eli5 – Why is the heat pump more efficient?

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We have a heat pump for heating. How is it that it’s cheaper to use it than just turn the oven on to warm the house? Also when the heat pump is off it still circulates air.

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The oven is designed to cook food and retain its heat inside. A heat pump is designed to spread the heat throughout the house and circulate the air. The blower stays on after the heating unit shuts off to use up any residual heat that’s still on the unit for both efficiency and longevity. It also helps ensure there’s no hot spots in the house by recirculating the air in the house.

Moving heat takes less energy than creating new heat. Also, the energy used by the heat pump creates some new waste heat, which is added to the heat output.

Your oven is generating all of the heat. Every joule of heat you get from the oven required a joule of electricity

The heat pump isn’t generating the heat its mostly just moving it from inside to outside. The heatpump is just an Air Conditioner pointed the wrong way

It makes the side outside colder where it absorbs heat which can be brought inside then given off to the room on the hot side, the opposite of how you normally run your AC. If your temperature difference is reasonable then the heatpump can add 3 joules of heat to the room while only consuming 1 joule of electrical energy so it can make the house just as warm with 1/3 the electrical energy

The system will also run the fan because heat at the ceiling does you no good when sitting on the couch so you want to keep things mixed to keep things the target temperature at your level instead of at the ceiling fan’s level

A heat pump takes heat from the outside air and pumps it inside. This means that you spend less energy on producing heat than the amount of heat energy you’re adding to the inside. Even if it’s cold outside, there’s still heat in that air you can pull out to bring inside. Also any waste heat created in this process is still useful.

Let’s assume our power comes from natural gas. Natural gas power plants are about 50% efficient at turning gas into electricity.

If we use a space heater to heat our home (which is by definition 100% efficient) we are still only getting 50% energy efficiency from that initial gas.

If we use gas to heat our home directly, that’s about 80% efficient. Better than a space heater.

If we use electricity to run a heat pump. We don’t measure the efficiency of a heat pump, but rather the coefficient of performance (CoP) which has a minimum of 1. That is because if it gets down to 1, it’s exactly the same.as a space heater. It’s pulling in no heat from the outside and turning the electricity only into heat. In most conditions, the CoP is about 5, so the heat pump is providing 5x the heat as it consumes in electricity. So with out 50% efficient power generation, we are getting 250% the heating in our homes as the natural gas had in initial energy.

Even with a CoP of 2, you are still matching the efficiency of a space heater.

You can calculate it based on the conditions CoP = Th/Th – Tc where Th is the temperature of the hot side and Tc is the temperature of the cold side, both measured in Kelvin

This doesn’t break conservation of energy because we are simply stealing heat from the air outside.

Lets walk through a very simple system.

You want a warm house. You put some water outside on wide shallow pool, with a dark stone bottom.

This dark stone absorbs sunlight, and heats up quickly. The water touching the pool bottom warms quickly as well. This allows the water to quickly warm up to temperatures higher than the air temperature.

You then collect the hot water, and carry it inside. Your house is well insulated, and so the water cools, brings the air in your home up in temperature compared to outside, and generally keeps it there.

While this bucket of hot water is cooling you take the already cold water in a second bucket outside, and put it into the heating pool.

You only spend energy moving the water into and out of your house. All the energy to heat the water comes from the sun.

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Oven approach, or in this case “water heater” approach.

Cold water is brought into your home. Your water heat warms it up, and this is actually a very, very energy intense process. Heating water is one of the most energy intense processes humans do on a regular basis.

Then the hot water is put somewhere else in the house (like a shower) which heats up the new room. The cold water is now removed from the house..

Notice that two of the steps (move hot water into home area to be warmed, remove cold water) are identical to the heat pump approach. The oven, or direct heating, approach has an additional, very energy intense step of “heat the water” using electricity or flame.

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Heat pumps are always more energy efficient than direct heating. If the temperatures of the exterior are at a point that the material used in a heat pump to shuttle the heat back and forth (in our case ‘water’) won’t be able to shuttle enough heat to keep the house to the desired temperature. Even if it’s -20F outside, it might not manage to keep the house at 70F, but probably 40F.

Direct heating can also have this problem (small furnace, large or drafty house…it’s to cold). And direct heating can be easily implemented with a space heater or two to supplement what the heat pump can manage. And this heat pump plus space heaters will be far more efficient than just furnace and/or space heaters.