Welcome to Backyard Chickens (and more importantly to the Ducks sub-group!)!
We currently have six silver Appleyards (large breed) in an elevated duckhouse accessed by a pretty long ramp. The duckhouse is ~4'x8' and the attached run is 12'x8' (with an additional 4'x8' under the duckhouse). We rarely lock them in unless a serious blizzard with gale force winds is going to coincide with negative (F) temperatures. We've been in the single digits a lot the last couple weeks at night, and our guys will all go into the duckhouse for most of the night (although they will get up to get a drink or a snack sometimes). During warmer temperatures (but even below freezing), they are more inclined to be up in the middle of the night and in the run. Regardless of temperature, they generally will all end up in the duckhouse sometime in the early morning hours during the winter. During other seasons, sometimes one or more of the girls will make a nest in the run, but usually they go into the duckhouse to lay their eggs.
I would not be worried about 20F temps. We will often see our guys hanging out during a snowstorm in the only corner of the run that gets snowy...lol.
A couple things to consider:
We did institute a routine early on where they free range supervised in the evening, then get a soup for dinner, and then get a mealworm "flock party" in their duckhouse. When called, they come running back to their duck pasture for soup and then usually hurry into the run and up the ramp for the flock party. This behaviour, which we continue to reinforce, has been very beneficial.
The only other thing I will add, which is NOT relevant to your situation at present, is that we added a Sweeter Heater last winter to the duckhouse. A Sweeter Heater is radiant heat and does not have the same risks associated with a heat lamp. I only mention this because I wonder what role it plays in encouraging the ducks to go in/stay in. They don't all use it, and sometimes none of them use it, but we have a girl who is molting right now, and she's been using it on single digit nights. It's also nice for eggs and gives the older ducks a bit of a boost. Is it necessary? Absolutely not, but we do a bunch of things for our domesticated animals that are not necessary but may provide benefits.
Good luck and keep us posted!