The secret life of boilers – how your boiler misbehaves

31 March, 2020

George Bennett

Reading time: 4 minutes

Part one of our blog series called ‘The secret life of boilers’ by George Bennett examines how your boiler misbehaves.

This is part one of our blog series, The secret life of boilers, by George Bennett. Have you ever considered how your boiler misbehaves?

Click, clack, whirr, tick tick tick, whoosh. The familiar and comforting noises that announced that the boiler had sprung into life and was about to give warmth round my home. But what is really going on inside the casing of the boilers that are in most of our homes, how can we get the best out of them and what might we do with them in the future?

Boilers have come a long way since the days of pilot lights and open flames, modern boilers’ innards are sealed from the room, and they can ‘modulate’ their output, giving more or less heat. But understanding what the heating system is doing from the lights, digital displays and noises can be difficult.

Let’s start with the traditional thermostat, the box on the wall in the hallway or living room normally. The thermostat checks the temperature and decides if it is what you requested, either through a direct setting or a daily/weekly programme. If it sees the temperature is too low the ‘click’ happens and the boiler is alerted that heat is needed. At this point the boiler takes up the baton and goes through a set start up sequence, also known as ‘firing up’. A carefully programmed sequence of pump running, fan whirring, gas valve opening, and electrodes sparking will result in gas burning inside the heat exchanger and resultant hot water pumping round the radiators. All you have to do is sit back and wait for the warmth to fill the house until the thermostat senses the room is warm enough. Things are going well, the radiators are hot and the rooms are warming up. But wait, the boiler seems to have switched itself off, and a quick look at the thermostat shows that the house is not warm enough. Over the next hour the boiler goes through this confusing cycle of firing up and switching off. This is a common behaviour of boilers across the country (Bennett et al. 2018) and is not good for the life of your boiler (switching things on and off generally wears them out) or for your gas bill (cycling lowers efficiency) (Bennett et al. 2016).

The cycling that so often occurs is a symptom of mismatched and contradictory forces at work in the heating system as a whole of which the boiler is just one part.

Firstly, imagine the link between thermostat and boiler. All the thermostat did was tell the boiler to fire up, not how hard to work: faced with this ambiguous signal the boiler does the best it can and aims to heat the water going to the radiators to the temperature set in its own controls (often set via the boiler), and because it knows you are cold, it tries to do this as quickly as possible. Often this is a safe and sensible strategy but if radiator valves are closed, or the internal temperature is already close to what you want, then the boiler may heat up too fast to be able to give all the heat to the house, and then decide the best option is to switch off before components are damaged. If you tried to boil milk with such a simple strategy, it would undoubtedly boil over. Boiler safety systems prevent the situation getting out of control, but the result is confusing.

More modern thermostats are capable of not just switching the boiler on with the familiar ‘click’ but actually tell the boiler how hard to work, overcoming the thermostat / boiler misunderstanding just described.

There are many more contradictions in heating systems which can make it seem like the boiler is misbehaving – luckily boiler efficiency is relatively robust to the imperfections in our central heating systems. However, as the need for low carbon heating increases then we should take more care about getting the most from our boilers and preparing our heating systems for high efficiency. We can already insist on radiator balancing during a boiler service, upgrade the heating controls by replacing simple thermostats, install thermostatic radiator valves and consider replacing old radiators with larger ones.

The whys and the wherefores of how all these little things add up will be part of this blog series. When heat pumps and next generation heating systems permeate our homes, then we will be well prepared to get the best out of them too, saving our pockets and the environment.

Part 2 of the series investigates how to burn less natural gas at home.


Banner photo credit: Alexander Raths on Adobe Stock