What is your location?...I don't understand the need for 15 KW heat strip for a 3 ton heat pump. A single stage 10 KW heat strip yields about 35 KBTUs alone for heat. I thought you were getting the 824 thermostat not the much cheaper 803 which is a good basic programmable HP thermostat.

On airflow, you need ductwork sized minimum 400 CFM/ton for both supply and returns. For a 3 ton condenser, minimum 1200 CFMs. Extra would be better especially for return. I would hope for a 3 ton, you have more than one return for better inside airflow and comfort. It is preferred to have outside condenser, inside air handler, and heat strips on separate circuits. Of course sometimes this is not available or practical depending on your electric service and cost to modify/upgrade. Trane at one time had a prewired staged 10/15 KW heat strip. With the 803 thermostat, that will not allow you to control heat strip operation. It would be wrong to make a hard and fast comment without making a visual inspection by a competent HVAC tech with electric/wiring experience.

If you are certain you have a 5 KW heat strip on a circuit by itself, I would cut it off entirely and see if you actually need it. A 10 KW should be adequate for supplemental heat, to temper air on a true defrost call, and for emergency heat when needed.

IMO

Thanks for your helpful replies.

My location is the mountains of southwest Virginia.

For most cases, a 10KW heat strip will be enough for the finished part of my house (main floor and second floor), which is almost 2000sf. I also have a 1400sf unfinished, usually unconditioned basement that has 3 registers that usually remain closed. I open the three basement registers only when it is extremely cold here, I wanted to have 5KW more available to help heat the basement or in case the outdoor unit fails.

I tested the functions of the two breakers today, and the first one does control the AH and 10KW of heat and the second one seems to control the other 5KW. I obtained the idea about the functions of the two breakers from reading another discussion page on HVAC. The temperature at the registers with just the first breaker on was 81 degrees and the temperatures of the registers with both breakers on was over 92 degrees. So I will keep the second breaker off unless there is an emergency.

The installer was supposed to replace a malfunctioning 850 with an 824, but showed up with an 803 instead. I will have him change it to an 824 when I have my first service.

After reading the TEM6 manual, I believe that I need to control the air speed for the auxiliary heat with jumpers 7 and 8.. Setting both to OFF should give (depending on conditions) about 1298 CFM, which should be enough for the 10KW, but is still a bit shy of what is needed for 15KW.

81 degree supply temp output at what outdoor temp?...depending on outdoor temp and with or without a heat strip activated, then that's quite poor performance...suggests a bad charge or even incorrect wiring for the 10 KW heat strip....either way i would call dealer to have this checked out...just so you know....

IMO

tigerdunes thanks. The outside temp was in the mid-30s. Those temps at the register were just with the emergency heat on; the outside unit was off.

your post begs the question why was the outside condenser not running?...not normal...

IMO

I set the 803 thermostat to Emergency heat just to see what the temp difference was between the two situations (both breakers set to On vs one breaker set to On) and that confirmed that turning off the second breaker only allowed 10KW of the 15KW heat strip to operate.

OK then with the HP condenser operating with the 10 KW, you should be getting well over 100 degree supply output probably in neighborhood of 60 KBTUs+...

IMO

Thanks. With the temp outside of 52 degrees and raining and indoor temp of 67, I bumped up the requested temp setting to 70 degrees to engage both 10KW aux heating and outside XL16i. Temp at register was 115 degrees (with aux heat on, CFM set to 1298).

sounds good...too bad you don't have the right thermostat and outdoor temp sensor to lock out the 10 KW heat strip...not necessary for that outdoor temp...

IMO