Feedback on temperature differential v/s efficiency for 2-stage cooling

Feedback & Discussion
1

@ziebelje

I put this in the uncategorized category since I didn’t easily see a section where Beestat users could get some assistance on configuring their HVAC systems using data from Beestat.

Anyway, I have a 2-stage 5-ton A/C. The temperature differential was configured to be 2 degrees. And the second stage kicks in after 20 minutes. I’m fairly satisfied with comfort, and indoor humidity.

Yesterday morning, I changed the differential to 1.5 degrees, without changing anything else. On the Beestat graph below, I’m just comparing the runtime on the night before last v/s last night. The outdoor temp in both cases was about 77F.

Screen Shot 2024-09-06 at 10.52.57 AM
Screen Shot 2024-09-06 at 10.52.57 AM2326×796 89 KB

On the night before last, the system ran a total of 5 times during my Sleep profile (purple bar). It ran for 100 minutes in Stage 1 and 71 minutes in Stage 2. In Stage 2, the compressor pulls about 1.33x the Amps pulled in Stage 1, so I make that a total of “194” Stage 1 equivalent minutes.

Last night, it ran a total of 6 times for 150 minutes in Stage 1 and 22 minutes in Stage 2. Using the same calculation as above makes that a total of “179” Stage 1 equivalent minutes.

So that’s about 8% less in terms of power consumption, although there is one extra compressor start.

There was one addition difference. The night before last, indoor RH was about 52%. Last night it was 54%. In either case, there was no noticeable difference in comfort.

For a system like mine, what would be better? The higher differential, which reduces the number of times the compressor turns on by 16%, or the lower differential, which reduces energy consumption by 8%?

I hope other users will chip in with their thoughts. Thanks!

1 Like
2

This is good for the Feedback & Discussion category. Interesting analysis…I feel like you need more data before making any concrete decisions. What’s the actual energy cost here and how much money is 8% saving you?

1 Like
3

For sure!

I’m going to run it like this for the month of September and compare it against August. Weather tends to be relatively similar between August and September here.

Energy cost is 12.5¢ per kWh. And A/C use is approximately 75% of my monthly electric Bill between June and September. For example, I used 2487 kWh in August, of which 1900 kWh was A/C + air-handler (from my Sense meter). 8% of that would be about 152 kWh. Or about $19.

4

This is a good experiment and good results. I’ve also done some similar experiments as well. My opinion would be the lower energy cost option is better than reducing compressor starts as the compressor is designed to start multiple times an hour and per day. Even if you only started the compressor once a week, the unit would probably still need replacing in 15 to 20 years because something else would fail on it or it would leak refrigerant. In general, running longer at a lower cooling stage is more efficient than shorter runtimes on a higher cooling stage.