Modern gas and oil boilers have to meet minimum efficiency standards – in practice that pretty much guarantees that they have to be condensing boilers. But what does that mean exactly?
Condensation is the change in state – or phase – of water from vapour to a liquid, a process that happens once the water vapour loses enough heat to the surrounding cooler environment. You often see condensation on the inside (and sometimes the outside) of your windows, as these tend to be the coldest surfaces in your house.
The exhaust gases from your boiler contain a lot of water vapour, being one of the main byproducts of the process of combustion of natural gas. Contemporary boilers are able to capture much of the latent heat energy tied up in the water vapour that would otherwise be lost from the flue, by condensing the vapour into water (this is then drained away outside), with the heat released from the condensation process then used to help heat the water for your heating system.
This is all well and good of course, however the boiler needs to operate in a condition that allows condensation to occur, otherwise the heat will be lost and the boiler will operate less efficiently. Unfortunately, many boilers are not set up optimally, resulting in wasted energy and elevated gas or oil bills.
The key parameter that determines whether – and how well – a boiler can condense is what is known as the return temperature. This is the temperature of the water returning to the boiler once it has passed around the central heating system or the primary circuit to the hot water cylinder. If the return temperature is too high then condensation cannot occur and latent heat will be lost as vapour out of the flue.
In this case, the magic number is around 55degC, give or take a degC – condensation will not occur at all above this point, and boiler efficiency will increase from 88% towards 100% the further the return water drops below this temperature. As a result, it makes sense to keep the return temperature as low as possible; in reality, boilers often run far too hot, typically turned up to MAX (75-80degC) with the return temperature rarely if ever dropping the boiler into condensation mode.
At this point we need to look at the type of boiler you have. If you have the ability to control the heating and hot water temperature separately with two dials or digital controls then that’s good. The heating temperature can be turned down as low as possible for the weather conditions whilst still providing enough heat. This will be a matter of trial and error but 40degC can be enough in autumn/spring, and no higher than 60degC in the depths of winter. This process can be automated if your boiler has, or can be fitted with, a weather compensation controller. Weather compensation reduces the flow temperature of the boiler in milder weather to maximise efficiency whilst still providing enough heat output; when the weather gets cold then it raises the flow temperature to counter the additional heat loss.
Cheaper boilers often only have a single temperature control (if that!) and that’s somewhat problematic as you have to ensure the water is hot enough to heat your cylinder up to an appropriate temperature, usually at least 45degC, meaning that the boiler should be producing hot water to at least 50degC for sufficient heat transfer to occur to satisfy the cylinder thermostat. If there are any concerns around the risk of legionella, for example there are vulnerable people in the house, then the boiler should produce hot water to at least 65degC to kill off any potential bacteria, although the risk in practice is generally extremely low.