Condensing boilers- do they do what it says on the tin?

2nd Mar 2017


For twelve years; since April 2005 when revisions to the Building Regulations came into force, stating that all replacement gas or oil boilers must be a condensing type, there has been debate - do they actually do what it says on the tin and condense in-situ?

If you ask a registered gas engineer they will advise that frozen condensate pipes and the plume produced by the boiler - especially when the condensing boiler was first introduced- is simple evidence that the boiler is condensing. Today manufacturers have made improvements in their designs and instructions to resolve the issue of frozen condensate and there are also various solutions to manage boiler plume.

Latest boiler heat exchangers are more efficient!

Whatever your opinion, findings released in a report in November 2013, from the Department for Environment, Food and Rural Affairs (DEFRA) entitled: UK’s Carbon Footprint 2004-2011, indicate that the UK‟s total carbon footprint, including other greenhouse gases (GHG) fell by 14 per cent between 1997 and 2011, with a peak in 2004.

It is my opinion that our united approach to condensing boilers has been fundamental in reducing our carbon emissions. Non-condensing boilers lost valuable warm waste as the gases travelled through the boiler's flue and escaped into the outside atmosphere, taking valuable heat with them. They varied in efficiency from around 70-80%, depending on their age and the design of the boiler. 

Modern condensing boilers extract additional heat from the waste gases by condensing the water vapor produced to liquid water, thus recovering its latent heat. A typical increase of efficiency can be as much as 10-12%. 

Manufacturers use the latest 3D design techniques to produce high performing heat exchangers. Software is available that can replicate the water flow patterns in the heat exchanger to optimise the design, even before it is put to the test in an appliance. These heat exchangers play a large part in the boilers performance and todays designs are more efficient, even without the added benefit of the useable heat that is extracted by a secondary heat exchanger using the waste gas.

Ecodesign and performance testing

European committees have produced performance standards for testing condensing boilers which allows for the comparison of different manufacturers products and provides the information for the energy efficiency labels. These performance tests are independently checked by a recognised notified testing body in their laboratory. More recently the government department BEIS have decided to use the data from the European testing standards in their Standard Assessment Procedure for new build premises, recognising that they are the most suitable method for measuring the efficiency of a gas condensing boiler.

So what about when the boiler is part of a system?

The lower the return temperature to the boiler the more likely it will be in condensing mode. If the return temperature is kept below approximately 55°C (131°F), the boiler will be in condensing mode and working at its most efficient.

What else helps the boiler to condense?

Many boilers now operate on a wider modulation ratio which can improve efficiencies and comfort while reducing wear and tear on components. The modulation ratio is a boiler’s ability to reduce its maximum output to its minimum output, in line with system demand. For example; a boiler has a maximum output of 30kW and a minimum output of 7.5kW its modulation ratio is less than 4:1. A boiler with a maximum output of 30kW and a minimum output of 3kW has a wider modulation ratio of 10:1. A wider boiler modulation ratio reduces constant on/off cycling which can improve efficiency and boiler longevity.  Combi boilers can also benefit as the stability of domestic hot water (DHW) delivery can be improved by the modulation ratio.

Heating controls also play their part in ensuring that the return temperature is below 55c and have developed rapidly over recent years using the latest technology available. Manufacturers have developed more advanced controls such as load compensation that reduces the water temperature to the radiators as the property reaches the required temperature and less heat is required.

Weather compensation is also available which monitors the outside weather using an outdoor sensor. With this information the control adjusts the temperature of the hot water going to the radiators. If the temperature outside drops then the temperature going to the radiators will increase. Likewise, if the outside temperature increases (here’s hoping) then the feature will ‘tell’ the boiler to supply less heat, and so enabling energy efficiency gains along with comfort levels.

Heat requirements in different properties will vary, this results in different system designs which may also include thermostatic radiator valves. Manufacturers have established that a variable speed modulation pump is a good way of maintaining a steady pressure for these systems which helps to maintain the efficiency of the system and has the added benefits such as reducing noise and the electrical energy the pump uses.

It’s not often that outside temperatures are below freezing- believe it or not

It is also noteworthy that most UK heating systems will be designed to an external winter temperature of -1° or -3°C whereas average winter temperatures are well above freezing level. This means that the boiler is rarely required to run at high temperatures, taking into account modulation and effective control it will certainly be the case that a condensing boiler will operate in condensing mode.

We have seen much in the car industry about performance testing but the heating industry does review its testing criteria at a European level and includes independent testing of the appliances. What we have to remember is that the installer plays a big part in specifying the right product for the application, ensuring the system is fully clean and balanced as well as controls that can save the consumer money by sending the right information to the boiler. Consumer education is also something that we have to continually work at to explain that radiators don’t need to be excessively hot to maintain the required comfort level and that lower temperatures will save them money.

So in answer to the original question- do condensing boilers condense?

Yes they do. Technology continues to progress and Installers have embraced new technical developments and the fundamentals of a well maintained system.  We need to ensure that we keep working to educate consumers so that they understand how to achieve the highest possible energy saving rewards which has made the condensing boiler such an outstanding success.