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.


  1. Hi Stuart - I would add a couple of comments on this subject.

    When I looked at one of the well known combi diverter valves there was a mode of operation whereby if the solar cylinder was not at temperature it would mix hot water from the combi with tepid water from the cylinder - which adds an even higher chance of legionella contaminated water passing through to the taps, as this won't have even passed through the combi so there is even less chance of safe temperature being reached. This is not how you have described in your explanation which makes me think that they may have revised the design since then - it was some time ago. However there may also be some designs out there which do not match the logic you describe?

    Anyhow there is also a third method for using a combi boiler which I have never heard anyone else mention before. You simply block off the dhw output and use the combi as a system boiler to heat a thermal store. You can nearly always use the solar controller to trigger heating the thermal store using some of the functionality supplied as standard on many controllers. Hot water is then supplied via the DHW coil in the thermal store, and the heating circuits are also taken off the thermal store.

    Of course this does fall fowl of the RHI regulations on the solar thermal system only supplying heat to DHW rather than space heating - yet another reason why this reg needs changing. Its such a simple solution, and does not require any complicated valves that may fail, and I can't see that it would risk taking the combi boiler out of warranty either.

    Finally - another possibility rather than an STA database would just be to force manufacturers to sell combis that can accept pre-heated water! It's not exactly a high cost implcation given nearly all other boilers are designed to modulate. The only thing which makes manufacturers not add this functionality is cost - and who wants a cheap as chips boiler made to the tightest possible budget? OK don't answer that I know : )

  2. love the blog detail and simplicity....I have a 3kW PV array and a Worcester Bosch 25CDi are my components compatible, would this project be suitable for a DIYer? willing to be a case study volunteer or are there installers in Gloucestershire? thanks Neil

    1. Hi Neil

      I checked the compatibility of your boiler on the HHIC database Worcester Bosch list and it seems to me that this model can accept hot water up to 60C with the addition of a solar accessory.

      Installation instructions for Pod are here:

      So you can decide whether your DIY skills are up to scratch for this!

      Alternatively you can get in touch with Viridian Solar directly we can put you in touch with installers or go through the installation for your area.
      01480 831 501


    2. Thanks for this Stuart. I have the same query as Neil Cook so will contact Viridian for more info, especially a guide price for this kind of system!
      Bob (Surrey, UK)

  3. Hi i can not find any info on why i need this solar accessory i have the 28cdi and i spoke to someone at the plumbers merchants he said i should be able to heat the cold feed from another source so i phoned worcester and they said the same oh but you should use this solar accessory but couldent tell me what it does do you know of any info on how to fit one i understand i need to mix the water 60 degrees before the boiler any help would be appreciated


    1. You only need to mix the water to make sure it doesn't go over 60C, the boiler will accept any temperature up to that point and top up to the required heat.

  4. I have been in contact with Worcester Bosch recently as I'm just having a new boiler fitted and wanted to make sure it would be possible to add a solar preheat to it later on.

    The 'solar accessory' consists of replacement brass parts for the plastic cold water inlet on the boiler so it can cope with 60C and it also adds a temperature sensor so that if the preheated water is within 5C of the boiler DHW set temperature then it won't fire up at all, preventing short cycling and effectively passing solar heated water straight through to the taps. If it's more than 5C below the set temp then the boiler fires up to top up to the set heat.

  5. I realise his is an old article but I've only just found it. Does anyone know of any UV irradiation systems that could be used with a solar heating/combi boiler system to control legionella, rather than relying on pasteurisation. I presume something could be fitted in to the pipework but I was wondering if there were tank fitted options?

  6. all gobelldygook, to me, why cant you talk english , i need to separate my solar from my combi because, the boiler wont fire up untill the temp is down to 18 degrees, so when its at 40 degrees, i have to run all the water off to get some hot water, can i take my tank away from going in to the combi boiler and just have a single tap in my bathroom that i can turn on when its hot enough...

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  9. hey, thanks for sharing this info as an experienced service engineer in one of the best Combi boiler company i have seen most of the people worry about price of combi boiler this info will help them to purchase suitable boiler the way your explaining is good keep going

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  12. Excellent! Just provided me with nearly all I want to know ! Now just need to know what sort of heater goes in the tank and how I control this. I also have a Samsung battery in system.
    Many thanks

  13. This is excellent - just the info I have been looking for. Thank you.

  14. Im looking for a system where i can just raise the temp of the incommimg water supply via an electrical heater to feed the boiler, so if the water temp entering is even slightly higher than supplied should save me on gas as boiler doesnt have to work as hard to get to 60deg . is there anything out there ? i dont want to store hot water in a tank to feed etc