Heating on low and constant better?

FZwanab

I think if you do go away during winter it is advisable to leave the heating come on at night for an hour to stop pipes bursting and costing you a lot more to repair. Thanks for all the ideas so far, you've all gone into great detail about the issue.

John_Pierpoint

Let us set the conditions:

Heating input: Natural Gas AND SOLAR AND DIURNAL (day v night).
Heating equipment: A burner that is anything from 60% to 95% efficient.
It may be made from chunky cast iron (big thermal mass) or have a heat exchanger that looks like corrugated cardboard (virtually no thermal mass).
Ventilation: Balanced horizontal flu or large extractor pipe called a chimney.

Type of house: Anything from Georgian, via early 1970's (low thermal mass, poor insulation, lots of draughts, BIG windows) right through to the current "L" building standard and beyond (eg terraced houses with S. facing windows built into a pile of earth near Nottingham).

Method of distributing heat: Radiators: Radiation & air movement.

Objective: Comfort at minimum price. Note lack of comfort is most noticeable when the occupier is sedentary (watching the box) and not generating internal heat. It is also noticeable when one part of the body is cold relative to the rest (Draughts, cold windows, cold feet etc) It is NOT directly related to the temperature on the thermometer or thermostat.
All "bodies" sit in an equilibrium of radiation received v radiation emitted.
The historical open fire solution was to sit on a wing chair or settle and adjust its position in front of the glowing fire until comfortable (even when the temperature of the room was perhaps only 10 degrees). Similarly I can sit in the conservatory reading the papers in the sunshine and feel comfortable in (say) only 15 degrees.

Loss of heat: Air changes, Conduction, Radiation LATENT HEAT.

Let us start at the extremes: The 1970 house versus the house built into a hill side facing south.

1970's house has not choice. The big windows are just slotted into a standard design and point to all 4 points of the compass. So any heat added in the day by the SE-S-SW facing windows is lost by the ones facing the other way. The windows might still be single glazed (Not good well draught stripped double or treble glazed with low emitting coatings) and anyway the walls leak energy and the internal "cardboard" walls have no thermal mass (There is virtually no heat stored in the air). The house is badly sealed and the high air changes carry off more of the valuable heat. In this sort of house there is no choice but to have a huge boiler feeding oversized radiators and put it on full blast when people are at home, running the radiators as hot as possible. The boiler is only 60% efficient running continuously. Its design is actually worse than that as a lot of heat is required just to heat up the boiler and as soon as it turns off that heat and the heat inside the house gets sucked away up the chimney. There is another problem with this sort of house: the symptom is black mould, which indicates that the surface of the windows and the walls get so cold that condensation develops.
This has three down sides:
1. Those big hot radiators are put on the outside walls, probably under the windows, so a lot of the excessive radiator heat simply goes directly to the outside of the building.
2. The room needs to be run several degrees hotter (thus increasing the rate of heat loss) to compensate for the loss of balance between the radiation of the "body" and the radiation coming back from the windows and walls.
3. There is also a latent heat effect: Heat a kettle on the gas and leave it boiling: Where is the heat going? The water is not getting hotter all that heat is being used to evaporate the water into steam. If the house has condensation dampness, the heat as well as the water is being redeposited on the walls and windows, where the heat readily escapes. Meanwhile, when the central heating crashes back into action a lot of heat is used trying to (re)evaporate all that water deposited on the windows and walls.
This only gets a convection cycle going where the windows and walls are acting like the freezer box in a fridge. (Yes the water turns to ice on a cold night but there is a lot less latent heat in ice turning into water than water turning into steam).

Now let us consider those South facing "class L++" modern homes built into
the side of a mound. They have massive insulation & thermal mass and low air changes. This allows any of the day time radiation of the sun to be trapped & stored for the night.
In fact if you are prepared to wear "long-johns" you probably don't need any heating at all, as the temperature almost never drops to 15 degrees. The internal surfaces do not get cold to produce the "cold nose" effect that encourages running the temperature at a higher setting. Modern high tech. buildings probably have some sort of heat exchanger on their ventilation system.

