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Mini Split Heat Pumps: Not Significantly More Than You Reasonably Need To Know


							
							

TL;DR? Bouncing off a literally unreadable schizopost? Just read the graph that should be displayed above to see the prices of various heating options (colored lines or dots) compared to heat pumps (somewhere in the blue area). Better units or milder climates move right on the X-axis, shitty units or cold climates move left. The blue area shows the realistic range of costs with electricity between 12 and 24 cents per kilowatt-hour, and poor to very good efficiency. I'm lucky to be at the bottom right where it's cheaper than gas even at last decade's low prices.

Are you in the market for a new heating or cooling system? Or buying or renting new construction that came with a mini-split system? Or have you just heard the endless advertising and propaganda about MINI SPLIT HEAT PUMPS SAVING THE WORLD and wondered if you were being sold a bill of goods?

The New York Times tells us they are The Future. They will Save The Planet while saving you money for being green! The price might seem high, but this listicle of intangible benefits will convince you it’s the right thing to do (or else)!

If you’re as cynical as you should be, one of your hands is now guarding your wallet while the other flips the safety on your Browning. Don’t shoot just yet, I'm only the messenger!

This post is an effort to make some use of the autistic research I did before buying and installing my own, now that I’ve confirmed its performance and the rebate check is safely cashed. It's a bit of a rough draft that I hope can be improved with feedback, but I wanted to get it out as a response to Haroldbkny's question from a few months ago.

We can skip the basics: you’re here, you know how refrigerators and air conditioning move heat using the latent heat of phase transitions between liquid and gas. It’s literally just that but flipped to heat up the inside and cool down the outside instead. Simple as. Let's get right down to the costs.

Coefficient of Performance (COP) and Running Cost

COP is your heat/work ratio. Electric heating elements give you 1 watt of heat per watt of electricity, making it the shittiest way to produce heat short of burning charcoal in your van with the windows taped up. A decent heat pump can move 4 or more watts of heat for every watt of electricity in the right conditions, for an efficiency of 400% or COP=4.

(The commonly stated HSPF efficiency rating is an American measurement that’s basically just estimated seasonal average COP multiplied by the 3.412 BTU (British Thermal Units)/watt ratio.)

Because electricity is almost always more expensive per watt than other fuels, you need this over-100% efficiency to be competitive. See the above chart for average current prices, or play around with cost calculations on this page. (Maybe 10milBTU would be more readable?)

Short version: fuel prices are currently high enough that heat pumps are almost always the cheapest option unless your electricity is very expensive, but this is likely to change in the next decade (right before posting I got an email from my utility announcing a 6% rate increase every year from now on). They are the cheapest option in a mild climate with low electricity costs, but in a harsh climate with high power costs they can be 40% more expensive than gas even at current record gas prices. However, they handily beat oil and propane at any kWh price. Electricity price matters far more than efficiency of the unit or the climate.

Let me know if you’re interested in European prices, which are literally off the scale of this graph (€73/Mcf vs $18/Mcf for gas, with heat pumps’ cost zone well above the cost of US fuel oil).

While this naive cost comparison leaves heat pumps looking very good, especially with CurrentYear fuel prices, there’s a big problem the NYT doesn’t like to mention: both the output and COP of heat pumps drops with air temperature due to physical law.

High end cold climate mini splits are now misleadingly advertised as “maintaining 100% capacity down to -5F,” but this is using rated rather than maximum capacity; it still loses output, it simply started with almost twice what it’s rated for (21kBTU vs 12kBTU, for a Mitsubishi FS12). It also doesn’t mean that the efficiency stays high, and everyone pushing cold climate heat pumps talks about “capacity” in a way that deceptively insinuates that cold temperatures have no effect on the system's efficiency.

Heat pumps are just inverse heat engines that use work to move heat from a cold temperature source (Tcold) to a warm temperature sink (Thot), rather than generating work by moving heat from hot to cold like a steam engine. It takes more work to move heat from a 0F source to a 100F sink than it does from 47F to 100F. Think of it lifting the heat like a bucket of water from a well: the deeper the well, the more work you do pulling the rope. The colder the outdoor air, the more work the heat pump has to do for every ounce of heat.

