Gas, propane, or heating oil furnace vs heat pump calculator

Data sources and calculation notes:

This calculation uses the following values for fossil fuel energy and carbon content:

• 1 CCF of natural gas = 103,600 Btus of heat and 11.27 lbs of CO₂
• 1 gallon of propane = 91,452 Btus of heat and 12.43 lbs of CO₂
• 1 gallon of heating oil = 137,381 Btus of heat and 22.61 lbs of CO₂

This data is published by the US Energy Information Administration (EIA) and is based on the direct emissions from power plants in each region as well as transmission power line losses.

How this calculator works

If you currently heat your home with a furnace, this calculator will do an approximate comparison of the annual operating cost and carbon emissions of your furnace to an air source heat pump.

It works by asking for your annual fuel usage, fuel cost, furnace efficiency, and electricity price. It converts your fuel usage to a quantity of heat in BTUs that the fuel will produce when burned. After accounting for the efficiency of your furnace, we can then approximately know how much heat the furnace puts into your home. The calculator then estimates how much electricity an air source heat pump would need to produce the same amount of heat.

This is a nifty shortcut to the problem of estimating the correct heat pump capacity for a home, which requires a careful energy audit and equipment selection. That is still the correct approach, and you should expect a good HVAC contractor to do a complete analysis of your home (such as a Manual J load calculation) to ensure that the equipment they select is appropriately sized for your heating and cooling needs. However, this requires specialized software and equipment such as a thermal imaging camera, so it’s not something that an average homeowner can easily do themselves.

This calculator avoids this issue by using your current heating bill to estimate your heating needs. By using your current heating costs as a starting point, this calculator can give you a decent comparison between your furnace and a heat pump in just a few seconds. This is a useful starting point and a reasonably good estimate given the minimal effort it requires, but it’s important to understand the limitations to the approach. (This will also help to make you more knowledgeable when interviewing contractors.) This is because there are many factors that affect the operating cost and efficiency of a heat pump, including:

• The coefficient of performance (COP), which is a measure of efficiency that varies with the outdoor temperature
• The local weather, especially the air temperature, during the heating season
• For central heat pumps, the blower motor consumes electricity, which will vary with the run time and amount of back pressure in the system (due to things like narrow ducts and clogged filters)
• The balance point of a heat pump, which is the temperature below which the system switches to less efficient “strip heating” or an external furnace
• Whether or not you have a time-of-use plan with your electric utility

What is COP?

Out of all of these factors, estimating a heat pump’s coefficient of performance (COP) during its operation is probably the most tricky, and it also has the biggest impact on efficiency and operating cost.

For a deep explanation of COP and other heat pump terminology, you should read my article on heat pump terminology, but the short explanation is that an air-source heat pump is less efficient at heating the colder it is outside. (A ground-source heat pump doesn’t have this shortcoming because the ground temperature doesn’t change very much during the year.)

Air temperature changes not only from month-to-month but also during the course of a day. This means that a heat pump will be more efficient at heating when the air temperature is higher during the day and less efficient when it’s colder at night. There are also seasonal changes during the heating season, and the additional complication that some years may be deviate from seasonal averages.

All of this makes accurately estimating the operating cost of a heat pump quite difficult, as it ultimately comes down to how the heating system is used. For example, if you like it colder when you sleep, you’ll save money compared to a home that has its thermostat set higher a night, not just because the heating system is running less, but because the COP is worse during colder nighttime temperatures.

A qualified contractor will use historical climate data and the published COP for a heat pump to generate a reasonably accurate estimate of the yearly operating cost for a system. Because this data is based on climate averages, you should expect the actual operating cost to vary from year to year. That said, a professional estimate will still be the most accurate.

Understanding HSPF

Because the COP of a heat pump can vary hour-by-hour as the outdoor temperature changes, another rating system was created to make it easier for consumers to compare the efficiency of heat pumps. Heating Seasonal Performance Factor (HSPF) is similar to COP in that it describes heating efficiency, but it instead is calculated for an entire heating season. Here’s a definition from EnergyStar.gov:

HSPF2 is the total space heating required in region IV during the space heating season, expressed in Btu, divided by the total electrical energy consumed by the heat pump system during the same season, expressed in watt-hours.

This means that HSPF can be used to make a reasonable estimate of the efficiency of a heat pump during the heating season, as it’s designed to account for varying efficiency due to outdoor temperature changes. The HSPF number itself tells you how much heat a heat pump will generate for every watt-hour of electricity it consumes. A heat pump with an HSPF rating of 10 will generate 10 British thermal units (BTUs) of heat for every watt-hour it uses. A Kilowatt-hour (1,000 watt-hours) is a more familiar unit that you will see on your utility bill, and so a more useful conversion is that a 10 HSPF heat pump will turn 1 kWh of electricity into 10,000 BTUs of heat.

