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Comment by echelon_musk | Hacker News Reader

> Comparing the power consumption of a [broken] 30 year old refrigerator to a brand new one

Yeah, the title is misleading. The article says that one of the compressors on the old one was running constantly - if you applied the same failure mode to the new refrigerator, the difference would be significantly less.

I would expect a refrigerator that has EC motors running the compressor(s) and fan(s) to be around 2-2.5x as efficient as one with fixed speed motors, based on what I know about variable frequency drives and three-phase induction motors. For those, 80% speed uses 50% of the power, 63% uses 25% of the power. For an 1800 rpm motor that is 1440 rpm and 1134 rpm. VFDs work well for most applications with variable torque (fans and pumps), but applications requiring constant torque (saws, grinders, etc) are better served by fixed speed starters.

> variable frequency drives and three-phase induction motors. For those, 80% speed uses 50% of the power, 63% uses 25% of the power.

You’re presumably thinking of the “Affinity Laws”, which, according to Wikipedia (and plenty of other sources), “apply to pumps, fans, and hydraulic turbines. In these rotary implements, the affinity laws apply both to centrifugal and axial flows.”

This is, IMO, one of the worst kinds of science writing. Wikipedia, and plenty of other sources, make little mention of when the do and don’t apply or, relatedly, why they’re true and why they can’t always be true.

They generally apply to situations where a pump is pumping fluid through something like a filter or a long pipe where the pipe is a closed loop or at least the ends are at the same elevation (e.g. a swimming pool pump, except when pumping from a pool into a higher hot tub). So you have no actual work being done by moving fluid, and you can run the pump slower, and thus move less fluid per unit time, thus reducing friction in a manner that the pressure that the pump needs to overcome goes all the way to zero as the flow rate approaches zero.

But the affinity laws are not really anything fundamental about pumps, and they certainly do not override conservation of energy.

Now consider a refrigerator. The compressor is pumping refrigerant from an (approximately) fixed low pressure to a fixed high pressure. (The fluid goes back from high pressure to low pressure via a capillary tube or expansion valve or similar lossy device -- it gets its pressure increased in the gas phase and decreased in the liquid phase.) There's some friction, but after subtracting friction, the pressure is independent of flow rate, and thus the work done per unit flow is independent of flow rate, and the pump power scales linearly with flow as opposed to super-linearly as the affinity laws suggest.

Also, the compressor is a positive-displacement pump, and the affinity laws don't even pretend to apply to these.

(A well pump is another common system where the affinity laws will lead to nonsensical results. If you want to size a well pump properly, you need to know the height that you're raising the water, the output pressure you need, and the range of flows that you want. And then you look at the actual measured performance curves of the pumps (and their drives) that you are considering, and you pick something appropriate.)

All that being said, variable-speed fridges exist, and they're kind of nice in that they try to run continuously and quietly instead of alternating between full-power (and loud) and all the way off. And they are probably a bit more efficient because there's less friction and because the motors are likely to be more efficient three-phase designs instead of the not-actually-amazing single-phase motors you'll find in older fridges.

> variable-speed fridges exist, and they're kind of nice in that they try to run continuously and quietly instead of alternating between full-power (and loud) and all the way off.

Modern continuous variable speed compressor fridges drive me absolutely crazy. They sound like two ceramic plate rubbing together with some maddening flutter.

Some also add incredibly annoying high pitch whines. That seemingly nobody seems to notice but me. In the same vein as coils whine from power supplies and other modern electronic.

Old bang bang fridges are loud, on lower frequency, and with a sound that is more consistent and stable. Not varying one second to the next, which I find easier to ignore.

I have started looking at how reasonable it is to move the compressor of my expensive and low quality 2025 fridge across the wall into the garage (refrigerant capture and refill, brazing new lines etc).

It's not just modern electronics. I used to be able to know, the minute I walked in, if there were any CRT TVs on in someones house. Flyback coil whine was a constant presence.

Same. Sometimes I'd be sitting in a different room not exactly "hearing" anything except a vague sense of heightened perception and unease, and eventually decide to get up to check the TV room on the other side of the house and sure enough, someone left the TV on. The feeling of utter blissful silence afterwards in comparison was incredibly relaxing though, I do kinda miss that part...

VFDs need really good grounding. Make sure you have a solid earth ground or you can get arcing across the motor bearings. This makes a sound that’s often described as “fluting,” and I think that might be your problem. If it is, you need to fix that before your bearings are trashed and you have to replace the motor.

Wait, the compressor and drive is a device or a pair of devices right next to each other, assembled in a factory, and made out of metal. Why does the quality of the connection between the hunks of metal and the planet have any effect?

VFDs can produce nasty waveforms, and there are cases where “grounding” could be a big deal, but I think that the wiring of the ground terminal of the power supply is only relevant at all when it’s involved in the connection between the drive and the motor. So, for example, if you have a VFD that is far away from a motor, then you would want to make sure the VFD and the motor’s grounds are connected to each other and maybe even that the VFD’s supply neutral (average of the phases) is reasonably close in voltage to ground, keeping in mind that there may not be an actual neutral wire connected to the VFD, and that the motor’s ground is well connected to the VFD’s ground. By modern standards one should use actual VFD cable and terminate it properly.

https://www.southwire.com/medias/sys_master/related-pdfs/rel...

Fascinating, I will investigate.

The outlet are grounded with a thin non insulated copper wire secured to the nearest water copper pipe, itself also bounded to the iron gas pipe (this is 1950 electrical). I am not sure I can call this a solid earth ground.

