
Heat Pumps Work Miracles - mhb
https://dothemath.ucsd.edu/2012/06/heat-pumps-work-miracles/
======
danimal88
I build controls for many heatpumps ([https://flair.co/pages/mini-splits-and-
window-units](https://flair.co/pages/mini-splits-and-window-units)) and follow
the space pretty closely. They are truly magical with respect to pumping heat
from the cold outdoors into a warm indoor space.

That said, the efficiency gains on the energy consumption side are admirable.
From a global warming standpoint however, heat pumps are actually still pretty
bad because of the
refrigerants.([https://www.drawdown.org/solutions/materials/refrigerant-
man...](https://www.drawdown.org/solutions/materials/refrigerant-management))
This is not an issue with more traditional electric heaters (resistive or
otherwise).

The real calculus for those concerned with global warming (I understand this
was not the topic of the writeup) is the net effect of these systems over
their lifetime - aka, Energy Saved and its GHG effect via leveraging a heat
pump vs the GHG effect from the release of the refrigerant. Perhaps the real
win would be a refrigerant recovery business that is...air tight :)

~~~
fallingfrog
There is a such thing as a solid state heat pump which operates without
refrigerants or any moving parts. They are not as efficient as regular heat
pumps, though, at least currently. But maybe that could change.

[https://www.researchgate.net/publication/270646164_Efficienc...](https://www.researchgate.net/publication/270646164_Efficiency_of_a_thermoelectric_heat_pump)

[https://www.energy.gov/eere/buildings/downloads/high-
efficie...](https://www.energy.gov/eere/buildings/downloads/high-efficiency-
solid-state-heat-pump-module)

[https://arpa-e.energy.gov/?q=slick-sheet-project/high-
effici...](https://arpa-e.energy.gov/?q=slick-sheet-project/high-efficiency-
solid-state-cooling-technologies)

~~~
vanderZwan
You reminded me of Lonnie Johnson's _" Johnson Thermo-Electrochemical
Convertor"_, or JTEC for short. On their webpage they say they are aiming for
85% Carnot efficiency for converting heat to electricity, and it can run in
reverse to become a heat pump[0][1].

Sadly, I can find no third-party measurements of how well it performs in
practice, so I guess it's still in the R&D phase. I did find a nice talk by
Johnson about the project though[2].

[0] [http://www.johnsonems.com/about/](http://www.johnsonems.com/about/)

[1] [http://www.johnsonems.com/benefits/](http://www.johnsonems.com/benefits/)

[2]
[https://www.youtube.com/watch?v=F6FUk2E9-68](https://www.youtube.com/watch?v=F6FUk2E9-68)

------
fastbeef
Modern heat pumps are truly a marvel, especially when you get one designed for
heating and not just an AC-unit running in reverse. They're very common in
Sweden and can easily warm an entire house even at -20 C.

Whenever this topic appears on HN I want to get on my soapbox and yell
"INSULATION PEOPLE, INSULATION!". Most houses I've seen outside of the Nordics
have shit-all insulation and that's depsite it working both ways - house stays
cooler in the summer. It's such a small cost to add during construction and it
pays off in a year or two, tops.

Modern, newly built houses in Sweden, way above the arctic circle, require
less than 5000 kWh per year to heat and generate hotwater.

~~~
floatrock
> It's such a small cost to add during construction and it pays off in a year
> or two, tops.

Classic market failure: operational costs don't factor into home buying
decisions prominently, so whatever small added construction cost is borne by
the homebuilders (the cost of added insulation doesn't quite result in an
equal increase of selling price).

Short of instituting a carbon tax that would make operational costs of heating
suddenly more prominent and priced accordingly, this is a great case for
efficiency standards in homebuilding. Suppose your home came with an "energy
star" HVAC rating...

~~~
leguminous
You can actually get your home Energy Star certified.

