That sounds normal for an undersized system.
There is a temperature, called the "balance point" for every heat pump and house combination. This is where, as you have found, the HP runs continuously to offset the heat loss of the house.

The heating requirements in St. Louis are moderately hard for a heat pump while the summers are not so bad. Typically, HVAC installers size the systems for air conditioning load so they end up too small for adequate heating. When this happens, it's quite common to have a balance point in the mid-30s.

Your Trane XL16i is a super efficient, two stage heat pump. Because of the two-stage operation, they should have upsized it by one full size, so that it would be able to supply more heat with the second stage and leave the first stage for air conditioning. I'm really battling with all the HVAC folks to change their conventions because they're doing nobody favors by undersizing two stage heat pumps.

Right now, you're stuck and you will have very high electric bills because of the backup heat requirements. A couple questions:

How old is your home?
How well insulated is your home?
How air-tight is your home?
Are you insulating your windows (heavy blinds help a lot)?

With most older construction, you can save enough energy by really beefing up the attic and sometimes wall insulation and by air sealing, that you can do the same thing as increasing the size of the heat pump. You'll use less energy, the temperatures will be more even, and you'll be more comfortable.

However, if it is a new construction, with plenty of insulation and relatively tight, then, it's quite likely that the only solution will be to replace the unit with a larger one.

moose

from your post, it is obvious that system is operating differently from when first installed.

reasons would just be speculative so I suggest a check-up from installing dealer.

1.refrigerant charge
2.possibly running in single stg and not 2nd stage when demand is there
3.problem with aux heat engaging when required
4.faulty thermostat

the idea that HP runs non stop in above freezing outside temps is simply not normal.

IMO

The XL16i had a run of reversing valves that would not switch over. Sometimes it would happen a couple months after it was installed, and sometimes it would take a year or so when it would stick. If it is the 410A system, the reversing valve wouldn't get enough pressure in the first stage to switch over, and it would get stuck between the cooling and heat position.

Thanks for all the responses. I just tried to post this response but it doesn't look like it went through. If it did, I apologize for the duplicate.

My house (1900 sf) is 17 years old and is fairly tight. One room gets a little drafty from a set of large window, fireplace, and door leading to the garage. The HP is a 3 ton unit charged with 410A. I looked at Lennox before selecting the Trane and received bids from a total of 4 contractors (2 Trane; 2 Lennox). All 4 contractors recommended a 3 ton unit. The contractor who built the home installed a 3 ton Janitrol system. It ran pretty good but I did notice a reduction in electricity usage with the Trane system.

Had the installing contractor out yesterday for a look. He thought the defrost cycle was too short because there was some ice still on the unit at the end. I have noticed cold air blowing for about 3 minutes during the cycle; cycle time was about 3-4 minutes. Contractor acknowledged that the air was too cold. He confirmed that one bank of elements came on during the cycle. He has it now set for two banks to kick in during the defrost cycle. Don't know how hard that will hit the electric bill. He checked the stat and indicated it was OK. He indicated he needed a warmer day to check the charge. Don't know what to think of that.

Temp now is low 40's and the HP is cycling. Air flow is low so I'm assuming it is running on the first stage. My general observation is the unit seems to run non-stop when the temp is in the mie-30's and lower and will cycle with temps in the upper 30's and higher. Makes me wonder if the HP has the capacity to heat the home.

what size and number of heat strip elements and how they are wired for defrost function and normal HP operation with auxilliary heat strips?

2nd stage of HP operation should be checked to see if it is operating correctly.

defrost function needs to be checked...

system should be checked out by an XL16i Trane HP pro.

IMO

There are 3 sets of elements. Beyond that I don't know the size or how they are wired for defrost function and normal HP operation with auxilliary heat strips. Is there something from the model number that will give me element size?

size of elements determine BTUs.
a 5 KW element produces about 17 KBTUs

your system needs checking out....

IMO

Thanks for the input. I will be following up with the installing contractor after the holidays. They indicate they would contact Trane to get their input.

Do you know if Trane certifies their contractors. The guy who installed mine also installed systems for two of my neighbors. One system is identical to mine. That homeowner doesn't seem to have the problems I've experienced. This would suggest equipment and/or installation issues. After having their service techs out I wonder just how much they really know about the newer systems.

The "three banks" of heat are probably 5kw each for 15 kw total auxillary. All 15kw should energize during the defrost cycle to temper the cold air. For a 3-ton system using 1200 cfm, each 5kw of strip heat is +17F from the supply air temp. If the defrost cycle produces 45F air, then 15kw of aux would add 51 degrees, so the net air temp is 96F. 10 kw of aux during a defrost cycle would have an exit air temp of 79 degrees which is cool to the touch.