What’s the most efficient way to run my central heating boiler?
Do I set the temperature to a high value i.e. >70 degrees with the idea that the house will get to its thermostat setpoint quickly?
Or is it better to run it at a lower setting, say <65 degrees and for it to cycle on and off more often?

Can of worms that, dependent on the rest of the system and the house (build, insulation etc). Experimentation is the way forward but the outside temperature variation has a far greater effect so coming to definitive conclusions is difficult.

The heat exchanger in your boiler will be more efficient with the water at a lower temperature. Selecting a lower temperature will also mean the burner changes to the lower gas flow rate sooner, the lower flow rate being more efficient than the high flow rate. Lower water temperature also reduces temperature yo-yoing due to having very hot water in the radiators when the thermostat switches the boiler off.

I’ve now removed the central heating in my home but the last year I used it I found the lowest temperature possible, 33°C, resulted in the lowest gas consumption and the most regular house temperature.

33 degrees? Was that during a Provencale winter or an english Summer? 😉

An Aquitaine Winter which is about 5°C warmer than the livable bit of the UK on average. The house is quite well insulated.

Edukator, is that rule regarding being more efficient at lower temperatures good for condensing and non-condensing boilers? I was always under the impression that non-condensing ones were more efficient at the upper end of the temp scale.

Also, is that right about being more efficient with a lower flow rate? On my boiler I can reduces the max modulation, never thought it would increase efficiency. Might have a play.

It’s just based on the efficieny of heat exchangers which in general tranfer more heat from the burning gas to the fluid at lower gas flow rates and with a lower temperature fluid being circulated.

Whether your boiler is condensing or non-condensing, lower flow temperatures will be more efficient since the boiler is doing less work to maintain the system temperature above ambient.

The flip side will be needing larger emitters to give lower flow temperature adequate opportunity to heat the space. Upping flow rate also helps with this, buts its less important the higher flow temperatures go.
You’ll likely need to run the system longer to allow the system to deliver enough heat to satisfy your comfort needs, but your consumption should still be lower.

Ah, I see. Never knew that.

So, if I run the boiler at 50c for longer it will use less gas than running at higher temperatures for a shorter period of time?
(This is assuming the same room stat setpoint and good insulation)

Yes.

In addition improving heat exchanger efficiency circulating lower temperature water has other advantages:

When the burner switches off the heat exchanger starts cooling the water in the system with the help of its external source of cool air.

If you have thermostatically controlled radiators the water in the radiators is cooler than the water being circulated through the heat exchanger and pipes which are often poorly insulated and run under the floor or through the roof space. Circulating very hot water you are thus maximising losses from the circulatory system while the thermostically controlled valves limit losses where you most want them – through the radiators to heat the house.

Its also better to heat the structure of the property, by heating the volume of air contained within. So letting it cool will take more energy to reheat it. So keeping it at a lower reasonable temp then top up is better than hot cold hot cold.

Be careful running some boilers on low temperatures though as condense can from in the heat exchanger and rust them out.

But generally good advice except as mentioned the lower the temp the bigger the emmitter required.