Why your new gas boiler isn’t efficient.

March 19, 2020 - 6 minute read

Why Your New Boiler Isn't Efficient

‘A-Rated’ condensing gas boilers were made mandatory by UK law in 2005, and approximately one million domestic gas boilers have been fitted each year in the UK since this change. The government proposed this as a massive step towards energy efficiency. Having fitted my first condensing gas boiler in 2003, I look at why your new gas boiler isn't efficient.

During the combustion process, gas is mixed with oxygen and burnt to produce heat, carbon dioxide and water (in the form of steam). Unlike older boilers, a 2020 condensing boiler is capable of recovering at least 93% of the energy from the combustion process. Simple right? Sadly Not! I wish it was a case of fit and forget, but it's slightly more technical than this.

Condensing gas boilers need the return water temperature from the central heating circuit, below the dew point of the flue gases to be efficient. Changing phase from steam to a liquid (condensing), releases the additional energy present in the steam (called latent heat). This energy is lost to atmosphere if not captured by the boilers heat exchanger, and normally seen as large amounts of water vapour or 'plume' exiting the flue. Many people believe the 'plume' to be a sign of high efficiency, but if this is excessive, you are witnessing the latent heat being lost to the outside air. The latent heat is not to be sniffed at either. A condensing boiler running without condensing, would be allowing at least 8% of the energy from the combustion process to escape out of the boiler flue.

Looking across social media platforms, lots of installers are retrofitting condensing gas boilers to existing heating systems, without maximising the efficiency of the new condensing boiler. Most boilers I see are being ‘swapped’ in a day, with very little thought about changes in radiator output, flow temperatures or correct flow rate balancing. The fact is, a condensing gas boiler with a return temperature of 50°C would only be circa 89% efficient (89p recovered for every £1 you spend), however the majority of condensing boilers have return temperatures much hotter than this, due to mismatched flow rates and high return temperatures. A condensing boiler needs the return temperature to be as low as possible, and at 30°C a condensing boiler would theoretically obtain 95% efficiency. Condensing boiler manufacturers typically state two outputs in their manuals; one at higher temperatures (70/50) and one at lower temperatures (50/30) to reflect the energy available at different flow and return temperatures.

Boiler manufacturers knew of the issues we would face when retrofitting condensing boilers and designed them with a larger 'temperature differential’ between the flow and return than non-condensing gas boilers. This is what we call the ‘Delta T’ (ΔT). Condensing boilers are typically designed to a ΔT of 20°C. This means the water flow rate through the heating system is much slower, pump sizing can be smaller, and pipework can be smaller verses a boiler built pre-2005. Having less flow rate allows the water to stay in radiators longer to exchange the energy the boiler has put into the system. When fitting a condensing boiler to an existing ΔT11 system, low flow rates can cause suspended magnetite sludge to literally fall out the water it's in and a need to ensure existing heating systems and radiators are cleaned properly is paramount.

When discussing heating system efficiency, heat emitters have a big part to play too. Radiators are the most common heat emitter in the UK. Cheap, simple and reliable. However, radiators are traditionally manufactured and tested to standards that existed long before condensing boilers. If you ever look at a radiator data sheet, it will give the radiator output in Watts or BTU's and reference ΔT50°C. This basically means the radiator needs a flow temperature of 75°C to the top, a return temperature of 65°C from the same side bottom connection (what we call TBSE or Top Bottom Same End) and at a room temperature of 20°C would give the stated output. The problem is, none of this applies to condensing gas boilers! A condensing boiler needs its return temperature below 54°C to obtain any of the latent heat I wrote about of before. So, if we take a 1000W radiator at ΔT50°C and adjust the radiators output for 70°C flow, 50°C return and 20°C room temperature (ΔT40°C), it would only be 750W which is 25% less heat output. This could be a problem, when the room at -3°C outside needs 1000W to heat it. But remember, a return water temperature of 50°C still means the boiler is only 89% efficient, which is some way away from the boiler manufacturers stated outputs of 93% ErP.

So how do we make things better?

