Air Conditioning Part 2

This is a continuation of our blog on air conditioning.  We will focus on the next three components of the A/C system which includes the liquid receiver tank, the air conditioning compressor and the condensor coil.  Each of these components is an intregal part of a properly running system.

The first of these components is the liquid receiver tank.  This tank is located in the heating and air conditioning compartment, and this location varies depending on the type of coach you have.  It acts as a reservoir for the liquid refrigerant that is in your system.  It assures that only liquid refrigerant will be admitted to the expansion valve, evaporator, and manifold tubes.  The sight glass in the receiver glass provides a way of determining the amount of refrigerant in your system.  With the A/C system running at a stabilized temperature and the coach engine operating at high idle, the refrigerant liquid level should be maintained ao that the ball is about in the middle of the sight glass.

The next component is the condenser coil.  The fin and tube-type condenser coil is usually mounted behind the evaporator and heater core in the heat and air condiditoning compartment.  Air flow across the condenser coil is provided by electrically driven fan motors.  For obvious reasons, properly operating fan motors are crucial to achieving maximum air flow and performance.  It is important to keep the evaporator, heater core and condenser coil free of any dirt and debris in order to get the most air flow and cooling capacity/heat transfer from your unit.

The final component we will discuss in this section is the air conditioning compressor. The compressor acts as an engine for the air conditioning system.  The capacity of the compressor is determined by piston displacement and clearance, suction and discharge valve size, compressor speed, suction and discharge pressure and the type of refrigerant.  The compressor suction pulls gas from the evaporator.

The compressor compresses the gas to higher pressures and puses it to the condenser which has colder air running through it.  There it takes the heat from the gas to condense to a liquid form and moves to the receiver dryer which then goes to the expansion valve.  The expansion valve, as discussed in the previous blog, controls the flow of liquid to the evaporator.

We will continue our blog next week with additional components in the system followed by basic operations of the system and symptoms of failures within the system.

Please remember that this information does not replace the need for a qualified mechanic.  We do offer technical support on

Continuation of Heating System

Now that we have discussed the components of the heating system and their general operation, we will pull it all together into what the entire system does and it optimal operation.

It is important to periodically check your system, not just for defects, but for regular maintenance items.  Some examples of maintenance items would be things like filters that you might have in the floor vents, dirt that may have accumulated in and around your heater core area, both intake and return water lines, and, of course, any electrical connections.  Each specific coach should have a schematic indicating the flow of water through the heating system.  

For most coaches, the interior of the coach is heated by an underfloor, forced air system using hot water.  Controls for the temperature may differ from one coach to the next.  They can be either air or electrically operated, depending on the manufacturer preference.  In both cases, these control function to control the primary flow of water through the system.

The grad-u-stat which is used for controlling the coach temperature is located in the right recirculating air duct, which is located under the passenger seat.  Some coaches are equipped with a special temperature control rheostat.  This rheostat is usually mounted on the driver control panel.

The hot water used in this system is supplied by the engine cooling system.  It is then moved to the heater core compartment by way of the supply line using both the engine water pump and the water booster pump.  From there it warms the air moving across the core by way of the circulating fan and blower motor.  Outside air for ventilation or defrost is admitted to the heater or is drawn in by the blower motor.  The water modulator valve and the water circulation pump are may be located with this same compartment. 

After circulating through the heater core, the water returns to the engine by way of the return water line.  Warm water from the booster pump is routed to the defroster heater core which is located at the front of the coach in the driver's compartment.  Warm air from the heater compartment is directed by way of a damper/vent to defrost the windshield and warm the driver's area. Once it goes through this heater core, it also returns to the engine. That completes the cycle of the heating system.

Naturally, when you have mechanical items that must work in unison for things to operate properly, there is always room for failure.  These failures/errors can cause under-heating problems, over-heating problems or a combination of both.  For example, the grad-u-stat setting can cause either situation. If the setting is too low, if it is out of calibration or if the unit is sticking, it will cause under-heating.  If the setting is too high, it can cause over-heating.  With using just that one part as an example, you can surely see how critical proper operation and function of each part can change the functioning of the entire system.

