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Comment by Avamander | Hacker News Reader

So off-peak pre-cooling, did I read your comment right?

That has been talked about quite a bit actually. Technology Connections made a nice YouTube video (https://www.youtube.com/watch?v=MFEHFsO-XSI) about it.

I'm a bit surprised though that off-peak pre-cooling isn't more common or even widely incentivised, especially with the rise of smart home and IoT stuff. All I've seen is reduction (so less cooling) during peak, if I'm not remembering wrong, people were quite annoyed with that.

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I work in HVAC and it is very common for larger buildings. They get huge incentives for shedding load during peak hours. Larger buildings also have more tools at their disposal beyond simply the "thermal inertia" of the building itself:

1. Ice thermal storage. You freeze ice at night and melt it during the day to cool your building. Only used for large buildings - check out Chicago's district cooling for an awesome example.

2. Adiabatic and hybrid cooling towers. These are HVAC equipment that can switch between regular and a "wet mode" where they evaporate water to cool the AC coil and reduce electricity usage.

3. Better automation tools. Building automation systems are fully programmable (e.g., switch schedules when outdoor temperature is past a certain threshold), unlike most programmable thermostats which can run a fixed schedule but not much else.

I recall watching a program on green construction on KQED and the Federal Building in SF and how it was built.

From Wikipedia:

> The building was designed to be a 'green' building consuming less than half the power of a standard office tower. Utilizing natural light to illuminate 80 percent of the building helped it achieve worldwide recognition as the first Federal Building to be certified under the USGBC's Leadership in Energy and Environmental Design (LEED) criteria. Its southern wall is draped with translucent panels of perforated stainless steel (3 by 8 feet in size), intended to accumulate solar heat and thereby create an upward air flow, which in turn causes cooler air to enter the building through sensor-controlled windows, achieving an air conditioning effect. The result has been criticized as unsatisfactory by employees working in the building, which has received low workplace satisfaction ratings.

And an article on it - https://www.govexec.com/pdfs/green/080108gsa.pdf

There are some neat things they tried there.

In Toronto there's a giant cooling system for downtown buildings that uses cold lake water pumped downtown from 83m underwater:https://en.wikipedia.org/wiki/Deep_Lake_Water_Cooling_System. It's being expanded to offer even more cooling with a low carbon footprint.

Very cool! Sort of reminds me of seasonal thermal energy storage: https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storag...

I spent nearly a decade working across the street from plant #1 without knowing what it was, it just seemed to be an out of place (for the comparatively tiny building it was on) massive cooling setup. When I needed to think, I'd often stare out the window and watch the cooling towers in the summer. It was kind of hypnotic, specifically all the running water. From the angle I was at, I could see pretty well into the cooling towers from above.

Knowing very little about HVAC, when I worked in higher education I was surprised to learn that some colleges centralize heating and cooling for their entire campus. For example, Harvard has a chilled water plant which cools 75 buildings: https://www.energyandfacilities.harvard.edu/utilities/chille...

Interesting article from the Crimson (student newspaper) from 1978 about their recent-at-the-time move to computerize the heating and cooling systems: https://www.thecrimson.com/article/1978/1/12/the-great-chill...

Edit: fixed unintentionally humorous typo, whoops

You know Harvard's endowment is getting too big when they go out and buy Ganymede.

FYI, when that webpage says "tons" they mean the capacity to freeze a ton of water in one day. So Harvard's CWPs can move enough energy to freeze 20,500 tons of water a day - absolutely humongous!

It only works on well-insulated buildings. There are a lot of older legacy structures where the temperature equalizes very quickly once you turn off the climate controls.

My home's HVAC system is getting old and when I had the HVAC technician out for the summer inspection I asked about a new system (mine was installed in 2008). He said a new one will be a lot more energy efficient and (depending on the system we choose) would have more settings than off and blast cold.

I mentioned that this one struggles to hold 75 when it's much over 100 degrees outside and was told it doesn't matter what system I buy, that will always be true. The systems can only pull down the temperature around 25 degrees Fahrenheit. Is that true? If so, how do stores hold their temperature so low?

> The systems can only pull down the temperature around 25 degrees Fahrenheit. Is that true?