The theoretical maximum (Carnot) efficiencies for those temperature gradients are COPs of 5.6 and 10.5 respectively, and you can’t do better than that without the thermodynamics police arresting you for building a perpetual motion machine (a heat pump powered by the steam engine it makes heat for). So ignore anyone who says that new heat pumps are "just as good" in cold weather. (Edit: holy fuck fix all three of these paragraphs)

The formula is Carnot COP=Thot/(Thot-Tcold), so efficiency drops as the weather gets colder or the outlet temperature goes up. Thot has to be much higher than room temperature for comfort, airflow limits, or radiator sizing for liquid systems (the radiators on air to water systems get ridiculous to bring the hot side down from the 160F used by normal central heating to the 120F of heat pumps). Still, Thot policing is important for heat pump efficiency: radiant floor heating can go down to 95F if you have money to burn on the install, but on an air unit turning the fan up and keeping coils clean to reduce condenser temperature helps too.

Tcold is the outside air temperature, and this being so variable is the big downside of using an air source rather than a ground source with a stable temperature. The colder it gets the more work is needed for every BTU moved, and at the same time your house needs more BTUs to stay warm.

Here are two top of the line 12kBTU mini splits for comparison: one a regular Panasonic, the other a Mitsubishi cold climate hyperheat. Both have much higher maximum BTU output than their rated capacity: 21k and 18k max vs 12k rated. You can see that both lose maximum capacity at lower temperatures, but the Mitsubishi retains its rated output. Both lose COP as temperatures drop, although the Mitsubishi’s magic Japanese regenerative vapor injection compressor holds on better (but sacrifices efficiency at low output, making it a poor choice for mild climates).

Modern heat pumps already work at almost half of theoretical efficiency, which is pretty good, but they’re not magic.

COP and cost aren’t linear: doubling the COP halves the cost. If resistance heat would cost $100, a COP 2 heat pump will cost $50. Doubling again to COP 4 will give $25, or twice the efficiency gain for half the savings. An impossible COP of 8 would save only another $12.5. Most of the savings are gained early on the curve, which is something to keep in mind weighing the cost/benefit ratio of a more efficient but more expensive unit. This explains the shape of the curve in the pricing graph. The latest Mitsubishi FS Hyperheats maintain a COP above 2 down to -13F, so consider this an absolute floor when comparing running costs.

Note that at a COP below 2.5 heat pumps have little advantage over gas from an energy conservation standpoint: a 40% thermal efficient gas power plant sending power to a 2.6 COP heat pump with 10% transmission loss uses exactly as much gas per BTU as a 95% efficient gas furnace. In cold weather events such simple cycle peaking turbines will be generating the marginal watts for all this new electrical demand. (The math for a 30%ηth german brown coal-burning power plant is left as an exercise for the reader, but needless to say there is a significant toll paid.)

2.6 is around the COP of a Mitsubishi unit at 17F, which is a newsworthy cold snap for me but Tuesday for much of the country. This will probably still work out in your favor due to pricing, but it’s a thumb in the eye for the “ending fossil fuels” justification.

The drop in COP during extreme weather poses a problem for the grid similar to being overwhelmed by summer air conditioning, which I will discuss in the culture war sibling post. Here I’ll just say that it’s a very good reason to have a backup system that doesn’t rely on the grid, because it will cause problems in future. Being slapped with “peak demand” fees on a cold winter night requires an alternative even if the power stays on.

Finally, how often your home is occupied makes or breaks heat pump cost-effectiveness. Their efficiency depends on modulating to the load, and a system that can rapidly ramp up the heat right before you get home would be too large and expensive. If you work from home or are retired, they’re a great choice. If you only come back to your house to eat and sleep for 9hrs a day, they will waste an enormous amount of energy heating your home while you’re away, or working in inefficient turbo mode to raise the temperature when you come home. A furnace will be much more efficient in that use case.

This is almost never mentioned, as most of the people pushing heat pumps are either from the laptop class or rich retirees building custom “zero net energy” mansions. The current move to mandate heat pumps rather than gas for the working poors is likely going to hurt them for no benefit, but what else is new.