[https://www.energystar.gov/newhomes](https://www.energystar.gov/newhomes)

Some states have even offered tax credits for doing so.

~~~
floatrock
Thanks for the link, didn't know about that!

------
torpfactory
It's worth noting that the temperatures at which heat pumps function well is
more of a design choice than a fundamental limitation. You can make a heat
pump which functions down to -20C with HFC refrigerants, maybe lower with some
other types. However, you may choose to optimize 'mild weather' performance by
forgoing low temperature performance. Modern heat pumps work pretty darn well
across a range of temperatures.

What we really need are heat pumps which do not use HFC refrigerants - they
are a pretty significant cause of global warming, and there are other options
(CO2 being one of the best, in my opinion).
[https://www.drawdown.org/solutions/materials/refrigerant-
man...](https://www.drawdown.org/solutions/materials/refrigerant-management)

------
thinkcontext
Heat pump water heaters have efficiency of 3x+ vs .9 or so for gas or electric
resistance. If you have have an electric resistance water heater get a heat
pump model. It costs more upfront but will pay for itself in not too many
years, especially considering many places have incentives. In the US natural
gas is so cheap the economics are not nearly as good, it takes a long time to
pay back the difference but does eventually. With even a modest carbon price
obviously this would be different.

If you are going to replace your furnace or AC get a quote for replacing with
a heat pump instead, either central or multiple mini-split heads. Heat pumps
have gotten a lot better in recent years, particularly being able to work in
cold weather down to 0F or below, expanding northward the places they are
practical.

~~~
heatpumper
I can tell you that my heat pump set up at my condo is absolute trash here in
Northern Ohio. Running 24/7 it is unable to get my condo above 62 deg F. It
blows cold-ish air under an outside temperature of 40 deg F. Windows are new,
doors are weather stripped. I'm having it run it in supplemental mode just to
get it above 65 deg F.

~~~
ethagknight
My HVAC is utter crap ≠ HVACs are crap. This is a great example of a worthless
anecdotal comment that distracts from the point. Give us some more detail like
model number or install date? Is it a modern unit not coming close to its
advertise performance? Is it appropriately sized for your condo?

------
donkeyd
Big issue in the Netherlands (and probably more countries) will be noise
pollution though. Houses here are built really close to each other and the
combined noise could be an issue. Neighbours are already complaining about the
noise from my parents heat pump.

~~~
toomuchtodo
Do you have any centralized district heating in NL similar to in Sweden?

~~~
donkeyd
In some newly developed areas we do, but most of the heating in the
Netherlands runs on natural gas, since we have so much of it.

~~~
Yetanfou
...or rather _since we had so much of it_. The Dutch government is, after all,
making noises about the country going 'off the gas' to become 'gas-free'
around 2050. As to whether this really will happen and what it actually will
entail - are they going to tell industry to stop using gas as well? - remains
to be seen but heat pumps are being pushed as one of the alternatives to gas.
Given that nearly all central heating in the Netherlands is water-borne and
most systems are designed for a high working temperature - between 70 and 90
degrees - the switch to a heat pump will need more than just a simple swap of
the prime heater. Low-temperature (possibly fan-assisted) convectors can be
used to replace high-temperature radiators, these can also be used to cool
down the house in summer. More insulation is needed in most Dutch houses, even
newly built houses sometimes don't have more than 10 cm of foam insulation in
the roof and less in the walls.

------
cdubzzz
The most recent season of This Old House, Jamestown Net-Zero House[0], is
worth a watch for folks interested in learning more about home efficiency
techniques in general.

[0] [https://www.thisoldhouse.com/watch/jamestown-net-zero-
house](https://www.thisoldhouse.com/watch/jamestown-net-zero-house)

~~~
SketchySeaBeast
I'll add that to my must watch list - I live in an old (1930's) home that
leaks heat like crazy, so I'm interested in possible retrofits, though I'm a
bit concerned that it'll be a monstrous task.

~~~
antisthenes
Pretty much anything in a pre-1980 house will a nightmare to retrofit. Houses
in the 1945 to 1965 era (in the US) especially were mostly built as quickly
and as cheaply as possible.

The concept of energy efficiency didn't really exist before the 1973 oil
crisis as oil use per capita was steadily going up and up until then.

Energy Star, for example, was only established in 1992.

Energy efficiency is very very recent.

~~~
SketchySeaBeast
Oh yeah, my home was built strictly to keep the frozen death out, I have to
throw a big furnace at it to be livable while it exhausts everything through
its impossible to insulate roof.

------
lxmorj
I had a "geothermal" heat pump put in to my house via Google's Dandelion and
it works great so far. It's been -30F at my house but the ground sourcing
renders that moot. As far as I can tell their innovation was going vertical so
they didn't need to dredge up a huge loop of land. The pump has two channels
that go down 300ft each and the fluid runs down and back up each bore hole.

It provides heat (working great so far), hot water (but I already had an
electric inline heater, so I can't speak to the slow-refill issues), and A/C
in the summer. I'm curious to see how well the AC performs!

