So, You Want To Decarbonize Your HVAC: An Interlude
I recently got a great email from a confused subscriber, who read my "So You Want To Decarbonize Your HVAC" series. He's paying attention and found this article from the American Gas Association, which raised some very reasonable concern in his mind regarding the transition to heat pumps.
Check out his question and my answer below.
"Randy" wrote:
I enjoy the articles/newsletters. I stumbled across the following.
https://www.aga.org/natural-gas-or-a-heat-pump-where-you-live-matters/
Obviously, they have a less than unbiased opinion on gas vs heat pump. However, they *claim* at least to have used government data. What's up with this? I am confused.
Here's my reply:
Hi Randy!
I'll start by saying: yes, I'm skeptical of the source, but I wouldn't dismiss it out of hand. It is a bit cheeky to describe a heat pump as "great at heating to 70 degrees if it's 70 degrees out" haha! But the point is essentially true; it takes more watts to move enough refrigerant to satisfy the thermostat when you have a bigger "delta T" (difference in temperature).
The devil's in the details with this, and there are a lot of details.
Heating load: The article says "As a rule of thumb, the higher the average annual heating bill, the greater the savings from natural gas." This definitely can be true, depending on rates (more on that below). But what they're referring to, obliquely, is the heating load. Your home's heating load might be high, and this could mean that a furnace is cheaper to operate with your home the way it is. But your home doesn't have to stay the way it is. By air sealing and insulating, we can reduce your overall gas bill, and bring the cost of running a heat pump within parity of running a furnace. Air sealing and insulating before installing a heat pump will be crucial for many Michiganders.
Rates: The other part of the multiplication -- besides the number of BTUs or kWhs used (aka heating load) -- is the cost of those fuels. Michigan is one of the most punishing states for electrification. Holland BPW is way more affordable than Consumers. And bringing down the initial investment with huge HBPW rebates definitely helps the ROI! But there's one more issue, which is beyond my pay grade... but I know enough to be dangerous. The rates we charge for fuels are not "optimized" properly. It's not just that they're the wrong numbers. Utilities can, and do, offer different rates for different use patterns. Industrial users, for example, don't pay the same rate as us residential users. There should be a heat pump rate, to properly distribute the costs of owning and operating a heat pump in the winter. This would improve the ROI calculation.
Machine efficiency: The article compares a 96% efficient furnace with a 9.0 HSPF heat pump. That's a good heat pump, but they come even better -- 11 or more HSPF. The more efficient the heat pump, the better the run costs. Also, many folks are not starting with a 96% efficient furnace. It might make more sense for many people to compare a super efficient 11 HSPF heat pump to the cost of continuing to run an 80% efficient furnace. The distinction here is important; they actually have a separate category of heat pump called a CCASHP ("cold climate air source heat pump.") Whereas many heat pumps can do your heating down to 20 or 30 degrees -- and all your cooling of course -- only a really nice CCASHP will be well-designed to efficiently take care of your heating needs down to -13 degrees.
Year-round costs: The article doesn't include air conditioning. This space conditioning is also worth including in the payback calc. A brand new heat pump will tend to have a better SEER rating than whatever AC you currently have. So even if you are paying more for the heat pump during Jan/Feb, you're probably lowering your costs in the shoulder seasons and especially in the cooling season. SEER 16 is considered high efficiency, but there are heat pumps with SEER ratings of 24 or even 26. Many old AC units sitting around from the 90's have a SEER rating of 10-13.
Sizing: This is a biggie! Heat pumps are not being sized properly, and so they're not running very efficiently. The HVAC industry is scared to size heat pumps smaller, because the standard practice for so long has been to just throw "something big" at any old house. Heat pumps don't operate very well when they're oversized because they have minimum outputs they can't operate below.
Might be a bit more but we have to do it: There are a lot of hoops we need to jump through to get really, really good at sizing and installing heat pumps as a society. Ultimately there's just no way we can meet our climate change goals without getting really, really good at heat pumps and installing a bunch of them.
Fortunately, the results are better: The good news is, heat pumps are much more comfortable! If we can dramatically improve the comfort of many homes, a lot of people might pay a couple hundred more bucks per year.
Two important studies from Michigan: 1. heat pumps are more expensive (no air sealing), 2. heat pumps are less expensive (with air sealing).