The Lemon Fool

What is the best (economical, practical, efficient) way to programme a central heating system? Is it best to have it on all day at a moderate temp (e.g. 18 C) or is it best to have it programmed to be off when you are out and then programmed back on when you are in (either at 18 or maybe 20).

Surely that depends upon how long you are in and out

For me the maths is simple. A house at 18C all day loses more heat than a house at 14C (which is about what our drops to during the day) so you must be using more energy to keep it at 18 than 14. When we get home it rises back to the set 17C in about an hour, when the boiler throttles back.

The first hit on Google seems to agree
https://www.moneysavingexpert.com/utili ... ing-myths/

Paul

Have it the temperature you want when you're in (possibly having it come on before you get home so it's heated by the time you get in).

But other than that you just need frost protection only.

This is obvious when you consider that over the long term, you only pay for the heat that escapes the house, through the walls, roof, windows, floors etc. So if you're heating the house when you don't actually need the heat, you're wasting money.

Scott.

swill453 wrote:...you only pay for the heat that escapes the house, through the walls, roof, windows, floors etc.

This should be printed in bold letters on every gas & electricity bill. C.

There is an interesting counterview by former Telegraph columnist Jeff Howell (which is also referred to in DrFfybes's link).

If the wall temperature in a room falls below dew-point – and dew-point is not a fixed temperature but varies with humidity – the thermal conductivity of blockwork and brickwork rises because of the damp. In other words its insulating property gets worse and heat-loss increases. And the most likely situation for this to happen is when a room that is warm (and hence the air can hold a higher level of moisture) cools markedly and the moisture then condenses on and within the walls.

In addition, simple physics would seem to imply that when a cooling wall hits dew-point, there is a small temporary rise or a lull in falling temperature, but when a damp wall heats up there is a corresponding lag in warming as the moisture in it rises above dew-point - i.e temperature hysteresis.

Exactly how significant these effect are is not clear; one would hope that the Building Research Establishment would have done some measurements.

stewamax wrote:There is an interesting counterview by former Telegraph columnist Jeff Howell (which is also referred to in DrFfybes's link).

If the wall temperature in a room falls below dew-point – and dew-point is not a fixed temperature but varies with humidity – the thermal conductivity of blockwork and brickwork rises because of the damp. In other words its insulating property gets worse and heat-loss increases. And the most likely situation for this to happen is when a room that is warm (and hence the air can hold a higher level of moisture) cools markedly and the moisture then condenses on and within the walls.

In addition, simple physics would seem to imply that when a cooling wall hits dew-point, there is a small temporary rise or a lull in falling temperature, but when a damp wall heats up there is a corresponding lag in warming as the moisture in it rises above dew-point - i.e temperature hysteresis.

Exactly how significant these effect are is not clear; one would hope that the Building Research Establishment would have done some measurements.

That is excellent. All understandable and leaves me too wondering if the effects are significant when compared to the simplified model.

Chris

stewamax wrote:There is an interesting counterview by former Telegraph columnist Jeff Howell (which is also referred to in DrFfybes's link).

If the wall temperature in a room falls below dew-point – and dew-point is not a fixed temperature but varies with humidity – the thermal conductivity of blockwork and brickwork rises because of the damp. In other words its insulating property gets worse and heat-loss increases. And the most likely situation for this to happen is when a room that is warm (and hence the air can hold a higher level of moisture) cools markedly and the moisture then condenses on and within the walls.

In addition, simple physics would seem to imply that when a cooling wall hits dew-point, there is a small temporary rise or a lull in falling temperature, but when a damp wall heats up there is a corresponding lag in warming as the moisture in it rises above dew-point - i.e temperature hysteresis.

Exactly how significant these effect are is not clear; one would hope that the Building Research Establishment would have done some measurements.

Would the humidity in most of a house be high enough for this to be a measurable effect?

Slarti

Slarti wrote:Would the humidity in most of a house be high enough for this to be a measurable effect?

This depends on the size of the rooms and the number of moisture-generating occupants, plus ventilation, badly-vented showers and so on. The easiest criterion would be to look for black mould at the bottom edge where a window frame joins the pane.

In Winter when all this matter most, the outside skin of cavity walls will usually be damp and often saturated anyway, so the amount of heat loss through the inner wall (plus air circulation within the cavity and/or the quality of the cavity insulation) is a significant factor.

Air temperature vs dew-point graphs are worth a look (vide e.g. Wikipedia article on dew-point)

stewamax wrote:If the wall temperature in a room falls below dew-point – and dew-point is not a fixed temperature but varies with humidity – the thermal conductivity of blockwork and brickwork rises because of the damp.

The dehumidifier stops any of that nonsense much more efficiently than heating a place.

UncleEbenezer wrote:The dehumidifier stops any of that nonsense much more efficiently than heating a place.

Probably true for a room with good cavity insulation or well-ventilated air-gap.
Probably less so where there is damp entering from the outside.

Howell's point was (to paraphrase him) that the inner skin of a cavity wall needs to be kept above the dew-point temperature. For a well-ventilated or dehumidified room, the dew-point will indeed be lower.

But, as I said, the effect needs experimentation by the BRE or similar to see whether or not it is significant.

What is the best (economical, practical, efficient) way to programme a central heating system? Is it best to have it on all day at a moderate temp (e.g. 18 C) or is it best to have it programmed to be off when you are out and then programmed back on when you are in (either at 18 or maybe 20).

If we look at comfort levels, a lot of what you do will depend on the construction of the walls of the building.

Thick heavy uninsulated stone walls take an age to heat up and benefit from almost permanent heating during the colder months.

Well insulated walls respond more far more rapidly. I think having internal insulation using insulated plasterboard would be the wall type to respond to heating.

Others have pointed out the benefits of keeping all rooms at a moderate temperature - If I'm away for a few days in winter, I'll leave the heating on with the thermostat set to 10ºC. Were I away for a longer period, I'd be inclined to lower that to 5-7ºC and open a few trickle vents to ensure fresh air moves through the house. I also leave all of the internal doors open.

No one has mentioned local heating. If I'm in the home office for the day, I'll leave the heating off an use a electric fan heater to keep one room toasty.

The dehumidifier ideas are excellent, while a bit noisy, they suck the damp from the air and provide a little bit of heat too. Within reason, the drier the air, the easier a room is to keep warm as the dehumidifier removes moisture from everything, the plaster, furniture, carpets even the mattress and bedding. Positioned in the right rooms, you'll never feel clammy again.

On that note, you can get some very discrete air conditioner/heaters which got a mention on the DIY board.

https://www.coolyoudirect.co.uk/solutio ... ditioning/
While I have no connection to this site, and the prices seem a bit keen it will give an idea of what is possible with only an electric point and a couple of holes.

B.

My experience tells me that, in very cold weather, it is no more expensive to leave the heating on 24/7 at the usual temperature setting. The walls act like a storage heater. If you try to cycle hi/lot or on/off, the energy required to return to the status quo ante is very similar to that used to maintain temperature, but the comfort level is lower.

By very cold, I mean sub zero all day. In more normal times I leave the heating on from 0630 to 2300 at its normal setting. The room and radiator thermostats do the work.

TJH

Thanks everyone. I'll continue with the on/off routine, but interesting points about damp and heating up the walls. I thought there was potentially more to it than the obvious.

C