One of the biggest mistakes many builders make is to install a heating system fueled by propane or oil heat without considering an electric heat pump. In most cases that choice is costing the owners hundreds (or even thousands) of dollars a year in higher energy bills.
The graph at the right compares annual fuel costs for heat for a typical a new 2,000-square-foot Massachusetts home. A similar graph comparing the cost to operate different types of water heaters is shown below. The relative costs of the different fuel options will be similar for similar climates, but it’s important to note that natural gas is not available in many places.
Given these numbers, why are builders still installing propane and oil heating systems instead of electric heat pumps?
“I always thought electric heat was less efficient”
This is one common response I hear from builders when suggesting electric heat. This response is a product of history. Historically, heating with electricity meant resistance heaters. Resistance heat was common until the development and wide adoption of small, safe, and efficient fossil-fuel fired heating equipment.
Today, heating with electricity usually means using a heat pump, and today’s heat pumps are two to four times as efficient as a resistance heater. Heat pumps are completely different products from resistance heaters, and we shouldn’t be biased just because they use the same fuel source.
Is this a passing trend?
It’s impossible to predict exactly how energy prices will change in the next 10 to 30 years, but the U.S. Energy Information Administration (EIA) predicts that electricity prices will rise at an average annual rate of 0.4% between now and 2040, while oil prices will rise at 0.6% per year and propane prices will rise at 0.5% per year (excluding inflation).
This means that according to the EIA, heat pumps will only look more and more favorable in the coming years.
Some people don’t trust electric heat
Electric heat depends on the grid and the grid doesn’t always work. These are a few options for dealing with grid intermittency:
• Backup generator. This has the side benefit of running the lights and appliances during an outage, but it will cost at least a couple of thousand dollars.
• Backup heating system. The options here include a fuel-fired furnace, a boiler, or a wood-burning stove. Since it’s a backup system, efficiency can be sacrificed here to save money. A low-efficiency backup furnace system might cost about $1,000.
• Tough it out. If you’re confident in the grid and willing to throw on a couple of extra layers when there is a power outage, this is obviously the most cost-effective option. If there’s a risk of losing power for days at a time in the winter, however, you’ll want a backup system to avoid frozen pipes.
Heat pumps aren’t as efficient when it’s cold
When an air-source heat pump is working, it is actually pulling heat from the cooler outside air. As the outside air gets colder, the heat pump gets less efficient. For instance, today’s heat pumps will have a coefficient of performance (COP) of around 4 at 50°F and a COP of 1.5 at -10°F. This means that heat pumps will perform better in more temperate climates — although they don’t do that bad in cold climates.
The paper Climate Impacts on Heating Seasonal Performance Factor (HSPF) and Seasonal Energy Efficiency Ratio (SEER) for Air Source Heat Pumps (Fairey et al.) provides recommendations for accurately modeling the efficiency of air-source heat pumps in different climates. These recommendations are taken into account in the algorithm used to calculate the energy bill results shown in the graphs included above and below. High efficiency levels on temperate days more than make of up for low efficiencies during cold spells.
The issue of low-temperature efficiency is less important for ground-source heat pumps, because ground temperature is a lot more consistent than air temperature.
In addition to providing cost savings compared to propane and oil, heat pumps have some other important benefits:
• They are safe. Since there is no combustion in the house, there is less risk of air quality problems from carbon monoxide.
• They are convenient. No fuel tank or delivery is required, as is the case for propane and oil.
• They are clean. Fossil-fuel-fired equipment (especially oil-fired equipment) needs to be cleaned periodically. Cleaning is not necessary for electric equipment.
Air-source or ground-source?
Ground-source (or geothermal) heat pump costs vary significantly. In genera,l a ground-source heat pump will cost at least $10,000 more than an air-source heat pump but will be more efficient.
An energy modeling analysis and a financial analysis are especially valuable here, since this decision has a large financial impact.
Ducted or ductless?
The discussion at the beginning of this article assumed that a house has a forced-air heating system with ducted distribution. A heating seasonal performance factor (HSPF) of 8.5 was assumed for the air-source heat pump option, which is the equivalent of a mid- to low-efficiency unit — above the Federal minimum of 7.7 HSPF but well below high efficiency units like the Carrier Infinity 20, which boasts an HSPF of up to 13! The Goodman SSZ14 is an example of a commonly used unit; it has an HSPF of 8.5.
Ductless systems are more cost-effective and energy-efficient than ducted systems. They alleviate the need for costly, leaky ductwork, but they aren’t as aesthetically pleasing as ducted systems and it can be tricky to ensure conditioned air is properly distributed to all rooms of the house.
Relatively small homes that are well-insulated for their climate zone are the best candidates for going ductless. Builder Carter Scott’s homes are an excellent example of how to use ductless minisplits effectively in a relatively cold climate. His relatively small homes are superinsulated, and can be heated with a couple of minisplit units. For homes that aren’t as well-insulated, ductless heat pumps can also work great in auxiliary applications — like a bonus room, addition or apartment, where it’s desirable to have a separate, small heating and cooling system.
Heat pumps are the technology of the future
By allowing us to heat with clean renewal energy generated by photovoltaics, wind turbines, or a hydro plant, electric heating will be an important and necessary step in cutting greenhouse gas emissions.
Also, while fossil-fuel fired heaters have become about as efficient as they can possibly be, we are not very close to the theoretical limits of heat pump efficiency. So, heat pumps are only going to get better.
Nick Sisler is a co-founder and Engineer at Ekotrope, which provides software and energy consulting for builders. Ekotrope’s software is a RESNET accredited HERS and IECC Performance rating tool. Nick holds a bachelor’s degree in Mechanical Engineering from M.I.T.