# Simultaneous heater/air conditioner second opinion **URL:** \ **Category:** Maintenance/Repairs\ **Tags:** heating, airconditioning\ **Created:** [June 21, 2008, 4:56pm UTC](https://community.cartalk.com/t/simultaneous-heater-air-conditioner-second-opinion/15646 "2008-06-21T16:56:19Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Geschlechlichinginie](https://avatars.discourse-cdn.com/v4/letter/g/4491bb/32.png) [@Geschlechlichinginie](https://community.cartalk.com/u/Geschlechlichinginie)\ **Post date:** [June 21, 2008, 4:56pm UTC](https://community.cartalk.com/t/simultaneous-heater-air-conditioner-second-opinion/15646/1 "2008-06-21T16:56:19Z")
From the need to introduce issues of durability and reliability, I hereby proffer the following second opinion about the Magliozzi Brothers’ answer to a listener’s question about using the air conditioner with the heater. Though the caller’s wife is technically right about using the air conditioner to condense moisture before heating air for the purpose of drying the car’s interior and occupants, in some cases it is actually unnecessary and even undesirable to operate the air conditioner and heater at the same time. In the situation the caller described, cold winter weather, the difference in temperature between the outside air and the heated air from the car’s climate control system (without A/C) is sufficient to dry the interior of the car and all within. Second, many cars have a thermostat that prevents the air conditioner clutch from engaging when the temperature under the hood is below a certain point, typically 50 degrees Fahrenheit. Using the air conditioner in recirculate (or MAX A/C) mode with the temperature control on full heat will cause the refrigerant to over-pressurize the system, resulting in refrigerant loss. I’ve been there and done that. Though I don’t know the maximum heat setting that would not cause this, it is better to be safe than to be sorry. If you want to dry out during damp weather, it would be best to use FRESH air (from outside the car) when simultaneously using the heater/defogger and air conditioner. In fact, most cars’ climate systems actually engage the air conditioning compressor automatically when the function selector is set to blow air directly at the windshield. For similar reasons, I would also use only fresh cooled air to cool down the car after it has been sitting the sun, and then use the recirculate function when the interior is cooler than outside. Otherwise, even though the recirculated air would be cooler over time rather than be constantly hot, I prefer not to gamble. I haven’t noticed that recirculating air through the system cools any faster than running fresh air through it. If it does, I’m willing to wait a few extra minutes to make the system last longer. Besides, I have an acquaintance that had to have A/C repairs from using the recirculate function to cool down a hot car. Also, It is wise to use the air conditioning compressor throughout the year to keep it lubricated and to prevent the seals from drying out. Along with refrigerant, there is oil in an air conditioning system. ---
**Author:** ![oldschool](https://avatars.discourse-cdn.com/v4/letter/o/e19adc/32.png) [@oldschool](https://community.cartalk.com/u/oldschool)\ **Post date:** [June 21, 2008, 6:01pm UTC](https://community.cartalk.com/t/simultaneous-heater-air-conditioner-second-opinion/15646/2 "2008-06-21T18:01:19Z")
Could you explain this “full heat refridgerant loss over pressurize condition” a little farther. AC systems along with a low pressure cut off switch have a high pressure cut off switch. But lets say the pressure went high where did it leak from? Any idea how high the high side went to cause a leak. Any further info on the friends AC repair (what component?) ---
**Author:** ![tardis](https://avatars.discourse-cdn.com/v4/letter/t/f0a364/32.png) [@tardis](https://community.cartalk.com/u/tardis)\ **Post date:** [June 21, 2008, 8:47pm UTC](https://community.cartalk.com/t/simultaneous-heater-air-conditioner-second-opinion/15646/3 "2008-06-21T20:47:58Z")
I would also add that almost all auto climate control systems in cars run the AC all the time and add in heat as needs to get to the correct temperature. All car ACs that I have seen recently have low side under-pressure and highside over-pressure switches. Automatic systems switch to recycle at first to most quickly cool the car. After that they switch back to fresh air. ---
