HVAC operation and problems

Old 02-09-2010, 04:09 PM
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Default HVAC operation and problems

there is always alot of questions about the hvac system and problems that we have with it. Thought it would be nice to do a write-up that would cover the operation of the system,how it works,issues that come up and how to repair them.
The write up is written assuming you do not have any mechanical experience but want to learn and do the repairs yourself,it will be very basic and specific.

The a/c side of the system is by far the most complicated and contains the most components,I will cover it first and in depth,then the heat side last.
The write-up will be done in 2 sections,first how the system works then the issues that come up and how to repair them. This will make it easier to understand as there is alot to cover.

Before we get started there is a couple of things to understand and safety issues to cover.

1. Refrigerant is the cooling medium for the ac system,not freon,freon is a trade name that Dupont uses to describe a group of refrigerants they developed,I will use refrigerant in this write up.

2. This write-up is about the hvac system and how to repair it,however when it comes to repairs that requires you to drain it to replace a component I strongly recommend you have a shop do the work,The reason for this is simple,most do not have the tools or ability to do the work right.first it is against the law to allow refrigerants to escape to the atmosphere knowingly,if you do not have an evacuation and recovery system you cannot do this legally.
The only component that can be replaced without draining the system is the compressor,it has service valves that allow you to isolate it from the system,however if the compressor goes bad it will always put debris and contaminates in the system,so it would have to be drained and flushed anyway. With that said there is still alot you can do yourself and will be covered here.

note: Do not for any reason mix refrigerants,R-12 and R134a are not compatible. In 1994 Chrysler started using R134a in there cars,if yours is a 94 or newer it has R134a.

This part maybe a little dry but if you want to know how the system works read on,if not skip to how the system works or trouble shooting.

Temperature changes and how they take place.

Latent heat is a big part of the cycle,latent means (hidden)latent heat is heat that changes the state of the refrigerant without changing the temperature.
for example water boils at 212 degrees,to change the same pound of water to 1 pound of steam you have to add 970 BTU's,even though you have added 970 BTU's,the vapor or steam is still 212 degrees,latent heat is the heat required to change a substance from one state to another without changing the temp.
The different phases of latent heat are.
A. latent heat of fusion,this is where water is changed from a liquid to a solid.
B. Latent heat of condensation,this is where the refrigerant goes from a vapor to a liquid,this takes place in the condenser.
c. Latent heat of evaporation,this is where the refrigerant goes from a liquid to vapor,this takes place in the evaporator.
This will be covered more later
D. Sensible heat,is heat that can be measured,the heat in your car is sensible heat.
e. Specific heat,is heat that is know,example is the amount of BTU's to change 1 pound of water at 212 degrees to a vapor at 212 degrees is 970 BTU's,since we know this,it is specific heat
f. Superheat,superheat is any substance like water for example that has been heated above the saturation temperature of the substance it was created,example is when refrigerant leaves the txv it is mostly moisture, except some flash gas, as it goes through the evaporator the refrigerant gains heat and becomes moisture free,at that point the refrigerant is 100% saturated,the refrigerant will continue to gain heat above it's saturation temperature and become superheated.
This is very important because any moisture that reaches the compressor will damage the compressor.


How heat is transferred

Heat is transferred in 3 different ways
1. Conduction. conduction heating is heat transferred through a solid,example is the heat from a stove burner through the pan to the water.
2. Convection heating is air moved through a space,example would be the air across your radiator through the fins to remove the heat in the water is forced convection cooling
3. Radiation is the transfer of heat from a a hot to a cold through space,the sun heating the earth is a good example of radiation heating.

Pressure changes

There are two distinct pressure changes that take place in the a/c cycle.
The first one takes place at the compressor,where the compressor takes suction on the evaporator at about 5 lbs and compresses the refrigerant to about 200lbs,the second takes place at the txv (thermal expansion valve)the txv is always located at the inlet to the evaporator.the pressure enters at approximately 200lbs and drops to approximately 35lbs and 35 degrees temperature.

How the system works

there are 4 phases to the air conditioning cycle
A. compression. The compression phase starts at the inlet of the compressor and ends at the inlet of the condenser.
B. condensation. The condensation phase starts at the inlet of the condenser and ends at the inlet of the receiver/drier.
c. Storage. The storage phase starts at the inlet of the receiver drier and ends at the inlet of the evaporator after it goes through the txv.
D. Expansion. The expansion phase starts at the outlet of the txv and ends at the inlet of the compressor completing the cycle.

