Using the heat when it's cold... mpg drain?

[moorecomp]

Originally Posted by jamesqf

And just think what having what would be in effect an electric heating coil in your ductwork, when you're using the fan for A/C :-)

Which is exactly what they are! Have you even seen what any of these look like! Wire resistors, nicrome wire. Lot of heat, low resistance, high current flow. Google is your friend. All kinds of reports of melting and even a few fires.

Here is one from a 1996 Ford Escort:

[PaleMelanesian]

My Honda one looks very similar to that. Still, the fan draws significantly less power on low fan speed than on high speed. It *must* be resisting / restricting current flow, not just burning up voltage.

[NiHaoMike]

Nowadays, it is common to use electronics to control motor speed, which is very efficient. One easy way to tell is by the presence of a digital control. If it uses buttons or a knob that will turn continuously, it is electronic. PWM is very easy to implement, but inverter drive is the most efficient.

It should be very easy to make a retrofit PWM fan controller. Just a potentiometer varying the duty cycle of an oscillator (~15kHz for high efficiency) and then use a MOSFET and Schottky diode as the motor control stage.

Beware that if you set the fan speed very low with the A/C running, it can freeze the coil and possibly slug the compressor. Therefore, add a comparator to lock out the A/C compressor below the original low speed. One LM339 can operate as the oscillator, PWM comparator, and A/C lockout comparator. Add some resistors and capacitors and a few transistors for the MOSFET driver and A/C lockout relay driver.

[Mustang Dave]

It *must* be resisting / restricting current flow, not just burning up voltage.

It absolutely IS resisting current flow. A resistor in series with a motor will cause a drop in current draw. Unless Ohm's Law has somehow changed since I earned my Electrical Engineering degree. (It hasn't.)

[NiHaoMike]

Originally Posted by Mustang Dave

It absolutely IS resisting current flow. A resistor in series with a motor will cause a drop in current draw. Unless Ohm's Law has somehow changed since I earned my Electrical Engineering degree. (It hasn't.)

That is true if the motor is a resistive load. But it's not. Motors draw more current to start than to run. If enough resistance is added in series, the motor will not start. Then it can draw more current than if it was running normally at full power. Obviously, that does not intentionally happen in our case. But with age, the motor bearings might tighten up such that it will fail to start on reduced current. The result is usually a blown fuse, but it's possible for the motor to be damaged by prolonged operation in "locked rotor" mode.

[Mustang Dave]

Originally Posted by NiHaoMike

That is true if the motor is a resistive load. But it's not. Motors draw more current to start than to run. If enough resistance is added in series, the motor will not start. Then it can draw more current than if it was running normally at full power. Obviously, that does not intentionally happen in our case. But with age, the motor bearings might tighten up such that it will fail to start on reduced current. The result is usually a blown fuse, but it's possible for the motor to be damaged by prolonged operation in "locked rotor" mode.

True. A running motor is not a purely resistive load.
A motor draws the most current when it's not turning. That goes for universal, induction, and synchronous motors. Many high-current motors use resistors in their startup circuitry to prevent excessive current draw.
As for an automotive HVAC blower motor - if it burns up, it's relatively cheap to replace.

[NiHaoMike]

The real problem is if the air handler fan stops working when the A/C is on. If left running too long in that condition, it can slug the compressor. That's particularly a problem with a capillary tube.

[jamesqf]

Originally Posted by Mustang Dave

As for an automotive HVAC blower motor - if it burns up, it's relatively cheap to replace.

Yes/no. The motor is cheap, but getting to it may not be. It's like replacing a light bulb in your instrument cluster: 50-cents for the bulb, four hours labor to get to it :-)

[bgd73]

the heater does have a magic trick.

fluids in your engine go by the heads, there is an energy created, easily summed yup with "static". water is aperfect ingredient for dispersing it. the heater on can actually increase mileage.

It is like the old school trick for early large micron ECUs. Turn on the radio, tunes up the computer. The hertz in the speakers is helping disperse the clock energy in the computer. I live in very very cold climate. I learned most cars like classical high pitched violins.... The winter time really demands this tiny clock stuff.
Warm and humid, disperses this very fast. It leads to why some people call their car a pig in the spring...it is cleaning out a long stagnite keeper called winter. There is even deionization.
Heater on is good. I run mine in the middle of summer sometimes. it even helps the core, and disperses strange goops internal to the system..

[taredog]

Wow. Quite a discussion. So how much does FE suffer if your charging system is loaded with an extra 30-80 watts? You guys are really worried about your carbon footprints

Running coolant through the heater core will lower the engine's operating temperature some. We used to use that fact in my younger days when we were driving POS cars that needed radiator work and we were running straight water since anti freeze cost money. Turning the heater control (assuming it worked) to high would sometimes allow the vehicle to get where we were going without overheating too much. Assuming the heater core didn't leak.

Back to the subject, a lot of cars as stated by Frank Lee do turn on the ac compressor when in the defrost mode. Some even when in the heat mode. All to dry out the air.

In the mid '80s I had an 81 Mustang in Scotland for a couple years. Petrol was about 98 pence a liter if I remember correctly. $/gal I don't remember exactly but it was a lot more than in the US. Since Scotland is a place you need your defroster a lot, I decided to try to disable the ac in defrost. The easiest thing would have been to pull the connector off the clutch at the front of the compressor. I didn't want to disable the ac as there are maybe seven days in the summer that it is hot there (to a Scot) and some of the local lasses liked to go for a drive to cool off. Some even wanted a ride out of the deal I pulled the HVAC control out of the dash and swapped the vacuum hose from the defrost position to the heat position. This made the blend air door put all air to the defroster vents in the heat position. No air to the floor. This created a vacuum leak at the hvac switch defrost position that I believe I dealt with by using a rubber ear plug. This did improve my dismal FE somewhat as the defroster was on most of the time I drove there.

This 81 Mustang 2.3/auto rarely got better than 20mpg and it was a total sloth as to acceleration. Never did figure out why.