My heat pump: a personal story about a broken heating industry | Hacker News Reader

My heat pump: a personal story about a broken heating industry

greenallianceblog.org.uk

32 pointsMatumio95 comments

The author doesn't mention brands but I found that the majority of the cost difference was due to it. I got two Daikin installed and it costed me 15k$ CAD. Simiilar models in Chinese brands run at half that price. Mine also run at 100% efficiency at -15C where most just run 100% at -5C. If you want -25C ones they become much more expansive though.

I got quotes from 4 vendors. None were technical enough for my liking they all just told bullshit marketing stuff. One tech did a better job and convinced me, but I still think like the author that they don't really understand the science behind all of it.

But this is nof the only industry, I DIY renovated my whole house and I can tell you everything is over the top these days. Another quick example is electricians still wire light circuits like in the incandescent days (max 8-12 bulbs per circuit) where bow LED consumme a fraction of it.

If you’re talking about residential wiring, most electricians will (in my opinion) correctly wire rooms to breakers rather than device types. It is infuriating to discover that every light on a story is on the same circuit when you want to replace a fixture at night.

For dedicated lighting circuits in the USA, the good news is that the 2023 NEC has provisions for 10 amp / 16 gauge circuits in recognition of efficient LED lighting.

I think the reason lights are on a separate circuit is that you don't want the lights to go out when a circuit breaker trips. That may be in the code, but I'm not sure.

Boy, how I wish my breakers were that nicely organized. They’re, at best, regions of my house. Every time I need to do some electrical work it’s a complete guessing game.

It's worth taking the time to flip the breakers one at a time, then go around and test outlets so you can write the information down.

Good activity for a slow weekend afternoon when you're not under time pressure for another project, and while it's daylight and not unbearably hot or cold. :-)

The last example may be code, not sure the code allows you to put 50 lights on the same circuit. At least not cans that could hold a normal light bulb. Perhaps wafer lights can get around that as they are fixed load.

If you're planning ahead for LED light, IMO it's generally better to get a dedicated power supply and wire up DC LEDs, as they're much longer lasting than retrofit bulbs. You save on wiring, too, without any safety risk of someone filling the circuit with incandescents.

I've had a heat pump in my home in the US for the past five years, and it's worked pretty well, but one of the things I've found frustrating is that either the pumps or the installers assume you need to run every indoor unit simultaneously.

I have a 24,000 BTU outdoor pump and one indoor unit for the first floor and another unit for the master bedroom. After COVID, we started using our second bedroom as an office, so we wanted an additional indoor unit.

I knew that a third indoor unit would be beyond the capacity of our outdoor pump, but we would never need to run all three indoor units at once, so I hoped it would work.

The installer said that they can't connect a third unit to our outdoor pump. They claimed that the system maxes out at two even if only two need to be on at once. I couldn't tell if it was bad design by the outdoor unit or if the installer just didn't want to sell me a single unit instead of replacing my whole system, but I was disappointed.

The indoor unit gets directly connected to the outdoor unit for refrigerant, power, and communication. It's likely then that your outdoor unit only had 2 hookups.

24,000BTU = 7kW, to save others from the conversion.

There's no thermodynamic reason that it couldn't work the way you want it to, but the physical connections and electronics controls need to be designed to accommodate that, either with direct connections on the outdoor unit or via a 'branch box' that serves as a connection point for both the refrigerant and electronic control circuits.

It's most likely that the installer was correct in that the outdoor unit you had maxed at two and did not have a smooth expansion path.

I think we could bring the price of these down tenfold if they were sold like dishwashers and other appliances.

Make a building code that has a universal plug and play interface so they can be connected like a washing machine.

The costs explode when you have to buy through installers.

(Yes, there are lots of reasons they are not plug and play but let’s figure it out!)

They pretty much are plug and play, you just need a 220V recpricial. They even come loaded w/ refigriant, so it is just cut copper, connect, done.

Every time I’ve bought one there’s been three employees at my house for 8-10 hours.

I. guaranteee you they were useless. It helps to have another person to mount it if not on the ground, but you can do it yourself. I've installed over 10+ myself.