Equipment cost, installation cost, and rebates:

So far I’ve only talked about high end $2400+ units, but I bought the cheapest $1100 white label chinesium piece of shit made by Midea that had an inverter and a decent US warranty (22 seer/11.5 hspf 5yr parts). Part of this was a rigorous cost-benefit calculation: the extra $1300 for a Mitsubishi would save only 20 cents a day on average, for a silly 18 year payback time. But the bigger part was that I am a money-grubbing bastard and wanted to make a profit on my power company’s crazy $1500 rebate. That tiny bit of free money gives me a warmer glow than the actual heat output every time I look at it.

If you also live in a very mild climate or are only going to use a mini split’s heating mode for the shoulder season, I’d strongly recommend at least looking at cheaper, non-hyperheat units, which home depot will deliver for free.

If you get one of these after this December, you’ll also be able to get a 30% federal tax credit thanks to the inflation protection act. Right now it’s only $300. Many states and local utilities also have their own rebate programs with various requirements, so check before buying a unit. Some aren’t even worth it, especially if they’re limited to specific models or contractors. The rebate I got was an exceptionally good deal.

If you don’t DIY it and get a contractor to install it for you, expect to pay like $2500-5000 just for them to bash a hole in your wall and connect the indoor and outdoor units. It’s ridiculous and there is no excuse: HVAC in this country is just piracy. If it makes you feel better, at least you’re not in Britain (unless you are, god help your soul), where you’d be paying higher rates for work that looks like it was done by literal monkeys, according to nonsensical gold-plated government standards that were also written by monkeys who have a banana stake in the companies they’re subsidizing.

If you do DIY, you can just pay a few hundred to a single HVAC tech who can check your work, fix any flares you fucked up, do a proper (and vital) nitrogen purge and vac on the line set, and most importantly sign for the warranty and any rebates. I wouldn’t advise doing this part on your own with a Harbor Freight vacuum pump like some youtubers, as a fuckup will leave you both warrantyless and with many hours of work to be done by a professional who will charge you a punitive DIY-cleanup rate. There are "fully DIY" units from Mr Cool, but they are leak-prone (designed for temporary field use in Afghanistan, IIRC), older models, and the extra cost is about what a HVAC tech would charge anyway.

Takeaways:

If you live in a mild climate and have cheap electricity, mini splits are a good buy, pretty much period.

If you have no access to gas, mini splits can be a good alternative no matter your climate. Pay attention to any peak demand surcharges your power company has, and expect them to be introduced soon if they don’t.

If you have a garage or workshop heated with electric resistance that you would like to get AC for, consider getting a mini split for it instead. Heating for less than half the cost will quickly pay for itself. There are even one-piece window units now with no install required.

If you have a long shoulder season before it gets seriously cold, mini splits can handle it incredibly cheaply and delay lighting up the furnace. Worthwhile if you do a cheap install and/or needed to add or replace AC anyway, or to hedge against fluctuating gas prices.

Always have a backup, ideally something like a fireplace, boiler, or something else with locally stored fuel. Keep old oil furnaces if you can, even if it means giving up on an enticing “fuel-switching” rebate (or fudging the application...). Propane works too, especially if you also have an emergency generator running off it.

If you’re thinking about giving up your gas connection, probably don’t unless the base charges are unsustainable. Eventually you won’t be able to get a new one, the electricity/gas price ratio will climb again soon, and I expect gas to be better managed and more reliable than the electrical grid in future.

If you’re paying over 25 cents a kWh and have access to gas at any price, for god’s sake don’t even think about it. You have no idea how many angry north-easterners complain about their bills doubling after paying $30,000 for heat pump installs because they trusted the New York Times and didn’t think their green indulgence would have ongoing costs.

Try not to go with multi-splits that have multiple heads per outdoor unit. They are notoriously shit. Seriously, one head in the living room does every room in my house, so don’t go overboard or let an HVAC contractor go nuts putting one in every closet for $3k each.

Many people repeat a mantra that mini splits “require well-insulated houses.” This is nonsense: a BTU is a BTU no matter what makes the heat. The Japanese were the first users of mini splits, and they think well-insulated means having a second layer of rice paper on the wall. The only real benefit is flattening the heating curve to reduce the range of loads the unit needs to work at. Some insulation is good, but it suffers the same diminishing returns as COP: twice as much for half the benefit with each doubling.

Also, no heat pump made recently has an electric heat strip for backup. They simply aren't used any more.

