BMW A/C Question

[Gepetto]

Also it is that continuous process which helps keep the HVAC system itself, as dry as possible, at the same time. When we switch it on and off, we can cause moisture to sit in the unit, at a warmer temperature, and that is where the bacteria usually grows.

How much effect do modern aircon compressors have on fuel consumption?

I ask because I see no real need to have the compressor operating when the ambient temperature is, for example, in the mid twenties, and I have the windows/sunroof open. Also, I view the primary function of air-conditioning to be that of cooling, whilst there are obvious benefits in filtering and drying, so therefore see no need (or benefit) in running my aircon during colder weather.

Is there a long term benefit in terms of efficiency? I would understand if constantly turning the unit on/off had a definite detrimental effect, and there possibly is, if, for instance, parts were not receiving correct lubrication due to inactivity. However I have the view that any moisture remaining in the system is surely a design issue, and should not be allowed to occur due to the risk of the bacteria you mention.

 

[Scania]

How much effect do modern aircon compressors have on fuel consumption?

I've not checked in my car (although I leave it on all the time) but in my 11plate Scania artic it will increase fuel consumption by roughly 0.5 MPG which does not sound a lot until you appreciate it has a 13litre engine and averages roughly 8.5MPG!

It's common to find trucks that have warm / non functioning A/C in the spring after winter months of inactivity where on my last regular use one, where I never switched it off, the A/C was still icy cold whenever I needed it to be, in short, running it constantly helps to keep things working in my experience albeit with a consumption hit.

 

[Django x2]

I ran a fairly good comparison over the same 4 hour journey with air con on and air con off

Air con off 47.8mpg
Air con on 47.0 mpg

That was sat at 72mph and both journeys clocked an average of 58mph, but, as the jump windows in the BMW for digital MPG are stupid (47.8 is the next one up from 47.0) then to be 100% accurate, or, as near as dammit, I would have to do the fill and brim method

 

[OllyM]

I've always wondered why it does that with the MPG readout - perhaps it works internally in L/100km and converts to MPG which gives the stupid jumps?

 

[SDK^]

How much effect do modern aircon compressors have on fuel consumption?

F Series BMW's have an ECO PRO driving mode which reduces the use of the A/C when it is on. Along with the wing mirrors and rear screen heated glass. This reduces the MPG impact of using the systems.

 

[[TW]Fox]

F Series BMW's have an ECO PRO driving mode which reduces the use of the A/C when it is on. Along with the wing mirrors and rear screen heated glass. This reduces the MPG impact of using the systems.

It's like you answered a totally different question

 

[Clarkey]

Never found any noticeable difference. I did find a gain of nearly 2mpg on my last tank since I increased the rear tyre pressure though, I had a larger gain than that during the time I had smaller PC2s on. On this basis I will say tyres have a much more profound effect on mpg than AC does.

 

[Tom_ed1987]

AC compressor sized for a typical 5 seat car uses ~3hp when engaged.

Personally I hate the feeling of dry skin I get with it on all the time so only use it when it's either hot or very muggy

 

[Gepetto]

Some quite surprising (to me) findings there. I had thought the aircon sapped more power than that.

The reason I asked was because my two BMW's have had a tendency to harbour the bacteria that Spartacus mentioned which in my 320D resulted in a horrible smell at times. So, if the constant running of the compressor ensures this doesn't happen I'll consider it.

The smell has only affected the two BMW's I have owned and is not something I've witnessed in other cars, but it was definitely a factor in getting rid of the 3 series, and I don't know if the issue has been remedied in newer cars.

 

[Clarkey]

Just leave it on, you'd have to be a pretty hardcore penny pincher to consider turning it off?

Bacteria growth is a problem common to any AC system, certainly not isolated to BMW.