When sized correctly that is true. If you install an oversized system it can cool down even more (which answers your store question: they have an oversized system). An oversized system will cool the air too fast and not deal with the humidity correctly. There are other problems with an oversized system as well that someone who knows HVAC better than me can talk about..

> There are other problems with an oversized system as well

(Not an expert but I've dealt with HVAC enough) Short cycling is another problem. The system cools the room air faster than the air can absorb heat from the objects in the room and the structure itself. This causes the thermostat to cut out prematurely. The air then quickly warms from absorbing heat causing the thermostat to cycle on. The room again cools too fast and thermostat cuts off. That constant on-off cycling can damage the refrigeration compressor. Plus like you said, the humidity is not controlled so the room is cool and humid which is much less comfortable (to me at least.)

Wouldn't variable speed systems alleviate some of the issues with short cycling?

It seems like this is something that automation could help. A variable speed system could learn what the neutral setting is for different indoor-outdoor temperature gradients and cool the space slowly enough to avoid short cycling. But then I dropped out of thermodynamics when I was in school, so I really have no idea what I'm talking about.

Edit: After looking, I think everything is already in place. Our thermostat also measures humidity, so there's no reason it would run the A/C at a level that would make condensation a problem.

One note I remember from my research, variable speed systems are tightly coupled to the thermostat so you may not be able to use an easily automated thermostat. My thermostat shows humidity (Mitsubishi MHK2), but as far as I can tell that doesn't factor into when/how hard the AC runs.

If you size up a system with variable compressor and fan the system will use the correct amount of power to maintain whatever temp you want. This is more common with inverter compressors, as far as I can tell.

That's gotta be bullshit. I live in a 1930s house with little to no insulation. I have curtains on all of the windows to block heat and my ground floor can maintain 72°F all day during 100°F weather. Same with my upstairs office using a window air conditioner. (The rest of the upstairs is a lost cause.)

There might a reason for that though. There are dampers in the basement air ducting I can use to redirect airflow, so I'm running a system designed for 1,300 sqft to cool an area that's half that. My office window unit is overkill for the room I'm in.

I've optimized this using data from home assistant, which continuously monitors the outside weather and temperatures (plus occupancy, light level (lux), light bulbs, etc) for every room.

Seems about accurate, but the variable speed systems are a big QoL upgrade regardless. Just being able to have AC on lower speeds has helped a lot with humidity even when the house doesn't necessarily need cooling. We upgraded last summer from a 3 ton unit to a 3.5 ton unit (max size our ducts would support). It's still struggling to hold 75 when over 100 outside and some upper rooms hit 80, but that's more down to our ducts and layout of the house. The lower humidity and constant (yet inaudibly low) air flow makes it feel much more comfortable at 75 than with the old system.

What I want is an oversized system with variable speed but they seem reluctant to sell me an oversized system.

This is from 2016, but an oversized system can be more prone to having the condenser freeze.

Ask This Old House (https://www.youtube.com/watch?v=-33wjzNxgSw)

The first time I had my condenser freeze, I called the HVAC company we use and they had me shut off the system for 30 minutes and it started working again. When their tech came out, he found the refrigerant pressure was low so he corrected that and let us know that when it's hot out (105+) don't ask the AC to keep the house at 75 because it will run non-stop and freeze again. That's when I started thinking about buying an oversized system.

Thanks for posting this video. It's pretty good, but it raises some questions for me as well (like was the 4 ton system in the video a variable speed system?).

Besides that you actually want your unit to struggle a little in order to have enough run time to control humidity.

It’s actually a problem if you have an overpowered system.

When is the humidity a problem? If I have my thermostat set to 73 and the system is maintaining that through the day and night, will humidity be an issue? A variable speed system in a typically insulated home will likely be running at a very low level most of the day.

I can see it being a problem if I come back from vacation and change the thermostat from 85 to 73 and ask the A/C to get there ASAP, but even then the controller could know how much moisture is in the air and make sure the temperature drop is slow enough to deal with condensation.

No that's not true, you could oversize the system or improve your insulation. The condensing temperature is typically 45 F and the evaporating temperature is typically 120 F - in layman's terms if the building were perfectly insulated, you could get it to 45 F as long as the external temp didn't go above 120 F.

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