At the same time, since people saying it's difficult and such - I should start selling this as a service :P

TFA said some homes need new piping, and Ia believe some need new radiators too?

Like many things: "it depends". If your current system can heat the house on the design day with 55°C/130°F water, you don't need to change piping or rads. (My house was in this situation as the original system was designed for gravity flow and so was massively oversized for a modern pumped system.)

If your house has minimum sizing of baseboard radiators and the design day flow temperature is 82°C/180°F, then you need to update at least the emitters and maybe the piping.

In my country (Ireland) you more or less have to use professional installers for anything non-trivial if you want to get any sort of state grant/refund to cover the outrageous costs (with VAT piled on top). I did a retrofit, I can't imagine how complex it might be for a self-build.

I admit dealing with old housing stock is a complicated problem. it seems they want you to partake in the bureaucracy to keep the cowboys out, which is not unreasonable given the problem at hand.

There are pressurized lines running coolant gas between outdoor unit and indoor heads, they cant be sold/installed like washing machine.

https://www.ac-heatingconnect.com/contractors/checking-charg...

Why don't they run an ethylene/propylene glycol solution loop (with refrigerant to-water heat exchanger integral to the compressor unit) like industrial chillers? There's a tiny loss in efficiency due to the extra heat exchanger and higher pumping losses, but you get less leakage, less refrigerant, your lines are unpressurized, etc. A lot more of the installation process could probably also be done with low-skill labor.

I imagine it would also be easier to retrofit such a system into houses with existing in-floor radiant heating and/or baseboard radiators.

> but because my installer looked at the data, it needed no major new pipework

How about if we start with this? What data, and how do you collect it? The site links to [1] which looks like it has some great info. But any potential adoptee will still need to translate that into some actionable steps to gather the data needed to know how to size the heat pump system

I too was a bit disappointed that the data in question wasn't explained. The only thing the article has made me do, is become even more wary about heatpumps!

I don't get the impression that I should be more wary about heat pumps, but that if I have the data collected in advance I would be ready to switch when the time comes. Armed with that data, you could also do some advanced prep work so that you can purchase a smaller system that will still maintain high levels of comfort. And any prep work would likely improve the performance of the existing system you have.

Right... but what data? What am I looking for?

Adjust the boiler so that the output flow temperature is 55°C (or 130°F) or lower. See if your house remains comfortable on the coldest day you expect to have.

Note that if you have a boiler not designed to operate with condensing flue gases, this test will eventually damage the boiler [over the course of months, not hours]. If you have a plastic exhaust pipe on your current boiler, you're totally fine to do this.

More writeup here: https://news.ycombinator.com/item?id=39144329

I do appreciate the writeup, but I feel as though there has to be a better way :)

In my area, the most common HVAC system is going to be a gas-fired forced-air furnace along with a central air conditioning system. What do you do there?

Air to air heat pumps are a pretty straightforward retrofit there. (Including central units with reversing capability; heat pumps don’t have to be per-room minis.)

In some ways, that style of central heat pump is less talked about because it’s usually a straightforward replacement.

Ducts may need to be enlarged in heating-dominated climates though.

1. Heating Degree Days 2. Net heat delivered (BTU delivered * efficiency) 3. Calculate BTU used per heating degree day 4. Calculate your heat load (BTU/hour) 5. Multiply by the ASHRAE sizing factor (1.4).

That's enough to allow you to size/resize a gas/oil furnace, as well as see what sort of costs you'll run.

To compare directly to a heat pump is difficult, as you also need to pull in your electricity cost, gas/oil cost, and if you have demand pricing, what sort of temp you keep your house at.

I will say though, going from something like a 9 SEER AC/heatpump to 17 is huge. 17 -> 20? Not so much.

Your last point is excellent, especially if you have a modulating/condensing boiler. (Do you have plastic exhaust piping and a small plastic line that drips water from time to time when the boiler is in operation? You do.)

The lower you can make the return water temperature, the more efficient your condensing boiler will run. The way to do that is to lower the leaving water temperature (and accept/relish the corresponding longer run times).