The specifications used to calculate HSPF are based on climate zones¹ 4 and warmer. You might immediately recognize a problem with this: there’s a huge difference in average outdoor temperature between Climate Zone 1 (hot and very humid) and Climate Zone 8 (Subarctic and Arctic) in the United States.

To compensate for this, the Electric Dwelling calculator asks for your zip code so that it can identify your climate zone. If you live in Climate Zone 5 or colder, the calculator reduces the HSPF by 15% to compensate for worse performance in colder temperatures.

The bottom line is that using HSPF in this way to generate an estimate of the operating cost of a heat pump is easier than using COP ratings and monthly average temperatures, this approach still can only give you a rough approximation at best. The problem is similar to EPA fuel economy ratings. While your car might achieve a fuel economy close to its EPA rating, its real world fuel economy will depend on your driving habits and operating conditions.

Regardless of these technical challenges, HSPF is still a very useful specification because it quickly identifies the efficiency of a heat pump. All the average consumer really needs to know is that a heat pump with a higher HSPF rating is more efficient that one with a lower rating.

What is the balance point of a heat pump?

Another factor in calculating the operating cost of a heat pump is the balance point. The balance point is the temperature below which the heat pump will switch over to a secondary source of heat because it’s too cold outside for the heat pump’s outdoor unit to extract enough heat to maintain the desired indoor temperature. If it’s cold enough, a heat pump could run continuously and still not be able to supply enough heat to keep your house warm. That’s what the backup heat source is for.

In my own home’s setup, I kept my old gas furnace as the backup heat source. When it gets really cold outside, the thermostat will automatically switch over to the furnace for heating, and switch back to the heat pump when it warms up outside.

Other setups will use electric resistance heating, also known as “strip heating”. Electric resistance heating is the same simple technology you can find in a toaster, electric furnace, or space heater. The advantage of strip heating is that the equipment cost is cheap and it will work at any temperature, but the disadvantage is that it is inefficient. The COP of an electric resistance heater is slightly less than 1 (after accounting for blower motor losses). In contract, the COP of a heat pump in mildly cold weather is typically around 3 - which is three times better.

Your HVAC installer will set a balance point that is appropriate for your equipment, but you won’t be able to exactly calculate the operating cost of your heat pump unless you know how many hours out of the year the temperature will drop below the balance point. The Electric Dwelling calculator doesn’t try to include the balance point in its estimate.

The balance point for a standard heat pump is usually a few degrees below freezing - say, about 25°F. If you live in a cold climate and expect a lot of days below this temperature range, you can choose a cold climate heat pump which are designed to work well even in subarctic climates. With a cold climate heat pump, the balance point can be set much lower, such as around 5°F.

Comparing the carbon emissions of a furnace with a heat pump

Another reason for the growing popularity of heat pumps is that they are usually better for the environment, both in terms of health and safety in your home but also climate change.

Even when the electricity that runs your heat pump comes from a power plant that burns fossil fuels, a heat pump is so efficient that its net carbon emissions will usually be lower than even the highest efficiency gas furnace. If you use heating oil, as many older homes in New England do, a heat pump will certainly have lower carbon emissions because the New England electrical grid has relatively low emissions, and heating oil has high carbon intensity.

Finally, if you currently have an electric furnace, which is common in Florida, a heat pump will always have lower carbon emissions.

The Electric Dwelling calculator compares the emissions of your fossil fuel furnace to a heat pump based on the mix of power plants in your local electric grid. This will be a good basic estimate, but if you are hardcore about reducing your environmental impact, you can help by programming your thermostat to operate the heat pump less when the grid is “dirtier”.

In general, the electric grid will have lower emissions during your electric utility’s off-peak hours. For example, where I live in Buffalo, clean hydroelectricity from Niagara Falls supplies a greater proportion of the electricity supply during overnight hours compared to the day when gas power plants kick in to meet demand. This means that my heat pump will have a lower carbon footprint at night than during peak hours in the early evening.

Meanwhile in California, there is so much solar power attached to the grid that the cleanest electricity is often the hours around noon when its sunniest. If your goal is to minimize emissions, you could make your California home extra warm (or cool in the summer) and then “coast” on that conditioned air so that you don’t have to use your heat pump during the evening peak when grid electricity is the dirtiest.

Bottom line: this calculator is useful for a quick analysis, but don’t solely rely on it to make a decision

If you’re just starting to think about making the switch from a furnace to a heat pump, this calculator can be a valuable tool to understand the impact on your wallet and the environment. However, for all the reasons outlined above, it can only give you an approximation. The best analysis will be from a professional HVAC installer, and be sure to get multiple quotes.

Despite the shortcomings, knowing how this calculator works will help you better understand how heat pumps work, which in turn will help you make a better decision about whether a heat pump is right for your home.

References