Thank you for the info!

If your electrical service is also bonded to the gas and water pipes (it likely is), that grounding arrangement for the receptacle is fine.

It isn't bonded at the panel. The panel ground is not used on any of the 1950 circuits. Neutral is bonded to the panel ground. And yes this does means neutral and ground pins at the outlet have a few volts across, since the path between them goes through the literal earth. The water and gas piping coming out of the earth at the opposite side of the house from where the electrical panel is located.

> Some also add incredibly annoying high pitch whines. That seemingly nobody seems to notice but me.

That drove me crazy for about a week trying to figure out what the noise was coming from... Pinhole water pipe leak? Cat stuck in the flue? Once I realized what it was, I didn't mind it much. It is better than loud old compressors suddenly kicking on and burrr'ing away then stopping.

> I have started looking at how reasonable it is to move the compressor of my expensive and low quality 2025 fridge across the wall into the garage (refrigerant capture and refill, brazing new lines etc).

It would be worth looking into commercial refrigeration as well, you can get a refrigerator with a remote condenser and I’m sure you could find used equipment. Either way you’re going to have to run refrigerant piping and plumb in condensate drains.

I hate fridge noise, any kind. I’d like to prioritize quiet running when buying, but for the last two purchases I haven’t had confidence in the research I did. In the end, one was decent and the other was fair.

However, in preparation for writing this comment I discovered Quiet Mark, which seems promising. https://www.quietmark.com/

Variable-compressor fridges will be more thermodynamically efficient, as the heat transfer happens more gradually, so temperature differences across the heat pump will be lower (e.g. because the condenser will have more time to gradually transfer heat to the room, it won't get as hot). And coefficient of performance for a heat pump is higher if the temperature difference is smaller. Another way to achieve this might be with a variable-displacement compressor (which is how modern car AC systems work, rather than cycling on/off).

Unlikely. The thermal timescales are long enough that the fridge turning on and off doesn't mean the temperatures vary wildly. That's why they can do it in the first place!

Central heating on the other hand... I'm definitely never buying a boiler without opentherm.

I think the comment you’re replying to is referring to steady state performance. At lower refrigerant flow, the temperature difference produced by the system will be lower and thus the thermodynamic efficiency will be higher.

(A fridge is producing a temperature difference between the hot gas exiting the compressor and the cold liquid/gas mixture coming out of the expansion valve. The former will be quite a bit hotter than the outside air and the latter will be quite a bit colder than the air inside the fridge. The smaller the value of “quite a bit” the higher the Carnot efficiency would be.)

This reply is amazing, thank you! This gives me a much better understanding of the work done by a chilled water pump in a closed loop vs a refrigerant pump with an expansion valve in the loop and in which situations the affinity laws apply to.

I’m just a dumb electrical PM who knows enough to be dangerous, and I only know how things like heat exchangers, pumps, and fans work on a very basic level so this is illuminating.

This is the kind of comment that makes HN special and precious. Thank you.

VFDs and inverters are nice, but have you ever looked inside of an AC unit with an inverter and asked what it would cost if the electronics has a problem? Simple dumb single speed stuff looks like it costs more to run, until you have a problem, which you will. On simple single speed fridges and AC that don't do anything special maybe you need a motor cap or motor or something every parts house has for little money and easy to swap; the moment you introduce more complexity is the moment you bend over for GE or Mitsubishi or Samsung to gouge you for basically any part.

VFDs also fail about 2x to 2.5x as often as anything else in the system. In small commercial applications they often end up bypassed within a few years of installation of that equipment. They're also unusually good at catching fire due to the design requirements of the part.

Properly sized multistage A/C systems are a much better idea.

You do see drive failures, but they’re less common than they used to be, almost every drive my guys install is an ABB drive and they’re pretty reliable.

You are correct about soft-starters being a lot simpler and requiring less maintenance, as it’s more or less just another contactor inside a regular across-the-line starter with some extra control wiring to handle the extra contactor. Adding an inverter, rectifier, and solid state electronics does make the complexity much higher.

I suspect a cheap fridge is still a fixed speed compressor. Same with AC units - only the nicer ones are “inverter”

Given modern minimalistic build standards, the modern compressor would probably overheat and fail if run continuously 24/7. I am reminded of the old simpsons bit where they make a tent in front of the refrigerator.

https://youtu.be/1t9BdMuzV64

Coincidentally, that episode is also 30 years old, and right before the clip starts, the fridge has just broken. So a fridge of that era cannot run 24/7 either.

Here's the full clip: https://www.youtube.com/watch?v=ioyU_sZufC8

It broke because they had the door open, causing it to run 24/7, much like the fridge in the op. I couldnt find the whole clip.

It's still functional though, the point is comparing 'keep running the old tired one' vs 'replace with new', not 'how well were they made then vs now'.

I feel like a more appropriate - or at least a more interesting - comparison would be “fix the old one” vs “replace with new.” Replacing an old broken product with a new unbroken one is changing two variables, not one.

Now, if the author would like to break their New refrigerator and report back, I’ll take it as an interesting result.

Can you even fix the 30 year old one... that's still '95 when these things weren't very fixable.

But at the end of the day the question is what is the likelihood the old fridge will be in a semi-broken state.

Usually the older ones are easier to fix. It's never going to be new of course, and even stuff like the insulation will be degrading with time.

I'd argue that in general, refrigerators are one of the few devices that came out of the energy efficiency mandates as better products.

"Broken"? It kept things cold.

Save us dang

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