On a Facebook Page I occasionally follow, someone asked about whether they should switch their heat pump to natural gas for the winter. Having installed a heat pump a few years ago, I reviewed my notes, and went looking on the Internet for the App I had used to calculate the economic balance point of my heat pump. Unfortunately, it was no longer available, and there were few other tools available online, and most used odd units of measure. In my research, I learned a bit about converting between various units of measure for forms of energy, and decided to create my own worksheet using Excel.
An air-source heat pump (the most common) works like an air conditioner, or refrigerator, but in reverse - it extracts heat from a cool place (making it colder) and moves it to a warm place (inside your house). Just as your fridge makes the freezer cold by dumping heat into the room, so does a heat pump. Most heat pumps are 2 to 3.5 times more efficient at creating heat than resistance heating, such as a baseboard heater. Unfortunately, as the temperature drops, the ability of a heat pump to extract heat from the air decreases. A heat pump which is 350% efficient (making 3.5 times as much heat as would resistance heating using the same amount of energy) may decrease to less than 200% as the temperature drops below freezing.
Let's get some terms out of the way. There are three different 'Balance Points' with respect to a heat pump:
- Thermal Balance Point or what I call the Mechanical Balance Point. This balance point is achieved when the heat pump can no longer meet the heat demands of the building, and a secondary source of heat must be used to meet the heating demands of the building. In many cases this may be natural gas or propane heat. These heat sources do not as efficiently convert the fuel to heat as does the heat pump.
- Comfort Balance Point. A heat pump delivers large volumes of warm air, and for longer periods, than does a furnace. For those used to the blast of hot air from a furnace, sometimes the air delivered by the heat pump seems insufficiently warm. Or it takes a long time to heat the space from a cooler temperature. In these instances, folks often want to use the hot air from a combustion heater to warm the space sooner.
- Economic Balance Point. This is the temperature when one fuel becomes cheaper than the other. For instance, in locations where natural gas is cheap and electricity expensive, it may be cheaper at cold temperatures to use inefficient, but cheap natural gas heat at less than 100% efficiency than to keep using electricity.
When I first installed our heat pump, electricity was inexpensive, and gas sufficiently costly, that my thermostat was set to the Thermal Balance Point; I used electricity to run the heat pump until it could no longer meet the heat demands of the house, at which time, more expensive natural gas was used to add the additional heat needed to keep the house warm. Since that time, electric costs have risen, and natural gas prices have dropped, so I needed to review my balance point setting. My heat pump is a 'package unit' with both the heat pump and furnace unit contained in one cabinet. You often see these on commercial building roofs, as they supply both heat and air conditioning in the one unit. This unit was chosen at the time as it was cheaper to install than a horizontal furnace, needed for my low-ceiling crawl space, and air handler unit would have been. The downside is that the furnace section is only about 80% efficient, rather then the 98% efficient furnaces available today. It replaced an old 65% efficient furnace in the crawl space.
So I spent an evening on the 'net trying to figure out my new balance point. Having perused a number of sites, I determined that somewhere about -5°C was my new Economic Balance Point. However, now my interest was piqued to make it easier the next time - thus this spreadsheet. On page 2, COP you can enter information about the efficiency of the heat pump you have, and generate a graph of efficiency vs. temperature. On page 3, Costs, you can enter various information about your heating system, and determine the Economic Balance Point for your home, based on BC utility inputs.
The spreadsheet also allows you to play around with numbers a bit. I learned that with current prices for utilities, and my 81% efficient natural gas furnace, the balance point is about -3°C if I can stay below the 1600 kWh bi-monthly penalty limit. So, it makes sense to switch to natural gas soon enough to avoid going over that limit. If, on the other hand, I had a new 98% efficient condensing gas furnace, the heat pump would best be set to come on at about 2°C, leaving the gas furnace to heat the house any time the outdoor temperature dropped near freezing. Since the average winter temperature here in Creston is about -3°C, there are many days when the temperature rises enough to switch to the heat pump, and of course many days in the summer, when air conditioning has value. Fortunately, my Thermostat makes these adjustments straightforward.
You can find my spreadsheet at this link.
Here is another spreadsheet I found subsequent to publishing this post. It contains mostly US data, but includes some Canadian climate info as well.