Heat Pumps: 270% Efficient or Hooey?

Contributed by Canuche Terranella:

We get asked all the time: “Are heat pumps really 270% efficient or is that a bunch of hooey?”

We are excited about heat pumps.  They save energy, they save money, they are more than 100% efficient (is that even possible?), they seem to work like magic and they are one more real world example of the Ideal Gas Law.

Figure 1: My inner chemistry geek loves this stuff!

Hopefully this blog will dispel some of the magic for you.

The equations can be complex (and are usually ignored) but the science can be simplified and understood.

First remember that air is made of molecules that vibrate at a frequency that we can measure as temperature. All molecules are moving (even relatively cold ones) and the energy of that motion we call heat.  Heat naturally flows “downhill” from warmer areas to cooler ones.  What that means at a molecular level is that particles moving faster collide with those moving slower so that the higher energy parts of the collection of particles goes down overtime.  So far so good, right? So how does this turn a 33 degree rainy night into warm and cozy couch time in a house with a mini-split Ductless Heat Pump?

The next piece of the puzzle has to do with how many particles you have in your particular space.  The analogy of a dance party comes in handy here.  When the dance floor’s empty that one guy can really move without getting the party started but as you add more people to the dance floor and the party reaches a critical mass, the energy of the room changes.Likewise when you add more mass of particles to the same box the energy of the space goes up.

The last piece has to do with how much space you have for the party. Ever been to the karaoke party in the really big room with a small group? Feels better to crow in the crowded booth doesn’t it?  Smaller space with the same number of particles will increase the movement of the particles.  More particle movement means more heat.

Overview: Remember that the cool wet air outside still contains particles that are vibrating with heat energy.   If you can get the energy of enough of those particles together in a tight space you can harvest heat from them.  A heat pump is a device that simply moves heat from one location to another. Since heat pumps simply transfer heat instead of burning fuel to create it, they have an eco-friendly efficient edge over their competition.  Heat pumps, when properly installed can provide 1.5 to 3 times the heat energy than electric energy it consumes (the baseline energy consumption is the energy a traditional electric heater would use to create the same heat).  Over a 12 month period using a heat pump can cut the amount of electricity you use for heating compared to conventional methods by 30%-40%, adding up to huge dollar savings in the long run.

Heating: Heat pumps often contain a refrigerant in liquid form that is pumped through coils in the outdoor unit.  Next a fan pulls outside air over the coils which absorb the heat energy in the air, turning the liquid refrigerant into hot vapor.  The hot vapor is than fed through a compressor which increases the pressure and heat of the gas, allowing the vapor to flow into the indoor coils. The heated air is pumped throughout the house as the refrigerant cools and flows back outside to start the process over again.

Cooling: The simplicity of a heat pump is that it can act as a heating and cooling device, cutting out the need for two separate systems. In cooling mode a heat pump simply reverses the process.  A reverse valve is used to turn the system from heating to cooling. The refrigerant is now pumped through the indoor coils absorbing the heat and expending it outside.

Extra Efficiency of a DHP: Duct losses can account for more than 30% of energy consumption for space conditioning, especially if the ducts are in an unconditioned space such as an attic. Therefore, a ductless heat pump (DHP) moves the heated refrigerant through insulated tubes to the room where the heat is needed instead of heating the air in one central location and pushing it through long metal ducts to where the heat is needed.

But sometimes the air blows cool? Since the DHP head is located in the room where the heat is needed it only heats the air slightly above the target temperature set at the thermostat as it does not need to overcome the heat-loss of the ductwork. When this air blows out of the heat unit directly on a person it can even feel slightly cool as moving air feels cooler on the skin than still air.  However the house stays warm, as the heat pump continually regulates the air temperature.

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