If you’re the least bit handy and drilling a hole in your wall isn’t terrifying, consider doing a DIY to save money. We can talk about it if anyone’s interested. Rest assured: if a lazy retard like me can do it, anyone can.

If you’re in a cold climate with no access to heating at a reasonable price, consider getting a shovel and digging a ground source heat pump trench; at least the exercise will warm you up, and water-to-water units are mono-blocs that don’t require refrigerant pipe connections. Downside: the guy you need to hire for hookups is worse than an HVAC tech. He is, may Allah forgive me for uttering the word, a plumber. Pump overhead is not factored into COP, and water-to-water units are not as heavily reviewed, so be careful.

Get a surge protector for all this stuff and hook it up yourself (and double check your panel and house grounding: I didn't realize how shit mine was). This shit’s expensive and sensitive to large surges. Take good care of the equipment generally. The indoor unit getting dirty and restricting airflow lets Thots get out of hand, and you saw how badly that affects efficiency in the earlier graph. Keep your filters and the outdoor coils clean.

Anything you can cost-effectively do to make yourself less reliant on the electrical grid is probably a good idea at this point. Shit’s going to get crazy, and you should expect electricity prices to rise and reliability to fall significantly to where the current overwhelming advantage of heat pumps vanishes.

I haven’t discussed solar, but it is a realistic opportunity for people in southern states. Inverters are an option, but there are mini splits that work directly from DC on the market (it just skips part of the AC-DC-3phaseAC conversion, IIUC) It requires either a battery bank or thermal storage, so wouldn’t be cheap, but there are people doing it in the Arizona desert, and 6-9kBTU mini splits are common RV retrofits.

If you live in the UK or Germany, uh… I’m sorry. If I was living there and couldn’t afford a plane ticket out, I’d get some of those charcoal burners and maybe tape for the windows.

If you’d like to hear more, head on over to the companion Culture War thread post (coming soon) where I can rip and tear until- calmly and rationally discuss possible disadvantageous trade-offs being made in the area of energy policy regarding mandatory electrification while neglecting the consequent demand growth.

If there's any interest in additional detail about the technical side, and where the technology could go from here, I'd be happy to do a followup post with all the stuff I had to cut for clarity/sanity. I didn't even get to post a single Temperature-Enthalpy diagram in this one :(

CCing @wlxd and @haroldbkny , who wanted to hear the results and asked the original OP question, respectively. Hopefully this reassures Harold about his leaning towards a gas furnace.

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Great writeup! I would appreciate a lot more posts on this forum of this nature instead of the latest celebrity racism gossip nitpicking.

Also... does anyone know a similarly good writeup for people being kept hostage by the EU green commissars?

In summer, I get annoyed at my retired parents for setting their small house two-room mini-split to 72F. They refuse to believe that’s the temperature of the air coming out of the things, not a thermostat temperature the device will smartly adjust toward.

Set it to 60, please, to mix with the 90F New Mexico air and approach non-sweat temps, please. You’re ALREADY energizing the coil, you’re already spending money, just turn it down twelve degrees!

And turn off the two standalone fans, just let convection move the cold air into all four rooms. You’re using up all the electricity you’re “saving” from 72F air instead of 60F.

Seriously. I lived there for five years, I know how to cheaply heat and cool that house.

It should be the room air temperature, because the indoor unit reads the intake air temperature and tries to get it down to the target temp (or uses the remote's thermometer, if you have a "follow me" setting). That's odd and maybe a sign of something wrong.

Some of these things do have weird behaviors, which I'll post a bit more about at some point. Mine goes to the lowest output once it gets 1 degree from the set point, rather than waiting to exceed the set point before throttling back. So if I want a 67F room it should be set for 68F.

This model gives no way to read current room temperature. The remotes are dumb infrared output-only devices, sending the entire desired settings on each button press. both remotes are identical, and work for both units, independently. I have a hard time believing it is intended to do anything other than have a desired output temperature, since I’ve seen absolutely no indication of any thermostat-style behavior.

What's the model? If you look in the top you should be able to see the temperature probe in the inlet path, under the filters

If you live in the UK or Germany, uh… I’m sorry. If I was living there and couldn’t afford a plane ticket out, I’d get some of those charcoal burners and maybe tape for the windows.

Plane to where? Also, Germany seems unlikely to get to "charcoal burners" stage.

