Heating science: How physics can save you money this winter
We break down how a heater warms a room, and what can you do to reduce your heating costs.
There’s not a lot to a plug-in heater, yet they’re essential appliances. We examine the science behind our heater test to find out how a small metal box can warm a room many times its size, and how you can reduce your heating bill this winter.
To ace our test, a heater needs to warm a room quickly, then keep it warm using as little electricity as possible. However, it’s one thing to warm our test room, and another to keep your old villa toasty. In theory, heating a room of cool air with a small heater is quick and cheap – in practice, heat loss and draughts mean your poor old electric heater has to work very hard.
Our unfurnished test room measures 4.9x3.5m, with a 2.4m high ceiling.
We start our test at 8°C and warm the room to 18°C. In theory, we’d need to supply 525kJ of heat. Using a 2.4kW plug-in heater, it would take 3 minutes 38 seconds (see How much energy and time does it take to heat the room?).
Our calculations assume we’re heating a perfect room, where all heat supplied to the air in the room stays in the air in the room.
However, our test room isn’t perfect.
Faults in our science
In practice, it takes a lot longer than three-and-a-half minutes to heat our room – the fastest 2.4kW heater in our test took 25 minutes. Several models didn’t manage to get the room to 18°C after two hours running full bore.
Our calculations include a few over-ambitious assumptions. To start with, we’re not just heating the air in the room. In reality, to make a room toasty, a heater also has to warm its walls, ceiling and floor.
Our worst assumption, by far, is that all the heat we put into the room stays there. Our test room doesn’t have windows, its door is sealed shut, and its walls, ceiling and floor are all insulated, but heat still leaks out.
In your home, you’ll lose this heat and much more through windowpanes, and in air that escapes through gaps around your doors and windows.
How much energy and time does it take to heat the room?
U = cpm∆T
U is the amount of energy needed to heat our room.
cp is the specific heat of dry air (1 J/g °C). This means it would take a 1000W heater one second to raise the temperature of 1kg of air by 1°C.
- m is the mass of air in the room. This is the density (for air, 1.275kg/m3) multiplied by room volume. Our modestly sized room contains 52.5kg of air.
- ∆T is the temperature increase we want to achieve (10°C).
We need 525kJ of heat to make our 8°C room a toasty 18°C.
∆t = U/P
∆t is the time it will take to heat our room.
- U is the energy required to heat the room (525kJ).
P is the power of the heater (2.4kW).
It would take 218 seconds to raise the room temperature by 10°C.
How science can save you money
You can do two things to warm up your room quicker:
- stop heat leaking away (reduce U – energy required)
- use a more powerful heater (increase P – power supplied).
Stopping heat leaking away is the place to start: insulate and stop draughts, so the heat you’ve paid for stays where you need it.
Heat rises, so start by insulating the ceiling, then walls, then the floor. Don’t forget your windows – if you don’t have double-glazing, drawing curtains or dropping blinds (when practical) will cover bare glass, which does little to stem the flow of heat.
Stopping draughts prevents warm air leaving the room through gaps around the doors, and cooler air entering the room through gaps around the windows (the cooler air pushes warm air out).
Use a more powerful heater. Choose a model that suits your room. The largest plug-in model that you can use in a normal socket is 2.4kW. We’ve got a calculator to help size a heater to your room in our electric heater buying guide.
However, if you have a draughty, uninsulated room, you might find even a 2.4kW plug-in heater isn’t enough. The best option (after fixing the insulation and draughts) is a more powerful fixed heat source such as a heat pump, wood burner or flued gas heater. If you can’t install a fixed heater, you can use more than one plug-in heater to increase your heating power, but the running costs will soon stack up.
There’s also a third trick that’ll save you money:
Move air around the room. Turbulent air heats up much more efficiently than still air. To move the air, choose a heater with a fan (such as an oscillating tower), or turn a ceiling or pedestal fan on to a low setting.
The second part of our test measures how much energy is needed to keep the room at 18°C.
It takes no energy at all to keep our perfect room warm. That’s because we assume no heat leaks out and, if no heat leaks out, we don’t need to replace it. The room never cools.
But our homes are far from perfect. Even our fully insulated test room with its sealed door and windowless walls cools down quickly when we turn the heater off.
If a heater ran full-bore, a well-insulated room would get unbearably hot. So, to maintain a constant temperature, all but the most basic models have a thermostat. Tip: Don’t buy a heater without a thermostat – any upfront saving you make will soon be eaten up by its increased running cost.
Once the room is warm, a thermostat turns the heater on and off. It’s a simple warmth-operated switch: when it warms up, it opens up an electrical circuit to turn the heat off. When it cools a little, it turns the heat back on again.
Thermostats that respond to small temperature changes and turn the heater on and off frequently are more efficient and mean the heater uses less energy to keep your room warm. The best thermostat-controlled heaters in our test kept our test room to within half a degree of the desired temperature. The worst allowed it to vary by 5°C.
Goldair vs Goldair: Panel vs Panel
Panel (or convector) heaters are the simplest of all heaters: they consist of a ventilated metal box, a heating element and a switch to turn the element on and off. They work by convection: the heating element draws cooler air in through vents in the base; the air rises and warms as it passes over the heating element; then returns to the room through vents at the top.
We compared the two similar 1500W Goldair panel heaters we used in our Plug-in heater features article. The cheaper GPH350 (left) has an analogue dial thermostat and two user-selectable power levels, while the GPPH620 model (right) comes with a digital thermostat, five power levels, and “inverter technology”.
Inside, the two heaters have identical heating elements contained in functionally identical metal boxes. In our test room, they would perform similarly in our heat-up test, and create a similar (not very good) heat distribution in the room. Neither have fans to help circulate the air, so warmer air would pool above the heater, while the other side of the room stayed cooler.
However, you might find a difference in how well they maintain a set room temperature, because they use different types of thermostat.
Low-rent analogue or high-tech digital?
The cheaper Goldair model uses an analogue thermostat, controlled by a rotary knob that can be set anywhere between its arbitrary “low” and “high” settings. The pricier model has a digital thermostat that can be set anywhere between 5°C and 35°C in 1°C increments.
Don’t swoon over that hi-tech digital readout. Since the thermostat is inside the heater, we found 18°C in the room, measured 3m from the heater, corresponded to 21°C shown on the heater display. Whether analogue or digital, it can take a lot of trial and error to find a thermostat setting that achieves the perfect level of warmth.
If you want to control the real room temperature, you need a remote thermostat, such as the Heatermate.
Our testing found no correlation between the thermostat type and its performance. In fact, the best we’ve tested was an analogue thermostat fitted to an inexpensive Living & Co oil column heater (a model no longer available), while the worst was a digital thermostat on a Dyson fan heater.
If it comes to preference, a digital display and buttons can be more intuitive to use, while a dial is a simple set-and-forget option.
Inverter like a heat pump
The cheaper Goldair panel heater has two heat settings. This switch cuts the heater power in half (from 1500W to 750W). If you’re heating a smaller room or trying to create gentle background warmth, using the lower power setting may make the heater more efficient (as it doesn’t overheat the room too much before the thermostat can respond and turn it off).
The “inverter technology” on the GPPH620 heater refers to automatically selected variable power (it has five power levels that can also be chosen manually). This tech is usually found in heat pumps and means the heater reduces its power output when it gets closer to the desired thermostat setting, so the room doesn’t overheat. This should make this model cheaper to run.