With my outdoor reset properly tuned, my Nest thermostat will run 22+ hours of every day and sometimes 24 out of 24. The outdoor reset control is the primary temperature control on the house and the thermostat serves more as a limiter than as a control.

This has the effect of using the lowest possible water temperatures (most efficient), which ends up being the most comfortable for the occupants because there's very little overheat/shutoff/overcool/turn-on cycling.

When I was trying to determine whether an air-source heat-pump could work to replace my old (like 1950s old) gas boiler, what I did was reduce the flow temperatures leaving that old boiler to see what flow temperatures would be required to satisfy the heat loss at various outside temperatures. (This is basically discovering the required outdoor reset curve on a mod-con boiler.)

I found the minimum temperature to maintain 71°F at various outside air temperatures and then looked to see whether the ASHP could provide that flow temp (sometimes called "leaving water temp"). (This is also what the article's author did.)

In a case like that, I'm inherently proving that the pipes are sufficiently sized as well. Now, the downside is that it requires time and effort on the homeowner's part, requires a working old boiler, requires a winter season of variable temps, so it's not a practical way for a contractor to bid the job, but for HN readers, you can do it that way. (Note also that if your old boiler is not a condensing boiler, you will be damaging it by causing it to condense acidic water from the exhaust. I was OK with that, because I knew it was going to be replaced the following year.)

If I found that the house struggled to maintain temp, that method doesn't tell me whether increasing the pipe size would help (though having a larger than desired delta-T coupled with a low flow rate is a strong hint).

For a contractor bidding/design situation, there is a universal hydronics formula which is BTU/hr = 500 * delta-T (in °F) * gallons/minute. Pick the delta-T (often 20°F) and the heat load, and solve for gallons/minute. Look up pipe sizing tables for maximum velocity, then using the pipe size and total developed length, figure out the pump you'll need. Then check the emitter sizing to ensure that it can emit the heat required (to "cause" the delta T) at the lower flow temperatures characteristic of an efficient heat pump system.

Ultimately, I proved to myself that a heat pump could work down to an outside air temp of about 18°F [which is slightly above our 99th percentile design temp] with flow temps of 135°F, so an air source heat pump could work with slightly reduced comfort on about 2% of days or could work all the time with supplementation with a 9kW [30K BTU/hr] electric boiler.

What killed the project is no heat pump installer was interested in doing the work (as reflected by outright declining to bid, while bidding a 4-hour gas boiler swap, or by bidding so high that they might as well not have bid, while also cheerfully bidding a 4-hour gas boiler swap). So my house still burns gas for heat, which matches the author's experience:

> this speaks to the third lesson in my story. The boiler market is focused on cheap and quick installation.

Innovation in building and tech is going too fast for these small installation type companies. Zigbee, sensors, switches, motion sensors, per room heating, insulation, decentralized heat recovering ventilation, heat recovering sewage pipes, (noisy?) heat pumps, CO2 sensors, particle sensors, hybrid heating systems, house batteries, solar panels, heat panels, car chargers, vehicle to grid, smart appliances, variable electricity rates, dc subgrids…

An enthusiast can make a home so much better, more comfortable and efficient. But non of it is easily obtained the way we obtain our old gas heating systems.

And it drives me crazy, it’s so complex, it costs me so much time, but I just see so many “professionals” contradicting each other, I hate that I have to make all these choices.

This article doesn't mention one of the main points heat pump gurus have been banging on about for ages - temperature compensation. Essentially, the colder it is outside the hotter you run the flow temperatures. You ideally want this setup so that there is very little intervention from an external thermostat, the balance of flow temperature and heat loss can be set such that one perfectly offsets the other.

It's the secret sauce which can take the heat pump from expensive toy to practical replacement for gas. The payoff time is still too long but at least it exists!

Does anyone else find it interesting how overrepresented the topic of heat pumps are on hacker news lately?

They operate at up to 400% efficiency and represent serious low hanging fruit for household carbon footprint reduction and the main barrier is education, so no I don’t think they are overrepresented.