Well, the majority of their "green" energy right now is coming from burning wood chips, so in some ways they've already regressed past charcoal lol

Just did a job with a guy who was a wealth of information about these, having installed 10 of all different makes over 15yrs. He confirmed my suspicions about Mr Cool DIYs: it was the only one he'd had leak, because of insufficient sealing on the factory-filled line set.

His son also switched from propane to a heat pump, and it sounds like his before and after fuel costs were right on my graph: about 1/3-1/4.

I was about to comment on ground source heat pumps, but you beat me to it.

You can do some DUMB shit with those; including just dropping a couple hundred feet of tubing down a well shaft.

I find the main advantage of them is they are decently efficient DIYable single room COOLING AND HEATING, which is sick.

I have a blind spot on vertical ones because they're definitely not an option here; my well wouldn't qualify as "marginally acceptable" as a water source in most places, and you'd have to be impossibly lucky to get a shaft with enough water layer and flow to work as a ground source. In a place like florida they'd be amazing, but in florida you could just warm your pool instead.

Geothermal in general has that "as much art as science" problem. There's research asking "wait, how much heat could pump from the ground below a city without causing problems?", and there's not much in the way of hard numbers yet, especially in the cold climates where a ground source is important.

That's one reason I think co-gen will be a big deal. Having a huge volume of power plant condensate to work with takes out the guesswork, and there's no technological advances needed over current high-temp cogen systems.

In yet another victory for thermodynamics; it turns out that a large thermal mass is just fucking better for the shit we want to do if you can max out capacity.

It's like that old tmblr post; that no matter what you are doing the most efficient form will always be whatever the train equivalent for the field is.

that no matter what you are doing the most efficient form will always be whatever the train equivalent for the field is

is it entire interesting part? Or have they got interesting examples?

A conceptual train.

Imagine you are, for example, lifting mass into orbit, or moving people between floors in a building, or cooking food, or digging a big hole.

What is the most efficient way to do all those things? Orbital elevator theoretically/BIG ROCKET currently, Escalator, BIG pressure cooker, BIG bucket wheel excavator.

The more the spirt of the thing is akin to a train, the more efficient the thing is, and also the more RIGHTOUS the thing is because trains are fucking sick

I'm curious what your house looks like that one head in the living room heats the whole place reasonably evenly. One of the best advantages of mini split systems is generally that it's easy to achieve balance by placing multiple heads or units in a building, so you don't overheat one area just to get heat to another, or you can leave marginal areas of the house less heated than occupied ones. Though that might just be a reflection of my town being relatively rural, with large weird houses being the norm.

Maybe "house" oversells my trad hovelcube a little. It's much smaller than the American average, but then so are many castles. There are a lot of "reasonably sized houses" in the pacific northwest for some reason, and it's of typical construction for there.

If you have a ton of doors you can't leave open, adding a second unit or a multi-split might be the best option, but the airflow is high enough to push heat up the stairs and keep my 2nd floor within a degree of the 1st.

Hey, when we're talking heating, my buddy just bought a 3,000sqft home with a weird multi split level first floor. He's spending $600/month on oil heat already and it's only November. My ranch with a heat pump and a woodstove has me feeling bad even talking to him. Small house small costs. I don't even want to think about what my parents pay in heat.

Better a small home you can keep nice than a big house than goes to shit. I see that too often around me.

Great post, I didn’t learn much new stuff, but it was a very enjoyable read. You have a very good point about high potential for grid instability on very cold days. I have a natural gas cooking range that can do something like 30-40k BTU combined, should be good enough for emergency situations, assuming regular air changes to avoid annoying high CO2 build up.

I hadn't even thought about using a cooking range for heating, that's clever. Wouldn't even be too inefficient if you had heat recovery ventilation.

Geothermal looks so cool, I'm pretty sure I'll be in my house for at least the next 10 years so it's tempting. My back yard is full of roots though and I'm worried I couldn't dig the trench without killing my shade trees, which I love. I'd probably rent an excavator though, I've done enough digging to know I never want to do it by hand again.

Depending on where you are and how high your water table is; you may be able to get a vertical shaft drilled for fairly cheap (except in the places where they take 2/3 of your liver, your kidneys, and your first born on the way out.)

Our condominium just installed geothermal recently, just finished in the autumn. Came at a very good time, one might say.

