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Topic Title: The least expensive way to electrically heat a room
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Created On: 29 October 2013 08:05 AM
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 29 October 2013 08:05 AM
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jencam

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What is the least expensive way to electrically heat a room? An electric heater is 100% efficient where the electrical energy consumed and the heat produced is equal to the voltage multiplied by the current multiplied by the time it is switched on in seconds. However, there is a second factor of the thermodynamic temperature of the heater. Is a heater with a higher thermodynamic temperature more expensive to use in practice than a heater with a lower thermodynamic temperature assuming that the electrical energy consumption of both is identical?

I have an inbuilt electric heater with radiant elements that looks like it was installed in the early 20th century. It is no longer used but when it was powered up it was extremely hot at close range. Would this be more expensive to heat a room with than underfloor heating with the same electrical energy consumption?
 29 October 2013 08:42 AM
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ectophile

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I suppose the least expensive way is to drill through the wall to an adjacent property, and steal your neighbours' electricity. But I will assume you want a legal way!

As you say, all electric heaters are pretty much 100% efficient, so for every kilowatt-hour of electricity you use, you get a kilowatt-hour of heat in your room. That's true whether you have a heater that gets really hot, or just pleasantly warm.

You may want to look at how the heaters deliver the heat, though. Some, such as halogen panel heaters (or your old radiant heater) are very directional, so you may feel hot sitting in front of one even if the rest of the room is still cold.

Others, such as oil-filled radiators, just deliver background warmth. They are good for keeping the chill off a whole room, but not so good for keeping you warm.

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S P Barker BSc PhD MIET
 29 October 2013 12:43 PM
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kengreen

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strangely there is a fact that realised which counts heavily against electric heaters except those perhaps which employ a fan.

A house is much more comfortable to live in if you have several small heaters distributed about (at least) the lower floor. The term "central heating" refers to a system of a centralised "calorifier" from which heat it is dispensed via a water system or a hot air system.Today we believe that there is economy in a distributed system such as electrical heaters in each inhabitted area because the power can be switched on as and when required; the fallacy lies in the time-constant of each area which means that the room has first to be heated above ambient temperature and continues to be "warm" for only a short time after the power is switched off. Meanwhile the furnishings and possessions are replenishing their stored water from a house filled with damp air!

The above is of course an argument for overnight electric storage-heaters which are powered only on cheap rate tariffs - they are rendered futile however by the Iidiotic design copied over and over again.

Ken Green
 29 October 2013 02:48 PM
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rogerbryant

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Another issue here is that of comfort. If you assume that you are not trying to achieve a specific temperature at a specific point (20°C at the thermostat on the wall for example) but simply trying to feel comfortable then underfloor heating may give the best result and high level radiant the worst. The downside is that underfloor heating is probably the most expensive to instal.

Best regards

Roger
 29 October 2013 04:59 PM
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jarathoon

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Make the room smaller, with lower head height, with wall that have a lower heat capacity. Seal for draughts and allow enough ventilation to maintain life via a simple heat exchanger to heat the cool incoming air, using the warm outgoing air.

For people that live alone

If you create a insulated wooden structure containing just an arm chair, light, televison and digibox; the light, televison and digibox will provide more than enough heat to keep you warm. No need for a jumper.

James Arathoon

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James Arathoon
 29 October 2013 05:02 PM
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jarathoon

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Just remember to heat the rest of the house enough so the pipes don't freeze!

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James Arathoon
 29 October 2013 07:19 PM
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kengreen

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Very funny - now let's be practical.

My memory goes back to a time before many of you were born and I recall whole blocks of dwellings in which underfloor heating was installed as specified. It caused considerable surprise when the occupants of those apartments began importing portable electric heaters;the complaint was that their feet were too hot while their upper bodies were cold.

I have never tried it but I believe that there is a fundamental error involved: a point source of heat invokes a circulation system which interchanges the hottest air at the top of the room with the coldest air close to the floor and it is this that makes for comfort. Heating the floor creates, if anything, a chaos of rising hot air without any means of pulling down the layer of cold air beneath the ceiling. In short underfloor heating creates a local temperature inversion when compared with "normal" methods of heating.

a properly placed point source with its room circulation will reverse the flow of cold air which descends from every window - even with double glazing this down-flow is a major source of draft.

