On the subject of air conditioning units working in reverse as reversible air source heat pumps to produce hot air to help warm up the cabin before any engine heat is available, (Mohamed, Issue 333), I started digging into this and found the following information regarding electric cabin air heaters. So I am now wondering if many cars have this feature of which owners are unaware? All Mk 5 Golf and Jetta Diesels have them; (and later Golfs, and many other cars, including some Fords, I have now discovered. Doc.) they are called PET heaters and are basically a 1kW electric heater positioned upstream of the heater matrix. A Mk 5 Golf technician’s self-study programme says:

“An electrical auxiliary air heater is installed in diesel vehicles. The electrical auxiliary air heater is a combination of a PTC heater element with an integrated control unit. It is installed downstream of the heat exchanger and provides additional heating of the vehicle interior after cold-starting the engine. The electrical auxiliary air heater directly heats the air which enters the vehicle interior.
Activation conditions: The electrical auxiliary air heater is activated:
• in the case of the Climatronic and Climatic systems, automatically via the CAN data bus.
• if the occupants set the heating output to over 90 per cent at the operating unit.
• if the water temperature is below 75 degrees Celsius.
• if the engine speed is higher than 500rpm.
• if no load management system is active.
• if the ‘Econ’ button is not pressed.”

Can we be sure that Mohamed’s Citroën and his other cars did not have this feature, rather than the clever heat pump air-conditioning unit that he describes?
Harry Gent

Hmmm. I don’t like feeling ignorant Harry, so I’ve had to do quite a lot of research to update myself on this, and I’m still feeling somewhat ignorant. Funny how manufacturers want you to know some things, but not others! PTC ceramic chips have been available for many years as low wattage sources of electric heat, and are manufactured from a barium titanate material, plus other control components, to provide the desired resistance/temperature characteristics. When electrical voltage is applied to a PTC chip, heat is not generated at a constant rate. These chips have a built-in temperature limit such that as the PTC chips heat up, they reach a design temperature at which the heat output decreases drastically and in this case ceases to heat the incoming air.

But now see this from Nissan: “A heat-pump cabin heater has been adopted for heating an electric vehicle (EV), using less power than (in) conventional models. It greatly improves power consumption when the heater is being used. Nissan Leaf is the first mass-produced vehicle in the world to employ a heat-pump cabin heater. Conventional air-conditioning systems used an electrical heater, but since the use of the heater directly relates to power consumption, actual driving range was significantly reduced. A heat-pump system, meanwhile, heats the cabin using the temperature difference between a refrigerant and the outside air, obtaining a heating effect (by) other than consuming electricity, and making it possible to heat the car cabin with less power than conventionally.” This system has now been used in Renault’s Zoe EV, too. Apparently, depending on ambient temperatures, electric power drawn from the batteries can produce as much as three times as much cabin heating (or cooling) power, using the heat pump principle, by drawing heat from (or discharging heat to) the ambient air. If Nissan’s claim is correct, then Mohamed’s Citroën did not have a true air source heat pump, and I shall have to do some more investigations into this and hopefully clarify things at a later date. My thanks to Mohamed and Harry for opening this fascinating subject.
Doc