There is a basic trouble-shooting guide that can be made available upon request.  Just send us an e-mail to busfixx@gmail.com.  We will be happy to send it to you for reference.

Remember that this blog is for informational purposes only and is not intended to take the place of a qualified mechanic when one is deemed necessary.

Sources: RTS maintenance manual, Eagle manual, GMC manual

Coach heating continued

Two systems, using forced air, are used to heat the interior of the coach.  One is the defrost heater which is used to heat the front of the coach and the driver's area.  The other is the underfloor system that uses the evaporator core, heater core and air filter screen in conjunction with blower motors for circulation.

Hot water from the engine cooling system is circulated through the supply lines, using the water booster pump, to the heater core.  The "Heat - Air Cond." control switch and temperature range switch control the flow of water through the coil of the front heater core. The heater control valve increases or decreases the flow of water through the defrost heater core.  After going through the heater core, the water is returned to the engine via a return line.

Gate valves, which are located in the heater supply and the return lines, provide a way of isolating the heating system from the engine cooling system. This allows for one system to be drained while leaving the other system intact and undisturbed.

Draining of the heating system is accomplished by the use of a drain cock which are located in the front heater system supply and return lines. Additionally another drain cock is located in the front heater core which is used to drain the front heater core and lines.

In addition to water circulation, adequate air circulation is also required for the system to work properly.  Blower motors, in the heating/air conditioning compartment, draw some air from the outside, through a filter screen.  Air that is recirculated is also drawn into the same compartment by way of ducts that are located under the seats (return air).

The blower motors draw the blended air through the heater core, evaporator core and filter screens. It is pushed into the coach interior.  When the "Heat - Air Cond." switch is in the "Heat" position, the blended air is not affected by the evaporator but heats up as it passes through the heater core.

The heated air enters the coach through ducts that are on each side of the coach at the floor and directly adjacent the windows. The heat warms the floor and the feet of the passengers. Ducts along the left side of the driver's area, warm the driver through adjustable vents.

The defroster heater assembly performs two functions. The first is that the heat is used to keep the windshield defrosted. It also recirculates air to provide heat to the front of the coach. The front heater core warms recirculated air that has been drawn through the dash compartment opening by the front blower motors.

The engine water pump and circulating pump motors use the pumps to provide water circulation to the main heater core, the driver's heater and sidewall heaters. In some cases there are also return air shutters which are used to control the recirculated air and outside air.

This blog is for informational purposes only.  It is not meant to take the place of a qualified mechanic when one is deemed necessary.

Sources: MCI 102D, GMC 4108-4905, RTS06

COACH HEATING

In most cases, coaches are “comfort” heated by way of hot water, forced air system, utilizing circulated coolant.  Heated coolant, generated by the combustion process, is circulated through the system.  Fresh and re-circulated air is used to heat the interior of the coach.

Hot water is forced through the heater supply line by the engine water pump. An electric water circulating pump and motor assembly pushes the coolant through the main heater core supply line and the driver’s heater supply line.  In some cases there might be an optional sidewall heater supply line.

Gate valves, one in the heater supply line and one in the return line, provides a means of isolating the heating system from the engine coolant system. This allows for one system to be drained while the other stays intact.

Once the coolant circulates through the main heater core, the driver’s heater core, and any sidewall heating lines, it is returned to the thermostat housing on the engine by way of the heater system return lines.

Most of the components are the same regardless of the coach system.  Each system has a heater core.  It is made of a fin and tube design, much like a radiator. Warm water from the engine circulates through the tubes of the heater and provides the means of heating the air forced through the system by the blower motors.

The blower motor portion of the system consists of motors and blower fan wheels. Each blower motor draws outside air through a filter screen into the heating and air conditioning compartment.  The driver’s compartment has its own blower motor, separate from the main motors.

A hot water valve assembly is included in each type of system as well. Some have a water modulator valve which is air operated. This valve is a spring loaded, piston type valve. Movement of the piston actuates the valve’s stem, opening and closing the valve. In other coaches, there is a solenoid water valve. It is electrically operated but performs the same function. This valve is normally closed but when energized to open, it controls the flow of coolant as dictated by the heat control circuit.

