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How to benefit from solar hot water with a combi boiler

A combi boiler provides central heating and hot water. Hot water is heated on demand as cold water flows through a heat exchanger in the boiler on its way to the taps.  This type of boiler accounted for 77% of all new boilers sold in the UK in the last 12 months and around 50% of all homes now have a combi boiler.

With combi boilers being so common you'd think that the heating industry would have worked out how to combine solar hot water systems with them. However, until very recently there wasn't really a good answer to the question of how to overcome some tricky technical challenges that crop up when you do this.

Solar energy lands on the solar panels all day long, whereas hot water use is concentrated in the morning and evening.  Solar heated water needs to be saved up for later use and this is done in a hot water cylinder or heat store. The solar energy available varies from day to day and through the year so it's also necessary to be use the boiler to bring the water to a high enough temperature on those days without enough solar energy.

Of course, it’s possible to re-configure the combi boiler to be like a conventional boiler (See image 1 below).  You install a hot water cylinder and heat it from the boiler  as an extra ‘zone’ on the heating system.  Solar energy (from either a solar thermal system or excess electrical energy diverted from a PV array) is added into the cylinder and the cylinder thermostat tells the boiler when it's needed to 'top up'.

This approach to adding solar to a combi boiler is really difficult. The intervention in the heating system is significant and there's often no space to put a hot water cylinder (because the house has a combi boiler).

A much simpler approach would be to send water that has been pre-heated using solar energy to the combi boiler.  A hot water cylinder is still needed to accumulate the solar heat through the day, but it can be smaller because it doesn't need a boiler-heated volume inside.

There are two tricky technical issues that need to be addressed if you’re going to take this approach. This is the case for both PV and solar thermal water heating.

1. The maximum incoming water temperature the boiler can accept

Some models of combi boiler may not be able to accept water above a certain temperature, either because there is insufficient control of the flame to keep the outlet temperature safe or due to the temperature rating of materials used on the cold water inlet side.

A component called a combi-diverter valve can be used to work with all but the most limited boilers. The valve either by-passes the boiler (if the water is hot enough to go straight to the tap), or adds cold water to keep the temperature below 28C, which is within the specification of many combi boilers.

You can check your boiler model using this online database of combi boiler inlet temperatures.  If your model isn’t listed you’ll have to call the manufacturer and while you’re at it please nag them to join in with this splendid industry initiative from the Solar Trade Association and the HHIC.

It may seem crazy to produce solar heated water and add cold water to it only to heat it up again in the boiler, but doing this does leave more heat in the solar store for later use.

2. Controlling the risk from Legionella bacteria

Legionella is a naturally occurring bacterium. It is present at safe levels in drinking water but can multiply if that water is held at warm temperatures (20C to 45C).  If you inhale droplets of water containing high levels of the bacterium (particularly if you’re ill or elderly) it can cause Legionnaire's disease.

In a standard hot water cylinder Legionella risk is normally controlled by setting the controls to heat the water to 60C at least once a day,  a temperature that kills off any bacteria that might have multiplied in warm water.

A combi-boiler normally raises a flow of cold water to a set temperature of around 55C after which it quickly passes on to the hot water outlet.  There is very limited risk of Legionella because the water didn’t spend time sitting at warm temperatures.

Due to the variability of sunlight a hot water store heated only with solar could spend periods at the warm temperatures in which the bacteria grow. 

Feeding solar heated water to a combi boiler introduces a risk that the water feeding the boiler could have spent time at temperatures that allows the Legionella to multiply.  The short heating cycle of the combi boiler cannot be relied upon to deactivate Legionella.

If the combi boiler cannot be used to control Legionella risk, then alternative means are necessary.

One approach would be to run an immersion heater in the solar store for a couple of hours each night when you can be sure the water is going to stand still long enough to reach the temperature for the required time.

This approach is a problem for solar energy for two reasons.  First, the solar store starts the day hot. If there isn't enough water use in the morning, then the cylinder's capacity to take in solar heat during the rest of the day is cut (because it’s already hot).  Second, the electricity used overnight to heat the cylinder to 60C is high in carbon emissions and expensive – offsetting the point of having solar.

A better Legionella control strategy, used in products such as the Viridian Solar Pod is to keep the volume of water in the solar cylinder static, and instead heat fresh cold water in a heat exchanger as it flows past on its way to the combi boiler. The problem of water sitting at warm temperatures for extended periods is completely avoided, and thermal pasteurisation is unlikely to be necessary.  (See image below)

It is my hope that the emergence of a new class of products that deal with the technical challenges of preheating water for combi boilers in combination with greater information from the boiler manufacturers will open up solar water heating to benefit even more people.

If you want more detail, check out this article on Stuart’s solarblogger website.

More information about Solar Thermal on YouGen.

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