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

System

In for a penny etc. If I look at boiler efficiency graphs, it is clear that greater boiler efficiency can be achieved if the boiler is operating in the condensing range. However, if, say, boiler efficiency is 87% at the Dew Point then boiler efficiency only drops to 86% at 80C return flow temperature. Conversely, with a 65C flow temperature and a 45C return temperature, the boiler efficiency is about 92%.

That said, there are other factors that also come into play. Firstly, most homes with a replacement condensing boilers do not usually have big enough radiators to cope with a constant low flow temperature situation; hence, for comfort reasons, the boiler is not always running in the condensing range. Secondly, people never talk about the cost associated with a constantly running pump. The pump on my recently installed 24kW boiler has a part/full load consumption of 72/96 watts. I think that I am correct in saying if a modern boiler is left in frost protection mode the pump runs continuously. Thirdly, we shouldn't forget that many condensing boilers are too big for the property in which they are fitted. This, in part, is down to the ignorance of property owners and the wish for a greater profit for the installer. The problem being that on part load the minimum boiler output is too big for a static situation when the property is up to temperature. The boiler therefore cycles more than it needs to.

All the above are factors which can affect the overall cost of running a condensing boiler. Even though my wife and I are at home most of the day, I am not persuaded by the 24/7 argument. Neither are Honeywell nor Nest. Both systems offer economy modes when the occupants are not at home.

Blackbeard_of_Perranporth

Swipe wrote: »

5C? What do you live in, a barn? The coldest I've seen is my bedroom at 6C and I have no heating in there at all, even during the day.

At that temperature condensate will form on the inside of your house causing damp and other problems. But then, I do not have condensate in my house!

Cardew wrote: »

There is no question that in vast majority of cases the longer your heating is off/or turned down, the lower the fuel bills.

Why, yesterday on first frost of the winter, my furnace was on overall for 17 minutes in the morning and 5 in the evening. yet my neighbour in a similar property blasted their furnace for a good half hour in the morning and then it was off, the house never reaching its intended 21C!

Cardew

Hengus wrote: »

I think that I am correct in saying if a modern boiler is left in frost protection mode the pump runs continuously.

Interesting, I have a fully central heated converted stables with a newish condensing boiler.

This building gets little use. Last winter I left it on frost protection mode - which is 5C.

As it uses little gas or electricity I don't check the external meters that regularly but when I did hardly any gas had been used but a lot more(than normal) electricity had been used.

I went in the building and the pump was running although the temperature was 9C. Anyway after ringing the UK factory(Veissmann) they got the information from Germany that once the temperature drops below 5C the pump continues to run until the temperature exceeds 11C(obviously the boiler is not firing above 5C). So in my case the pump would have been running for some months.

The German technical department stated that they believe this logic is used by all modern boiler manufacturers - probably some obscure
EEC regulation from Brussels!

System

@Cardew. I have an Dutch boiler (ATag). Full frost protection is selectable:

Danger of frost

If there is danger of frost damage to the CH installation, it is advisable to let the pump run
continuously.

By pressing both - buttons at the same time for 8 seconds the pump can be switched on
continuously.

If the pump is set to continuously this is displayed by (Icon) .

Boiler frost protection

If the (T1) flow sensor registers a temperature of 5°C or below in the boiler, the boiler will fire up. The boiler keeps on firing until the temperature reaches 10°C (measured on the flow sensor) and the boiler switches off again.

In this mode the symbol will flash.

Interestingly, I also follow the Automated Home Heating forum as I have Evohome installed. This has been the subject of an ongoing discussion:

Quote Since having the combi fitted with Evohome my electric month on month has more than doubled. Gone up from an average 160kwh to 340 a month.

Any ideas? Unquote

Most posters accept that electricity usage would increase but no one could explain the increased amount. Even permanent frost protection (pump running) would not account for the size of the increase. The poster is now looking at getting an energy monitor.

reeac

Blackbeard_of_Perranporth wrote: »

At that temperature condensate will form on the inside of your house causing damp and other problems. But then, I do not have condensate in my house!

Why, yesterday on first frost of the winter, my furnace was on overall for 17 minutes in the morning and 5 in the evening. yet my neighbour in a similar property blasted their furnace for a good half hour in the morning and then it was off, the house never reaching its intended 21C!

So your supposedly 24/7 running amounts to only 22 minutes per day. I think that you're on to something big here ...better patent it pronto.

orrery

lstar337 wrote: »

Intelligent control that costs far more in outlay than any energy savings can ever recover. I don't see them as a valid money saving option. They are cool toys though.

Not at all. The old Honeywell wireless controller I took out (sets a time and a temperature rather than time periods) implements a rudimentary proportional control based on how close to the target temperature you are i.e. uses proportional within about 1C of target.