Ken Green
 29 October 2013 11:04 PM
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jarathoon

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If the room is well insulated with large area radiators (as with an under floor heating system), then very low grade heat can be used (35 DegC or less).

According to thermodynamic theory the cheapest way to heat the room electrically, is to use electricity to power a heat pump system.

However to make it maximally and reliably efficient in our climate you will have to build a massive underground heat store that you can stuff with heat energy during the summer months, by circulating solar heated hot water through it somehow.

I am not sure what the cheapest way of constructing the heat store is, but it needs to be large for a house.
You house perhaps needs between 30 and 90 kWh of electricity to heat it each winter each day; lets call that 60 kWh per day on average for six months; that's an electricity bill of £1640 (assuming a standard unit of electricty is 15p).

If you build a heat pump system with a co-efficient of performance of 3 or 4, then the bill reduces by a factor of 3 or 4. But over the 6 winter months you will be removing 11,000 kWh (approx 39,600 MJ) from your underground heat store and you don't want the temperature to lower too much over this time, so the heat store will need to be large.

(Specific heat water = 4.187 kJ/kgK, water density = 1000 kg/cubic metre)

Extracting 40,000 MJ out of 1000 cubic metres of water lowers the temperature by roughly 10 degrees Celsius.

In the summer months you need to refill your heat store with energy again, otherwise it will gradually drop in temperature over the years; the co-efficient of performance will gradually falling in unison.

If you only need to heat one well insulated room electrically the heat store can be much smaller.

However if you keep a heat store large you probably won't need a heat pump; in this case you can just use an ordinary pump to circulate the hot water direct from the heat store via an under floor heating system.

So in the end that's probably the cheapest way to use electricity to heat a room: use it to power an efficient water pump (having pre-stored all the energy during the summer months in a well insulated underground heat store). If the water gets too cold to heat the room effectively, use a heat pump for the remainder of the winter.

James Arathoon


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James Arathoon
 30 October 2013 01:07 AM
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kengreen

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hi James,

I thought you would be along with your encyclopaedias and endless figures.

There is a vast heaTt store beneath every house every street every town every park which is being exploited already in at least one experimental installation in Cornwall - namely the Earth's mantle. All that you have to do is to drill a couple of holes and use a pressure pump to force water down one of these and again to return up the other. But, as always in practice, there are difficulties - who was it who said that there are no free meals?

These sort of discussions remind me of Arthur Askey's song about the earthworm: "wriggle, wriggle,wrIiggle, squirm squirm squirm". I think that the IET should expect new ideas (like Barnes Wallis' bouncing bomb and many others) and stop these word games.

By the way can you tell me the correct plural form of the word "Encyclopaedia"?

Ken Green
 30 October 2013 07:54 AM
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jencam

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According to the second law of thermodynamics the amount of electrical energy required to increase the temperature of two identical rooms by an equal amount is the same regardless of the type of heater used. In practice it is more complicated as heat flow has to be factored in. Underfloor heating will heat up a room by conduction and convection reasonably equally whereas concentrated point sources of heat will result in a thermal gradient. An infra red camera can be used to identify how equally a room is heated up or whether large thermal gradients exist. The poor thermal conductivity of air explains why a soldering iron maintains a temperature of around 400 degrees C. If the tip is placed in contact with a piece of aluminium it acts as a heatsink and the temperature decreases even though it still emits the same amount of thermal energy.

The most awful electric heating system that I have encountered was ceiling mounted radiant heaters in a 1960s apartment bock that looked similar to small fluorescent lights and would glow red in the dark at night. Heating was very uneven with an intense beam if you stood directly below them and cold spots everywhere else. The positioning of furniture was critical. If a black leather chair was located below a heater then it would get red hot in a short time despite the air in the room always feeling cold even after the heaters were switched on for hours.

There was a time when my son attempted to heat a room using a hot air gun of the type used for stripping paint. His assumption that it was 1000W meant that it would have been just as effective as a 1000W fan heater. What happened in practice was that it produced a concentrated stream of searing hot air then just a few feet away the air was still freezing.
 30 October 2013 07:13 PM
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OMS

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According to the second law of thermodynamics the amount of electrical energy required to increase the temperature of two identical rooms by an equal amount is the same regardless of the type of heater used


Indeed - but the human animal wants comfort not a number - so the comfort temperature will differ radically from the air temperature depending on how the body receives radiant, convective and conductive heat paths.

