That’s fair. Thanks for
That’s fair. Thanks for engaging on the topic and sorry for opening the extra can of worms. I feel better that we’ve at least established that the reference is for single zone systems. Perhaps on the hypothetical ASHP article you can share an example of a mid-efficiency model getting cop2 at 0F and also something better, rendering that the mid-efficiency option. I still suspect those units may be the peak and not the middle. With respect and appreciation, Bob.

@Bob, as noted in my post, 2
@Bob, as noted in my post, 2.0 COP @ 0F or thereabouts is typical of ‘mid efficiency’ heat pumps. Of course higher efficiency models are available. If you have the expanded performance tables for a heat pump, you can easily calculate the COP at any BIN temperature, although that’s getting way beyond the subject of this article.

BTW, 2-pipe multi-split heat pumps such as the M-Series you linked, by definition, are less efficient than single-splits. Moreover, smaller systems are generally more efficient than larger systems. Therefore, multiple small 1:1 systems will always be more efficient than large 2-pipe multi-splits, and may even cost less depending on specific models and # of zones.

Thank you for the response,
Thank you for the response, and David too of course. First, let it be known that I’m a huge fan of ASHPs, just an even bigger fan of whole truths. I completely get that they’re a great option and perform well in cold climates as well. And I’m not a fan of natural gas (ran into way too many unsafe homes as an energy advisor years ago, and follow Josh Fox’s twitter for too long). Anyway, not to take merit away from split systems, but the whole truth is that if you’re pointing to an ASHP system that has a COP of 2 at 10°F, it’s probably a single zone system (one compressor, one zone/AHU) – and that might be a supplemental installation in some homes north of DC, but it’s not as common as homes with 3-4 air handlers per compressor (multi-zone). It’s tough to find the COP numbers published for each unit with a variety of OATs, but here’s an example (http://meus1.mylinkdrive.com/files/M-Series_Engineering_Manual.pdf) the COP is 2.4 at 17°F. Perhaps I can dig through some old files and find the data to show COPs at more temps. Anyway, if you’re talking about single zone systems, then we’re on the same page, just going for full truth. Or are you talking about ducted systems?

JC, at least the savings are

JC, at least the savings are better than I showed last week with my incorrect calculations. As David Butler said in the comments of my last article, a 2×6 wall with advanced framing can be done for about the same cost as a standard 2×4 wall. What I’d recommend would depend on to whom I was making the recommendation. Someone who’s more interested in reducing their carbon footprint and is willing to pay a little extra for a better enclosure would probably be willing to go for the wall with exterior continuous insulation, maybe even a 2×6 wall with exterior insulation. But as I said in the last article, you’d need to look at what other opportunities might be available. Windows and ceilings and ducts might be present better opportunites for saving than beefing up the walls.

1. Thanks, Bob!

1. Thanks, Bob!

2. Well, there’s a lot packed into those two questions. The calculation is based on the equation for conduction, but the actual physics involves conduction, convections, and radiation. All three modes are in play as heat moves from the warmer to the cooler side of a wall. And no, it’s not just what happens through the cavities. The heat flow through the framing is included above, but not through fenestration. In the first article I mentioned using a framing factor of 23%.

3. What David said in his reply. I was just looking at a paper about heat pumps in Vermont and, yes, the COP drops with the temperature, but even at 0° F, the COP is still at about 2. But as David said, there’s still a whole lot of heating hours that happen at higher temperatures.

@Bob, Allison’s article is
@Bob, Allison’s article is specific to warm climates. However, I wanted to point out that heat pumps still make a lot of sense in cold climates if there’s no access to cheap natural gas. Heat pumps are gaining traction in places like Maine and even Alaska.

Keep in mind that the colder the climate, the more heating hours there are above freezing, where heat pumps shine. But even at 0F, a mid-efficiency heat pump produces around 40% of nameplate capacity at a COP of ~2.0, so it still costs less per BTU of heat delivered than any alternative to natural gas. It’s just a matter of choosing the most economical supplemental heat (electric vs. fossil fuel), which depends on relative local energy costs.