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Comment by sparker72678 | Hacker News Reader

Heat Pumps are great, and the latest generation of variable speed units are really cool and our most efficient ones yet.

Still, for heating, especially in really cold climates, a Heat Pump is going to have a hard time competing against other approaches, in efficiency, efficacy, and cost.

The latest gen of "ductless" or "mini split" units are awesome *AC* units (efficient, quiet), and I'd definitely recommend checking them out for cooling. If you're somewhere that it doesn't get too cold in the winter (lows in the mid-20s at the lowest), you might be able to tolerate using one for heating, but any colder and you're going to need another solution.

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The laws of heat engines are governed by the relative temperature on the hot and cold side. It's easy to push a little water uphill, but beyond a little it becomes a progressively more farcical exercise.

Cold and humid seem to be the worst combination from reviews I've seen for minisplits, because the second biggest problem is heat transfer, and transferring heat through ice is very inefficient. Every time the ice begins to insulate the coils, you have to run the system backward to heat the pipes above freezing and melt off the buildup. Not only does that lower the efficiency of the system but eventually it limits the duty cycle, because you can't be thawing and pumping at the same time. And so you are attacked from all sides by both physics and logistics.

From what I've heard, heat pumps with ground loop heat exchangers maintain a 450-500% efficiency rating all year round, regardless of how cold or hot it is outside. Here's a variable speed one that claims COP of ~5: https://www.waterfurnace.com/residential/products/geothermal... (I think that's only when it's running at 30%, though, drops to 4.5 when at high speed).

Installing a ground loop is a _significant_ expense, oftentimes well beyond the reservation price for residential buyers.

In my neighbourhood, ground source is triple the cost of air source. Got quotes for both about 3 yrs ago. We don't have the option for horizontal so ground source would have been 3, 50 meter deep bore holes arrranged in an equilateral triangle 7 meters between points.

Yeah, the ground loop is a really big chunk of the overall expense, I think it was $20k of the cost by itself for our most recent quote, but it's expected to last 50-100 years, so it's amortized over a loooong time period of significant energy savings and increased performance over an air-source heat pump. Home buyers probably won't properly account for that when you're reselling, since it's not visible in the same way a remodeled kitchen is, but hopefully they'll start to as it becomes more common and/or energy prices increase.

Also, there are a lot of tax credits - 26% of the entire cost this year as a federal tax credit if you install this year (decreasing to 22% next year), and a number of state and local incentives bring the cost down significantly. And there are obviously financing options available.

If you combine it with a large solar array, I think the biggest thing it brings you is reducing uncertainty about future bills - you're locking in your expenses, a little like what buying a house does vs renting.

Conveniently, the USDA publishes a map specifically suited to this question:

https://en.wikipedia.org/wiki/Hardiness_zone#/media/File:201...

Anywhere in yellow (8a) or warmer can use a heat pump, no problem -- the best heat pumps go down to about 0 F (-20 C) intake temperature. That's about half the country. With good construction it should work down to 7a (light green), and this transition zone includes DC/Philly/NYC. The darker green and blue regions need more advancements in the tech to rely on them.

But it's mostly the US that has this problem. By contrast, practically everyone in Latin America / Africa / Southern Asia should have no trouble switching to heat pumps (if they need heating at all, i.e. Zone <10):

https://upload.wikimedia.org/wikipedia/commons/a/a7/World_Ha...

Since many of these countries are industrializing during the efforts to mitigate global warming, hopefully they will get heating right sooner rather than later.

> the best heat pumps go down to about 0 F

Mitsubishi makes a line that still has reasonable COP numbers down to -13F or so. It's a clever trick: the actual problem at these very low temperatures is that the refridgerant in the lines starts to get slushy as it approaches its own freezing point, but it still functions. So they actually steal a bit of the heat that would otherwise enter the heated space, and use it to slightly warm the refridgerant so that things keep ticking along.

You can do two things, if so inclined: I’m in the midst of having a heat pump system installed in line with my existing gas furnace, the so-called “dual fuel” approach. The heat pump should suffice for most cool days, with the furnace available for when it’s truly cold. (Colorado front range; we don’t get the same winters that we used to.)

Air heat pumps yes, but ground heat pumps can work in very cold climates.

Surely mid 20-s, 25°C ,is really hot? Where is that a winter low.

Fahrenheit not Celsius.

Fahrenheit.

In places that use a silly system called 'Fahrenheit'.

New Orleans here. Mini-splits are fantastic.

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