Should I Leave My Heating on Low All Day?

June 26, 2021

Here at Heatsy we’re all about energy conservation and trying to waste as little as possible while simultaneously trying to get out as much as possible.

The question, Should I leave my heating on low all day? Is one which has been hotly debated through the ages. It’s right up there with life's most important and philosophical questions, why are we here? If a tree falls in the woods and no one is around to hear it does it make a sound? And of course, why did the chicken cross the road?

Just like these head scratchers the answer to, Should I leave my heating on low all day? Isn’t a straight forward yes or no, it depends.

Forget about the boiler and heating for now and let's focus on you and your lifestyle. If your home is empty for most of the day and you generally only use the heating for an hour in the morning and an hour in the evening then leaving the heating on all day is probably wasteful, if however, you have a house which is constantly full, people are always turning the heating up and down and the boiler is going on and off every few hours then it’s a bit more complicated.

Think of your car, you start the engine, put your foot down, and the engine starts, you hear the engine revving, you work your way through the gears, you reach your desired speed then everything calms down and you quietly cruise along.

Your boiler works in a similar way, when it receives a demand for heat and it starts from cold, it goes straight to high fire furiously trying to heat not only the water but also the cold components inside the boiler ,the first 30 - 60 minutes are when your heating is at its least efficient, then it reaches its desired temperature and modulates down to a nice calm low flame to just maintain the current temperature - it’s what happens next which is debated.

When you reach the end of your current heating period should you let the boiler go off and all the heat it's produced go away or, set the controls to maintain the heating at a lower temperature until the next heating period begins?

Well, if you have an old boiler, you’ll likely have old controls that operate as an on / off switch, without the ability to automatically set a low temperature in between heating periods.

You could fit modern controls but in honesty the improvement when fitted with an old boiler is negligible, you would be better off waiting until you replace the boiler and get new controls at the same time. In this scenario the best thing to do is open all the radiator valves fully, turn the stat on the front of your boiler down to about 2 or even 1 and set your room thermostat to 19 degrees, providing you have some form of insulation the rate at which the heat escapes should be lower than that at which it is being produced and the house will gradually reach a nice comfortable temperature.

If you have a newer boiler with energy efficient smart controls you might notice that you don’t have an off period, you have on and set back. Set back has replaced the off period for one important reason, the boiler is at its least efficient when it comes on from cold so, we want to avoid the boiler needing to come on from cold.

It's also a lot easier to raise the house temperature slightly than it is to heat a cold house and the boiler itself suffers far less wear and tear so should last longer with fewer breakdowns.

Depending on whether you have a smart thermostat, open therm control, load compensating thermostat or weather compensating thermostat (to read more about each stat and which is best for you click here) you’ll have slightly different features and energy efficiency gains but whichever you have if it’s modern and not a traditional on / off stat you should be able to set a setback temperature.

The temperatures you’ll want to set your thermostat and boiler to will be partly specific to your home and heating system and partly set to your personal preference, we advise as a rule of thumb to have your heating periods set to 19 degrees and your set back period to 15 degrees so your heating timetable could look something like:

  • 8am - 10pm 19 degrees Heating period
  • 10pm - 8am 15 degrees Setback period

Contrary to popular belief having your heating ‘on’ all the time doesn’t mean the boiler will be on all the time, when the heating period ends, and the house is at a comfortable 19 degrees the heating demand will be satisfied and the boiler will turn off but crucially it will not be allowed to go cold.

When the temperature in the house drops to the setback temperature of 15 degrees the boiler will come on but instead of going straight to high flame to try and quickly heat everything from cold it will go straight to a low flame to just maintain the current temperature and when a new heating period begins it will slightly increase its flame to slowly bring the house back to the desired 19 degrees and back to a nice low maintenance flame to maintain the desired temperature, eliminating the need for a high flame and ensuring the boiler is always operating at its most efficient.

The temperature you want to set the boiler stat will depend on the age of your heating system, old systems were designed to have a delta T or temperature loss across the heating system of 11 degrees while modern systems and boilers are designed to operate at DT 20.

The reason newer systems are designed with a DT 20 is because they want to extract more heat for the system and a return temperature of anything above 40 degrees will mean your boiler will struggle to condense as the water is too hot and the boiler will not operate at its most efficient.

Therefore, simply buying a new boiler with new controls and fitting them to a 30-year-old heating system is often not enough to fully benefit from the new technology, it’s like taking a Formula 1 car to the supermarket on a Saturday morning and telling everybody the cars useless because you can’t fit the kids and the shopping in.

If you want to fully benefit from your new boiler, you’ll need to put it in the environment it was designed for, that might mean installing slightly larger radiators, increasing the pipe size below the floor, it will almost certainly mean thoroughly cleaning the heating system, then you can see how that ‘one day boiler swap’ when done properly quickly becomes a two, three or even four-day heating installation.

The design of the system plays a major factor in the efficiency of your heating if you have a modern Vaillant ecoTec plus boiler, installed on a properly cleaned and upgraded heating system with amply sized radiators being controlled by modern weather compensating thermostats such as the Vaillant Senso range or the Vaillant VRC 700. The system is properly commissioned, and the correct settings chosen your new Vaillant boiler will happily run all day with a flow temperature of between 40 - 60 degrees and a return temperature of 20 - 40 degrees. your boiler will constantly be in condensing mode and your home will be effortlessly comfortable.

Another key factor in how you choose to use your boiler is the insulation of your home, keeping the heat we have already created is more efficient and so preferable to going through the process of producing the heat again.

Cavity wall insulation, loft insulation, double or triple glazing, draught proofing, thick insulated wall paper, carpets instead of wood floor all these home improvements vastly improve your homes green credentials and help it to retain much of the heat your system produces which means leaving the system on a setback or ‘low temperature’ all day will really be more efficient,

At Heatsy we’ve been in the heating industry for a long time and have seen many changes, some more welcomed than others. Recently we’ve been seeing a lot of terrible installations with grossly oversized boilers chucked on the wall and if the radiators are hot there seems to be no consideration for the efficiency, suitability, or life of the boiler, how could there be when the quote was generated by a computer program.

Thankfully though we’ve noticed a recent increase in engineers and companies trying to increase their knowledge on subjects like hydronic heating design and energy efficiency to be able to offer their customers a level of service most ‘boiler swap’ companies can’t compete with.

We for one welcome the latter.

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Please remember, we’re heating experts so if you need your boiler repaired, a new heating installation or if you’re struggling with anything we’ve discussed here feel free to get in touch with us now so we can see how our solutions might help you.

Thank you for reading.