Home heating turns out to be an incredibly complex and regional issue – how much heating is needed, the practicality of heat pumps, and grid energy emissions vary significantly in the US. Not to forget your home variables – what services are available, ducting, current system efficiency, etc. Given that keep in mind there’s no one answer that works everywhere.
While home heating a complex and fairly mundane topic it deserves a lot of attention – some back of the napkin math suggests home heating and cooling in America is responsible for at least half a trillion pounds of CO2 annually.
We’re going to look at the three main categories of heating systems – electric resistive heaters, heat pumps, and gas furnaces.
This category encompasses baseboard heaters, built-in electric forced air heaters, space heaters, and electric furnaces. While this encompasses a big group they all fundamentally work similarly – electricity is passed though a resistive material and heat is generated. They are highly efficient and cheap, but don’t let efficiency fool you into thinking they’re environmentally friendly or cost effective.
Heat pumps fundamentally move and compress thermal energy from one source to another. The three major subtypes are air source, water source, and ground source heat pumps – depending on where they draw the thermal energy. Heat pumps at first glance almost seem to defy the laws of physics as modern units can be 200-500% efficient – but don’t worry, that’s just because they’re only counting the electricity input, not the ambient energy it also uses.
Gas furnaces are probably the most common form of heating in homes, generally using natural gas to heat air. Gas furnaces at first glance appear the least efficient (80-98%) but you may be in for a surprise later if you expect them to come in last.
Its also worth note if your furnace is from the 80s or before, you could be running an old 55-72% efficient furnace.
So Which is Most Environmentally Friendly?
For the sake of discussion we’ll look at the Western Washington market because of familiarity and its actually fairly middle of the road for heating needs. According to one study the heating load here is 582 therms for a fairly typical 2000sq foot home. We’re factoring 11.7lbs CO2/therm of natural gas and 1lb CO2/kWh from PSE utility power.
|Estimated Annual lbs CO2||Estimated Annual Cost to Run||Estimated Purchase Cost|
|Electric Resistance||17050||$1705||Don’t do it!|
|Standard Efficiency Gas Furnace (80%)||8512||$549||$3500|
|High Efficiency Gas Furnace (98%)||6948||$448||$4250|
|Air Source Heat Pump (HSPF 8)||8000||$800||$4500|
|Air Source Heat Pump (HSPF 15)||3820||$382||$6000|
Quick note on heat pumps – unlike the other systems that work with a fixed efficiency, heat pumps will becomes less efficient as temperatures drop. This does make such estimates a bit like nailing jello to a wall, unlike the simple calculations for gas heating.
Unlike older heat pumps can’t couldn’t work well below 40F, new cold climate units can work below 0F and still be significantly more efficient than resistive heating. This is why I left off ground source heat pumps – the additional cost is nearly impossible to recoup given the improvements in air source heat pumps.
Needless to say for anyone using electric resistance heating, a heat pump can radically slash energy bills and environmental impact. However if your home isn’t ducted, ductless systems can cost a fair bit more to install. As always check for rebates – PSE offers $800-1500 rebates for replacing resistance heating with heat pumps.
The high efficiency gas furnace is worth note because it has a modest cost upgrade from a standard furnace, utilities often offer rebates ($350 from PSE), and the up-front difference pays for itself in just a few years. Saving 1500 pounds of CO2 seems modest, but its about the same as cutting beef from your diet.
Why Efficiency Isn’t Directly Comparable
You might understandably be perplexed why an 80% efficient furnace is more cost effective and has lower CO2 emissions than a 100% efficient electric heater. The problem is the efficiency of each heater is only measured at the unit – it fails to account for the efficiency of the powerplant, distribution method losses, etc. For example a coal powerplant is around 35% efficient, then you can have 8-15% losses in power transmission, etc.
While its hard to believe an environmentalist would push petroleum, right now a high efficiency gas furnace is a decent investment both on cost of ownership and environmental impact. This is especially true in areas of the country with predominantly coal power as the CO2 output of electric resistance and heat pumps can be about double the above estimates.
Having said that buying a new high efficiency furnace to save $100/year over an working 80% efficient furnace doesn’t make much sense, so this is more of a thing to consider when its time to replace the old furnace.
On the flip side as utilities add more renewables to the mix, the heat pump you install today will keep getting greener. Not to forget a solar power system could eliminate the CO2 output while lowering the operational cost of the heat pump. Given the heat pump also cools, it can be an appealing option.
And of course for those who utilize electric resistive heating, a heat pump system is well worth consideration as it could yield huge reductions in power bills and CO2 output. The savings here warrant consideration of replacement sooner than a failure of the existing system given the possibility of saving over $1000/year on heating bills.
Bottom line, when its time to replace your heating system, don’t just get the same thing you have today – check out a high efficiency furnace or air source heat pump.