1. The compressor,shown below,separates the low side from the high side,it takes suction on the evaporator and discharges to the condenser,the pressure on the suction side is approximately 35lbs and 200lbs on the outlet,after the refrigerant leaves the compressor and goes to the condenser where it is condense back to a liquid,this is done by removing latent heat and changing it from a vapor back to a liquid. After it leaves the condenser it goes to the receiver/drier where moisture is removed and stored for use. After leaving the receiver/drier it enters the evaporator through the txv,the refrigerant looses it pressure and temperature at this point it is a vapor with small particles of refrigerant in it,as the refrigerant goes through the evaporator the refrigerant gains heat and becomes superheated,this is very important because if the refrigerant was allowed to go back to the compressor as a liquid it would damage the compressor.
The following is pics of each component and more explanation of the system,the first pic is of an 89 compressor and the second is a pic of a 99,the suction and discharges are opposite,the best way to tell which one is suction or discharge if you do not have a manifold or gauges is to feel the lines,the discharge will be hot and the suction will be cold,if the system is working properly,also the suction lines are larger and discharge is smaller,this is because anytime a substance goes from a liquid to a vapor it takes up more space,the line has to be larger on the suction to handle the same volume.
The suction is circled and the power supply of the compressor.
The pulley on the compressor is always turning,there is a coil on the clutch,when a/c is selected the coil engages the pulley to the compressor and starts the system operating.
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This picture is of a compressor from a 99,notice the line on the right is smaller than the left,it opposite from my 89.

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2. This is the condenser,it is located in front of the radiator the purpose of the condenser is to take refrigerant at high pressure in a vapor state and condense it to a liquid at high pressure. The refrigerant runs through the tubes of the condenser from top to bottom. The refigerant like I said enters as a vapor and about halfway through it goes through latent heat of condensation and turns into a liquid.
One thing to understand is that a condenser is not just a tubed and fins,all condenser are made up of 2 components,a heat exchanger,(the tubes and fins located in front of the radiator)and a fan,the electric fan located behind the radiator,shown below. The electric fan is not known as radiator cooling fan,it is a condenser cooling fan designed to be used to condense refrigerant. The electric fan does help with cooling for the engine when the temp gets to 217 degrees and the a/c system is charged and you are below 35mph,above 35mph the electric fan does not good for cooling the engine but it will run if the a/c is selected no matter what the car speed is.

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3. This is here the a/c condenser fan not the auxiliary cooling fan for the engine coolant,that is a secondary purpose.
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4. This is the receiver/drier,it's purpose is to remove any moisture in the refrigerant and act as a storage point for refrigerant before it is used. There is silica in it to remove moisture. Also you can see the sight glass on top of the receiver,this is here so you can check the system visually for presence of moisture and refrigerant charge,this will be covered further in the trouble shooting section.
Also in the picture is the low pressure cutout,this switch will keep your ac from working if the system charge is low,it will also keep the electric fan from coming on if the charge is low.
On later models the receiver/drier is located against the firewall on the passenger side.
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5. This is the txv(thermal expansion valve) It's purpose is to take refrigerant from high pressure to a low pressure,approximatley 160lbs to 35lbs,refrigerant at 35lbs is also about 35 degrees,the txv is also know as a metering device,it meters the refrigerant into the evaporator. This type of txv is called an h-block style,it used by Chrysler and ford,it is not as common as the externally actuated txv shown in the next photo but does the same thing. All txv's are located on the inlet of the evaporator,this one may look like it is on the outlet but the way it is mounted the valve is actually located on the inlet and the sensing tube is on the outlet. The way the txv works is it is placed in the stream of the refrigerant and senses the temperature,the txv is charged with a small amount of refrigerant,as the temp goes down the pressure in the valve will lower and allow the valve to close,when the temperature comes back up the pressure in the valve will increase and open the valve allowing refrigerant to enter the evaporator and cool the air across the evaporator. There is also a screen on the inlet to stop any debris from the receiver/drier,primarily the silica from drier entering the compressor and causing damage,if you see frost on the txv it is clogged at the screen causing a restriction,the pressure drop takes place after the txv not at the inlet.
The red is the inlet and blue outlet.

