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Comment by Scoundreller | Hacker News Reader

Maybe I’m dumb, but aren’t compressors pressure-agnostic? They just add pressure, right?

Other than needing to be a bit stronger to keep from bursting against the higher delta with the ambient atmospheric pressure like the rest of the components.

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In this case there is a higher delta between the system and the atmosphere as well as a higher delta between the hot and the cold side, which is what requires a stronger compressor.

I had a HVAC guy mention that seals are problematic always with non hermetic compressors. Everything larger than a fractional HP system has a shaft seal between the compressor and the motor. Small systems being a pain because there is less refrigerant and no way to monitor the amount.

There is a way to monitor it, but you'd need to put in T-Junctions and add a pressure sensor on the high and low pressure sides. You'd then need to rig up some system to read those pressures. But _you_ would be unfortunately the one that has to do it because I don't think there are any off the shelf solutions.

Power for compressible fluids is approximately proportional to the relation of pressures not difference.

The ideal model of everything is everything-agnostic.

Refrigeration stuff is generally soft copper, and modern refrigerants are already working up in the hundreds of PSI. So I can see that getting expensive or requiring a sea change in materials.

If they’re using copper, changing materials would probably save money. Copper is comparatively expensive. I suspect your premise that they are using mostly copper is incorrect, but then I write software for a living, what do I know.

Copper is (very) effective for conducting heat. Aluminum is also pretty effective but obviously has different material properties than copper. It might be a good fit here; I don't work in the space.

Aluminum generally has fatigue limits that mean it will fail eventually, releasing refrigerant (which is normally an environmental disaster)

what qualifies as a disaster? the amount of refrigerant in a consumer unit just doesn't seem like a lot even if 100% of it was released. if it was a continuous supply, then maybe it could get disaster level, but these are closed systems with a finite amount. at these levels, it seems to me that just driving one's car would qualify as an environmental disaster.

The issue isn’t just the 1st one that breaks the issue is the other 100,000+ that do.

If a typical large split system has about 10Kg of R410A (GWP = 2000x CO2) that's about 20 metric tonnes of CO2. Compare to 5 tonnes for an average passenger car per year. So a leak is like four years of driving a car.

Is it a disaster when an invisible odorless gas escapes and no-one notices? Or when a tree falls in forest? Not on its own, but our entire crisis is a pile of sand grains each too small to count.

My real point was accident vs disaster. Whether anyone knows about it or not (someone will know when the AC doesn't work and the repair tech sees it being empty). If one house looses coolant, then I'd call it an accident. If a vendor has a major problem during manufacturing so that the majority of units fail, then maybe we can use bigger words.

The newer refrigeratants that require higher pressures are much better for ozone and have a lower global warming potential.

Pentane and isopentane, R601 being one of them (of course flammability is a problem) and R744 which is just CO2.

aluminium also has the problem that it is a dissimaler enough metal from most other plumbing hardware (being either made of brass, copper or steel) that it will result in the buildup of a lot of gunk because the aluminium disolves.

That’s galvanic corrosion, but it’s usually an issue in water. I don’t know that it would be a problem with refrigerants, likely not.

Copper has long been the choice for piping because it's soft enough to bend without cracking. The obvious #2 choice, stainless steel, is often even more expensive.

(I don't like the standard HN analysis technique of assuming that another field has made a really basic error that can be spotted by an autodidact from outside)

While I generally agree with your response to the perceived arrogance - shouldn’t we all ask if we can do better than assuming someone else is an expert and has fully optimized a solution? As long as we approach new domains with a sense of humility and respect I think it’s desirable to always be looking for better solutions.

> assuming someone else is an expert and has fully optimized a solution?

I wasn't offering my own singular expert [0] opinion, but rather pointing out what I've observed the entire industry has seemingly converged on. Despite being subject to the hyper-cost-optimization of the consumer market, every refrigeration appliance I've seen still uses copper heat exchanger tubes and copper piping.

I would welcome someone chiming in saying something like "actually I work in HVAC and things are moving towards aluminum heat exchanger tubes and stainless piping" or "that's already the case for most new building-scale systems", because I'd learn something new. But "just asking questions" based on one single material property isn't particularly helpful.

And yeah, copper has gotten expensive. That has helped some new technologies displace it (eg PEX), but copper is also still used where it's needed (eg PEX and PEX fittings over 1" nominal (which is equivalent to 3/4 copper) are prohibitively expensive for some reason).

[0] In fact, I'm not a refrigeration professional, and have yet to pretend to be one. I just tend to look at how things are built.

First off, I was upfront about my ignorance. Secondly, copper is way more expensive than stainless steel, unless I’m missing something. More than 8x according to random websites on Google.

I’m sure there is a reason copper is used, but if the industry has to find an alternative, it would likely be cheaper.

Stainless sucks at heat transfer, and has to be welded. Copper can be soldered (low temperature), or I believe refrigeration lines are commonly brazed (higher temperature).

Raw material cost doesn't tell the whole story. Stainless is harder to work with, therefore costs more to manufacture. And even if it ended up being less expensive in the long run, there would a lot of capital investment to recover.

Stainless steel not conducting heat would be an advantage for linesets - you don't really want the heat/cold to be released in the lineset... it's released in the condenser/evap coils. Those coils are already often made out of aluminum, so dissimilar metals aren't really an issue as far as I can tell...

Flexible stainless tubing is more expensive than copper flexible tubing.

Also, copper flexible tubing is super easy to source.

FWIW there does seem to be a certain blind spot about PEX as an alternative in the US.

A friend redid his whole house with it himself to great success. He mentioned one of the contractors he got a bid from while deciding his plan, when asked about PEX, said "Well that just saves you time and money" in tone of dismissal.

I think there's something similar going on with ductless heat pumps in the US. They're a fantastic option in my area but still uncommon. Part of that seems to be the installers make more from other options.

I very much agree with you about the tendency of HN's community to assume other industries are simply obvious idiots, rather than realizing they're just ignorant of that industry's reality.

Personally, having lived in places with ductless heat pumps they suck.

Lots more maintenance (they need regular cleaning or they build up mold), they don’t circulate air through the house so rooms get their own weird microclimates and get stuffier, and it’s a constant hassle tracking down remotes.

Central is way more comfortable and lower maintenance, IMO.

My house with central forced air already has rooms with weird microclimates due to differing amounts of exterior walls and/or windows. I’ve had the thought that I could use ductless heat pumps to reduce the temperature differences between different rooms in my house.

Even if you do need to make thicker pipes, strength in a metal generally increases with the square of the thickness, so it doesn't cost twice as much in materials to double the strength.

This isn't true. For pipes, double the pressure requires double the wall thickness, which requires double the material.

However, I don't think raw material cost is a big part of the cost of refrigeration systems anyway.

The “soft” copper tubing they use in refrigerant lines is not only very cheap (it’s not 100% copper) but is very much over built for pressure it is likely going to be subject to. It’s way cheaper, in this instance, to simply sell a line set that works with 99% of systems instead of having dedicated factory lines and consumer confusion from having different grades of registrant tubing.

> For pipes, double the pressure requires double the wall thickness, which requires double the material.

Double the wall thickness requires more than double the material.

Gearing on the motor driving the compressor, compression ratio, etc. all have a huge impact on wear and tear, power requirements, etc.

These all change based on the pressure delta.

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