**Author:** ![Beadsandbeads](https://avatars.discourse-cdn.com/v4/letter/b/c57346/32.png) [@Beadsandbeads](https://community.cartalk.com/u/Beadsandbeads)\ **Post date:** [June 22, 2008, 2:26am UTC](https://community.cartalk.com/t/simultaneous-heater-air-conditioner-second-opinion/15646/4 "2008-06-22T02:26:57Z")
? ??In the situation the caller described, cold winter weather, the difference in ?temperature between the outside air and the heated air from the car’s climate ?control system (without A/C) is sufficient to dry the interior of the car and ?all within. Second, many cars have a thermostat that prevents the air ?conditioner clutch from engaging when the temperature under the hood is below a ?certain point, typically 50 degrees Fahrenheit.? If they don?t have a low temp thermostat, the pressure switch will prevent the compressor from running. ?Using the air conditioner in recirculate (or MAX A/C) mode with the temperature ?control on full heat will cause the refrigerant to over-pressurize the system, ?resulting in refrigerant loss. I’ve been there and done that.? I don?t see how that could happen, will you explain? ?I haven’t noticed that recirculating air through the system cools any faster ?than running fresh air through it. ? You would if it is very humid/hot outside and dry/hot inside. The energy to dehumidify can easily exceed the Entergy required to cool hotter air. ?Besides, I have an acquaintance that had ?to have A/C repairs from using the recirculate function to cool down a hot car.? I doubt it, what happened? Tardis, all the systems I am familiar with use AC in the defrost, Max/recirculate, fresh and defrost modes. In the floor mode, there is no AC. In electronic/thermostat systems that is true as well as in manual systems. In manual systems you can dial in heat from the blend door or, rarely, water valve any time you want. ---
**Author:** ![crankwalk5](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@crankwalk5](https://community.cartalk.com/u/crankwalk5)\ **Post date:** [June 22, 2008, 2:30am UTC](https://community.cartalk.com/t/simultaneous-heater-air-conditioner-second-opinion/15646/5 "2008-06-22T02:30:59Z")
condensed version please. ---
**Author:** ![Craig58](https://avatars.discourse-cdn.com/v4/letter/c/3da27b/32.png) [@Craig58](https://community.cartalk.com/u/Craig58)\ **Post date:** [June 22, 2008, 2:41am UTC](https://community.cartalk.com/t/simultaneous-heater-air-conditioner-second-opinion/15646/6 "2008-06-22T02:41:13Z")
A couple of clarifications: Use of the AC will not affect the removal of frost from the exterior of the window. The use of AC will normally help to remove moisture (fog) from the inner surface of the window by providing hot/dry air instead of hot/damp air to the defrosters. The use of AC with heat will help remove moisture from the interior of the car and help prevent the windows from fogging (on the inside) while driving, especially in “recirculation” mode. In very cold weather, the AC system is designed to shut itself off prior to damaging itself, so the AC is probably not engaged when you turn on the defroster on a 10F day anyway. ---
**Author:** ![bloody\_knuckles](https://avatars.discourse-cdn.com/v4/letter/b/ee7513/32.png) [@bloody\_knuckles](https://community.cartalk.com/u/bloody_knuckles)\ **Post date:** [June 22, 2008, 2:52am UTC](https://community.cartalk.com/t/simultaneous-heater-air-conditioner-second-opinion/15646/7 "2008-06-22T02:52:35Z")
Stuff and nonsense. I operate my defroster WITH the A/C all winter every winter in all my vehicles. It clears the windows faster and keeps them clear. I pass people like you on the Thruway with foggy windows all the time. Furthermore, in almost half a million miles driven in the snow belt of New York State I have NEVER had an A/C failure nor heard of one in any of my friends vehicles due to cold weather. Lastly, failure to use the A/C system all winter long leads to drying of seals and loss of refrigerant. The majority of A/C failures in this region that are attributable to refrigerant loss are caused by this problem (as related by a mechanic friend and my repair shop).