Sunday 9 November 2014

Solar for Combi Boilers

How to combine solar water heating with a combi boiler

A combi boiler provides central heating and hot water.  Hot water is prepared instantaneously
and on demand as cold water flows through a heat exchanger in the boiler on its way to the outlets

Combination (combi) boilers that provide hot water on demand have become more and more prevalent in the UK.  Data from HHIC shows that 77% of all new boilers sold in the UK in the last 12 months were combis. According to the English Housing Survey 2012, 49% of all homes now have a combi boiler.

Housing developers love combis because they don't need to give up valuable space to a hot water cylinder in small (sorry, "executive starter") homes.  In addition, eliminating the cylinder means that the overall complexity and part count in the heating system is reduced which improves reliability and lowers costs.

Manufacturers of hot water cylinders argue that the flow rates that combi boilers can provide can be inadequate, especially in multi-bathroom homes, but judging by the combi’s dominance of new installations their reputation for producing only a trickle of hot water may now be undeserved.

Challenges of Combining Combi Boilers with Solar Heating

Given that combi boilers are so prevalent, you'd think that the heating industry would have sorted out how to use solar hot water with them.  However, until very recently there wasn't a good answer to the question of how to combine solar with combi boilers.  This is because combining the two raises some technical challenges.

Solar energy arrives over the course of the day, whereas domestic hot water use is intermittent and concentrated in the morning and evening.   Consequently solar heated water must be saved up for later use in a hot water cylinder or heat store.  Since the amount of solar energy that is available varies from day to day and through the year, it's also necessary to be able to bring the solar heated water to a hot enough temperature with the boiler on those days when the light levels are not high enough.

So one approach is to re-configure the combi boiler to behave like a conventional boiler.  A new heating 'zone' is added to the central heating system with separate timer control and controlled from a thermostat on the cylinder.

Solar energy (either from a solar thermal system or a PV array and excess energy diverter switch) is added to this cylinder and the cylinder thermostat tells the boiler when it's needed for a 'top up'.

(1) Reconfigure the heating system to use the combi boiler with a hot water cylinder

This approach to adding solar to a combi boiler can be difficult to implement in practice.  There's often nowhere to put the cylinder (since the house has a combi boiler).  Even when you can find a space for the cylinder, the intervention to reconfigure the heating system can be significant.

A second approach is to send solar pre-heated water to the combi boiler.  A hot water cylinder is still needed to accumulate the solar heat, but it can be a bit smaller because it doesn't need a separate boiler heated volume inside.

(2) Sending solar pre-heated water to the combi boiler
There are two issues that need to be addressed when taking this approach.

1. Combi Boiler Inlet Temperature

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

A combi boiler that cannot regulate the flame down sufficiently would produce too-hot water when the inlet water arrives above a certain temperature.  This causes a safety cut out switch to activate, killing the flame.  The boiler would cycle on and off producing too-hot and then too-cool water.  Operating this way is not good for the boiler lifetime.

Some boilers have plastic components on the cold water inlet side and some of these will not be suitable for water above a certain temperature.  

A component called a combi-diverter valve is available to work with boilers that have such limitations.  One product consists of three thermostatic valves and produces the following logic:

  • Inlet temperature >45C, add cold water to 45C, by-pass boiler straight to taps
  • Inlet temperature 27C - 45C, add cold water to reduce to 27C, pass through boiler
  • Inlet temperature <27C, pass straight through boiler

It may seem crazy to produce solar heated water and add cold water to it only to heat it up again in the boiler (for the middle temperature range), but bear in mind that doing so means you're using less of the solar heated water and leaves more heat in the solar store for later use.

The combi diverter valve ensures that the hottest inlet temperature the boiler will see is 27C, so this can be a way to make solar preheated water work with any combi boiler, so long as you can check that this temperature is OK for the model in question.

