I have read many posts about the Furrion tankless water heater freezing/failing after attempts at "winterizing". There are many posts (and videos) on the net about the "correct" procedure to do this, including some from so-called reputable sources. After careful study of the unit, I have come to the conclusion that some of what is posted is incomplete at best, and some is downright wrong at the worst. (There are some who have it right, I think.) I took the liberty to annotate a couple of pictures that may help some people see the issues and perhaps understand the solution.
View attachment 48964
View attachment 48965
View attachment 48966
The are 2 places that may hold residual water, even after valiant attempts at winterizing:
1) The copper tubing in the lower portion of the heat exchanger.
2) The infamous "mixing" bowl.
Starting with the copper tubing in the lower portion of the heat exchanger, even after opening both the CW inlet screen and the so-called low point drain, water will remain in this lower portion. It is essentially designed as a big "water trap", as both the CW side inlet and the HW side outlet tubing are well above the remaining lower portion of the heat exchanger tubing. In my mind, the only way to purge this water is ...
1) Leave the Nautilus in City Water mode (eg do not bypass the water heater with the red valve).
2) Leave the CW screen in place. If you remove it, pink antifreeze will gush out making you think the tubing is clear, but alas it will not be since this opening is BEFORE the heat exchanger tubing.
3) Open the water heater HW side low point drain (the bottom most copper drain plug in the lower right of the front of the unit).
4) Pump antifreeze into the city water connection on the Nautilus until it flows out of the HW side low point drain. This will hydraulically purge ALL of the residual water out of the copper tubing.
Moving to the mixing bowl ...
1) Put the HW side low point drain plug back in. If you don't, none of the antifreeze you apply will push anything out of the mixing bowl. You will see pink antifreeze coming from this drain, but none of it will be going back up through the mixing bowl inlet pipe and purging the residual water there.
2) Open the nearest HW faucet (bathroom, kitchen whatever). You must purge the mixing bowl through this faucet.
3) Pump additional antifreeze into the city water connection on the Nautilus until 1-2 quarts of pink shows up in the sink with the open faucet. Don't count clear water, just good strong pink stuff. Unfortunately, you cannot do a true hydraulic purge of all of the residual water in the mixing bowl in this (or any other) way. This is due to the vertical inlet tube that empties into the mixing bowl near its top, creating a "dam" that holds water in the lower portion of the bowl. What you can do is use the mixing bowl to "mix" the antifreeze and the residual water in the mixing bowl to the point it is mostly antifreeze, and little water. The 2 are soluble, so they will mix and drop the freezing point of the residual fluid in the mixing bowl most of the way down to the antifreeze level, if not quite all the way.
Now you can remove both the CW screen plug and the HW low point drain plug if you want to drain some of the antifreeze out of the unit. There should be nothing but antifreeze left in the 2 "problem" areas at this point. Or, just leave it all in there.
Of course, an even simpler procedure to clear both trouble spots is to leave the HW heater in the system at the Nautilus (red valve up, City Water configuration), open just one HW faucet nearest the HW heater, pump the pink stuff into the city water inlet until a quart or 2 shows up in the open faucet sink. Don't count the clear water, just the strong pink colored stuff. This will purge all the residual water out of the heat exchanger copper tubing, as well as mix the residual water in the mixing bowl with enough antifreeze to "dilute" it to a low enough freeze protection point. I would do this LAST to avoid purging any residual plumbing system water back into the completed/protected HW heater.