Perhaps try a google for Dry resultant tempeerature and Comfort temperature

Underfloor heating will heat up a room by conduction and convection reasonably equally whereas concentrated point sources of heat will result in a thermal gradient.


Underfloor heating will actually have a large radiant fraction (and a substantial emitter surface with relevant view factors to the space) - it's why people feel comfortable with a low(er) air temperature. The thermal gradient can be quite low depending on the operating temperature of the floor.

Concentrated sources neeed not introduce a significant temperature gradient depending on delivery temperature and the method of distribution - most wet radiators are primarily convective and again quite low thermal gradients are possible

We regularly design underfloor systems with very low temperature difference to the space (which must be well insulated) - if you can maintain the floor at say 1C above the comfort temperture, then as the space temperature rises, to match the flor temp - heat transfer ceases - the system is basically self regulating and very efficient if putting the elecrical energy into say a heat pump for transfer to the floor warming fluid.

A lot of the negatives with older floor warming systems or ceiling radiant systems is the huge temperature differences needed to combat heat loss.

I'm working on a system at the moment that has underfloor heating and a similar wall heating system running fractionally above the room temperature - this provides a huge surface area and thus we don't need big temperature differences - a solar collector can provide most of the energy input


There was a time when my son attempted to heat a room using a hot air gun of the type used for stripping paint. His assumption that it was 1000W meant that it would have been just as effective as a 1000W fan heater. What happened in practice was that it produced a concentrated stream of searing hot air then just a few feet away the air was still freezing.


In the short term yes - but that highly convective and very directional air jet would distill down eventually to warming the room surfaces, there would be re radiation from those surfaces which sets up further convection currents and (other issues like noise aside) it would produce an equitable comfort temperature if it was matched to the room heat loss (which of course would also be varying)

Regards

OMS

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Failure is always an option
 30 October 2013 07:36 PM
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kengreen

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is not anyone going to deal with practicalities?

All the above posts have one snag in common in that they do not consider "the room" to be furnished?

Consider the effect of underfloor heating when a carpet is required? What is the effect of a large sofa plus a couple of armchairs? What is the effect of a full double-leaf dining table? And what will be the effect on the backside of a toddler navigating the floor without a nappy or other rear end covering.

I would love to see the mathematical solution to the bare baby's bottom; let me start you off with b|3 :-)

Ken Green
 30 October 2013 09:10 PM
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OMS

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We Ken, I'm a building services engineer and UF heating in all of the scenarios you describe is perfectly feasible - including babies bare bum

For infants/children, the sick, infirm or elderly, then control of the surface temperature of any emitter is important (and in some cases a legal obligation) - dealing with floor temperature whether carpeted, wood plank or polished concrete or tile is just part of the design thinking and the resukting floor and fluid temperatures

Do toddlers not put thier bare bums against radiators then - at least once?

Regards

OMS

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Failure is always an option
 31 October 2013 03:49 PM
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jarathoon

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OMS,

If your building is very well insulated and the radiators only need to be a few degrees above ambient, then in addition to scavenging heat from the sun, on sunny winter days, some of the heat could be extracted from a well insulated compost heap near to the building. You would perhaps need to dry store organic material (including newspaper) during the summer, suitable for wetting down and composting in the winter.

For a small anaerobic digestor, rather than capturing and storing large amounts of methane from the decaying waste (which has safety implications), you could store very small amounts of biogas which can be continually combusted under controlled conditions every hour or so to help keep the feeder tank to the underfloor heating slightly warmer than ambient.

A well designed cess pit might also be an extra source of energy for underfloor heating. Might need a three container system. One containing sewage in mid stew, one nearing end of stew, and one collecting sewage from empty having just been flushed. Once heat output and gas release has slowed the contents of the tank at the end of its stew can be flushed into the main sewage system.

I change my mind using electricity to operate an automated cess pit system for a large family, might compete as cheapest way to use electricity in the heating of a room.

