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

malc_b

I've posted on here maths showing how is it possible to have lower fuel costs with longer running, i.e. 24/7 heating being cheaper. As it is a bit difficult to understand this all in just text I've done a blog page here with pictures and cleaner layout which I hope will add understanding.

[Deleted User]

Cardew wrote: »

Sorry but don't agree!


Obviously the TRV is a 'soft control device' (as you term it) in that the water flow into the radiator decreases as the room approaches the temperature set on the TRV.


However the whole purpose of a TRV is to control the amount of hot water entering the radiator. Thus it will let in sufficient hot water to allow the radiator output to maintain the temperature set on the TRV.


So, as stated in my previous post, setting my TRV to '3'(20C) will maintain the room at 20C* regardless of the outside temperature. If I turn it up to '4' it will raise the temperature to 22C.


* If it cannot maintain 20C when set to '3' then it will not maintain 20C regardless of the TRV setting i.e. turning it to '4' or '5' will have no effect; the output of the radiator is insufficient. The solution as matelodave states is to get a bigger radiator or increase water temperature on boiler.

This is not correct, the TRV is part of a negative feedback loop. Suppose you want the room temp at 20C. The amount of heat required to achieve that temperature will be more on a cold day than a mild one, so that means the flow rate will have to be greater on a cold day than a mild one too. But in order to get a greater flow through the TRV there has to be a drop in room temperature. Therefore, if you leave the TRV setting constant the room temperature will still drop as the outside temperature drops, but it will drop by less than it would have without the TRV. Conversely, if you want the room temperature constant you will have to keep adjusting the TRV.

My TRVs take a temperature change of about 10C to adjust all the way from off to fully on, so if the outside temperature changed enough for the radiators to go from min to max the room temperature would change by 10C. The TRV reduces temperature variations, it doesn't eliminate them.

If you were to make TRVs with a higher gain (less temperature change to shut off the flow), there would be less variation in room temperature, but you run the risk of loop instability, and heating design would have to involve a careful stability analysis to prevent oscillation.

[Deleted User]

malc_b wrote: »

I've posted on here maths showing how is it possible to have lower fuel costs with longer running, i.e. 24/7 heating being cheaper. As it is a bit difficult to understand this all in just text I've done a blog page here with pictures and cleaner layout which I hope will add understanding.

This is far too simplistic. The whole issue revolves around the analysis of the thermal inertia of the building, and the associated time constant, along with the radiant heat loss from the body as the wall temperatures change.

Switching off saves heat, but not in proportion to the hours the heat is off for. I'm in the middle of constructing a spreadsheet to calculate the savings, and it looks as if the savings from switching off for 7 hours (29% of the day) are just a few percent, but I haven't finished it yet.

I had the spreadsheet working last week, but now it's stopped, and I can't remember which bits I changed in the mean time. It still needs the radiant heat losses putting in yet, though.

oldskoo1

From a very high level the more insulation and draft exclusion the smaller the extra cost leaving it on 24/7. The less insulation the larger the cost 24/7.

I calculated too and found 24/7 used about 10% more, not as great as you think but still whichever way you cut it it's more expensive running it 24/7.

If you have a well setup system you should have comfortable temps within half an hour. This might double in colder temps. Certain boilers that run weather compensation devices and smart pumps can improve the situation, as in increase the boiler output and flow to warm up quicker when it's 0c

Andy_WSM

jack_pott wrote: »

This is not correct, the TRV is part of a negative feedback loop. Suppose you want the room temp at 20C. The amount of heat required to achieve that temperature will be more on a cold day than a mild one, so that means the flow rate will have to be greater on a cold day than a mild one too. But in order to get a greater flow through the TRV there has to be a drop in room temperature. Therefore, if you leave the TRV setting constant the room temperature will still drop as the outside temperature drops, but it will drop by less than it would have without the TRV. Conversely, if you want the room temperature constant you will have to keep adjusting the TRV.

My TRVs take a temperature change of about 10C to adjust all the way from off to fully on, so if the outside temperature changed enough for the radiators to go from min to max the room temperature would change by 10C. The TRV reduces temperature variations, it doesn't eliminate them.

If you were to make TRVs with a higher gain (less temperature change to shut off the flow), there would be less variation in room temperature, but you run the risk of loop instability, and heating design would have to involve a careful stability analysis to prevent oscillation.

Mostly twaddle.

http://heating.danfoss.co.uk/xxnewsx/bc9b4d5a-bdb9-4d40-9691-43abf6a67fdf.html

Suggest you have your valves refitted. Mine are all on the return connection, going against the grain of standard UK fit as I had already worked out that would give a more stable temperature. I don't have room to fit my TRVs horizontally otherwise I'd have done that too.

orrery

Andy_WSM wrote: »

Mostly twaddle...

I'm afraid not - the points jack_pott makes are those any control engineer would make. The points you make are also valid but careful fitting on flow or return, or vertical or horizontal, can't fix the limitations of the device itself - only prevent making it worse.

The Danfoss page reflects an idealised situation (as any sales literature would) and glosses over other variations. As has now been explained twice, the valve sets a relatively consistent flow rate for a fixed room temperature but the flow rate required may need to be significantly different (by several degrees of control setting) on warm days and very cold days, so the regulation simply can't be that accurate.

They are probably correct in saying that TRVs will control temperature almost as well as a centrally located room stat, which will not control other room temperatures very well, but nowhere near as well as a room stat fitted locally.

richardc1983

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

PorkyPig

Hi
Does anybody know if the same applies to Air Source Heat pumps?
There seems to be conflicting arguments here too!

orrery

richardc1983 wrote: »

The problem also is that people are silly with the thermostat....

Good advice! I'd like you to come and talk to my Mum - I can't get through to her ("But I was cold, so I turned it up"). Her thermostats always end up set to maximum.

Two further points to add to that:
- some boilers have the facility to control the return (inwards) temperature, rather than the flow (outwards) temperature to make sure that it always operates in condensing mode (on mine - a Vaillant - you can turn this on in the settings, but you'd need to read the manual to find it).
- if you don't have a clever high-end boiler then most modern digital thermostat/timers (mine is a relatively bog standard Honeywell wireless one) have the ability to do proportional control as it approaches the set-point, and can turn the system on early (pull forwards) if the temperature is very low.

richardc1983

PorkyPig wrote: »

Hi
Does anybody know if the same applies to Air Source Heat pumps?
There seems to be conflicting arguments here too!

Yes these must be left on all the time however set them at a lower temperature when you go to bed as they have a very slow warm up when first turned on due to the lower flow temps. If you run your boiler with low flow temps you will also be able to follow the same method and look how comfortable underfloor heating is.