Ventilation is key

Properly cleaning HVAC units is time consuming, but the payoff is well worth it
By Robert Buchwalter

We have all been enjoying, or enduring, an extended hot and humid summer in North America. For those of us in the bus business, images of warm weather, picnics, baseball and fireflies are mixed with dreams of refrigerant bottles, compressors, filters, gauges and unhappy drivers. Our daily write up sheets include the expression “A/C not working” written boldly, often followed by more than a few exclamation points. It happens every year. But this summer’s record setting heat and drought have only increased the scrutiny paid to our HVAC systems, especially by our paying customers. The heat is on everyone’s mind in 2012 and we have to be on top of our game in servicing and diagnosing our HVAC systems.

HVAC brings to mind a number of diagnostic disciplines (refrigeration, electrical, heating) that we need to apply in repairing these systems, but let’s look at those letters intently. Traditionally, HVAC means Heating, Ventilating, and Air Conditioning. But for us, it actually means Hot Vehicles Aren’t Chartered. A bus with no air conditioning is clearly a bus down situation. Yet the letters themselves – HVAC – are incomplete. It ought to be HVACCDT, which stands for Heating, Ventilating, Air Conditioning, Controls, Driver, Technician. If we are missing something in anyone of those areas, we will have problems keeping our passengers, drivers, and supervisors cool.

Traditionally, a hot bus complaint rives us to inspect the refrigeration system: the compressor, condenser, evaporator, and expansion valve. But this month, I would like to focus on the V in HVACCDT. Ventilation is extremely important in maintaining a comfortable cabin, but it is often overlooked in diagnosis.

When it comes to ventilation, we usually look at the amount of air being discharged at the window sills and flowing into the cabin. If this flow seems to be less than normal, we adapt the mindset of trying to force more air into the cabin. But as we look to improve discharge airflow, and passenger comfort, I suggest we also look at the other side of the squirrel cages, to the return air side. Return air is equally important to discharge air. If you cannot pull the air out of the cabin, you can’t push it back into the cabin.

In the Prevost training seminars that Robert Hitt and I conduct, discussions about return air flow center on the Prevost H-Series coaches. The Prevost X-Series: XL, XLII, and X345 coaches have a simple return air system: a single basket placed above the evaporator compartment. The H-Series is a bit more complex. In the H-Series has a return air grill in the stepwell and two return airs on the driver’s side of the coach, at the front and rear. These are the starting points for the ductwork system that eventually delivers the return air to the evaporator compartment. Cabin debris can set up in these areas and begin to impede return air flow. Regardless of your OEM, you should explore, learn and be familiar with the return air system and be on the lookout for areas that can compromise this air flow. Wherever air flow is forced to turn a corner, it will slow and debris can drop out of the return air stream. These areas need careful cleaning based on your coach’s environment and operating cycles.

Blockage in the return air system, with the resultant loss of ventilation, can also cause problems for the cabin temperature sensors which are usually in the return air ducts. If we are not flowing the proper return air, these sensors will fail to obtain a true reading and the driver must over or under adjust the set point to obtain a comfortable cabin temperature.

In certain cases, it may be necessary to create your own inspection panels by cutting into the duct and directly inspecting areas of the return air system not normally viewed. You can also reach in and remove any built up cabin debris that will collect in these areas. Inspect the duct completely, as there are a lot of nooks and crannies where debris can lodge. When you do this, remember to leave a flange for attaching the closure panel you will have to fabricate to seal the inspection area.

There’s also another procedure that used to be a staple in the bus business: reverse cleaning of the evaporator.

Generally, the return air flow in a coach is pulled through the return air filter, evaporator and the heater core, then sucked into the squirrel cage fans and pushed back into the cabin. When we reverse clean an evaporator, we are going to remove the filter, completely remove the evaporator motor, housing, fans and ducting. Then we remove the heater core (shutting off our manual coolant control valves first). Once we’ve exposed the face of the evaporator, we are going to blow compressed air backwards through the evaporator fins and tubing, then use a low pressure garden hose to wash out the evaporator. As with the air, we are going to flow the water against what would be normal air flow direction.

Once this is done, clean out the area behind the evaporator and repeat the process for the heater core before reinstalling it.

This is a big job and, at least on a Prevost, requires 4-6 hours to complete, but I believe the results are well worth it. Take the oldest coach in your fleet and conduct this operation. I think the results, improved air flow and improved performance of the evaporator and heater cores will absolutely convince you it’s worth the time. BRM

Robert Buchwalter has been in the motorcoach industry since 1981 and joined Prevost Car in 1991. He has been one of Prevost’s two technical field instructors 1997. He conducts seminars at the Prevost factory in Quebec, customers’ shops, and transit agencies. He is a member of the BUSRide Editorial Advisory Board.