your engine warms up faster without heater??

[demarpaint]

Those were my thoughts and understandings all along. So people running the defrost or AC in the winter to lube the AC system aren't lubing anything if it is below 32F since the compressor isn't engaging. My thoughts about running the defrost or AC to lube the system are correct, if it's cold it won't matter because the compressor isn't engaging.

 

[eljefino]

My wife is one of those types who will turn the blower motor on super-high when the motor is cold and heat is scant. With the flow-through ventilation it just pumps tepid air through the cabin.

Myself, I'll have it on low. Even off, there's ram-air that forces some heat through. Coasting down a long hill in gear, the fuel gets cut out and I can watch the temp drop on the gauge.

 

[brianl703]

I think it's possible that engine heat might warm the evaporator coil to more than 32F so the compressor engages, but it will probably only run for a few seconds and then shut off again for several minutes.

 

[mechanicx]

I think the low pressure switch has to see quite a bit of static pressure to turn on- like 70 psi or more and so temperatures above 40F. However once the switch is tripped on it might not trip off till 32F. The switch can have a different on pressure and off prerssure.

Keep in mind that cold air outside air is usually always going past the A/C evaporator and the condensor as well obviously so the refrigerant has to rise to an overall pressure in the system to trip on pressure switch. Once the A/C is running the low side will drop into the 32-40 psi range and so a lower trip off setting makes since.

You want the A/C to come on in Defrost/Defog mode and do some dehumidfying even at temperature in the 50-60's because sometimes you can have steamy glass even at those low temperatures.

 

[rationull]

I've always been under the impression that to get fastest warm up time you should not only turn off the blower fan, but also turn the temperature down, but from some of the posts here I'm wondering whether that's correct. Do the temperature controls on manual HVAC cars tend to close off the coolant line to the heater core or just operate the blend door?

The reason I thought this is I'm pretty sure I've read instructions to turn the heat all the way up before draining the coolant when replacing it, which would only make sense if the heat needed to be up in order to drain all of the coolant.

I'm pretty sure I've noticed my Civic warming up faster when the temp is set all the way cold. I haven't done any extensive testing on our Mazda3, but that car seems to warm up faster than the Civic anyway for whatever reason.

 

[mechanicx]

Originally Posted By: rationull
I've always been under the impression that to get fastest warm up time you should not only turn off the blower fan, but also turn the temperature down, but from some of the posts here I'm wondering whether that's correct. Do the temperature controls on manual HVAC cars tend to close off the coolant line to the heater core or just operate the blend door?


With most all of them the temperature control just operates the blend door. Most heaters are constant flow. Some cars have a heater valve that blocks coolant flow but they are few and far between and I'm not sure manipulating the heater setting will change the operation of the heater valve. I think it opens whenever coolant is hot enough. So you are right turning the temperature down prevent airflow over the heater core. But the A/C evaporator still usually gets constant airflow.

Quote:
The reason I thought this is I'm pretty sure I've read instructions to turn the heat all the way up before draining the coolant when replacing it, which would only make sense if the heat needed to be up in order to drain all of the coolant.


I think if you had a heater water valve, the heater core could still drain, it just can't circulate until it's open. Usually you turn the ehater all the way up and the fan up while refilling the cooling system to encourage flow through the heater and get the air bled out.

Quote:
I'm pretty sure I've noticed my Civic warming up faster when the temp is set all the way cold. I haven't done any extensive testing on our Mazda3, but that car seems to warm up faster than the Civic anyway for whatever reason.


These cars probably do not have a heater valve and as you said leaving the heat turned down will prevent cold air from flowing across the heater core, so the coolant should warm up faster.

 

[Craig in Canada]

Originally Posted By: rationull
I've always been under the impression that to get fastest warm up time you should not only turn off the blower fan, but also turn the temperature down, but from some of the posts here I'm wondering whether that's correct. Do the temperature controls on manual HVAC cars tend to close off the coolant line to the heater core or just operate the blend door?


It depends, probably. My 1990 Integra manual system did both via two bowden cables (variable air flap, variable coolant valve). My 1992 Porsche semi-auto system had electric blending flaps and an on/off coolant valve (vacuum operated via solenoid IIRC). My 1998 BMW auto system has lots and lots of electric blending flaps and a pair of PWM variable flow coolant valves - the air flaps and the valve opening combine to set the vent temperature. It all depends on the system.

Regardless, if there's no or little air flow over the heater core, it isn't sinking any additional heat. The coolant loop will be a little longer and include more coolant with the heater valve(s) open.

"Winter" means different things to different people. If it's 0*F you aren't going to get away with no airflow or using the recirc setting if there are breathing humans in the cabin - you need at least a little bit of dry airflow to keep from fogging. In "quite cold" conditions in the 1.8L Integra I used to put the fan on but with the controls set to cold to keep the windows clear. On the BMW I've found that I set the fan to low but leave the temperature on auto - this will still gently heat the windshield but doesn't place a huge heat sink on the system. The Porsche never saw rain, let alone snow

Quote:

The reason I thought this is I'm pretty sure I've read instructions to turn the heat all the way up before draining the coolant when replacing it, which would only make sense if the heat needed to be up in order to drain all of the coolant.


Correct, the heater valve needs to be open to drain/fill the heater core.

Quote:

I'm pretty sure I've noticed my Civic warming up faster when the temp is set all the way cold. I haven't done any extensive testing on our Mazda3, but that car seems to warm up faster than the Civic anyway for whatever reason.


It's a combination of the temperature setting and the airflow. Any heat you're putting into the cabin, whether a little bit of scorching air, or max speed fan of warm air, is heat being taken from the engine.

I suppose some might think I'm a nut-job for taking this into account at all, but I don't feel my actions are extreme considering our vehicle currently does a fair bit of winter short-tripping (kids) that it didn't do in the past (pre-kids). It's been pretty nippy here lately (in the 10s*F?) and there's barely a trace of condensation emulsion on my oil fill cap despite the short trips. Different vehicles have different HVAC systems and may require different actions or compromises.

If "winter" for you is 50F, worrying about the heater load affecting engine warm up may be a little extreme.

 

[Ken2]

The auto air conditioning systems I've seen use a low pressure cut out switch to prevent the evaporator (the cooling grid in the dash) from getting cold enough to frost over, and it also shuts down the compressor when the charge leaks out. Here's the pressure/temperature table[\url]. I don't know the set point of the low pressure cut out switch, but it is mainly seeing the pressure relating to the temperature at the evaporator...does the heater warm the evaporator and thus raise the refrigerant low-side pressure?

 

[mechanicx]

Originally Posted By: Ken2
...does the heater warm the evaporator and thus raise the refrigerant low-side pressure?


No normally air is always first flows over the A/C evaporator whether the A/C is running or not, then depending on the HVAC temerature setting is diverted over the heater core. When the A/C is off the low-side pressure (actually total system pressure) is dependent on ambient temperature and whatever heat that soaks into the system from engine running. When the A/C is on the low-side pressure drops and is regulated by the low-side cut off switch, or in GM variable displacement compressors the compresor control valve.

 

[ThirdeYe]

My car has a cable connected to the climate control slider that manually opens a little door to allow heat in. I don't really notice it becoming warmer quicker if I have the heater on or it set to cold.