In older coaches, there may be a grad-u-stat.  This is a thermostatically air controlled valve that is sensitive to the inside coach temperature. Expansion and contraction of the element of the valve is caused by the increase or decrease in the coach temperature.

Every type of system requires some type of temperature control assembly. Some controls,  which are used, are electrically connected to the air operated main valve in the system and the temperature probe which is installed in the coach’s duct.  For more mature coaches, these controls are connected to the grad-u-stat which has a temperature range of 65 to 85 degrees.

All systems have a water circulation system that works in conjunction with an air circulation system.  Both systems need to be in full operating condition in order for the heating system to work at its optimal capacity. 

This blog is for informational purposes only. All information provided herein is not intended to take the place of a qualified mechanic, when deemed necessary. 

Sources: GMC 4108-4905, RTS, and MCI manuals

Summer will be here....

Summer will be here before we know it!  With that said, now is the time to checking the functionality of your air conditioning system.  This includes items such as blower motors, air conditioning compressor, filter dryers, filters, etc.

The air conditioning system is an integrated system that works together with the heating system. Typically this is an automatic system using many of the same controls.  In most cases, the system uses a thermostat that controls both the heat and the air conditioning.  When the climate control switch (thermostat) is OFF, the heater-air conditioning unit is not operational.

The cooling units consist of:

1. The evaporator coil of fin and tube type construction
2.  A refrigerant expansion valve
3.  Filter dryer
4.  Liquid refrigerant receiver tank
5.  The air conditioning compressor
6.  The condenser coil

Additionally there are blower motors, hoses, high and low pressure switches and, of course, adequate refrigerant.

The first thing to check is the evaporator coil.  The main evaporator is typically accessed through one of the baggage compartments.  There is another, smaller one mounted with the driver's heater core. The integrity of the fins and the tubes are critical when it comes to air and oil flow.  The air passes through the evaporator core.  The core cools the air and removes the moisture from it.  The air is then reheated by the heater core.  The evaporator coil must also provide a steady flow of oil to the compressor.  Proper lubrication going from the evaporator to the compressor is essential.

Liquid refrigerant and oil enter the evaporator at the expansion valve and pass through a series of tubes until the refrigerant is completely evaporated.  It is extremely important that both the refrigerant and the oil are at the proper levels when this process is started for the proper operation of the complete system.  The refrigerant boils, absorbs heat from the tubes and fins while the oil, which is saturated with refrigerant, foams as it is forced through the coil by the boiling refrigerant.

If the flow rate is correct and the oil started at the proper level, the oil goes back to the compressor where it is collected in the suction chamber and returned to the crankcase.  Some of the oil is pulled through the suction valves to provide lubrication for the rest of the compressor.

The refrigerant expansion valve is mounted to the evaporator coil and the refrigerant liquid line. The expansion valve will be accessed the same way the evaporator is accessed.  The expansion valve controls the flow of liquid refrigerant going into the evaporator coil.  The valve is designed to maintain the superheat on the gas returning to the compressor.  Superheat is measured is degrees and is necessary to prevent the refrigerant from returning to the compressor in a liquid form.  Superheat is the amount of heat added to a gas to raise the temperature above its boiling point.  Superheat is achieved by adding less liquid refrigerant to the evaporator than what is capable of boiling.  The gas then absorbs the heat from coil to increase a greater temperature - or superheat.

Expansion valve adjustments should not be required unless the valve power head has been replaced or damaged.  Any adjustments should only be performed by a qualified refrigeration mechanic using proper equipment.  Since the most common malfunction is a loose or improperly installed remote bulb, which causes improper  heat transfer, it is imperative that this installation is done properly.

The filter dryer is a disposable type screw-on filter that is mounted in the high pressure liquid line.  It is used to remove foreign matter and moisture from the refrigerant before it reaches the expansion valve.  Any time that work is done on a compressor or on the system in general, this filter dryer should be replaced as a precaution.

This is the first half of the components that make of the air conditioning system in a coach.  Our next blog will focus on the second half, followed by the general operation of the system.

It is our hope that you will check back to continue with our series on air conditioning in preparation for summer.  See you again soon!!