Let me know how it goes! Any chance you could snap a few pictures? I have zero local sources on ground source installs.

It does! I'm in a similar situation with shade trees, but seriously thought about lopping them all down and doing it anyways. It was tempting to put a big "geothermal!" mark at COP5+ on the graph, but it was already too crowded.

Had to cut almost everything about ground-source stuff for length, but I think it's the future in cold climates, and Europe is making a serious mistake trying to push air-source units rather than street-level ground source. There's a tremendous amount of wasted heat flowing down every sewer pipe without even having to do any digging, and ultra-low-temperature local cogeneration would make even a simple gas turbine about 260% efficient.

There's a tremendous amount of wasted heat flowing down every sewer pipe without even having to do any digging

I've thought about this before, and what I came up with was water-water heat exchangers in shower drains, between the outflow and the cold inflow. You'd want a thermostat-controlled valve to keep the shower temperature from drifting, but that'd be an improvement to UX even without the HX. Dishwashers could do the same thing to recycle heat from prewash->wash->rinse. (IDK if warm rinse would be more effective, but it'd use less energy to dry.)

Putting the recovery device as close as possible to the producer of warm graywater gets you the highest-grade heat, and also means your HX doesn't have to tolerate actual poo. In the individual house/apartment, drain heat is intermittent and unreliable, but conveniently correlated with the need for hot water. Building-level heat capture could smooth out the availability with a big buffer tank, but hot sewer water is diluted with cold.

In large apartment buildings, the owner could install a cold water pre-heater (to say, 20°C or so), using the most economical type of heat for the climate, which would reduce cooking energy (and time) in winter. People might naturally use less hot water for washing hands too. A couple of years ago, I measured the flow rate and hot-cold temperature delta of my kitchen faucet in winter, and now whenever I run the hot water waiting for it to warm up, running through the back of my mind is, "YIKES 13 kW!"

I'd like to read your write up of the geothermal material that you had to leave out. Any chance of a supplementary post?

Next post is focused on all the culture war you could see oozing from the seams in this post lol.

After that I was thinking about a electrical generation post. Unfortunately I don't have any firsthand experience with ground-source heat pumps, and there's so much propaganda around all things heat-pump that I wouldn't want to post my research as trufacts without independent confirmation. Maybe Stefferi could help with that.

The formula is Carnot COP=Thot/Thot-Tcold

Your meaning is clear, but some parentheses would not go amiss: COP=Thot/(Thot-Tcold).

Also, holy shit this is a fun post, at least for those of us who have a proper appreciation for autistic infodumping. Thank you for writing it!

I guess this is a good time for me to complain that division should be the last order of operation, because (1) ime that's usually what you want to minimize parentheses and (2) that's the way it works when you're writing equations in latex or by hand and drawing horizontal lines with a numerator and denominator.

I didn't find the meaning very clear, I don't get how sluts divided by sluts minus cold sluts makes any sense. But I also concede I don't have much experience using thots as a unit of measurement outside of STD questionnaires.

Still a fun a post for the parts I did understand, lol at Europe for having expensive energy costs, they did a good job of screwing themselves over on that.

I hope people find it useful! And thanks: that's the greatest/only compliment(?) my writing has ever gotten.

Haha, yeah, just adding clarity... I definitely didn't forget my default order of operations rules from kindergarten or anything... <_< >_>

You can also write the variables as Thot (from T<sub>hot<\sub>), but then I wouldn't get to laugh at "Thot policing".

Testing if we're using the new math markdown module

$$ COP = {Thot \over Thot-Tcold} $$

Apparently not, damn.

@ZorbaTHut is it possible to get a markdown math module on here? It'd be very convenient for doing formulas by text instead of having to link images. I don't know anything about modern website stuff, but it looks like some people use stuff like this https://www.mathjax.org/ tutorial thingy: https://jupyterbook.org/en/stable/content/math.html

I think this mostly comes down to developer time. We need to do a revamp of the markdown setup anyway, but once that's done, this is certainly possible . . . if someone wants to put the time into it. I don't think it's likely to be a high priority for a while.

So basically, talk a friendly coder into doing the work and I'll happily put it in, but I'm not going to promote it for quite a while :)

Yeah, I felt dumb for asking for it when we don't have single-line-spacing yet, my bad