Must a condensate drain line have a low point, a trap?

 Express the Subject another way, in a Google search with AI:

may gravity-drain condensate lines loop with a low point?

Yes, a gravity-drain condensate line can include a loop with a low point, as long as the overall slope of the line still allows for proper drainage by gravity; the low point essentially acts as a "trap" to collect any debris or sediment that might otherwise clog the line further down. 

This result is correct. A trap may exist. It is not commanded or desirable. Condensate is acidic and foul, and over years unflushed, can plug the line. In usual PVC, the fouling is not visible.

Many HVAC mechanics think a trap is MANDATORY, and will glue-in this PVC trap. They will then plumb this to a transfer pump, even where a gravity drain was easily achievable..

This is the starting condition of a situation I have improved. See the undesirable PVC trap in the upper drain line of an air conditioning heat exchanger. The high-efficiency gas furnace condensate drain dumps directly to the pan of an ugly condensate pump. The pump pushes foul water up and over thirty feet to dump outside through a drilled hole in a rim joist.

There was never a need for the PVC trap.

All will be rearranged except the supply registers and their approaching in-wall ducts.

Now toss the PVC pipes and pump, and dump as trash. See a convenient plumbing drain that will accept gavity drain of condensate via a new P-trap.

With this arrangement of joining the two condensate paths, a trap exists in the furnace condensate line. I know it is nonsense that this could alter the dripping-down in  the exhaust pipes or affect furnace performance, I will eliminate the trap.

As the minimum required for all drain piping, and as limited by availble elevation change, I maintained a steady 1/4 inch per foot downward slope to disccharge atop a P-trap

I then swapped out the furnace-drain plastic 1/2"  MPT x 3/4" barb fitting for brass, to stop a flow pouring along the barbs.Unbelievably, the barbs are formed as spiral threads. I can't just return the fitting to Home Depot. I have reported a manufacturing error, homedepot.com, Feedback, asking a product recall. I believe the pipe threads also leak unavoidably from roundedness and inability to develop sealing pressure of engagement. Cast polypropylene may be always inappropriate for fluid hose pipe thread fittings.

This is the homedepot.com product image. Do you see the spiral threading of the barbs?Defective, for all of a batch.

4.5 stars in 56 reviews. There is no means of offering my own review. But, I have called the manufacturer.

Here are rival and contradicting Google AI search results:

Must a condensate line have a trap?

Yes, a condensate line must have a trap, especially if the drain is under negative pressure, as it prevents conditioned air from being blown back into the living space and is crucial for proper drainage and system efficiency; essentially, a trap is considered a necessity for a condensate line on most HVAC systems. 

This is an incorrect rule for HVAC condensate. The search demonstrates current Google AI bias to the affirmative of a question; the supporting of preconceived notions that makes a searcher feel good and click more.

Household drain piping is vented to atmosphere and can not be under negative pressure here. Pipe internal total pressure must be equal, atmospheric, at both sides of the trap.

Must a condensate line drain without looping?

A condensate line ideally should drain without looping, meaning it should have a consistent downward slope to ensure proper water flow and prevent potential clogs caused by water pooling in a loop; however, depending on the layout of your system, a small loop might be necessary, but it should be minimized and designed to avoid trapping water. 

This is more-correct, and it is what I practice.

And yet, know that permissiveness of a trap in a condensate line is a very positive thing. With stiff hose material sold in a coil. straight-running at modest slope may be hard to achieve. A complete loop for reach flexibility, is allowed. Do not prefer, opaque stiff and fragile PVC, for trap avoidance. Prefer serviceable connections without glue.

Here is another gravity-drain condensate lines example with an upflow high efficiency gas furnace:

In both examples, the found A/C heat exchanger condensate line has a P-trap, and the furnace heat exchanger condensate line does not. Why then, a trap, ever?

Here, PVC drain piping serves better than flexible hose.

Here see a rare example of better efficiency of heat transfer to air, with intermediate hot water, Lennox Complete Heat. It is hydronic home heating. Want to know more about this integration of domestic water heating, and home heating. Wonder why this was discontinued by Lennox. Wonder how this relates to electrification, in future fossil fuel bans. Tentatively find that the integration makes sense with geothermal heat pumps. See that this unit is serving reliably.