Why our condensing boilers do not condense...
IN THIS GUIDE
How they work
Non-condesning boilers could lose upto 40% of the heat they generated via the flue, whereas condensing boilers can recover most of that heat. Condensing boilers work by capturing the heat at the point it is leaving the flue and turning it back into water (hence the term condensing). The water runs back down the flue and pre-heats the boiler (this is why condensing flues are fitted at a slight incline).
Condensing boilers need to operate at lower temperatures than non-condensing boilers in order for the heat to turn back into water at the point it exits the flue. When the temperature of the water in the central heating system as it returns to the boiler if less than 54 degrees C, the boiler will run in what is known as ‘condensing mode’. This is when the boiler recovers its lost heat.
Even this is a sliding scale however. At 54 degrees C the boiler is 87% efficient. The boiler does not reach 90% efficiencies until the return temperatures are around 45 degrees C.
All central heating systems work by sending hot water out into the radiators in one direction (called the flow) and receiving back after it has travelled around the radiators (called the return). A non-condensing heating system is set up to deliver heat to the radiators at 80 degrees C. As the water ‘flows’ through the radiators about 20 degrees C of heat is given off to the room. Water ‘returns’ to the boiler at 60 degrees.
For a condensing boiler to get 90% efficiencies. water must exit the boiler at a maximum of 65 degrees, impart 20 degrees of heat to the room via the radiators, and return at 45 degrees.
The problem is, most boilers are still set up to output at 80 degrees and return at 60 degrees. The boiler still works, and therein lies the problem, but it does not condense and it certainly does not reach 90% efficiencies.
Why are efficiencies lower in practice?
A 12-month study undertaken by the Energy Saving Trust of boilers in UK households found that A-rated condensing combi boilers were on average 83% efficient and heat only boilers 80.3% efficient. The report concluded that “the in situ performance of the boiler is significantly less than the rated SEDBUK seasonal efficiency”. There are many reasons for such a large discrepancy between actual efficiencies and stated efficiencies:
A Carbon Trust study into boiler efficiency found that “a significant number of them [boilers] were found to be substantially over-sized for the properties in which they were fitted and this is believed to be common practice in the UK”.
The report when onto highlight market failures that encouraged oversizing when it identified that “the average peak heat load of UK houses is around 6kW, but the size ratings of new boilers typically range from 10kW to 30kW”. In other words, oversized boilers send the wrong messages to consumers and installers that the smallest in the range must be for the smallest home. The smallest in the range will actually heat the vast majority of UK homes.
As a nation we have grown so accustomed to larger boilers, particularly combi boilers which show outputs of 24-50kW, that we now push for bigger boilers, just in case. However oversized boilers are at risk of wasting gas from excessive energy dumps of heat that cause the boiler the come on and off repeatedly.
When boilers fire up, they automatically ramp up to 70% of their maximum output. For a 21kW boiler (common is most homes) this would be around 15kW. Given that only 6kW is needed on a very cold day, the boiler is always going to overheat the system. When the radiators cannot get rid of enough heat the return temperatures to the boiler are too high for it to condense.
The answer in the short term is to reduce the maximum output of the boiler down to meet the maximise heat requirement of the property (e.g. 6kW). So when the boiler fires during periods that need less than 6kW, it is only to 70% of 6kW.
This is done by the installers, however it is probably not reduced enough. Moreover, the graduation towards bigger boilers means that the lowest minimum output of larger boilers can be between 8-12kW. This means they will always be too big for the heating systems they serve.
Basic and incompatible heating controls
Basic controls turn the boiler on when the room temperature drops below the thermostat temperature and switches it off when the room temperature is reached. These were used with non-condensing boilers. In reality the temperature of the room continues to rise even after the boiler has switched off because there is latent heat in the radiators that the thermostat cannot account for.
More sophisticated controls tell the boiler to reduce its output before the room temperature is met which means it meets the desired temperature exactly. These are known as load or weather compensation controls and save an extra 3-5% in gas when paired with a condensing boiler.
The Energy Saving Trust report observed that “No boilers are recorded as having load compensation”. In other words, all of the condensing boilers in the Energy Saving Trust study used basic on/off controls, which do not maximise the efficiencies of condensing boilers and would have been a significant factor in much lower average efficiencies of 83% and 80.3%.
Boiler Plus was introduced in 2018 and aimed to lift up the control standards for boilers. Load and weather compensation were included, but the regulations were significantly diluted by the inclusion of ‘smart’ on/off controls under the ‘automation and optimisation' category.
As consumers we all believe that smart controls are smart because they are efficient. But if they do not speak the same language as the boiler - so that they can vary their output up and down - then they are really only an on/off control with some remote programming.
Balancing ensures that 20 degrees of heat is imparted from the radiators to the room. The temperature of the radiator is measured on one side and then the other to check this is happening. If water departs the boiler at 65 degrees and returns at 45 degrees, this is a perfectly balanced system.
The problem is that systems do not stay balanced and radiators are often the wrong size. As soon as less than 20 degrees C is dissipated into the room the efficiencies drop.
Thermostatic radiator valves (TRVs) can add to our woes. Standard TRVs will shut down as individual rooms meet their temperature, but there is a delay in the boiler reducing its output (made worse if the boiler has not been, or cannot be, ranged rated).
How can I tell if my boiler is condensing?
Unfortunately your boiler cannot tell you the amount of time it spends in condensing mode. One of our recommendations to the UK Government’s consultation is for the aggregated efficiency of boilers to be digitally displayed on the boiler – see our blog on the Future Homes Standard.
In the meantime you can look at the flue emissions. Contrary to popular belief, a sign that a condensing boiler is operating in condensing mode is an invisible discharge from the flue. A boiler is NOT condensing when white plumes are exiting the flue. This is a sign that the energy is not being recovered as the heat is lost to outside.