In Part-I of the above Article, we tried to understand how the ‘Cooling’ or the ‘Refrigerating’ part of the System works.

From April 2000 onwards, most Manufacturers have switched over to ‘R134A’ as the ‘Mandatory eco-friendly’ Refrigerant - as opposed to the earlier Ozone-depleting ‘R12’.

To an average Car Owner, it should suffice to bear in mind which System his Car has. This is clearly stated in his Owners Manual + suitable ‘stickers’ in the Engine Compartment – such that while ‘topping-up’ no mistakes are made.

Suffice to say that ‘R12’ in a ‘R134A’ System can be tolerated to some extent but the other way round is a no-no. This is because the ‘latent heat of evaporation’ of R134A is ‘lower’ than R12 and therefore, R134A systems call for larger front-end Condensers and in-cabin ‘Cooling-Coils’.

To make things a little more complicated, most of the present generation Cars nowadays come OE with an ‘HVAC’ – Heating Ventilation and Air Conditioning - System. The major advantage of such a System is that it allows one to choose the most comfortable in-cabin Temperature without having to suffer the suffocating thermostatic ‘dead-band’ of an ‘AC System Only’ in the earlier Cars.

Now let’s see how this is achieved and what are its plus/minus points – as far as load on the engine and hence the fuel consumption is concerned.

A Typical HVAC FAQ

People often wonder or wish to know if there is going to be a difference in Load on the engine/fuel consumption when one keeps the ‘Red to Blue’ Thermostat at max blue or less. For example, what happens when one moves the Thermostat from the coldest to an intermediate position -

1) Does the AC Compressor run continuously or keeps cutting in and out – loading the Engine accordingly.

OR

2) Does it run continuously and thus load on engine more?

One would ask such a question when failing to notice any difference on the Load on the engine when the AC thermostat is set to less than ‘max-cool’.

Or in other words, the AC Comp doesn’t seem to cutout even when one desired less than max cooling from it. So - where does the extra cooling go when the Thermostat is set to low!