~~~
jonny_eh
I don't see Google mentioned anywhere on their site. What's the relation?

[https://dandelionenergy.com/](https://dandelionenergy.com/)

~~~
lxmorj
Alphabet spinoff, looks like:
[https://dandelionenergy.com/about/](https://dandelionenergy.com/about/)

------
jstapels
Maybe I'm missing something, but no matter how efficient a heat pump is,
doesn't it have a limit on how much heat it can pump as ∆T increases?

Even here in western NY we occasionally have to deal with -10 F (-23 C)
weather for a week and so that's a pretty big temperature delta if you want a
comfortable 68 F (20 C).

Combine that with the fact that gas is still quite a bit cheaper per unit of
energy than electricity (in my area) and a heat pump just doesn't make any
sense.

The ability to have a single unit heat and cool is definitely nice, but
without a geothermal-like ambient temperature to work with, I just don't see
this working out, even with all the insulation in the world.

~~~
thinkcontext
Elsewhere in this thread fastbeef says "They're very common in Sweden and can
easily warm an entire house even at -20 C [-4F]." So, it can work though at
-10F you would probably need supplemental heat.

Of course in Sweden residential natural gas prices are almost 4 times what
they are in the US so its a very different economic decision. Shows what a
carbon tax can do.

[https://www.statista.com/statistics/702735/household-
natural...](https://www.statista.com/statistics/702735/household-natural-gas-
prices-in-selected-countries/)

~~~
fastbeef
Keep in mind that 1 kW worth of gas gives out 0.9 kW, while 1 kW of
electricity entered into a heat pump gives out AT LEAST 3 kW of heat.

Also, no one uses gas for heating in Sweden. Very old houses down town use it
for cooking only.

------
baybal2
Question to other engineers here. We were designing a "smart wine chiller"
recently. Ran into a problem - small size sealed compressors are very, very,
hard to come by, and if you want one that is also quiet, prepare your arm and
a leg.

We ended up having the compressor custom made by local fridge factory.
Engineers there told us that designing such a small piston compressor and
hoping it to be reliable and efficient is a futile effort, but they still did
their best.

Is between piston driven compressors, and peltier coolers, is there just
anything on the market? More quiet and robust than small piston compressors,
and more efficient than peltier?

~~~
FiatLuxDave
Have you looked into compressors for mobile applications, such as a Danfoss
35F? I mean, technically its a reciprocating piston compressor, but the
smaller ones can be quite quiet.

[https://assets.danfoss.com/documents/DOC223886433214/DOC2238...](https://assets.danfoss.com/documents/DOC223886433214/DOC223886433214.pdf)

~~~
baybal2
Not small enough. The one we custom ordered is like 6x6cm (piston itself is
only 9mm,) and they even managed to make it run on DC.

The factory and their tech level truly surprised me despite them being
complete nonames, who mostly do fridge part kits for local production in 3rd
world countries.

This reminded me yet another time of just how underappreciated are those no-
name factories. They truly are the real unsung heroes of our globalisation
era.

~~~
baybal2
Adding to that, the most expensive single part in all at happened to be the
specialty BASF o-ring for the piston at $6 dollars...

Body, piping, insulation, every other part came at less than that

------
DoofusOfDeath
Slightly OT, but..

I live in a temperate climate. I've sometimes wondered if it would be
worthwhile to adjust house construction so that:

\- In the summer, the kitchen refrigerator's heating coils touch outdoors air
instead of indoors air.

\- In the winter, the refrigerator's cooling is provided, at least partially,
by the outside air.

\- In the summer (only), the kitchen oven vents into the outdoors rather than
into the kitchen.

\- In the winter (only), a heat exchanger recovers some of the heat from our
electric clothes dryer.

~~~
MrStonedOne
One idea i've had is to just make a central heatpump system.

Why does a fridge need its own compressor that vents heat back to the house
even in the summer?

Why can't your water heater use that same heat instead?

This all started because my brother used to redirect dryer air back into his
apartment during the winter, using a saw dust sock to keep the lint contained.

I started thinking about how heat could be better managed if everything used a
central ac/heat pump. heat generated by the fridge, your computer, the ac, etc
could be used by your water heater, your heater heater, your dryer.

Add heat recovery systems to your dryer, and your drains, and bam super
sustainability.

~~~
throw0101a
There are AC units that do this: pipe the heat to the hot water tank first.