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6. This just shows the outlet of the txv,this is where the refrigerant temperature is measured,the valve is on the inlet side and controls the amount of refrigerant entering the evaporator depending on the outlet temp.
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7. This just shows the typical style txv,this one has a remote sensing bulb that is routed to the tail coil,or outlet of the evaporator.
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8. This is the evaporator,this is what cools down the inside of your jeep. This is a spare I have using it to show what it actually looks like,the picture below shows where it actually sits in the jeep.
The way the evaporator works is the refrigerant enters the evaporator through the txv where the pressure and temp is lowered by the refrigerant loosing pressure and expanding,the temp is approximately 35 degrees when it first enters. Under normal operating conditions the air comes from the outside and is directed through the evaporator,as the air flows across the evaporator the hot air is cooled by (convection) heat transfer, the heat of the incoming air is transferred to the refrigerant and the air is cold entering the jeep. if max ac is selected then the door located under the right side of the dash will close(see next pic) and recirculate the air inside the jeep which will result in faster cooling,this happens because no outside hot air is being brought into the jeep, some think having the window open will result in air conditioning working better when you first start the jeep,not true,it may help cool the jeep off because the outside air is not as hot as the inside from sitting in the hot sun allday,but the fact is anytime the window is open while the ac is in max cooling it puts a higher heat load on the system,it would be the same as having it in normal mode,because air comes from the outside anyway,except having the window open even under normal operation is adding more heat than the system can remove.
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9. This is the door that closes when max ac is selected,it is located under the right side of the dash.

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10. This just shows where the ac evap is located in the jeep,if it ever needs to be changed the dash has to be removed,it will cost at least a $1,000 to have a shop replace it do the labor involved.
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Trouble shooting
safety notes. When diagnosing problems with any part of your electrical system do not bypass relay's,fuses,circuit breakers,etc,they are there for a reason,if you do you put yourself and car at risk of being damaged or destroyed,the following is a brief description of what these items are for.
A.Relays are for circuits that have high loads,such as ac compressor and and auxillary/condenser fan, Relays reduce the current when starting components with high start up loads,that start and stop frequently. To help better understand this(When the compressor starts it will draw alot of current until it gets to operating speed,but when it reaches operating speed the current is reduced,it does not require as much current to keep it running as it does to start it,the relay is for initial start-up,(it does this by using an internal resistor) so the circuit will not become overloaded and burn the wiring.
B. Fuses protect the circuit from overload either overtime or shorts and anytime the system is high enough to melt the fuse ,these can be caused by components going bad and drawing current higher than designed,or shorts. note:due note replace a fuse with a larger fuse ever,find the problem.
C. circuit breakers are for circuits that have temporary high loads,they work like fuses but have a bi metal strip that when overheated will open then close again once they cool down,they are used in systems like windows,seats,etc.
D. Sensors are electrical components that monitor the operating systems for proper operation,example is water temp,oil pressure,etc.
E. Switches are electrical components that operate systems at predetermined pressures and/or temperatures,example would be the electrical fan,refrigerant charge,like the electrical fan will come on at 217 degrees water temp.


Now that we have gone through how the system works lets go through some of the items that come up the most and how to repair them.

1. I am going to repeat myself here but it is important to understand not to mix refrigerants. Also mentioned before if any of the components go bad and need to replaced where it will require that the ac system has to be drained,I strongly recommend you have a shop do the work.
2. There are many things you can repair and that is what I will cover here,if any of the repairs requires removing the dash then please refer to the write-up in the tech write up section titled dash removal 1-6 so I do not have to list them all over again here.
3. I will go through the things that can go wrong and under it I will list corrective actions and suggested repairs.
4. Mine is an 89 and when I show a relay,switch,etc,it may not be the same place as yours but it will be in the general location and look the same.


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1. When you turn the a/c system on and the compressor or fan will not run check the following items.
1. Low refrigerant charge.
A.You need to have a gauge for this to check how much refrigerant is in it,you can get a cheap set at your automotive store or they come in a recharging kit.
2. Low pressure cutout switch is bad.
B. To check the switch you can disconnect the switch and jumper it with a wire,if the compressor comes on then it is either the switch is bad or the refrigerant charge is low. The pic below shows the low pressure cutout.
3. There is also a high pressure switch located on the discharge side of the compressor and before the condenser,it is there to protect the system from high pressure damage. For what it is worth my 89 does not have one but they sell them,there is no reason I can think of not to have one on all models but I have looked at several xj's and could not find one,the pic after this one is of a 99 and where it is located,just past the discharge of the compressor,compressors also have mechanical relief valves for system protection if the pressure gets dangerously high.
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this is the high pressure cutout on a 99 xj,the low cutout is on the receiver drier

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3. The relay on the right is the a/c relay,if the compressor will not start and system is charged the relay maybe bad.
If you suspect the relay is bad,use a known good one to check,do not bypass or jumper relays.

