New Windows with Low-E Glass

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Old 09-13-20, 03:23 PM
G
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New Windows with Low-E Glass

Last fall 2019, we replaced our windows with new Andersen units with low-E thermo-pane glass. Our old windows were reasonably tight but not thermo-pane - just regular double-hung with storms. I'm convinced that our a/c utility usage has gone down this summer 2020. I also note that the window sills don't seem as hot as I remember them on a sunny day. The only hard data that I have is the monthly kWh used this summer and last, but without a lot more work, I can't precisely adjust it for degree-days between this year and last. Could the low-E glass account for what I think I'm seeing? My 3-ton a/c unit will be due for replacement within a few years, or sooner. I'm thinking that if trends continue, I will downsize to 2.5 tons.
 

Last edited by gilmorrie; 09-13-20 at 03:56 PM.
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Old 09-13-20, 03:50 PM
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Like you I replaced all of my windows last fall. Before in the morning, the back of the house would be up to 10 degrees warmer than the front (measured), now only a couple of degrees (extensive morning sun). I used a thermo gun to measure interior surfaces. All showed significant reduction in temps (which obviously reduces the surface re-radiating the temp into the air). My A/C runs 3.5 hours on typical days compared to 5.5 hours previously as tracked by my crappy nest thermostat. It used to come on around 11 in the morning, now around 3 or 5.

I'm a big fan of low-e.
 
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Old 09-15-20, 11:40 AM
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G: You're probably more oversized than that. I'm thinking 2 tons would be plenty. When there's some hot weather time the outdoor unit: Time on + time off = Cycle length (typically 20 minutes @ 3 CPH). Time off/Cycle length approximates degree of oversizing. Outdoor temp needs to be more than "ASHRAE 1% Value" for your locale.
 
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Old 09-15-20, 11:45 AM
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And when you do replace the unit, make sure the bidders perform the required "Duct Survey" to identify any duct issues (Restrictions, leaks, substandard insulation, etc.). Fixing those can reduce cooling requirement by 100 to 400 SF/Ton.
 
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Old 09-15-20, 02:23 PM
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So Ferd42, how does that work? That formula indicates in order to be properly sized it needs to run constantly. So if it's 5 ton unit and the cycle length is 30 minutes and it runs for 24 minutes, it's still 20% oversized and I need a 4 ton unit?

What am missing?
 
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Old 09-16-20, 12:30 AM
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Constantly when outdoor temperature exceeds the 1% value with a good duct system is the goal. In the Real World that rarely happens.

It's unusual to see a 30 minute cycle, because most stats operate at 3 CPH (cycles per hour), meaning 3 cycles @ 20 minutes each. Given that, you've done the math correctly for your 30 minute cycle. As I said, it's an approximation. And if your duct system is substandard, you're more than 20% oversized.

I don't recall saying this here, but ALWAYS fix the ducts when you replace equipment. Ignoring this important step is to "WASTE ENERGY EFFICIENTLY" with your new, more efficient equipment. Oh yes, there's the wasted expense of that extra ton or half ton of equipment you wouldn't need, money that'd be much better spent on duct repairs.

Those wondering about the 1% value for your area, just surf "ASHRAE 1% Design" or some such. Here's one site ASHRAE climatic design conditions
Be sure to set for Inch-Pound (IP) at the upper right of your city's page. "1%" and "DB" (dry bulb temp) under "Cooling DB/MCWB" near the top of the page is what you need.

ACCA Manual J is the main source for MJ practitioners, here's a page on Texas.

 
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