* [https://www.rheem.com/innovations/innovation_residential/int...](https://www.rheem.com/innovations/innovation_residential/integrated_systems/)

* [https://techcrunch.com/2011/06/12/air-conditioning-water-hea...](https://techcrunch.com/2011/06/12/air-conditioning-water-heater/)

Once the water tank reaches its max temperature, then heat is dumped outside.

The practical problem is that you need to have the different parts work
together, and fridge makers don't don't to hot water makers.

~~~
MrStonedOne
>The practical problem is that you need to have the different parts work
together, and fridge makers don't don't to hot water makers.

You don't need a fridge or a hot water heater or an ac or a furnace at that
point, because the primary function has been offloaded to the central heat
pump. you need heat exchanger attached to water, heat exchangers attached to a
cold room, heat exchangers attached to a central air system.

You don't have to have the parts work together if the most expensive part of
each appliance has been centralized. At that point you've now reduced the
appliance to where to put the heat exchanger.

------
elmolino89
The most commonly used heat pumps utilize the external air as a source. Which
is quite silly on a sunny but chilly day. Even 2m^2 black & isolated panel
exposed to the sun will be tens of degrees warmer. Just putting the heat
exchanger in some micro glass house with a valve would increase the
efficiency.

------
gdubs
We have a heat pump and I love it, but for the longest time it was beyond me
how the thing works. When it finally clicked, it blew my mind. The idea that
there’s heat to be found, even on a cold day, is amazing. It’s actually what
made me want to understand thermodynamics more deeply.

------
Pulcinella
Also heat pumps work a miracle because the fluids they use are NOT perfect,
platonic fluids that obey the ideal gas law.

[https://en.m.wikipedia.org/wiki/Joule–Thomson_effect](https://en.m.wikipedia.org/wiki/Joule–Thomson_effect)

------
kev009
This was a really lucid and engaging piece on the mathematics of these
systems. I wish more math teachings would be done in long form like this
instead of the cargo culted textbooks that are put out by major publishers.

------
agumonkey
Talking about fridge, do you guys know if there's a use for heat collector out
of the fridge radiator ?

Right now it's mostly wasted heat, or IIUC even worse, in closed spaces it
forces more energy consumption from the fridge.

~~~
Scoundreller
It’s not a lot of heat in residential use cases.

Fridges for enclosed spaces should have a fan to push the air around.

The more space you give a radiating fridge, the more efficient it’ll be.

I do wonder if a small fan on a radiating fridge would pay for itself if
controlled by temperature probes.

------
wazoox
BTW the whole blog is a goldmine. Rim-filled of evidence of how our energy-
hungry civilisation can't last :)

------
tomohawk
Heat pumps based on air do not work economically below 40F. Neighbor has one
and his electric bill is out of control in the winter. Drop below freezing and
the system will pump heat out of the house to de-ice the external unit, and
electric resistance heat will kick in.

We have dual fuel, so we burn oil below 40F. Works great.

If you can afford it, putting in a heat pump that works off of ground will
eventually pay for itself.

~~~
willvarfar
Outside temperature definitely affects an air heat-pump's performance, but the
ones common in sweden stop being effective at -10c or even -15c.

I have a ground-source pump and its brilliant. But plenty of my neighbours are
happy enough with air-source pumps.

~~~
bklyn11201
Recent conversations with net-zero builders indicate that ground-source pumps
are being pushed away for most single-family residential projects in favor of
the new-generation, highly-effective, cold-climate air-source heat pumps.

Mitsubishi and Fujitsu seem to have a large lead in cold-climate air-source
heat pumps that are rated effective to -13F (anecdotal reports
greenbuildingadvisor.com indicating effectiveness down to -20F). Yet, talking
to builders, contractors, friends, and family, you will often still hear old
tales about how "heat pumps aren't effective below 40F". Heat pumps come in a
wide range of flavors. It's clearly possible to heat your house exclusively
using an air-source heat pump in most USA climates.

~~~
fastbeef
There's a huge difference between air-source heat pumps and rebranded AC units
running in reverse. We have a Mitsubishi FH35 6kw heat pump that easily heats
80 m2 (800 sq feet) down to -13 C this winter.

~~~
toomuchtodo
Do you recommend the model you quoted in your comment?

~~~
fastbeef
I do, it's fantastic! Just make sure to get a good installer for it.

~~~
jjtheblunt
As a fellow northern climate person, now in Arizona, have you any idea its
efficacy in the 45 celsius normal summer heat, for cooling?