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4. temperature sending unit can also cause the a/c to not operate,if it is giving a false reading or the sending unit is replaced with the wrong one the system will think the engine coolant is hot and will not allow the system to operate,the system does this because any added heat to the air coming into the radiator will cause poor heat transfer in the radiator.
A. You can check it by disconnecting the switch and jumpering it(note: there is a big difference between jumpering switches and relays,fuse,breakers,anytime you jumper a switch it should be for testing only and then returned to normal) you jumper the switch by using a piece of wire and placing each end of the wire in the connector going to the switch,if it runs then replace the switch.
B. The picture shows the switch on mine but this is for a 91 and newer,87-90 is on the side of the block.

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5. The condenser as discussed before condenses the refrigerant from a vapor back to a liquid,it cannot due this if it is not efficient.
A. If you get debris,bugs,mud,etc on the cooling fins and tubes it will not be able to condense the refrigerant and therefore the system will not operate as designed. One indication you have a problem is the system is fully charged but the air in your jeep is not cold it could be the condenser is blocked,there has to be sufficient air flow to condense the refrigerant,another indication is your jeep is overheating,if the condenser is block then you will not get sufficient air flow to cool engine coolant.
B. Another problem you can have with the condenser is the condenser fan,if it is not running you will not have sufficient airflow accross the condenser to condense the refrigerant at low speeds and idle.
C. between the compressor and the condenser is a high pressure cutout switch,if the condenser is clogged/blocked the refrigerant will not condense and the pressure will go up and the switch will shutdown the compressor,if the compressor is cycling continously this could be a problem.
D. If the condenser is blocked then you can remove it by flushing it with water from the engine side,in some case it maybe nessasary to remove the radiator to clear it.
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6. This is the a/c condenser/auxilary fan and is essential to efficient operation,it is possible for it to run but not at full speed if it is going bad. The main thing to understand,for good efficient a/c system operation you need good airflow accross the condenser.
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7.
A.The problems you can have with the receiver/drier is it is not removing moisture,the silica in the drie gets used and moisture in the system will cause the system to operate poorly and not cool.
B. If you ever notice ice accumilation on the outlet side of the receiver/drier it needs to be replaced,it indicates a restriction from debris like the silica and that your pressure drop is there instead of the the txv.

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8. The problems with the txv are most of the time they will fail open and flood the evaporator with refrigerant,this is not a good situation.
A. If the txv sticks open it will continue to supply refrigerant to the evaporator causing refrigerant in a liquid state to the compressor,if it is not repaired it will destroy the compressor.
B. If it does fail you will see frost on the txv and if it is bad enough it will freeze the evaporator,if this happens you will not have airflow accross the evaporator therfore now airflow in the jeep,this situation will remain until the ice melts weather it is in ac,heat,or vents.
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Last edited by BlueXJ; 02-14-2010 at 09:12 PM. Reason: grammar & spelling
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Old 02-12-2010, 02:18 PM
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Default HVAC Operation/Problems

The problems with the Evaporator are mainly do to airflow restrictions,but it will also effect heat flow as all air goes through the evaporator,if you have airflow problems it is possible the inlet can be block. if the txv fails open the evaporator can actually freeze and block airflow to the evaporator,the outside air has moisture in the air and will freeze and block the inlet if the txv fails or there is an airflow restriction.
To clear it remove blower against firewall and clear the inlet.
1. This is the evaporator circled in yellow
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2. This is the inlet to the evaporator and also the area any debris would be if it was blocked.
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3. this is the the blower/fan,this has to be removed in order to get to the inlet of the evaporator,there is 4 screws that hold it in place.
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4. This is the a/c thermostat,if your compressor is cycling continuously this could also be one of the problems. It monitors the air temp across the evaporator and will shut the compressor down if it gets to cold,it has a capillary tube that is attached to the evaporator and like the txv it will open and close depending on temp.
A.To check it requires removing it and placing in cold water and checking the resistance,if there is no resistance with the then place it in warm water,there should be resistance which means it closed.
B. If it needs to be replaced you need to remove the lower dash,see dash removal 1-6 in the tech write-up for procedure.


