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I live in Gilroy, and we've had 3 days of 105+ weather. When the house I live in was built, they put in a 3-ton unit, which was complete fine for the first 20

I live in Gilroy, and we've had 3 days of 105+ weather. When the house I live in was built, they put in a 3-ton unit, which was complete fine for the first 20 years my family lived here. I replaced it a few years ago with a 4-ton unit, and added extra blown-in insulation. It has barely kept up with the heatwave. Next is dual-pane windows, I guess. There are a lot of folks with less who are straight-out suffering. They re-opened the local library to function as a cooling center. Difficult to do during covid, but heatstroke is dangerous, too. Rough times.

amluto 6y ago
Fancy insulation, double pane glass, etc are great for the heating season. They reduce conductive heat transfer, which is (roughly) proportional to the indoor-outdoor temperature difference. Even on a 100F day, the indoor-outdoor difference with A/C set to 75F is only 25F, which is equivalent to heating to 70F on a mild 45F winter day. Try looking for major sources of solar heating instead. Windows and especially skylights are big culprits. So are inadequately insulated roof / ceiling assemblies. You can put solar film on windows, use indoor or outdoor shades, or replace glass with low SHGC (solar heat gain coefficient) glass to reduce heating.
- Baeocystin 6y ago
  One unexpectedly strong benefit of the panels we put on our roof was the lower cooling bill, even before we were allowed to switch on the panels and use the power generated. Our roof geometry only allowed for a 65% solution instead of the 80% we were aiming for, and this unaccounted for improvement almost made the difference.
brudgers 6y ago
Going from a three to four ton unit was a mistake. One that's easy to make because it's counter-intuitive. And because bigger units are more expensive there's a sales incentive to sell bigger units too. To be effective at cooling people an air conditioner needs to remove latent heat in the form of water vapor. Lowering the air temperature isn't enough. Oversized units short-cycle, lowering the air temperature significantly faster than they remove latent heat by condensing water vapor. The controlling thermostat turns the unit off quickly because the big unit quickly reduces the air temperature. The latent heat in the water vapor remains in the air. People still feel hot. Basically, a residential air conditioner should be running most of the time if it's properly sized. Oversized units will come on, run for a few minutes, shut down for a few minutes, run for a few minutes, etc. Double pane windows and blown insulation will magnify the effect because the air conditioner will reduce the air temperature more quickly and efficiently and leave more latent heat in the air. Note that latent heat in the air from water vapor increases with temperature. That's why we talk in terms of relative humidity. So oversizing is less of an issue for heating season and double pane windows and extra insulation help.
- Baeocystin 6y ago
  I know you mean well, but I promise you, it wasn't a mistake. :) It's not rapid cycling, it's barely keeping up. It has literally gotten hotter here over the years. That's what I was trying to get at with my original post- what was appropriately-sized in decades past is no longer enough.
- baddox 6y ago
  Don't some modern high-end residential HVAC systems have separate air conditioners and dehumidifiers to provide more granularity (and perhaps effectiveness or energy efficiency) between the two effects?
- andromeduck 6y ago
  My understanding is modern compressor tech has already resolved that issue.
chiph 6y ago
Wait - you don't even have dual-pane glass? That's like leaving a door ajar all day. You can fake a second pane by putting plastic film over the window frame and taping it securely around the edges so there's no leakage, capturing the air between it and the glass. This will act as an insulating layer. Air leakage = heat transfer. But you're better off replacing the windows with dual-pane units that have low-e coatings. Maybe the government or your utility has a program to help cover the replacement cost.
- Baeocystin 6y ago
  Yup. Currently have the plastic film deployed on about half my windows. It's obviously not true double-pane, but it makes a real difference. Well worth the $. For folks who don't know, once you've applied the plastic film, you shrink it with a hairdryer until it is smooth. If you take your time, it doesn't look bad at all, and is optically clear.
  - gdubs 6y ago
    3M makes a variety of films for solar reduction and safety; they’re pretty great. Only available through certified installers but we had them done to protect our historic single pane windows, and while we were at it we had some UV film put on our large west-facing windows.

amluto 6y ago

Fancy insulation, double pane glass, etc are great for the heating season. They reduce conductive heat transfer, which is (roughly) proportional to the indoor-outdoor temperature difference. Even on a 100F day, the indoor-outdoor difference with A/C set to 75F is only 25F, which is equivalent to heating to 70F on a mild 45F winter day. Try looking for major sources of solar heating instead. Windows and especially skylights are big culprits. So are inadequately insulated roof / ceiling assemblies. You can put solar film on windows, use indoor or outdoor shades, or replace glass with low SHGC (solar heat gain coefficient) glass to reduce heating.

Baeocystin 6y ago
One unexpectedly strong benefit of the panels we put on our roof was the lower cooling bill, even before we were allowed to switch on the panels and use the power generated. Our roof geometry only allowed for a 65% solution instead of the 80% we were aiming for, and this unaccounted for improvement almost made the difference.

brudgers 6y ago

Going from a three to four ton unit was a mistake. One that's easy to make because it's counter-intuitive. And because bigger units are more expensive there's a sales incentive to sell bigger units too. To be effective at cooling people an air conditioner needs to remove latent heat in the form of water vapor. Lowering the air temperature isn't enough. Oversized units short-cycle, lowering the air temperature significantly faster than they remove latent heat by condensing water vapor. The controlling thermostat turns the unit off quickly because the big unit quickly reduces the air temperature. The latent heat in the water vapor remains in the air. People still feel hot. Basically, a residential air conditioner should be running most of the time if it's properly sized. Oversized units will come on, run for a few minutes, shut down for a few minutes, run for a few minutes, etc. Double pane windows and blown insulation will magnify the effect because the air conditioner will reduce the air temperature more quickly and efficiently and leave more latent heat in the air. Note that latent heat in the air from water vapor increases with temperature. That's why we talk in terms of relative humidity. So oversizing is less of an issue for heating season and double pane windows and extra insulation help.

Baeocystin 6y ago
I know you mean well, but I promise you, it wasn't a mistake. :) It's not rapid cycling, it's barely keeping up. It has literally gotten hotter here over the years. That's what I was trying to get at with my original post- what was appropriately-sized in decades past is no longer enough.
baddox 6y ago
Don't some modern high-end residential HVAC systems have separate air conditioners and dehumidifiers to provide more granularity (and perhaps effectiveness or energy efficiency) between the two effects?
andromeduck 6y ago
My understanding is modern compressor tech has already resolved that issue.

chiph 6y ago

Wait - you don't even have dual-pane glass? That's like leaving a door ajar all day. You can fake a second pane by putting plastic film over the window frame and taping it securely around the edges so there's no leakage, capturing the air between it and the glass. This will act as an insulating layer. Air leakage = heat transfer. But you're better off replacing the windows with dual-pane units that have low-e coatings. Maybe the government or your utility has a program to help cover the replacement cost.

Baeocystin 6y ago
Yup. Currently have the plastic film deployed on about half my windows. It's obviously not true double-pane, but it makes a real difference. Well worth the $. For folks who don't know, once you've applied the plastic film, you shrink it with a hairdryer until it is smooth. If you take your time, it doesn't look bad at all, and is optically clear.
- gdubs 6y ago
  3M makes a variety of films for solar reduction and safety; they’re pretty great. Only available through certified installers but we had them done to protect our historic single pane windows, and while we were at it we had some UV film put on our large west-facing windows.