400% compared with what? and how?

They can shift 4x the amount of heat than the same amount of energy would generate.

400% is probably the ratio of thermal energy released by the heat pump to energy (of whatever kind) used to operate the pump.

That has to depend on the outside temperature.

And also the indoor temperature. My recollection from designing these in class about 20 years ago is they typically operate at about half the Carnot efficiency.

A resistive electric heater converts 100% of its energy to heat, by essentially wasting it. A natural gas heater converts about 95% of its energy to heat, by burning it. A heat pump converts about 3x its energy to heat, by acting as a sort of reverse air conditioner that’s cold on the outside and warm on the inside.

I take your meaning, and it's a good point. But it's still interesting, even if you swap the framing for "underrepresented outside of hacker news."

Unfortunately, in California the electricity price is 6x the price of natural gas.

This is a fair statement, but it represents an incomplete truth. Quick term run down, then some quick math.

COP: Coefficient of Performance. This is a ratio of how much you can augment the temperature for every unit of energy you put in.

Your best, most efficient top of the line gas furnace has a COP of 0.98. This means that, for every 1 joule of energy, you're getting 0.98 joules of heat energy. Let's say that, for easy math's sake, you pay $1 per joule for natural gas, so your operational cost for the furnace is $1.02 per joule. Yes, numbers are crazy town, but simple math for simple explanation.

A modern bog-standard heat pump has a COP of 4. The operational COP of the heat pump varies depending on the difference in temperature from inside to outside, but the vast majority of the state of California is going to operate at temperatures that can support the COP. Per your statement, electricity is 6x the cost of gas, so we'll say that every joule of energy going into the heat pump costs $6. This puts your operational cost at $1.50 per joule.

It's certainly not a "better deal", but the cost delta can't simply be measured by the cost of the energy going into the heating system.

> most efficient top of the line gas furnace has a COP of 0.98

That's a steady-state, best-case figure that is almost never achieved in practice. Very long run times and very low temperature operation is required to achieve those figures, and most homes are not setup to create those conditions. I'd not be surprised to learn that most 95+% efficient furnaces run at 90% realized.

This is super interesting. Do you have a source for this? I don’t doubt it, but I had never thought of it and it doesn’t surprise me that it is a misleading figure to begin with.

The difference (loss) between SSE and AFUE is intended to cover this effect, provided the equipment is sized correctly (almost never done and almost always at least 50% oversized, leading to short-cycling and greater spread between SSE and actual fuel use [meaning AFUE is overstated]).

That might give some terms to Google and find additional data.

Peak COP’s equivalent is closest to SSE; SCOP is closest to AFUE.

Not for equivalent heating with a heat pump it isn't, that's ridiculous. What are you citing?

FWIW: in Oregon, buying the "renewable" power from the utility: the heat pumps we installed 19 months ago for house and water heating are almost exactly even with the gas appliances they replaced. If we ignore the $8/mo fee we pay to keep the gas line attached (for a grill and two fireplaces), they win by a tiny bit.

California might well be more expensive. But a factor of six is silly, that's just not right.

It’s really not that far off from the truth, this varies slightly depending on the utility (I’m on a notoriously more expensive one in my area). Very recent gas price increases have also pushed gas closer towards electricity but not by nearly enough.

Source: https://shrinkthatfootprint.com/natural-gas-heat-vs-electric...

That source is essentially lying to you. It's measuring electricity as "heat" by simply converting the energy value (i.e. as if you were using 1950's radiant baseboard heaters). Modern homes use heat pumps that are (literally!) 3x as efficient.

It gets away with this by admitting the error in the text ("The only caveat is that we talk only about traditional electric heaters. Heat pumps are another story."), but the numbers remain a lie.

In fact if you look at it, that site seems to me to be a for-pay blog. Most of the articles are just product "reviews", with the remaining stuff a mix of bland overview stuff and articles like this that seem to push an agenda counter to the presented theme of the blog.

Be careful out there. The internet is a wild place.

