Why do electric heaters have selectable power options (efficiency question)?

[Geflugenmitt]

General query. I have owned a few stand-alone electric heaters in my time - eg oil-filled, convector and fan. They all have a thermostat to limit the upper temperature (a lot of the time this regulates the temperature of the heater rather than the room, due to its proximity to the heating elements, but that's a different issue).

What I am wondering is why most/all of these heaters also have a selectable power switch, typically 1, 1.5 or 2 kW. Intuition tells me to use the highest setting always, which brings the heater and the room up to temperature faster, and then regulate that with the thermostat.

So what is the rationale for having the lower settings? Is it supposed to be more efficient to burn 1kW, even if the heater then never turns off because it can't quite reach the thermostat temperature? I've experimented with my oil-filled rads, and on the 2 kW setting, once it's reached temperature, it spends less than 50% duty cycle, therefore burning less than 1kW on continuously. This should not be possible for the same room temperature due to the laws of physics.

 

[JohnD]

It reduces the size of the temperature swings.

 

[aptsys]

It reduces the size of the temperature swings.

That's down to the hysteresis of the thermostat, not the power level.

There's no real rationale behind it

 

[foxhole]

It reduces the size of the temperature swings

 

[Mottie]

Perhaps it reduces the size if the temperature swings.

 

[mattylad]

It allows the temperature to slide up and down as it swings, a roundabout thing.

Could it be that sometimes you may not want full on hot but perhaps just a little warm?

When I have my fan heater on at work (on a cold day) under the bench I sometimes have to turn it to the half setting so my gonads don't get roasted.

 

[aptsys]

It reduces the size of the temperature swings

How?

 

[ericmark]

The heater in my caravan has three settings 500W, 1000W and 2000W, (also gas option) in the main I use 500W so the combined load will not trip the incoming supply, the heat goes into a large cast lump which either allows air to freely circulate around it or we can switch on a fan which circulates the air the latter is 12 volt so could be a drain on battery when not connected to mains. Thermostat is on opposite wall.

The size of the heat sink means it really makes not a scrap of difference what the input is, once up to temperature it is rare to need over 500W. Only when exceptional windy and cold does one need the higher settings.

So with a fan heater there may be an advantage using the lower setting, but with a oil filled radiator it is unlikely to make much difference. However it would depend on the thermostat, with my central heating radiator the thermostatic radiator valve head has two sensors one for air and one for water, and the latter compensates the former so what it reads is near enough the room temperature.

Again in another house we had a Myson fan assisted radiator, and the hysteresis with that was quite marked, one felt the chill before it restarted.

I would assume with a portable heater it allows it to be used in many different rooms and adjusted to suit, so in a draughty poorly insulated small room the lower the setting the more uniform the temperature, and selecting an input near to what is required would help. But put the same heater in a draft proof well insulated large room and likely it will not make a scrap of difference.

In this house when the central heating goes off at 11 pm it is 5 am before you notice the temperature drop specially upstairs, the radiators are only just warm even on very cold days, except just after the TRV has changed the target temperature. This is of course the advantage of a modulating boiler and modulating TRV heads they run all the time but with very little raise in temperature over the room so you don't notice it switching off/on.

However the old mineral insulated hot plate had a large time between off/on periods where as the new induction hob has a very small time, the mark/space ratio is likely the same, but the time of each mark/space can be so short it seems as if analogue or that long you can see the ring heating and cooling. It is the same with any heater using mark/space control.

Since the less the thermostat switches the lower will be the wear and tare, and also the lower the current the less wear and tare, added to that likely even with best control there will be some hysteresis, I would always select the lowest output that will do the job, even if in that home with that heater it may make no difference to how well it heats, likely the lower the setting the longer it will last.

 

[ericmark]

How?

It takes time for heat to travel from the heater to thermostat, during that time the heater is still producing heat it just has not reached the thermostat, so more it produces per second the more it will over shoot, of course the difference between off and on will also affect the over shoot.

With heat pumps you don't want to restart the pump until the pressure has dropped, so with some thermostats there is also a delay built in. So with an analogue output as with modulating gas boilers you would get this curve,

with a digital control you would get this curve,

however if you use a solid state relay you can closer emulate a analogue control so more like curve as in real terms it starts to pulse before it reaches the target temperature and continues after so reducing the hysteresis, with mechanical contacts they would wear out too quick, only with solid state can it switch that often without failing.

 

[aptsys]

It takes time for heat to travel from the heater to thermostat, during that time the heater is still producing heat it just has not reached the thermostat, so more it produces per second the more it will over shoot, of course the difference between off and on will also affect the over shoot.

With heat pumps you don't want to restart the pump until the pressure has dropped, so with some thermostats there is also a delay built in. So with an analogue output as with modulating gas boilers you would get View attachment 160478 this curve,

with a digital control you would get View attachment 160479 this curve,

however if you use a solid state relay you can closer emulate a analogue control so more like View attachment 160480 curve as in real terms it starts to pulse before it reaches the target temperature and continues after so reducing the hysteresis, with mechanical contacts they would wear out too quick, only with solid state can it switch that often without failing.

This is a function of the control not the power output of the heater. Unless the heater has a large thermal mass, it is a non-issue for room heating.

 

[ericmark]

The diagram shows circulation, cycle time is important, if the heater can cool completely during the on/off cycle the flow of air is interrupted if however the cycle is fast 5 minutes or less it works as if it's analogue control however as the cycle time approaches ½ hour then circulation is lost.

So control and heat output are linked, with a semi-conductor control it really does not matter how bit the heater is, but with mechanical control one wants the heater to be just enough so most of the time it is on.