Does aircon normally run when heater is on?

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Sat 20 Jun 2009 13:59
Does aircon normally run when heater is on? - Pondlife
My car's heater control valve packed up about a month ago, which means hot water never gets to the heater matrix but the blower and a/c work fine. I'll be replacing the valve soon, but that's not what this post is about.

I would expect the a/c would only come on when it needed to cool, but no - it comes on even when it's trying to heat. Normally I wouldn't notice this, but without any heat from the matrix it makes itself known because I get a blast of freezing air.

This is not a big problem because I just turn the a/c off, but it seems such a wasteful thing to do. Why would a car be designed to use a/c when it was trying to heat? I know the heat is effectively free, but the a/c compressor must put some load on the engine. I guess I should keep the a/c off when I don't need it in future.

This is on a Jag S-type, which maintains a specified cabin temperature when the heater controls are set to "auto".
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Sat 20 Jun 2009 14:08
Does aircon normally run when heater is on? - Old Navy
If the aircon is on, or to auto, I believe the air is cooled and dried before it goes through the heater matrix. So the heater has the final say on the temperature output of the system. The aircon is as much a dehumidifier as a cooler. I am surprised your car has a heater valve, I thought heater systems worked by air blending these days.

Edited by Old Navy on 20/06/2009 at 14:12

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Sat 20 Jun 2009 14:16
Does aircon normally run when heater is on? - jc2
Very few heaters use "air blend" nowadays-even Ford which used it for years have changed to water valve.It makes for a smaller,cheaper,simpler system but control is not so good due to water-flow varying with engine speed.
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Sat 20 Jun 2009 14:28
Does aircon normally run when heater is on? - Old Navy
Full cycle eh, water valve, air blend, water valve. Obviously I am not keeping up with modern technology! :-)
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Sat 20 Jun 2009 15:10
Does aircon normally run when heater is on? - Number_Cruncher
>>control is not so good due to water-flow varying with engine speed.

MB and BMW use an auxiliary pump to keep flow rates up while the engine is running slowly.

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Sat 20 Jun 2009 15:47
Does aircon normally run when heater is on? - jc2
This will happen more and more due to manufacturers changing to slower and slower idle speeds because the idle speed has a very large effect on emissions and economy-it makes up a large percentage of the official drive cycle.With electronic control of the idle speed,slower speeds can be used more reliably.
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Sun 21 Jun 2009 02:14
Does aircon normally run when heater is on? - Lygonos
I reckon within 3-4 years the average engine idle speed will be 0 rpm ;-)
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Tue 23 Jun 2009 13:45
Does aircon normally run when heater is on? - softopdriver
I reckon within 3-4 years the average engine idle speed will be 0 rpm ;-)


as per current 'stop/start' technology.

just curious now, but how would you get such a vehicle to ever idle for workshop diagnosis purposes, there must be an over-ride to such a system?
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Tue 23 Jun 2009 14:14
Does aircon normally run when heater is on? - jc2
There has to be an override as the "idle" phase still forms part of the legal emission/economy drive cycle.You can advertize fuel savings with the idle cut-out but the legal figures must include idle.
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