Sure, although it did somewhat align with my real world experience moving from a new house to another similarly sized new house. All else equal, I saw my total energy bill increase drastically with the heat pump, even with an 8kW solar system. Something didn’t add up, maybe it’s not working properly. It raised a lot of questions for me.

That said, it’s arguably really tough to compare these two heating systems objectively and you can’t take the advertised rating at face value necessarily. It’s not apples to apples. Electricity billing is complex and includes many more variables and gas is billed completely differently which adds to the comparison problem. Any comparison online ends up making many assumptions so of course we need to take that with a grain of salt.

The entire world is going to switch away from using fossil fuels for home heating within the next 20 years, so you should expect to hear a lot about heat pumps going forward. In places like Europe, where gas prices have skyrocketed, the change is already happening with great urgency.

And as global temperatures AND wealth related comforts are adopted; the cooling side is also increasing in use.

My theory is that heat pumps are before or right after the "chasm" in the technology adoption cycle and that's an exciting place to be! Skeptics can make good points about how over hyped they are, how the benefits are less than people think etc. Believers can feel excited about being an early adopter. It's the perfect circumstances for the people of hacker news.

The other issue is that heat pumps were pretty awful in even remotely cold climates for decades, and there is some hangover due to bad experiences. But the technology has suddenly experienced a leap from being effective around -0C to -25C. It takes a while for industry to recognize that the technology has improved, and there are multiple generations that have done nothing but sling gas appliances and don’t want to upskill to know how to install heat pumps.

I have had a hell of a time trying to get a trade anywhere near my small town that will actually install one. They all just use the introductory meeting to spread FUD about heat pumps.

Same thing happened when I swapped from a gas water heater to a heat pump water heater. Lots of FUD, but I finally got the switch made and am so happy I recommend it to everyone.

I also switched from a gas stove to induction and the FUD around induction was total bullshit and I’ve never been happier with an appliance.

/endrant

> suddenly experienced a leap from being effective around -0C to -25C.

I was wondering what changed. Better refrigerants?

I believe so, yes.

Maybe everyone should just point out that it's now the preferred heating method in all of the Nordics. If heat pumps work there reliably, it clearly is a proven technology.

As a disclaimer, if you have an air source heat pump in a very cold environment you do need a backup heating method too, like direct electric or just a fireplace.

I don't know about official policy, but I think in Denmark district heating (fjernvarme) is still the preferred option where it's available.

Copenhagen's system is enormous, covering most of the city. It's powered by 69% biomass (wood and straw), 16% renewable waste, 8% non-renewable waste, 4% oil, 3% gas, 0% coal.

[PDF in English] https://www.hofor.dk/wp-content/uploads/2016/09/district_hea...

[Danish page] https://www.hofor.dk/privat/fjernvarme/bliv-klog-paa-fjernva...

What induction stove did you get? All the ones that have good reviews start around $3k which is way too high for my budget.

I bought a Samsung 6.3 cubic foot - NE63B8411SS. It was $1600 Canadian (on sale - looks to be $1900 now / regular) at Home Depot. The reviews I read were good, and it’s been fantastic. The only thing I wish is that the app would allow me to set the temperature to preheat without physically being at the stove - sometimes I’d love to get it started in advance. Give me a safety warning or something to remind me that I should have ensured nothing was inside the oven in advance…

Requiring initial physical interaction reduces the attack surface, even once the attackers have gotten a toehold.

Yeah, I know this is the argument but if I can open my garage via app, change my thermostat via app, unlock doors via app, etc etc etc, I don’t see what the difference is. If the response is that an oven can get hot, I guess it should be designed so that it can’t get so hot as to cause a fire, in which case it is as dangerous (in terms of potential for an attacker to cause damage/loss) as several of the other examples I’ve given.

I’ve been on heart pumps since 2014, and a brand new one for the last year. They are awesome 75% of the year and totally worthless during the cold months. Technically they work if you don’t mind luke warm air blowing in your face all day long. I also switched to an induction stove and i hate it. The bottom of my food cooks nicely…

I think the advances in technology are super recent, like the last 2-3 years in the really cold climates.

