Energy myth-busting: Is it cheaper to have heating on all day?

malc_b

Cardew wrote: »

Depends if the boiler is a condensing boiler or an old non-condensing boiler.


With condensing boilers the return water temperature should be low - below 55C - for the boiler to stay in condensing mode. So set as low as possible, whilst still keeping the CH warm enough for your requirements.


With a non-condensing boiler there is very little difference in efficiency at any of the set water temperatures.

Whilst I agree and you are correct for CH it should be pointed out that it is recommended that HW be at least 60C so you can't set CH too low if you are using it to heat HW.

richardc1983

I've updated the links below as they were not working for some reason and had broken when I put them on my post. They are now working and take you to the right articles.

richardc1983 wrote: »

Some professional advice for you here regarding efficient running of a modern condensing boiler system...

Having the flow temp (how hot the radiators feel) set higher means that the rooms will warm up faster. If your comfortable at e.g 21C setting the room thermostat higher e.g 30C WILL NOT mean the rooms warm up any faster as the temperature still needs to get 21C first. Only the flow temp of the radiators will affect how quick the rooms warm up to the set temperature on the controller or the thermostatic radiator valve (TRV).

Modern condensing boilers are most efficient when the return temperature back to the boiler is no higher than 55C. E.G if your flow temp (from boiler to the radiators) is 80C its very difficult to get that differential. Having your flow set no higher than approx 65C should see the return temp (from your radiators back to the boiler) and your boiler will go into condensing mode which is when your boiler is running most efficient. You also need to make sure your radiators are balanced properly and have the correct flow.

How to balance your radiators: http://www.homebuilding.co.uk/advice/DIY/how-balance-radiators - your looking for 12C difference on older boilers (non condensing) or as close to 20C as you can on modern condensing boilers.

Another efficiency feature on many of the newer higher end boilers have is something called load compensation/weather compensation which works by calculating the difference between outdoor temperature/room temperature against the set temperature. E.G if the room temp is reading at 16C and you have set the room thermostat to 21C when the system first turns on it will automatically set the flow temp to be high so as to warm the house up quickly, as the room temperature approaches the set temp on the room thermostat it automatically reduces the flow temperature to a much a lower temperature to maintain the set temperature. It is more efficient to keep the boiler running at a lower temperature maintaining the set temp than it keep turning on/off all the time and heating the radiators up from cold to hot for the set temp to overshoot meaning the boiler turns off completely. You don't have the temperature fluctuations or walk into a room and it feel stifling because of a high flow temp.

If you don't have the above feature on your system or you have an older boiler, whatever type you have you can do this yourself manually by turning the flow temp to high when you are warming the house up from cold and once the house gets warm and up to temperature turn the flow temp to as low as you can that will maintain the house at a nice background temperature you have set on the thermostat. You may have to play around with this and learn your system and how your house heats up to get it spot on as each house is different. A large old un-insulated house will not hold heat so your always going to have to run the radiators at a higher temperature just to maintain comfort so this will cost more so in this situation timed is better than constant. You will have to try it for yourself and see your results.

I do this in my house and I actually leave the system on constant. The flow temp is about 45C and the room thermostat in the hall is set at 19.5c (during day and at night) -18.5c (when in the house moving about and when in bed or out at work) the system runs probably about the same time as if it was on timed except the house is warm constantly and at a nice even temperature. There is always someone at home and I have tried both methods of having it on timed/constant and leaving it on constant actually saves money because we are running the boiler at a lower flow temperature to compensate. This means the house isn't cooling down to low temperatures so the boiler is just ticking over rather than having to come on and blast the house up to temperature which can take a while and the house still feel cold because all the walls, furniture and furnishings absorb that heat and have to come up to temperature also. Remember don't abuse the room thermostat and be sensible with it.

The problem also is that people are silly with the thermostat. If you use it on timed and while it is off say the house drops to 14C. When it comes back on and you set it to 21C the air temperature will quickly reach 21C and then it will cycle off however the contents of the house will still be at 14C and slower to warm up. People compensate for this by cranking up the thermostat to keep the boiler on, this then leads to overheating of the air itself and then people fight each other with the thermostat until the house warms up. These are the same people who feel the need to run the boiler at maximum flow temps so the radiators are scalding hot.

Much better to have a low flow temperature on the rads (enough to keep your house warm and even temperature) and keep this temp between 16 & 20C depending on whatever your comfort level is. Only do this as long as the house is occupied. If your away for the weekend then turn it to the frost stat setting eg 10C to stop pipes freezing but even if your at work or out for the day don't set it below 18C as it will take a long time to heat back up and comfort is compromised and then we go back to the people being silly with the thermostat.

I mentioned weather compensation and how to do that manually on your boiler if you don't have it. Works a treat.