James Arathoon

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James Arathoon
 31 October 2013 07:25 PM
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kengreen

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OMS,

Only if the installers of radiators do not know which end holds the bum.

Ken Green
 31 October 2013 07:45 PM
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OMS

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Indeed James - but you are thinking of a single residence scale - better to look at energy from "waste" at a community level or bigger.

Land fill gas generation, gas extraction from animal waste, energy from burning fat 'bergs in the sewage system, bio fuels from fat reclamation etc etc are all technologies that are in the pipeline as it were, or are there in reality producing real energy for use in real buildings.

We've just got agreement to lay ground loops for a ground source heat pump in close proximity to some buried high voltage cables which allows us to extract the heat lost to the ground due to I2R losses in the cable.

There's a million small scale things we could be doing in addition to the big things to change how we obtain and use energy in the UK

Regards

OMS

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Failure is always an option
 04 November 2013 11:39 AM
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HarryJMacdonald

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Several aspects to choose from - as has been alluded to already.

Hot air rises and very hot air rises quickly so radiators that create a small amount of hot air create very warm ceilings. In buildings like churches this a serious as it can be a long time before the heat filters down to floor level. Under floor heating creates a layer of air not much warmer than the rest of the room so the convection currents do not form. They are also pleasant to walk on. In our church students soften take off their shoes to warm their feet when they arrive!

If you search the internet for a recommended temperature for a playgroup or office you probably won't find it. What you will find is a requirement that the environment is comfortable and a statement that air temperature is a bad indicator of comfort. This is because radiant heaters, which heat objects like people and not the air can make a cold room feel comfortable. Just go outside on a sunny day in winter to see this.

Clearly pumping heat from elsewhere can give you 2 - 3 times the warmth per Watt, but finding the "elsewhere" can be a challenge unless you have half an acre of ground next door.

Also consider the occupation times of he room. For a bedroom, you might need heat for half an hour morning and evening. Probably not worth using night storage which at least partially heats it 24hours a day.
 04 November 2013 11:47 AM
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HarryJMacdonald

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A Canadian friend has refurbished a flat in Scotland has installed a heat exchanging ventilation system where all the air is extracted from the kitchen and bathroom and used to heat fresh air being ducted into the living room and bedroom. He is surprised this is unusal in the UK as it is the norm in Canada/
 04 November 2013 02:17 PM
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jarathoon

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Once you have improved the insulation on a house beyond a certain level, heating the air that comes in from outside for ventilation, becomes a big enough proportion of the energy bill to think about further. Some people like sleeping with the windows open (even in the depths of winter) so for them the problem is insoluble, and insulation beyond a certain point is valueless to them (unless of course they insulate their bedroom from the rest of the house to keep it colder!)

Last winter in Britain there was several weeks when the air temperature night and day hovered around freezing temperatures.

The question is how long do these periods of freezing air temperatures (day and night) have to be, to start justifying the expense of fitting mechanical ventilation systems.

Places right in the northern centre of the country with high elevation and exposure might qualify first, on these grounds I suppose.

I suspect houses in many places in the south with mild maritime climate, would benefit much less from this capital spend.

How do you compare the expense of a mechanical system of ventilation (and the cost of removing all the draughts) with one which attempts to modulate the natural ventilation, allowing more from midday to early evening, when the air temperatures are generally higher, for example?

In our ground floor lounge there is an air vent (originally sized for an open coal fire) through which I expect most of the air to ventilate the house comes (excluding the draughts up through the carpets, skirting boards, door frames etc).

Would it be better to try varying the flow of the air between night and day on this vent as an intermediate step first? There is quite a draw on this vent (with outflows of air on the upper floor), so throttling the flow on this vent at night may well just lead to higher draught flows in other places and have little net effect. Controlling the throughput of the upper floor vents between night and day might have a bigger effect, if the air leakage on the upper floor is less than on the ground floor.

I suppose I would need to measure the effect of throttling back air flow through the main ground floor vent had on the air outflows from the upper floor vents and visa versa.

James Arathoon

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James Arathoon
 20 March 2014 12:31 PM
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TRobins

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Surely you would want to look at the insulation of the room. There's no point in heating the room if it will leak out after a short period of time. Have you thought about using an underfloor dry system?
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