I am shocked you feel that way about induction. Actually shocked. The control over temperature and rapid boiling has been a life changer.

I’m curious which stove you have, my is pretty cheap.

I bought a Samsung 6.3 cubic foot - NE63B8411SS. It was $1600 Canadian (on sale - looks to be $1900 now / regular) at Home Depot.

My dad had a heat exchange loop buried in his back yard. I believe he had it buried below the frost line.

In any case, unless you're building on permafrost, you can bury your heat exchanger deep enough to have your heat source at least 0 C in the dead of Winter. Though, for most of the US, the time to recoup the extra cost of the heat exchange loop is pretty high/never (counting the time value of money).

I think there’s a bit more to this where you need to ensure the ground makeup can handle the exchange so that it is effective. My previous discussion was all about air heat pumps rather than ground based. As I understand it, air is around 1/3 - 1/4 the price and today’s technology is more efficient, but I am not an expert.

It is literally the new hotness, and as the article says, getting clear info on it can be difficult. Our furnace--a gas tankless combi unit--probably has a few years of life left in it, but we want to plan for it's replacement, not wait until it fails in the middle of winter. Figuring out how well it will work in our location, with the pipes, radiators, etc, we have, seems to be not straightforward.

The northern hemisphere is going into the dead of winter, heating is on a lot of peoples mind

Given we are in a climate crisis and this one technically interesting solution ith a need for more innovation - the representation feels proportionate.

My personal experience, new home (construction completed circa 2017):

- MANY design errors, some I spot at project stage, I was told they are not errors, it turn out I was right;

- sizing is an issue because the climate may vary MUCH. Three years ago today in the morning there was -22℃ today +10℃ to be in comfort we need to been able to heat (or cool) in much above/below the average cases, so well... I'm not much against pay for extra margins;

- my sanitary hot water heater (Daikin/Rotex M2O EKHHP) break after 5 years, a leak from the sanitary water pipe heat-exchanger, two month for a fix, BUT being provident I've put aside a very small resitive heater classic boiler (50 liters) so I have had exactly ZERO "service interruption", in that case the problem is that something is cheap to back up, something is definitively not...

For instance I have one of the first thermodynamic VMC (a double-flux VMC with a heat pump heat exchanger instead of a passive one) and being in a new home I can't stay without a VMC for more then few hours or I have to open the windows no matter the outside temp. Well, I've also choose to have a backup but it's just a passive VMC since spending 4k€ to keep a secondary cold spare it's definitively wasted money. The small resistive water heater was just 150€ in total plumbing materials included, mounted by myself so a reasonable backup. Similarly the main heating (water-water heat pump) have no electrical backup, I've chosen AND I REGRET a small wood stove for modern homes (Jøtul Scan-66) and I keep a bit of wood (used in the spring to cook pizzas outside, so it remain moderately "fresh" since it's renewed any two/three years). I regret because for not much more I've should better chose a wood based boiler and a LARGE insulated reservoir to make hot water in few hours for an entire day and the small p.v. LFP batteries suffice for a pump. Long story short: most technicians do NEVER think about when something goes wrong and most do not even have at their own home the "new" systems they design so they do not really know them beside theory.

Reading the comments, and I find it amazing that people really do pay $5000-15,000 USD for a heat pump. They are super expensive in the USA at $2000-4000, but you can DIY in 1 hour. Really. The labor is truly fabricated.

In Asia, I've bought more then I could count, and they typically go for $500-700 for 2hp units, inverter, etc. I've debated bringing somme from Thailand or HK to USA because ven the cost in cargo on a plane makes it cheaper. Daikin for example are manufactured in Thailand, and also JP.

It would take me longer than an hour just to install the electrical circuit and disconnect box. Then more than another hour to bore holes in the brick wall, mount lineset covers, and run the lineset.

It's not a huge amount of labor and $5K+ for the labor portion is not justified, but if I sat on your sidewalk with a stopwatch and stopped you at 60 minutes, I'm willing to bet a pretty good sum of money that you wouldn't be done.

2hp is 1.5kW. That's 5 to 10 times smaller than the units being discussed here.

