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In a side-split on three levels, four including the basement, the heating and cooling unit is almost new (2 years), and the heating/cooling unit was only available at 35% more cubic feet capacity than the house. Hence it should be quite easy for it to handle the volume of air inside.

Both summer and winter I keep the air ciculation turning on perpetually, whether the heating and cooling unit is on or off.

The thermostat is on the main level, set to about 68F/20C in the winter and a little more, 70F/21C, in the summer.

Despite all these factors, each year in May and September I find it necessary to move my desk, in May downstairs and in September back upstairs. Otherwise the temperature is too uncomfortable for an extended sit-at-the-desk session. I found it easiest to just buy a second desk, and move everything else (monitor, monitor arm, laptop, paraphernalia, desk chair).

Is this:

  1. Normal or near-normal. Regardless of the roof and wall insulation, the rooms upstairs will have to heat up if the house (detached) is baked throughout the day from the sun. In the summer the temperature outside is regularly 80F/28C and sunny.
  2. Not normal. The description above indicates that the walls and especially the attic are not sufficiently insulated.

The bedrooms are upstairs, but the extra heat is not an issue, because the temperature outside is anyway cooler at night.

In this question I'm not even seeking a solution yet (there is already a huge leafy tree on the east side and another, even larger, on the west side). I'm first trying to understand what is to be reasonably expected for comfort inside at all levels.

Quantifying (following isherwood's comment):

I'm asking for a round-the-year issue, but mentioning specific temperatures may help. In this late May, and while the temperature outside is a pleasant 65F/19C, I haven't even switched on the A/C unit yet. Despite that, the upper southwest room (used as winter office) reached 78F/25.5C yesterday. The air circulation is on. Is this heat level normal? Could a room that's baked on two sides for the whole afternoon be expected to remain cooler? (Notice that it became much warmer than the outside.)

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    @isherwood You're right. Does the update give a better picture? – Sam May 22 '20 at 14:59
  • It's a start. The behavior you describe is attributable to solar gain. You therefore need to tell us about your siding and windows and any trees involved. The answer you've accepted is wildly speculative. – isherwood May 22 '20 at 16:27
  • You're using it as a home office. Does the temperature increase similarly on weekends, when you're not in the office? What if you're working at night? Do you close the door when you're working? The temp increase could be from the electronics and your own body heat. Turning on the room lights, my laptop, 2 monitors, and sitting down to work will raise my office 8-10 degrees in 3 hrs, day or night. – Eric Simpson May 24 '20 at 17:26
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This is quite unreasonable for a newly installed system with plenty of capacity.

You may want to check:

  • Duct work.

    • Ensure that everything is attached properly. We've had ducts disconnect in our crawl space. (It was after many years, but it's still worth checking for you.)

    • If you can't get to the attic/crawl space to physically examine the duct, ensure you're getting even airflow out of each supply vent in each room. You can use an IR thermometer to check the air temperature coming out of each duct to see how much difference there is in outlet temp. (These can be picked up for around $20 or less at your favorite "cheap imported tools store". They're handy for other things and just fun to have around, so it's money well spent.)

    • Ensure that flexible ducts haven't been crushed. (I just discovered a supply line in hour house was crushed between the strapping holding it up and the large cold air return that was in the same strapping.)

  • Insulation

    • Did you have the same issue before you replaced the HVAC systems?
    • Buy, rent or borrow an infrared camera (or pay someone to look for you) and look at the outside of the house on a hot day looking for cold spots where air is leaking out. (The same works on a cold day looking for hot spots, so this will work whether summer or winter is coming in your hemisphere.)

  • Windows

    • We just replaced all our single-pane windows with new double-pane windows last fall and the temperature difference in the whole house was fairly substantial.
    • If you have double-pane windows but they're older, they may be leaking air somewhere around the framing.
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  • Your suggestion of inspecting the ducts is very interesting. It didn't seem that the folks who do home inspection even look for that, but then they know a little about everything, and not a lot about any one particular specialty. – Sam May 22 '20 at 15:00
  • Silly question: you use the infrared camera inside the house, not outside; is that right? – Sam May 22 '20 at 15:00
  • I think it could work from inside or outside, @Sam. However, from inside, if the system is blowing air you're going to see hot/cold spots near the outlets that will throw off your readings. I'd think from the outside is better, but I'm not an expert. You may want to ask a whole new question about using an IR camera to check for air leaks. (Note an update to the "duct work inspection" section, too) – FreeMan May 22 '20 at 15:13
  • From outside would be fine. There should be a temperature difference of at least 10 deg Celsius between inside and outside. And it should be done at night time with no daylight involved after all exterior surfaces are cooled down from direct sunshine. In other words, best time is before sun rise. – xeeka May 22 '20 at 15:20
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I have seen what in my opinion are crappy jobs on multi floor systems. No single system at the same static level year round will ever heat / cool properly. I install adjustable dampers in the trunk lines both supply and output for the air handler. In the summer the upper floors are wide open and the lower floors closed down ~20% this pushes more cold air up where it is warmer. In the winter I reverse for the lower floors to be open and the upper floors throttled back. This uses a limited amount of natural convection or heat rising cool falling because of the density.

It may be possible to add trunk dampers this is the best and easiest way to adjust for summer winter temp changes , even a very well insulated home should have dampers in my opinion not only in the rooms and each room to be balanced but on the trunk lines to the different levels so the house comfort is similar on all levels.

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