floor heating, as they supply a constant flow of water at lower temperatures. Otherwise the heating of the under-floor heating pipes would cause repeated cycling of the boiler.

The advantages of Thermal Stores are:

  • They provide mains pressure water
  • They can be used by multiple systems
  • They have quick re-heating time periods
  • Require no loft tanks
  • They are vented and thus easy to install and maintain

Thermal stores are by far one of the best systems on the market. The only problem is that many who do not need to replace their water tank do not want the upfront cost, which is understandable. Those looking to replace or upgrade their system should consider Thermal Stores.

Obviously, a renewable energy system will replace some of the need for your boiler, thereby reducing your energy bill. However, you will always need a form of traditional boiler system in place as a back-up for your renewable energy system. This can be in the form of oil and gas to emersion heaters. Also, some renewable energy systems can only heat water to 45 to 55 degrees Celsius, and thus you will need another system to heat the water to 65 degrees to get rid of the legionnaires disease risk. Thus the ability to have a dual heating system is important when installing renewable energy systems. It is for this reason that Thermal Stores are the preferred option. However, that is not to say that your current system won’t suffice.

Thermal Stores in practice come in a variety of shapes, sizes and names. If you search for products you are likely to see Heat Pump Thermal Tanks, Solar Thermal Stores, Solar Heat Banks, Heat Pump Solar Tanks etc. However, in reality, they are just simple Thermal Stores that have been further developed. Some even now offer 3 or more input heat sources, allowing you to use Solar Thermal, Heat pumps and Gas/Oil in conjunction. These are ideal for larger properties that want a complete solution, or for business premises. However, the basics of Thermal Stores are the same.

However, Thermal Stores do require more space than traditional Water Tanks, and not every house has room for one. An alternative is to have a Twin Coil Water Tank, which is a mixture between the tradition Single Coil tank (except it has two coils and thus can take two heat sources) and a Thermal Store (in as much it can take multiple heat sources but is a smaller tank!).

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