Most of us live in something somewhere between these two extremes.
If by keeping the heating ticking over you avoid getting a cycle of condensation and cold walls, you just might save money by feeling more comfortable in the evenings with a room temperature that is a few degrees lower than it otherwise would need to be. You might also be able to get away with a smaller boiler, by working it longer in the day. A small boiler worked hard is more efficient than a big one delivering only 2/3rds of its potential and constantly cycling on and off. If your house has high thermal mass it will naturally smooth the diurnal changes.

However have you picked the "low hanging fruit": insulation and draught stripping by utilising the current government/power company subsidies will bay back better than fiddling with the time clock.

Personally, I have a "set back" system built into the boiler controls and live in a south facing bungalow with high thermal mass. I also have a wood burner for those dark January weeks when the overworked small boiler has trouble keeping up with the sub-zero external temperatures.
The internal temperature in the living room has never been below 8 degrees BUT when we go away (eg at Xmas) it takes 36 hours to get back to the normal 17 degree set back level.

As I write it is 10 degrees externally and 22 degrees in the conservatory with the heating off.

John

skintmostofthetime

Hate to add to all of this but - if you live in a detached house/ bungalow it will always FEEL harder to heat than a terraced property/mid floor flat etc. The more outside walls you have, the harder to heat EVEN if you have good insulation everywhere, if you don't have them - it's great because you are `using' your neighbours heat to keep your warm even if they don't have it high !! No one thought to ask what type of home was being heated.!! Oops - have I set off another debate?

Merlot

skintmostofthetime wrote: »

Hate to add to all of this but - if you live in a detached house/ bungalow it will always FEEL harder to heat than a terraced property/mid floor flat etc. The more outside walls you have, the harder to heat EVEN if you have good insulation everywhere, if you don't have them - it's great because you are `using' your neighbours heat to keep your warm even if they don't have it high !! No one thought to ask what type of home was being heated.!! Oops - have I set off another debate?

Your absolutely spot on there, I have a new build detached house and its freezing compared to my mid terraced house I used to live in, and I spend 3 times as much on gas heating this place. Blo*dy Laminate floor everywhere don't help I think, I got my coat on cause the heating has been on for 3 hours this morning and I can't possibly justify keeping the heating on all day. Maybe I just need to get off my backside and move around a wee bit more.


Can anyone tell me, I have a gas flamed fire in the lounge, does it use alot of gas?, it sure heats the lounge up quickly, but someone told me they practically eat gas.

Just read this thread, cor blimey its technical, its way over my head, what did you all do before forums arrived on the scene?

John_Pierpoint

Here is a short program of people talking sense, when faced with the government's wish list of carbon reduction hopes.

I expect that the "Friday" program will be available for a week.

https://www.bbc.co.uk/worldservice/programmes/business_daily.shtml

"Governments may be loud in their commitment to fighting climate change, but do some of their initiatives stand up?
We investigate one of them - a government target for building so-called zero carbon homes in Britain. But is the zero carbon home achievable?"

SwanJon

Merlot wrote: »

Can anyone tell me, I have a gas flamed fire in the lounge, does it use alot of gas?, it sure heats the lounge up quickly, but someone told me they practically eat gas.

I've been told that they use 3x as much gas for the same heat. The heat to the room comes from the glowing 'coals' not from the flames, and so they're even less efficient for short bursts.
Most of the heat goes up the chimeny.

tomstickland

The heating might not be on, but the house is still losing heat at the same speed when it switches off as it did a moment beforehand and continues to lose heat when you're out or asleep. Then you pay for a load of gas to replace what leaked away. The saving comes from the lower rate of heat loss as the temperature difference between indoors and outside reduces compared to when the heating was on. If this figure doesn't exceed the loss in efficiency of running a condensing boiler hard to heat the house up quickly again, no saving is made.

Yes, I agree. If you use electrical heating with 100% heating efficiency then it'll always be cheaper to turn if off when not in.

mech_2

John_Pierpoint wrote: »

You might also be able to get away with a smaller boiler, by working it longer in the day. A small boiler worked hard is more efficient than a big one delivering only 2/3rds of its potential and constantly cycling on and off.

Great post John. Lots of good stuff.

I'm not sure the above is quite right though. Especially if you want a combi boiler, it has to be sized according to the hot water requirements. And even if you only sized the boiler based on the heating requirements of the coldest day of the year, it would still be oversized for the majority of the heating season.

