How is a thermal store different from a hot water tank?

People often ask if they can use heat from their solar panels for heating as well as hot water. The short answer is no, because the solar panels heat your hot water tank which supplies hot water to your taps, not your radiators. The long answer is you could, if you installed a thermal store instead of a hot water tank, though this may or may not be a good idea. A thermal store is also a big tank of water it works in a different way so it can be used for space heating as well as hot water. Thermal stores can be useful in various situations:

• You can combine heat from different sources, e.g. solar panels or a wood stove as well as your boiler
• You can store heat to optimise use of your heating system - for example if it is inefficient to turn on and off according to demand (particularly useful for log burners but also for other biomass boilers or even heat pumps).
• It can store heat when power is cheap for use when it is expensive. For example, using a thermal store you can avoid running your heat pump at peak times and get a cheaper tariff (at least you will when we all have smart meters and time of use tariffs are available).

The size you need depends on what you want to do with it.

Thermal store heats water instantly like a combi boiler.
The main difference between a thermal store and hot water tank is that the hot water tank supplies hot water directly, while a thermal store heats water instantly, like a combi boiler. When you turn on the hot tap, cold water from the mains runs into the thermal store through a heat exchanger and comes out hot. Similarly for your radiators: warmish water comes back from the radiators and is heated up to go round again. (The radiator circuit needs to be completely separate because the water in there contains toxic additives to prevent corrosion.)

Thermal mixing in a hot water tank is bad, but in a thermal store it is a disaster.
Hot water tanks are usually fairly squat, to minimise surface area and hence heat loss but thermal stores are usually tall and thin. (They are of course both well insulated). The reason for the thinness is to minimise mixing of different temperatures at different levels; this is far more critical for thermal stores than for hot water tanks. To see why, consider taking a shower. Your hot water tank is probably heated up to 60°C but you only need 40°C for your shower. Hot water for your shower is taken from the top of the tank which is the hottest part (hot water is less dense than cold) and is replaced by cold water coming in at the bottom. If there is mixing, this cools the water at the top of the tank but it doesn't matter as long as the top doesn't get cooler than your shower temperature. Your thermostatic valve in the shower will mix in the appropriate amount of cold for the temperature you want. However, when it comes to water for your radiators, you really want it is as hot as possible, to get the most heat from the radiator. So preventing mixing is a critical issue for the design of a thermal store.

This is mainly achieved by design of the heat exchanger and baffles.
Fortunately, the main driving forces behind mixing in a hot water tank doesn't happen in a thermal store - the cold water to be heated doesn't actually flow into the tank, it only goes through a heat exchanger. The only way mixing will occur is by convection currents caused by temperature differences. To minimise these currents the heating coils are oriented so that the heat is supplied at the top (see diagram below). This means that the top part of the thermal store is warmed first and most and there is minimal tendency to mix. When the top of the store is as warm as the heat coming in then the parts lower down will start to heat up.

Thermal store showing stratification, hot at the top, and heat exchanger arrangement to minimise mixing.

Sometimes baffles are fitted at various levels to physically slow any currents that develop between top and bottom. (This explanation is somewhat simplified - see Stoves Online for more detail.)

A third mechanism, though I am not sure if it is used commercially, is to arrange that cool water comes in on the supply side at a level in the store which is at a similar temperature. The picture shows how this was done DIY by 'Peter in Hungary', using a ladder of pipe connections at 20cm intervals (described on Green Building Forum)

Ladder manifold brings cool water for heating into the thermal store
at the level corresponding to its temperature.
Image from Green Building Forum

A hot water tank for three showers: minimum 80 litres.
The size of the thermal store you need depends on what you want it for, which governs the temperature range between inlet and outlet and the amount of heat you need to store. For the shower case, the water coming in is at the mains water temperature, around 15°C, and the maximum temperature in the tank is normally set to 60°C. This gives you a temperature range of 45°C. One shower (6 minutes at 8 l/min) needs 1.4 kWh. A 150 litre store is normally recommended for three people [1]; three showers like this would actually need only an 80 litres tank, as shown in the table below (assuming there are no losses and the entire tank is heated to 60C to start with).

To store one day's solar space heating in April: 500 litres.
Now consider you have lots of solar hot water panels and you want to use your thermal store for all your heating needs in April, when the mean temperature outside is 9°C. A typical fairly well insulated house under these conditions needs 23 kWh/day (space heating assumptions given below). If you have a conventional boiler with radiators, the flow/return temperatures are typically 70/50 so the inlet to the store is at 50°C. Theoretically a solar thermal panel can supply at a very high temperature but obviously you can't have the water in your thermal store boiling so the maximum is around 90°C. This means to store heat for one day you need a 500 litre store.

To get 23 kWh from your solar panels, if you are using solar PV and putting all the electricity into the store you would need an 8 kWp array - up to 32 panels. Most domestic systems are only 4 kWp and you presumably want some of this for other things so you will get at most half your heating needs. If you had solar hot water panels you would need considerably less area because they are more efficient but there would be nowhere to put the heat in the summer when you are not using the heating.

For three hours heating in winter: 200 litres
Alternatively, suppose you want to use the thermal store with a heat pump to avoid using power at peak times in winter. If it is 0°C outside and you want enough to handle 3 hours: 7 kWh. The maximum temperature from a conventional heat pump is 55 C and the return from your underfloor heating could be as low as 25 C. Your heat store needs to be 200 litres.

Case	Temp. in (°C)	Max. temp. (°C)	Temp. range (°C)	Heat needed (kWh)	Store size (litres)*
Three showers	15	60	45	4.2	80
Solar heat, one day in April	50	90	40	23	500
Winter evening 3 hours, underfloor heating and heat pump	25	55	30	7.0	200

* heat needed/temperature range / 4.2 x 3600


Space heating takes more energy than your shower
As you can see, if you want to store heat for space heating as well as hot water, you need a thermal store and it might need to be 2-4 times as big as a typical hot water tank, depending on what you want to do with it. The details vary depending on both your heat source and heat delivery system, but fundamentally the problem is that space heating takes a lot more energy than your shower.

Phase change materials could reduce the size of the store needed.
There are other solutions to this problem, involving materials other than water, in particular phase change materials (PCM). This allows heat to be stored in a smaller space - such as the 'heat batteries' made by Sunamp. For the moment products such as this are expensive and not widely available but hopefully this will change.


Space heating assumptions
Mains water temperature is 15°C
Head demand for the house is 150 W/K
Internal gains are sufficient to heat the house to 15.5°C, the usual assumption for calculating degree days.
Heat losses from the store and pipes are negligible.

[1] Sizing a hot water cylinder (Hot water Association)