Does the homemade or cheap heat pump have a 300-400% efficiency?

I had a similar problem 18 years ago. I had bought a house with a heat pump connected to the water heater. When it failed, I could not find a local technician competent enough in both plumbing and air conditioning who would touch it. I ended up with two discrete systems.

I’d love for someone to make a portable standalone heat pump for a swimming pool.

Why not make it solar powered with fold out panels. With 400% efficiency 1500 watts of solar power could get you 6000 watts of heating.

Even better, have the cooling end of the pump be in your house. Cool your house while heating your pool

There are existing systems today that do this for residential pools.

It’s not uncommon for efficient commercial buildings like corporate headquarters to have a retention pond be the heat sink for their chillers in the summer. They’re building a pond for aesthetics anyway, so if it’s large enough this can be cost effective vs. digging geothermal wells.

There's a hospital near me built about 15 years ago that has a huge (10 acres?) and deep pond for exactly this. If you hadn't seen it being built, it would look just like a normal decorative pond.

Back when I worked at Goldman, I was told that either their HQ or their Jersey City tower over by the Statue of Liberty had a huge water thermal mass in the basement. They took advantage of cheaper electricity at night to use heat pumps to push the thermal mass whichever way they needed it to go for the next day's heating/cooling needs.

For heating e.g. a swimming pool a solar heater would be more efficient.

Why buy a large and expensive PV array at 15% efficiency to run a complex heat pump when you could instead use the sun’s thermal energy directly in a solar water heater at 70% efficiency?

That's what I was going to comment (although it's closer to 30% than 15% these days), It's low tech and a fraction of the cost, for a swimming pool all you need is a pump and some black pipes really

That probably flips the balance from "use solar thermal" to "use PV and heat pumps", especially in any environment where indoor cooling is simultaneously needed (which is a common case).

If solar PV is 25% efficient and the heat pump CoP is then 4.0, you get a gross efficiency of 100% (plus "pre-paid" air conditioning).

It depends on what you want to do with the heat pump. That solar water heater will not help you much when you want to cool your house in summer.

BTW, I am not sure why exverbody is exited about "heat pumps", which are just air conditionings that have been in use in Asia for decades and probably been installed a billion times

These exist. I am on the fence about buying the $600 one for experimentation and repurposing it to heat/cool garage. Many have heat/cool functionality. You can put the air side either inside or outside.

https://www.amazon.com/s?k=pool+heat+pump&crid=1LWMIT8JDL7S8...

My idea is to do some ground source heat pump supplemented by solar thermal water heating to get lots of free BTUs. The issue (for the $600 one) is it can only cool the water down to 46F. Recall that cooling water means heating air (inside). But heating water goes down to I think 23F. Decisions will have to be made.

I would love for this to take off, solar panel installation alongside GHCP. Since this is probably for a single family residence, toss in a battery and an electric vehicle charging interface - bob's your uncle.

Do you get sun? Putting some black-painted tubing (exposed to the sun) inline with the pool filter pump provides quite a lot of heat at only the cost of the materials. The filter pump already exists and is already circulating pool water :)

Geothermal is even better but can be cost prohibitive. Unlike a boiler, geothermal wells don’t “wear out” and with a steady supply of ground heat, are even more energy efficient.

I don't think your average London terraced house is going to be drilling 100m - 200m deep bore holes.

You could run a central line in the street, then connect to it like sewer and pull heat off of it that way.

It would be interesting to check the average public water temp as well, as long as the delta is closer than outdoor ambient air it would be more efficient than heat pumps. You could potentially treat public water like some GCHP use water wells.

Just drill until you hit a Tube line. Plenty of excess heat available there.

We pushed hard for geothermal. But the quotes for the wells we needed were around $40,000-$75,000, and all of the installers seemed highly uncertain and preferred air-based units. So that's what we eventually went with.

That's a wild price. I'm no expert on the topic, but a quick search results in price estimates in Finland for 15k€-25k€, and another source quotes for equipment 5k€-10k€, bore well 4k€-7k€.

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