For a condensing boilers I think it's especially undesirable to run a small one harder instead of a larger one. Unlike a conventional boiler there isn't loads of off-cycle flue loss because the operating temperature is lower and the heat exchanger has enough resistance to resist significant air infiltration from outside (I mean, that's why they have a fan, right?). Additionally, at part-load they can also reduce the burn intensity, so the boiler is on for longer anyway. With the burn rate modulated down, the heat exchangers become proportionally larger compared to the amount of heat that needs to be transferred, so it's more thermally efficient than at full load, even at the same flue gas temperature, even when it would still have been condensing.

As I understand it, it's most efficient to let a condensing boiler cycle gently and often, at as low a temperature as the capacity of the radiators can sensibly manage, so the return temperature is always low and the boiler condenses as much of the time as it can and operates at as low a modulation as possible. This suits having the heating on a timer less well than with a conventional boiler, especially on a retrofit system with less than ideal radiator capacity, because if you let the house cool much you have to turn the boiler temperature up to maximum or the house takes forever to warm up again. That can hurt efficiency by as much as 20%.

The internal temperature in the living room has never been below 8 degrees BUT when we go away (eg at Xmas) it takes 36 hours to get back to the normal 17 degree set back level.

That sounds similar to experiences I have had in my house. I went away for 4 days two weeks ago, letting the house cool right down to 9 degrees C. (The outside temperature for the first 3 days was 5 degrees then rose to 7 by the time I got back). On the fifth day I came back and ran the heating hard for 15 hours, burning nearly 250% as much gas as I would normally burn on a typical day of 7 degrees outside (without using any hot water) and I still didn't manage to get the house up to temperature.

This means I switched the heating completely off for 4 days out of 5 and made a 54% saving (and it wasn't even on for all of the 5th day). There's no way I could save that much switching the heating off for a portion of each day as it would never lose enough heat to bring the rate of heat loss close to equilibrium. Even when it was -2C outside, the house didn't go below 9 degrees C in 8 hours during the night, and floor and wall surfaces in the house were still warmer than the air.

There is nothing special about my house: it's a small 3-bed 1930s semi with single-glazed softwood windows with over-painted draft-stripping that doesn't work properly any more. It has some south facing windows at the back, and also a massive north-facing bay at the front of the house both upstairs and down and only the east wall has cavity insulation as they couldn't get it into the cavity at the front and back. The exterior doors are hardwood and don't quite fit the frames properly. I have no conservatory. I have undersized radiators with no TRVs. I have 4 inches of insulation in the loft. The downstairs floors are wood over ventilated crawlspace. I keep my thermostat at 18 degrees, which turns the boiler on at 17 degrees and off at 19, but it's 2 degrees warmer upstairs according to a room thermometer. The reason the house holds heat is the load-bearing wall that runs through the property from front to back. Pretty typical for this kind of house and this kind of house is numerous. There are also two partition walls upstairs that are cinder-block. I do have a 2 year old condensing boiler. That pretty much sums it up.

John_Pierpoint

I too went away for several days, calling in on my detached bungalow late on Saturday 17th when the living room temperature was still 16.5 degrees. I finally returned, to turn the heating back on, early in the morning on Monday 19th and the temperature had fallen to 14.2 during the previous two cold nights. I think my advantage is due to better insulation and the double glazed windows facing south.

I think you are spot on with your comments about condensing boilers; after all if you have virtually no thermal mass, relatively cold water in the heating system and no natural ventilation out of the flue, you should be pumping in 100% of heat and getting 90% plus left within the envelope of the building. However with luke warm radiators it is going to take for ever to re-heat the house from cold. This is another reason not to let the building get too cold as jacking up the radiator temperatures into the 50++ degree range reduces the efficiency of your condensing boiler.

The heat pump boys have a real problem with this as their kit is only designed to raise the temperature from inlet to outlet on the unit by a modest number of degrees:

http://www.kensaengineering.com/pdf/faq/Fact_Sheet_-_Heat_Pumps_at_Start_Up_for_Flat_Battery_Buildings.pdf

zarazara

maybe its best just not to let the house get cold in the first place, as , using the kettle analogy, it costs most to get the water from cold to hot , than from hot to boiling.
So, if you have very good insulation leaving the heating on all the time, set low, might be a good idea as it wont actually be "burning" much, but if the house is cold it needs to burn quite a bit to get to a comfprtable temperature.