Current heating theory works on the principle that we design heating system output based on the worst-case scenario heat loss of a property. In the UK, this is typically at -3°C but means the system is always oversized whenever the temperature is above this value (which is typically 50 odd weeks of an average year). Designing to a temperature like this, means we need to get things 100% correct to prevent energy wastage. There is a desperate need to accurately size our heat emitters and boiler to the actual load of the property without guessing. I'm pretty sure the majority of installers in the UK are guilty of oversizing at some point in their working life. It's not really installers fault though, when you think the smallest and most popular combination boiler in the UK is 24kW. To put that into context, 24kW to space heating is four times too much for the average domestic property at -3°C.

A simple fact is the heating industry has got lazy, and laziness means efficiency losses at a time when energy efficiency couldn’t be more important.

Discussing boiler and heating system efficiency on social media, often means installers views are not always aligned. However, when we start to think about how we make things better, those in the know all have a similar view. The methods discussed are weather compensation, load compensation and correct balancing of heating systems.

Weather Compensation works on the principle that the boiler has a sensor that detects the outside air temperature. As the outside air temperature falls, the boilers flow temperature is increased using a correctly determined ‘curve’ (set by the installer) to allow for the increase in heat lost from the building to be replaced. This means when outside temperature is warmer, there is less heat loss and the boiler runs more efficiently by significantly reducing its flow temperature.

Load Compensation works on the principle of internal temperature reference (instead of external temperature), with the most advanced systems accurately mapping the heat loss characteristics based on the rise and fall of room temperatures. When the set point internally is dramatically different from the room temperature, a typical load compensation system will call for the maximum designed flow temperature of the boiler (say 70°C if the system is a 70/50 designed system). As the room temperature nears the temperature set point, the boiler is told to reduce its flow temperature to maintain the air temperature and this could be a significantly lower flow temperature, such as 40°C.  Multi Zone Load Compensation goes one step further to Load Compensation. This system has multiple room sensors and normally smart radiator controllers. The radiator controllers are extremely accurate can be adjusted to different room temperature profiling (therefore preventing energy wastage in heating zones you are not occupying) and they can also vary the flow rate to each radiator, dynamically adjusting the radiator output to match the exact heat loss requirement of the room. 

Setting radiator flow rates (radiator balancing), prevents the boiler putting too much energy into the heating circuit. This is one thing always overlooked, but in combination with Weather or Load Compensation is the absolute best way to ensure energy efficiency of a heating system. The method of achieving balancing is to obtain the correct Delta T across the flow and return pipes of the radiators in the property, but this method can be hit and miss depending on how good the radiator valves are and room/external temperatures at the time.  Some manufacturers are making balancing easier to achieve, with products like IMI Hydronic Eclipse TRV’s. These radiator valves have built in flow rate setters and when used in conjunction with IMI Hydronic's HyTools App, you can quickly determine and set the correct flow rate to each radiator.

There is always scope to improve new and existing heating systems and with promotion of industry best practice, installer training, innovative products and consumer education, I believe there will be a demand for quality heating installers who can produce well-designed and efficient heating systems for many years to come. To quote Benjamin Franklin

“The bitterness of poor workmanship remains long after the sweetness of low price is forgotten”


Author - Richard Burrows, Director, The INTERGAS Shop


To find out more on condensing boiler theory, please see Heat Geek’s amazing reference website - Condensing Theory, How To Maximise Domestic Condensing Boiler Efficiency

The Heating Hub is a great consumer resource for all things heating and partner with us in promoting efficient boilers and systems. More details on our involvement and our work to help develop 'The Healthy Heating Standard' can be found here - The Expert Panel

If you are a heating installer and want to improve your heating system knowledge with training on hydronic design, please visit Heating Academy Northampton’s website for course dates - Heating Academy Northampton Course Dates

Need accurate and indemnified heat loss calculation software? Please have a look at the Heat Engineer website - Heat-Engineer.Com

The BetaTalk Podcast is a one of my favorite listens and Nathan regularly features manufacturers and the view of installers at the coal face of our industry. Please enjoy the BetaTeach Podcast Season 2 here - BetaTeach Season 2 Podcast

If you are a qualified heating installer and interested in efficient heating system design, specification or controls training, please come and join our Facebook group - The Delta T Forum