So, programming in a set of times in the morning with ever increasing temperatures (e.g. 05:00 18C, 05:30 18.5C, 06:00 19C, 06:30 19.5C, 07:00 20C etc, subject to the number of slots you have available) will bring the temperature up gradually and maintain proportional control (and therefore condensing mode on the boiler), unless the temperature is way out in which case it will just turn it on hard - exactly how you would want it. This approach allows a high flow temperature to be set so that both h/w and c/h is rapid, and a higher flow temperature can be set in very cold weather.

lstar337

malc_b wrote: »

Orrery I would agree with you that better controls are the way to go but it's not as bad as you might think. All boilers have a output temperature control so out water temperature is fixed to the chosen temperature. The return temperature depends on the flow speed, the radiator area and the room temperature. Basically the boiler is adding energy which the water is carrying away. The radiators are giving out energy, how much depends on their area and the difference between them and the room. The drop in temperature, out to return, depends on the flow rate and how much much the radiators give out. Slower the flow the larger the drop. In practice the difference is pretty constant in a well set up system.

Exactly malc. I have highlighted the text above as it was the point I tried to make to Orrery earlier. I keep my boiler condensing by turning down the output.

*Note: I understand this isn't an option for some. For a start, my boiler controls the hot water separate from the heating so turning down the heating doesn't affect the hot water.

As I know condensing mode works with a return of sub 50c, I set my output at 50c so the return will always be less.

I only heat for 2 hours a day in the worst of winter because my house is A rated for efficiency and has good insulation.

I have no doubt that heating all day would cost me more as it simply isn't needed when the house is empty.

orrery

malc_b wrote: »

Orrery I would agree with you that better controls are the way to go but it's not as bad as you might think....... In practice the difference is pretty constant in a well set up system. The only time you would see the difference head towards zero is if all the TRVs were hard off so radiator area was reduced to near nothing. If system is working correctly all the house radiator will be giving out heat and the TRVs just tweaking the flow rates to individual rads. In other words the main stat needs to switch off before all the TRVs have gone hard off. I think if that wasn't the case the rooms would be uncomfortable and they'd get very hot until the TRV shut hard off and then gte very cold while you waited for the main stat to turn on. And the boiler would be cutting in and out as it tried to deliver heat with no rads on..

You articulate the problem. If the TRVs are set to the right temperature, then as the rooms come up to temperature the flow rate is starting to limit. Rooms vary, and some are closing (remember that TRVs are very soft and will be progressively closing a degree or so either side of the target). As the boiler heats the flow water the return water starts to rise as the water manages to complete its cycle round the system and return that little bit hotter than it was. This continues until the boiler flow reaches target. When the flow reaches target then the boiler must modulate and, exactly as you describe, the return temperate will increase. When the boiler can't modulate any more it is likely that it will allow some over-temperature before shut down. I'd guess that most systems will be operating in this zone (acknowledging that the good people on this forum have extremely well adjusted systems) and most people won't be using condensing mode for much of their time.

So, my point is that - for the common man who has little understanding* of setting up a system, the very best solution to maintaining condensing mode is to have a proportional controller and boiler temperature set high. In my view that strategy is most likely to maintain a return temperature for condensing mode. I've spent a long time with a thermocouple on the return of the boiler (so I can see both flow and return temperatures at the same time) to come to that conclusion.

* The common man, or woman, has very little understanding of the subject. Most people can't understand what a thermostat does and will simple turn it up or down when they want it to be hotter or colder. Take a look at TRVs when you go into peoples houses - they are frequently all fully open.

Blackbeard_of_Perranporth

The question really is how efficiently do you control the temperature of your house together with its insulation. The method of intelligent thermostat control coupled with an efficient furnace provided by far the best method.

Some here regard only heating their furnace to 50C is efficient yet at this temperature the germs such as legionnaires remain.

Some ramp up and try to get their house to 21C but never make it because they switch off the furnace before their place gets warm.

Me. I leave it to my Honeywell thermostat time switch and it decides when it needs to come on because of the outside air temperature and the house. I have no cold rooms. I have no condensation either and my utilities are averaging £75 a month in a range from £70 to £80 in the last few years.

My neighbour pays more for the same property in the blind belief of two short blasts from the same furnace is an efficient way to heat the place. She has condensation problems and used a dehumidifier to try and reduce that.

Then again I am not an expert but I know how much I pay each month!

malc_b

Cardew I can't tell house much gas your house will use in various situations. The aim of the maths I've put together is give a better understanding. Going back to the car analogy, I'm trying to show how to get the best mpg out of a car to new driver, some rules to follow and why that is the case, i.e. highest gear but not struggling, 50mph not 70mph, correct tyre pressures, empty boot, brake use to minimum so predictive driving with large gap to car in front so you don't need to brake hard when he does.

The government rule is shorter is better but that just doesn't stack up. A typical house has a cycle of a day or more but that will depend on the house. It is something you can estimate. In winter, heating off overnight how cold does the house get? Mine drops from 20C to 15C I reckon so clearly it will take over 24hrs to get fully cold. I need to pump heat into it every 24hrs to get back up to 20C so the difference between 24hr or intermittent heating will be about 5%. The better insulated I make my house the longer the house takes to cool so the less difference between 24hr and intermittent.

That sounds wrong I know but look at it from heat lost. A well insulated house stays warmer so the heat lost is more constant.

The rule should be keep the boiler in condensing mode as much as possible. Running as short as possible and cranking up the flow temperature which you would have to to get the shortest run time is NOT the way to stay in condensing mode as rule.

Orrery I accept that you have measured your system and come up with a method that works for you. I'm not sure it follows it is the best method for everyone as it seems to be not the normal method of setting the max flow temperature low in order to get a low return temperature.