The HVAC School of Witchcraft and Wizardry

SPONSORED CONTENT: Phil Reynolds works in the product marketing department at PACCAR Parts. Here, he puts his spin on the operation of HVAC

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Welcome students to a wondrous journey delving into what some would believe is the dark arts of truck HVAC systems. Our first lesson: what is HVAC? Easy – it stands for heating, ventilation and air conditioning. That makes sense – so let’s see what else we can make sense of.

Heating for vehicles is achieved by sourcing hot coolant from the engine and running it through a heater core (think small radiator) inside the cab. Coupled with controllable taps for coolant flow and blower fans, we can control the amount of heat output for comfort.

Ventilation is purely our means of moving heated, cooled, internal or external ‘fresh’ air around a cab. This is done with ducts but also mechanisms to control the direction of flow, so we can choose between feet, head, screen or a combination. This control is also available for either recirculating the air inside the cab, or taking fresh air from outside to pass through the HVAC system.

These mechanisms have changed over time from simple cables to vacuum lines coupled between servos and engine manifolds, to modern servos electrically controlled for ease of operation.

Phil Reynolds

And so we come to the air conditioning system, yes the most intricate of our HVAC ingredients but one that once mastered in understanding will have you feeling cooler than Ron Weasley after winning the affections of Hermione Granger. Let’s start at the heart.

The compressor – our only mechanically driven part in the whole system and commonly the part that packs up when issues are present elsewhere in the system (ever had a compressor claim knocked back citing it was a consequence of another issue?). The compressor is basically a high pressure pump taking low pressure refrigerant gas from the evaporator (we’ll get to that bit) compressing it and delivering it as a high pressure hot gas to the condenser (that part on the front of your truck that looks like a thin radiator).

The condenser cools the hot gas, reducing the pressure and returning the gas to a liquid state. Next, it’s on to the receiver dryer taking the liquid refrigerant, filtering it and removing moisture – the arch nemesis of the AC system. Now, we have filtered and moisture-free liquid refrigerant heading toward your cab, ready to work its magic through the Thermal Expansion valve (TX valve).

High pressure liquid refrigerant is forced through the TX valve where it expands, dropping in pressure and temperature and evaporates (think of turning on your BBQ gas bottle and as the gas comes out from the high pressure liquid state within the bottle it expands to its gas state cooling as it does). It enters the evaporator core (like your heater core) and blower fans push air across the surface area of the core to exchange that cool refrigerant into cool air for your cab.

C:\GREGS FILES\4. OWNER DRIVER WEBSITE\Oct 2020\Paccar parts\HVAC-diagram-1.jpgOnce through the evaporator core, the low pressure refrigerant heads back to the compressor to start its journey again. There are hoses and pressure switches along the way to keep everything contained and in check. And that, for those still following, is the way an air conditioning system works!

Now that you are a little more knowledgeable on the HVAC system, you must take that knowledge and use it for good. If you do experience an air conditioning compressor failure, think about the impact on the rest of the components in the system.

Whenever you experience a compressor failure, or you are doing preventative maintenance on your system, it is always good practice to change the receiver dryer and the TX valve, which has low tolerance for foreign material. The amount of refrigerant and lubricating oil is also critical, so make sure you get repairs done from your authorised dealership, such as the PACCAR dealer network, which supplies quality, warranty-backed HVAC parts and technical wizards trained in the art of HVAC!

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