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So that means that Tesla's new heat pump is less efficient in cold weather than the old heating system?
Not for your kind of cold. It should always be more efficient. The relative efficiency will drop as the temperature drops.
And either side of deep winter where you still need heat it'll be more efficient.
In any case, cold-weather heat pumps still raise efficiency down to low temperatures, which is part of the reason why so many Canadians use them and raise their eyebrows when people say they don't work in cold weather.
No, in the worst case, a heat pump system is one unit of heat for one unit of energy, same as resistance heating. Only if scacanged heat from the environment was less than heat loss due to the compressor being external (versus the in cabin ptc heater) would it wind up worse.
I think the confusion when it comes to heat pumps lies in the word "efficiency". Heat pumps are never less efficient than resistance, even at sub-zero Fahrenheit. You're still getting at least 1 unit of heat for 1 unit of electricity, which is the best resistance can do. However, they do put out fewer BTUs as temps drop. So from the perspective of a person sitting in a car wanting to be warm, heat pumps feel less efficient than resistance because they take longer to warm the cabin when it's cold out.
I heat my house purely with heat pumps, with no backup resistance strips. I don't let the house get below my set point during the coldest months because it could be hours to bring it back to the set point (i.e. I don't use a nighttime temp and a daytime temp during Jan/Feb). But it holds the temps just fine. The rest of the heating season I do let the temps drop in the house at night.
That said, for a passenger car, by design you're going to want resistance heat for the initial warm-up in very cold temps then a heat pump to hold the temp, unless you don't mind freezing your lower body parts off for a while until the cabin temp raises.
I heat my house purely with heat pumps, with no backup resistance strips. I don't let the house get below my set point during the coldest months because it could be hours to bring it back to the set point (i.e. I don't use a nighttime temp and a daytime temp during Jan/Feb). But it holds the temps just fine. The rest of the heating season I do let the temps drop in the house at night.
That said, for a passenger car, by design you're going to want resistance heat for the initial warm-up in very cold temps then a heat pump to hold the temp, unless you don't mind freezing your lower body parts off for a while until the cabin temp raises.
Which is exactly what Tesla does; just not in the conventional manner of resistive heating elements.
Another problem I've found with heat pumps is if you want to cross the ambient temperature. Say the car is 120F, temps outside are 90F and you want to get the car to 75F. As the temperature in the car approaches 90F, the heat pump will essentially quit putting out cold air and getting the car below 90F is going to be very difficult if not impossible.
We have a whole house heating and cooling system, but two heat pump units in two rooms of the house. We have a home office on the SW side of the house upstairs that can be very hot (because of the computers in there) when the rest of the house is OK. I've had frustrating days when the air was mild temp outside, but the office was too hot to be in because of solar heating and the heat pump doesn't get the job done.
We have a whole house heating and cooling system, but two heat pump units in two rooms of the house. We have a home office on the SW side of the house upstairs that can be very hot (because of the computers in there) when the rest of the house is OK. I've had frustrating days when the air was mild temp outside, but the office was too hot to be in because of solar heating and the heat pump doesn't get the job done.
A heat pump is functionally no different than a standard air conditioner when in cooling mode. It’s possible that your unit(s) are undersized, in need of service or the ductwork supplying the warm room wasn’t appropriately sized for the current conditions (additional computers on the sunny side of the house). If you spend most of the day in that room, a separate, small mini-split system could save you quite a bit of energy while increasing comfort.
Side note: most of this home heating discussion is unrelated to the HVAC systems in Teslas, which are significantly more advanced. Maybe someday Tesla will build some trick new home system based on what they’ve learned from the cars.
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The problems I have are cooling the room on days that are cool outside (very little difference between the target temperature and the outside temperature). The only time I run the heat in the office is on very cold winter days, but the whole house heat is usually running and heating the room just fine.
Most of us are talking about our experiences with heat pumps at home because that's where most of us have experience with them. I did also study them a bit in thermodynamics at school, but I admit thermodynamics was not my strongest area of my engineering education (my degree is in Electronic Engineering).
Sounds like the heat pump wasn't working and it was just exchanging inside and outside air. In cooling mode it is identical to standard air conditioning only systems.
Something else is likely wrong. The refrigeration cycle doesn't work that way. As @Big Earl already pointed out, when cooling a heat pump is thermodynamically the same as air conditioner. If what you described is true refrigerators and freezers wouldn't work.
What is surprising to most people is the capacity of residential air conditioners/heat pumps compared to ones in cars. Cars have almost the same sized cooling as a typical residential unit but they're cooling a much smaller space. They're intended to change the temperature very quickly of a small space. Residential systems are typically sized for average consumption, not for a rapidly chilling down the the much bigger space. There are dehumidification (and other) reasons why systems are intended to run for longer instead of short quick bursts..
Think about the thermodynamics if the inside is 120 and outside 90. I would expect the heat actually wants to get out. Once the inside starts dropping below 90 which way does the heat flow? I would expect it to be in. To cool the space you have be moving the heat out faster than your gaining it. I'm ignoring the complications of ducting/airflow in many HVAC systems.
In summary, it's not a question of technology, it is question of capacity.
stopcrazypp
Well-Known Member
Yeah, the confusion is people ignore capacity. Systems are size for the given load. In terms of efficiency a heat pump is always more "efficient" than a resistive heater. The difference is capacity, which varies for a heat pump.
When the heat pump is operating in its most efficient mode, the capacity is typically significantly higher than the resistive heater. When in its least efficient mode it may have less capacity than a resistive heater, and people mistakenly say it's "less efficient". Many systems use a backup heat source for this reason. The ones that are designed to operate without a backup source (like Tesla's) will use other methods to generate more heat. As discussed in other threads, Tesla uses the compressor as an additional heat source (there are some other home heat pumps that do the same, like Mitsubishi's "Hyper Heat" system).
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It is less efficient than it is at a more optional condition, but not less efficient than resistvie. At peak, you might get 4 units of heat for every unit of energy (400% efficiency). At worse conditions, you may only get 2 units of heat for every unit of energy (200% efficiency). On pure aux resistive heat, you only get 1 for 1 (100% efficiency). Natural gas furnace may give you 0.9 units of heat per unit of energy (90% efficiency).
Max power input to the heat pump is fixed (mostly) capacity is directly tied to the efficiency at that operating point.
stopcrazypp
Well-Known Member
To clarify, in that part I meant to say "have less capacity" than a resistive heater, not that it is "less efficient". I have edited it to make the wording more clear.
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