Here is an article about the above points I have just mentioned...

http://energy-surprises.blogspot.co.uk/2012/09/getting-best-from-your-condensing-boiler.html?m=1

Currently using £7 a week for heating/hot water and a lovely warm house. Of course the energy companies will say turn if off so you then run your heating at full pelt to get it warm again. You then use more gas as you have to get the contents of the house back up to temperature. Timed meant the boiler had to be set hotter to get the house upto temp quicker and it used to cost about £18 a week to heat the house in winter.

Hope that helps

EssexExile

malc_b wrote: »

Whilst I agree and you are correct for CH it should be pointed out that it is recommended that HW be at least 60C so you can't set CH too low if you are using it to heat HW.

Surely hot water should be no more than 60 for safety reasons?

Smiley_Dan

*Delivered* hot water shouldn't be much hotter than that, but stored may be to harness uncontrolled energy sources, e.g. biomass, solar. Fit a TMV to get the right temp from the tank.

gracehoper

I'm wondering whether the advice to turn the heating on when required applies to all boilers, including newer ecological ones like NIBE (I have the F370 model)? The advice I've received from my landlord is that it's cheaper to keep it on all day.

BazChapman

Do you leave the engine running on your car the whole time in order to have it warm when you get in?
If you were going on a tour of the world for a year would you leave the heating on the whole time you were away?
How about a month, a week, a day? And what is true for a year, month, week, day is also true for an hour.
This has been known for 400 years, its called Newton's law of cooling, and it shows that it takes more energy to keep something hot than it does to warm it back up.

Smiley_Dan

Your last paragraph seems to contradict your previous ones.

gracehoper - that's a heat pump which provides lower grade heat. As a result it takes longer to emit the same amount of energy. So you have to run these for much longer. Sorry, I'm no expert.

Smiley_Dan

At the risk of raising my head above the parapet...

I think there are a few real-world things to consider which cloud the *interpretation* of the physics (they do not change the physics).

Overheating. A higher rated power source is more prone to expending extra energy as the heat curve the thermostat follows will be naturally more peaky. Improvements in technology try to avoid this, but this will always be a problem *to an extent* with higher grade heat versus lower grade.

Insulation. Discussions about Passivhaus are a bit academic in the sense there are so few of them; however, there are better insulated and airtight houses now (not as many as we should have, but still). This makes differences in cost increasingly negligible (I'm talking about highly insulated here, but not necessarily PH).

Heat generation technology. Some technologies work best at lower grades of heat, and are cheaper ways of generating heat. Heat pumps being an example, which have to be run for much longer than other heat sources.

Lifestyle. It matters whether the occupants are in all day, or work a 9-5. If they are in all day, that leaves more choice as to heating technology; this is one variable where low grade heat is more suited. If you have the heat on longer, it can also work better with houses with higher thermal mass. Whereas those coming home from work want instant heat; timber framed houses may be better there.

I am NOT disputing the laws of physics. They are clear. And in the general case, it is best to time your heating so it is only there when you need it. Furthermore, most of the posters who dispute the physics don't have highly insulated homes with heat pumps. However, the laws of physics are just one input to this; looking at the thread title, also important are economics. And there's also an implied notion of comfort; how much heating is enough?

PigletJohn

You said on the Myths page "Turning your radiators up and down using thermostatic radiator valves on the side of them affects how quickly the room heats up. If you have them on high, your radiators will emit lots of heat quickly until the set temperature is met and vice versa."


This is not true. Turning up thermostats does NOT make the room heat up faster and does NOT increase the temperature of the radiators or their output. It only causes the room to continue heating up after it has reached your comfortable temperature, resulting in the room getting too hot.


If a stat is set to e.g. 20C, it will run the heating (or continue heating the radiator) at full power until the room hits 20C; then it will turn off, and will come back on if the temperature drops below 20C (radiator valves may be able to partially open if the temperature has only dropped by a tiny amount). If you set it to 30C, it will still take exactly the same time to reach 20C, but then it will continue wasting energy until either you manually turn it down, or the room reaches 30C.

orrery

PigletJohn wrote: »

If a stat is set to e.g. 20C, it will run the heating (or continue heating the radiator) at full power until the room hits 20C; then it will turn off, and will come back on if the temperature drops below 20C (radiator valves may be able to partially open if the temperature has only dropped by a tiny amount).

I think that the problem here is that the TRVs are not as ON/OFF as you might like. Modern TRVs are likely to have a control range of about about 4C.

So, if you set your valve on a nice warm 20C day so that the rad is warm (i.e. to a 50% flow rate) then it will be fully open at 18C and fully closed at 22C.

Note that the TRV controls the flow rate depending on the sensed temperature. So, setting it higher (at least over that 4C range) will cause the flow rate to increase and therefore the rad to heat up more and the room to heat quicker. It also means that to maintain the exact same temperature, you may need to set the TRV slightly higher in winter and slightly lower in summer to achieve the same effect.

I'm not suggesting for a minute that you do mess around with the settings on a daily basis, but half a graduation higher in winter and half a graduation lower in summer is useful in maintaining the same temperature.