Unfortunately, boiler manufacturers do not routinely put the maximum inlet temperature of their products on their data sheets, at least in the UK.  My own experience of calling the 'help' desks of the boiler manufacturers is that it's often a struggle to get a definitive (or consistent) answer to this question.  If pushed, the safest (and default) answer is to say that the boiler should not have water above 'ambient' as an input, push again and you will get told 20C max - after all why would the manufacturer put themselves out on a limb for this?

Thankfully, an initiative by the Solar Trade Association (STA) and the Hot Water and Heating Industry Council (HHIC) has resulted in an online database of combi boiler inlet temperatures.  This list is new, and currently not comprehensive enough (special mention to Ideal for their derisory contribution - boo!).  I hope that over time more manufacturers will see the benefits of publishing this information and join in, and that those who have published will add data on more of their older models.

The STA and HHIC are to be congratulated on this initiative as it removes a significant impediment to the deployment of solar water heating.

2. Legionella Control

Legionella is a bacterium that occurs naturally in drinking water.  It is present at very low levels in drinking water but can multiply if that water is held at warm temperatures (20C to 45C).  If droplets of water containing high levels of the bacterium are inhaled it can cause Legionnaire's disease.  People with suppressed immune systems, for example the ill or the elderly, are most at risk.

Legionella bacteria can be de-activated by heating the water to above 50C, with the time taken to disinfect the water falling rapidly as the temperature increases above this level. 

A combi-boiler takes a flow of cold water, raises it to a set temperature of around 55C and then it quickly passes on to the hot water outlet.  There is very limited risk of Legionella because the water has not spent any time at warm temperatures before passing through the boiler.

Legionella risk is normally controlled in a standard hot water cylinder by setting up the controls to heat the water to 60C at least once a day (called pasteurisation), to deactivate any bacteria that could have multiplied in warm water.

Because solar energy is variable with the weather and seasons, it is not possible to guarantee that deactivation temperatures are reached every day in a cylinder heated only with solar.  Indeed, at certain times of the year a cylinder heated only with solar could spend periods at the warm temperatures in which the bacteria grow.

A twin coil solar cylinder will normally adequately control Legionella risk if the boiler heated section of the cylinder is heated daily to 60C for 1 hour, and the water is resident in the boiler heated section for 24 hours (which happens if the boiler heated volume is greater than the daily hot water use).

Putting a solar heated cylinder upstream of a combi boiler introduces a risk that the water feeding the boiler could have spent time at temperatures that allows the Legionella to multiply.  A paper by the Water Regulations Advisory Scheme (WRAS) confirms that because the combi boiler will only heat the water passing through for a short period of time, it cannot be relied upon to deactivate any Legionella that may have multiplied while the water was resident in a solar heated cylinder.

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

For a conventional cylinder, where the water in the cylinder goes to the combi boiler, the most common approach would be a thermal pasteurisation of the water.  In practice this would mean  fitting an immersion heater to the cylinder and running it for a couple of hours each night when water is unlikely to be taken from the cylinder during the pasteurisation.

Overnight pasteurisation is a problem for solar energy for two reasons:

(1) The solar cylinder then starts the day hot.  If there isn't a significant water use in the morning, then the cylinder's capacity to take in solar heat is greatly diminished, reducing yields.
(2) The electricity used overnight to raise the cylinder to 60C is high in carbon emissions and expensive - offsetting some of the benefits of solar.

An alternative Legionella control strategy, used in products such as Viridian Solar's Pod,  is shown in (b) and (c) in the diagram below - here the volume of water in the solar cylinder is static, and instead fresh cold water is heated as it flows through a heat exchanger.  The problem of water sitting at warm temperatures for extended periods is completely avoided, and thermal pasteurisation is unlikely to be necessary.

If the stored water passes through the combi boiler, then additional immersion heating
is likely to be required for control of Legionella risk

Installing solar water heating with combi boilers has been left in the 'too hard' box by the heating industry for too long.  It is my hope that the emergence of a new class of products to make this easier in combination with greater information from the boiler manufacturers will open up solar water heating to the benefit of even more people.