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Cottonwood
Roadster#433, Model S#S37
As far as I know, the seat heaters are purely resistive.
As for cabin heating, the Model S does use waste heat from the motor and inverter, but the EV propulsion system is pretty efficient, and below about 50˚F, 10˚C, there is not enough waste heat to keep the cabin warm. After that, the car uses purely resistive heating for cabin heat. Although the air conditioner could be turned around into a heat pump at better efficiency down to -10˚C or so, Tesla has chosen to keep the systems simple, and only use resistive cabin heating.
I thought heat pumps couldn't operate below ~32 F (0 C) because it would create ice?
Super Snake
Member
So how long does it take to warm up the car? Say its in the 20's and you have a 15 min ride, will it be cold the entire ride or will it warm up after a short time? Also how long does it take to warm the car with the smartphone app? I can picture us in a restaurant and programing the car to warm up before we are finished dinner. How much time should I allow for that to happen?
Unlike an ICE car, where the engine/coolant has to be warm before the cabin gets any heat, Model S's resistive heater begins heating the cabin at full strength as soon as you turn it on. In practice, you can just get in the car and drive: the interior gets comfortable quickly; for ultimate comfort you can turn on the climate control ahead of time. If you are plugged in, however, it's always best to turn on climate control and heat the cabin before you unplug and drive, because all that energy comes from the wall instead of the main traction pack: your Wh/mi will be significantly lower, especially for shorter trips.
Some diesel cars do this too. My 335d begins heating immediately with an electric heating element because Diesel engines take much longer to warm up than gasoline engines. Otherwise my entire commute would be freezing cold in the winter.
My previous house in the Phila area had heat pumps and it got down below 32 often in winter. There is still energy that can be extracted from air below 32 F, just less of it, so it is less efficient. There was a resistive element for backup and I usually switched flipped the switch on the thermostat to backup when it got below 35 F. An HVAC guy I knew told me it was more efficient and cost-effective to do so.
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Unless you have a 200+ mile trip before plugging back in, and are concerned about cutting it close, what's the difference? You are going using that energy one way or another - either up front from being plugged in or on the back end when recharging.
There's actually quite a bit of difference:
1. It's somewhat easier on the battery because heating is happening without driving power being used at the same time. This may lower total energy use a bit assuming that a higher rate of energy use causes a non-linear draining of the cells.
2. The cabin is warm when I get into the car.
3. I don't need to even turn the seat heaters on during my 25 mile one-way commute.
4. Ice on the windshield has mostly melted by the time I get in.
Ok - that HVAC guy is flat out wrong. He cost you a lot of electricity.
A decent modern heat pump can beat resistant heat to below zero. At 32 degrees, a typical COP is 3.0 which is the terminology used to mean 3 times more efficient than resistant heat.
Now a really old heat pump - perhaps a COP of 2.0 at 32 degrees. Now the compressor of course will wear out faster the lower temps you run it down to. Cost effective taking compressor life into consideration is a fairly variable situation. But "efficient" - no brainer
A decent modern heat pump can beat resistant heat to below zero. At 32 degrees, a typical COP is 3.0 which is the terminology used to mean 3 times more efficient than resistant heat.
Now a really old heat pump - perhaps a COP of 2.0 at 32 degrees. Now the compressor of course will wear out faster the lower temps you run it down to. Cost effective taking compressor life into consideration is a fairly variable situation. But "efficient" - no brainer
Interesting. In my case, my knowledge is comparing heat pumps to natural gas heat on my hybrid system for my house. Below 40F the system switches to gas. It's configurable, but the limit is 32F. My understanding is at that point I would be using gas to defrost the condensers which defeats the purpose.
Tedkidd
Member
Modern heat pumps work efficiently to -20f.
Most HVAC guys have no clue. How would they? They never track energy use. Imagine a car mechanic who never tracks even his own mpg telling you how to save gas...
Because you can install a box in a basement doesn't magically make you expert on energy efficiency.
+1
There are a WHOLE LOT of "it depends" in that. If you have a Carrier Infinity you can enter your fuel rates and it can figure it out for you, chances are it'll be colder than 30.
For smaller homes we recommend abandoning the gas meter altogether. Commiting to gas infrastructure is chasing hypothetical pennies with real dollars. The meter/billing fee handicap can not be recovered by "cheaper btu". And that "cheaper" is not likely to persist as gas consumption ramps, and solar prices continue to drop.
House size, equipment/enclosure load match matters, and enclosure deficiencies.
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The cabin heat is faster than an ICE car, I'd say 50% faster. The seat heaters are the real gem though, they get HOT and FAST. You won't even need the cabin heat unless you have kids in car seats.
I have had my S for about 2 months here in Minneapolis. With the exception of depleted range, which is not a factor at all day to day for me, the heat, warm-up, defrost situation in the S is superior to ICE cars by a wide margin.
Throw in the remote heating and a great situation gets even better.
Operating a heat pump at low temperatures does create ice, depending on the humidity levels. However, the heat pump can still deliver useful heat in that range, at lower rates than in warmer temperatures - it just has to do a defrost cycle to melt the ice from time to time.
Walter
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YMMV, always. But typically in the US, natural gas is 1/2 to 2/3 the price of electricity on a per unit energy basis - and the gas fired furnace may get ~90% of the gas converted into heat in the house while the heat pump can usually get ~3x as much heat as electricity (the COP mentioned in an earlier post.)
A modern heat pump will almost always be cheaper than any other form except solar thermal in terms of running costs - provided your situation allows it to be used effectively.
(California may be an exception - I know you guys have crazy electric prices, but I have no idea what your natural gas prices are like compared to the rest of the country.)
Walter
That's the best case. Good luck on actually getting one installed with that kind of efficiency. Most installers around here just refuse to put them in, so you end up with one that's about 50% efficient.
It is a carrier infinity system. I've not seen a way to enter fuel costs either in the advanced menu or the service hidden menus. I'm also on a TOU plan so the calculation is likely beyond the thermostat's ability.
Tedkidd
Member
What year is the system?
What sizes?
House sf?
You don't happens to have house leakage cfm50 number?...
What are your delivered energy rates?
Any temperature imbalances, noise or other complaints?
I have an infinity system with the infinity touch thermostats. I can enter fuel costs for electric and propane to display usage cost. But I am told it doesn't use those numbers in determining switchover from heat pump to propane.
My older infinity thermostats do not have a place to enter energy cost.
My older infinity thermostats do not have a place to enter energy cost.
Correct me if I'm wrong, but I thought all thermostats use standardized (mandated) signaling to the A/C-furnace-heat pump. If that's the case, there's no reason you couldn't have a different thermostat installed with whatever additional features (assuming someone makes one with what you want in it - in the extreme case you could hook a computer up and code it I suppose.)
Walter
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