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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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.
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.
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.
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