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Comment by criddell | Hacker News Reader

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?

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> 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.

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?).

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.

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.

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.

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