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5. This is the blower resistor located behind the glove box,if your fan does not come on or only runs in high this may be the problem.
A. To check it you need to remove it and check it for resistance across all connections,the way it works is when you switch from low to high you are diverting power to a lower resistance,on low there is less power supplied and high it has full voltage,so any resistance higher than normal for the selected position will cause improper operation. personally if you suspect it is the resistor i would just change it,very cheap.
B. To remove it you can either lay on your back and look behind the glove box and remove the 2 little screws that hold it or remove the lower dash,to remove the lower dash see dash removal 1-6 in tech write-up.
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6. This is the electric fan relay on an 89,the one in place,if electric fan does not run check relay.
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7. This is the dist box on a 99,this where the power comes from for the fan on newer models,located on right fender well.

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8. This shows the dist distribution box on a 99 which shows the relays and fuses.
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Most common problems.

1. Compressor will not start
A. check refrigerant charge.
B. Low pressure cutout switch bad
C. high pressure switch is bad
D. Electric fan will not start
E. compressor relay is bad

2. Air is warm
A. Low refrigerant charge
B. Moisture in refrigerant,check for bubbles in sight glass.
C. condenser inlet is resrticted.


3. Air stops when accelerating.
A. this is caused when the reservoir in the front bumper right side is not working,or there is a vacuum leak to the climate controller. The resevoir is used to maintain vacuum to the climate controller,cruise control,front axle if it has a vacuum actuated front axle. The reason for this is when you accelerate your engine vacuum will fall to around 0" of vacuum until you return to cruise,the reservoir has a check valve in in that prevents lose of vacuum to the the above mentioned components.
The following pictures show where the vacuum starts at on the intake,to the resevoir in the bumper,then to the lines that go through the firewall to the climate controller,the vacuum lines from the climate controller go the vacuum actuators shown below,the actuators work the same weather in ac or heat mode.
If you just follow the vacuum lines from the intake to the reservoir bottle and other components it is easy to see how it all works,providing it has not been hacked by someone.

4. Where vacuum starts.
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5. This is the vacuum reservoir in the right side of the bumper,to get to the reservoir you need to remove the corner of the bumper like this is here.
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6. This is where the vacuum goes through the firewall to the climate controller.
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7. This is the climate controller,the middle switch is the vacuum switch,the left one is the blower and the right one is the ac switch.
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8. These are the vacuum actuators that are behind the dash, left side of the heater core.
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9. This is the actuator for the max door,when max air is selected this door will close,what that does is recirculate inside air and eliminates outside air so cooling will take place quicker.
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Heat side

The heat side is much easier than the ac side. The heat side consist of the following components.
The flow through the system starts at the water pump,through the heater control valve and heater core,and to the thermostat housing
1. The Thermostat is very important,without it your engine will not heat up therefore you will not have hot coolant for the heater core.
2. The heater control valve.
In 97 Cherokees no longer used a heater control valve and use the blend door for temp control,coolant runs through the core at all times.
3. Heater core The heater core is circled in black. I is located behind the dash and if it need to be replace you will need to remove the dash,refer to dash removal 1-6 in the write-up section.
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4. This is the heater control valve,it is located on the firewall engine compartment passenger side. It is operated by vacuum from the climate controller.

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Things that can go wrong
1. The heat side is easy to diagnose. If you do not have heat once the motor is warmed up check the following.
A. Heater control valve,turn climate controller to heat and check if valve opens,if not check for vacuum to valve,if there is vacuum replace the valve. If no vacuum trace line back to source until you find either a blockage,broken or disconnected line or leak.
B. heater core is plugged,to check it make sure engine is cool and disconnect the 2 rubber hoses below and use a garden hose with a nozzle on it and check for flow,if it is restricted you can try and flush with a coolant flush kit,but you may have to replace it,if so go to dash removal 1-6 in the write-up section for procedure.
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That is all there is to the heat side,it is very basic and simple design and when working properly is a very good system. The main thing about the coolant system and heater core is to use fresh anti freeze and change it at least once a year. Anti freeze is not just for keeping your engine from freezing,it also has chemicals in it to prevent corrosion and deposits from attaching themselves to the engine and radiator passages,the best way to. Antifreeze does nothing for cooling,that is why there is the 50/50 mix,if you use 100% coolant you will have engine cooling issues.
I hope you find this information to be helpful,if you have any questions you can email me and I will assist in anyway I can.
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Last edited by BlueXJ; 02-15-2010 at 01:21 PM. Reason: Spelling errors
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