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I'll have to explain it step by step so that you have no chance to misunderstand what I'm asking about.

First, what a convector heater is. It's a plasma TV shaped box usually made of sheet steel. There's no plasma TV in there, it's hollow. A set of holes is made in the bottom side of the box. Another set of holes is made either in the top side or in the front side near the top of the box. A resistive heating element is installed inside the box near its bottom. The box is either mounted onto a wall or installed on small wheels or legs. The heating element is connected to AC mains so that it can draw electricity and produce heat which heats the air inside the box and causes convective flow of air through the box. So it's yet another way to produce an old boring electric heater.

That was an unregulated heater which would either underheat or overheat the room most of the time. So a heater is usually equipped with a thermostat which is mounted somewhere on the box. A thermostat may have a rotating knob and a bi-metal plate or it may be an electronic thermostat which has some electronic circuits and button controls to set desired temperature. Either way it cycles the heating element on and off. So the heater is either fully on and producing heat or it's off and it's cooling down to room temperature.

Now welcome an "inverter control" heater. It's the same heater as above but now it has some clever electronic circuit that can run the same heating element at different power levels. I cannot find the details of how exactly it's implemented (and if I could I'd rather not understand them) but it sound very similar to how LEDs are driven with a switched mode driver - exactly the right average power is applied to the heating element and so it's always "30% on".

Then there're two very similar convector heaters - nearly the same shape and size, same power and both with "inverter control", produced by two different manufacturers. Some dealer claims that a certification lab tested both heaters and one of them turned out to be seven percent more electrically efficient than the other. I don't know what exactly "electrically efficient" means but the dealer frames it such that the "better" heater will consume less electricity for the same amount of heat.

How could this be possible? If it were LEDs than the driver losses would be useless (and even harmful) heat. This time heat is useful and it doesn't matter where heat is produced - in the heater or in its control unit which is attached to the heater. Either way it should produce one kilowatt-hour of heat for every kilowatt-hour of consumed electricity.

How could one heater be "more electrically efficient" than the other? Is it technically possible?

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  • \$\begingroup\$ The only thing that sprung to mind is that the more efficient one keeps supply voltage and current in phase better by being closer to a purely resistive load. \$\endgroup\$ – ocrdu Sep 15 '20 at 10:11
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    \$\begingroup\$ The term efficiency is normally applied to power conversion devices, so that power output is power in a form other than heat, and the difference between input and output is lost heat. Since all the heat produced in a heater can be considered to be output, heaters by the normal definition are 100% efficient. "Some dealer" must have his own definition of efficiency, perhaps "more profitable." The testing lab may have determined that the better heater keeps the temperature more constant. The dealer may have somehow translated that to "efficiency." \$\endgroup\$ – Charles Cowie Sep 15 '20 at 13:02
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You are correct. An inefficient heater would just waste power as heat which would heat the room in any case.

Due to the slow thermal response of a room most heating systems work on a slow duty cycle by thermostat. So 30% power might be full power for 30 s on and 70 s off. Due to hysteresis in the thermostat this approach would cause some oscillation of the temperature around the setpoint. And this is really the only opportunity to improve "efficiency".

If some sort of dimmer controller is used on the heater and a PID (proportional-integral-derivitive) control algorithm is used then the heater could run at 30% continuously and avoid the oscillation. This might improve the efficiency a little for a given setpoint. You'd probably need to compare against the worst thermostat type to get a 7% improvement.

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Figure 1. Pavel Rojtberg's improvement to his meCoffee machine gives an idea of improved control available using PID relative to a thermostat. Source: rojtbert.net.

As to what "inverter" means in this context you'd need to talk to their marketing people. Generally it is used to describe a device to convert DC into AC but the heater is running on an AC supply to start with so there is no need for one.

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  • \$\begingroup\$ Is there a way for two PID controlled heaters each with "dimmer" to have 7 percent efficiency difference? \$\endgroup\$ – sharptooth Sep 15 '20 at 11:25
  • \$\begingroup\$ Not if they hold the temperature close to the setpoint. \$\endgroup\$ – Transistor Sep 15 '20 at 12:39

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