Heating and Cooling Systems

Your source of heat (for those of you in cold places) will depend on a few factors:

The most commonly available energy source
The price of that energy – typically as compared to electricity
How your house was built e.g. did they put in gas pipes or rely on electricity only?

Likely you are using a carbon-based fuel – oil or gas, else it will be electric The carbon intensity of that electricity depends on the sources of generation. Wood is also an option, but used as a supplementary heat source in most places (a nice insert or log burner for example).

Methods of Heating and Cooling

Heat Delivery

Chances are your heat is either delivered via air (through ducts to vents or registers) or through water (through pipes to radiators). A furnace (with fan) or boiler (with pump) will then provide that heat. Efficiencies will range from terrible to excellent depending on your product.

Moving heat through water uses less energy in the delivery process. Water holds 4x the heat per kg pumped vs. air blown, while hot water to a radiator is a more effective way to heat than a air through a register, due to the greater temperature difference between the radiator and the air. However, a ducted system using air does allow you to incorporate fresh air ventilation more easily. If in doubt, hydronic (water) means greater comfort, less noise and a lower cost of electricity per unit of heat delivered.

A heat pump is an air conditioner than can work in two different directions – extracting heat from the house in summer, then delivering it inside in the winter. How? Think of your refrigerator. It moves heat from inside to outside. A heat pump does this in summer (a/c) but in winter turns your refrigerator into an oven: taking heat from outside the house and bringing it in.

An air conditioning system relies on a refrigerant moving heat from one place to another (the hot inside to the cooler outside). The refrigerant boils when it takes on heat (inside), then condenses (outside) and sheds that heat to the outside air with the help of a fan.

How does a heat pump find heat in winter where ‘there is none’? Well it’s all down to the boiling point of the refrigerant at its operating pressure and the properties of air. Everything contains some heat as long as it’s above absolute zero (-273°C). If the boiling point is lower than the outside temperature, then even in cold winter conditions the refrigerant will boil. Boiling takes a lot of energy and so taking this now-absorbed energy inside means you can heat with it.

This cycle of boiling and cooling allows the system to move more heat than the amount of electricity consumed by the compressor. This is known as the Coefficient of Performance (COP).

A heat pump’s COP will vary with temperature however, dropping and dropping until it can actually be below 1 (thus making it less efficient that a pure electric heater). However modern ‘cold climate’ heat pumps can stay well above a COP of 1, down to -30°C, meaning they can work in all but the World’s coldest places. The unit’s output will likely drop with temperatures, so care is needed when choosing a heat pump.

https://www.energy.gov/eere/buildings/articles/cold-climate-air-source-heat-pumps-innovative-technology-stay-warm-winter

https://taf.ca/publications/lessons-from-a-heat-pump-retrofit-at-walpole-ave-a-case-study/

https://www.energy.gov/energysaver/heat-pump-systems

Air Source Heat Pumps

Ground Source/Geo-exchange Heat Pumps

This is complex!

As opposed to a place like Iceland where geothermal energy results from volcanic activity near the surface, energy form below the ground can be usefully extracted by a geo-exchange system.

The sun’s energy enters the ground via absorption, and remains, providing the solar energy gained exceeds the losses to the air due to lower air-than-ground temperature (think, not permafrost). Warm ground water - indirectly heated from the sun - can also contribute a source of heat.

Free energy from the ground is complex and therefore very site dependent, particularly as relates to ground water.

Most of the time, commercial-scale geo-exchange is energy storage, not free energy

i.e. trans-seasonal storage – we dump excess heat from air conditioning loads in summer time into the ground (warm it up), then extract that heat in the winter (cool the ground back down).

At residential scale, it can be a mix, or purely from the ground. A heat pump is needed to extract that energy.

Here's a primer for you.

https://www.epa.gov/rhc/geothermal-heating-and-cooling-technologies