Heater Use and MPG

[Edison]

I have seen the comment on this site and others that using the heater on a Prius lowers the mpg. I can certainly understand that being the case for air conditioning, which requires extra power to run the compressor, but on "normal" cars cabin heating is essentially free (except for the electricity to run the fan, which is tiny). That's because the heater takes its BTUs from the ICE cooling system, and those BTUs would be lost to the atmosphere, in any case, via the radiator. So, in most cars (contrary to common wisdom), there IS a free lunch -- cabin heat on a cold day. What makes the Prius different?

[tomdeimos]

Originally posted by Edison@Jan 27 2006, 09:14 AM
I have seen the comment on this site and others that using the heater on a Prius lowers the mpg.  I can certainly understand that being the case for air conditioning, which requires extra power to run the compressor, but on "normal" cars cabin heating is essentially free (except for the electricity to run the fan, which is tiny).  That's because the heater takes its BTUs from the ICE cooling system, and those BTUs would be lost to to the atmosphere, in any case, via the radiator.  So, in most cars (contrary to common wisdom), there IS a free lunch -- cabin heat on a cold day.  What makes the Prius different?

[snapback]198578[/snapback]

You are correct that the heat is essentially free for some cars. It is not true for cars with small engines.

My last car was a Diesel Mercedes 190D and that car had as much trouble heating up as the Prius.
Same for my first Saab with 3 cylinders.

Car engines have more losses from thicker oil when cold, and run richer. If the engine is small, the heater can easily need more heat than the car can supply.

This is even more true for Prius if conditions allow the engine to shut off at lights etc.

My records show my Diesel got 25 mpg in cold weather on my commute to work, and 38 in summer on the same commute. This was mostly caused by the cold weather and lack of engine heat. Some was from winter fuel.

My Saab was less affected than most cars due to being a 2 cycle engine but that car never had enough heat for winter, even though it came with shades to block the radiator off in winter.

With temperature readouts and proper radiator blocking I can keep the Prius warm enough down to 0 deg F, but I also minimize heater use as much as possible until the engine gets warm. Makes a big difference in mpg. If I just drove it with heater on auto, I could lose over 10 mpg easily, depending on temperatures.

If your commuting is on highway at 70 mph you will likely be less affected, than those with slow city commutes.

[richard schumacher]

The Prius engine does not normally run all the time, and so it is not always available to supply heat to the cabin heater core, which uses engine coolant. If the cabin heater calls for more heat the engine will run more to provide it. (One detail I don't know is whether there is a supplementary electric heater. I would guess that there is, but in any case the cabin heat ultimately comes from burning gas.)

[tomdeimos]

Originally posted by richard schumacher@Jan 27 2006, 09:45 AM
The Prius engine does not normally run all the time, and so it is not always available to supply heat to the cabin heater core, which uses engine coolant.  If the cabin heater calls for more heat the engine will run more to provide it.  (One detail I don't know is whether there is a supplementary electric heater.  I would guess that there is, but in any case the cabin heat ultimately comes from burning gas.)

[snapback]198585[/snapback]

There is electric heat. The wattage is so low you can hardly feel any heat from it, and once the coolant gets warmed up it shuts off. And as you say it all comes from gas, so either way it costs you mpg.

The only time heater use doesn't cost you is when you are generating enough heat to keep engine at normal operating temperatures while running the heater. You should try to keep engine at 163 deg F or better, or around there. I use 190 F as a max, since I don't want to waste power running the radiator fans. I also make sure not to block the lower part of the radiator, which cools the inverter.
You need a can view or some other device to control this all.

[parrot_lady]

I've started rule with my passengers... If I'm taking you somewhere within 5 miles or where it won't take long, I'm not turning on the heat unless I'm in danger of loosing windows. then I'm doing it.

Its helped too! my MPG went up last fill up by about 5 MPG. Not exactly summer time averages, but not bad either.

Amy

[Edison]

Originally posted by parrot_lady@Jan 27 2006, 11:08 AM
I've started rule with my passengers... If I'm taking you somewhere within 5 miles or where it won't take long, I'm not turning on the heat unless I'm in danger of loosing windows.  then I'm doing it.

Its helped too!  my MPG went up last fill up by about 5 MPG.  Not exactly summer time averages, but not bad either.

Amy

[snapback]198592[/snapback]

OK. So, it would seem that the solution to the heating/mpg loss problem is to keep the heat off until the car is fully warmed up. (This is not unique to hybrids; it's just that many of us Prius owners are very sensitive to our mpg, so we notice the difference.)

Unfortunately, Toyota, in its infinite wisdom, did not think it necessary to give us an engine (ICE) temperature gauge, but I would assume that two or three miles under normal load should do the trick.

Thanks.

[EricGo]

Originally posted by Edison@Jan 27 2006, 11:57 AM
Unfortunately, Toyota, in its infinite wisdom, did not think it necessary to give us an engine (ICE) temperature gauge, but I would assume that two or three miles under normal load should do the trick. Thanks.

[snapback]198606[/snapback]

If the car is not shutting off the ICE at stops, or you find it hard or impossible to glide in areas that were not a problem in summer, a cold engine is the likely cause.

Parrrot_lady -- GOOD FOR YOU !! I drive without the heat most of the time also, but I am not sure I would do the same in Chicago-land. That takes fortitude.

[SomervillePrius]

Trying to make my current 96 Maxima for FE while waiting for my new Baby I've started using a lot less fan (sure it's miniscule but it's fun to play around with). I've found that the inside heats up to bareable tempratures fast even here in Boston w.o a fan. I find that as long as the temp is above freezing you don't really need the heater. I leave the heat on MAX but turn the fan of. This pushes some warm air into the car (not that much at lower speeds) but it helps. It'll be interesting to see if I can fo the same in the Prius. Some of you seem to indicate that this really help FE so I'll probably do it when I'm alone in the car.

[Walker1]

Originally posted by tomdeimos@Jan 27 2006, 10:03 AM
There is electric heat.  The wattage is so low you can hardly feel any heat from it, and once the coolant gets warmed up it shuts off.    And as you say it all comes from gas, so either way it costs you mpg.

The only time heater use doesn't cost you is when you are generating enough heat to keep engine at normal operating temperatures while running the heater.    You should try to keep engine at 163 deg F or better, or around there.    I use 190 F as a max, since I don't want to waste power running the radiator fans.    I also make sure not to block the lower part of the radiator, which cools the inverter.
You need a can view or some other device to control this all.

[snapback]198590[/snapback]

How do you know what the coolant temp is? There's no temperature gauge in a Prius.

[parrot_lady]

Originally posted by EricGo@Jan 27 2006, 12:19 PM
If the car is not shutting off the ICE at stops, or you find it hard or impossible to glide in areas that were not a problem in summer, a cold engine is the likely cause.

Parrrot_lady -- GOOD FOR YOU !! I drive without the heat most of the time also, but I am not sure I would do the same in Chicago-land. That takes fortitude.

[snapback]198620[/snapback]

ROTFLOL... the first time I took my mom to the post office she was all set to complain then found out it was liveable-- temps have been mid 30's when I started this, and we've had a couple of really warm days where I've had to actually open a *gasp* window cause it got warm in the car.

And the cold steering wheel? Not a problem. I threw my bike gloves in the center console-- I like those over driving gloves because of the gripped palm and the fact that my fingers aren't covered 100% . I worry about slipping on the wheel otherwise.