Will my propane furnace prevent freezing when the boiler is off?

Is it an outdoor unit, or in a dedicated building?

TS

It seems like it wouldn't freeze, but I can't comment as I don't have any expierence with WAHX.

TS

if your pump is wired to run 24 -7 then as long as the water is moving you will be fine.

In the 5 years I've run my system I've never had a problem. I've went out of town at least a week every winter and just let the circulation pumps run and I have straight water in my system. I have an inline temperture gauge in my basement and a temp gauge on my boiler and the water temp when I get home is always right around 60 degrees. If you think about it, the water is flowing all the time, the boiler and all the lines are insulated, plus the water is flowing through the heat exchanger in the plentum. So there is almost no way it should freeze. Just my experience.

If you are circulating the water 24/7 and it is passing through your water to air HX you should be fine. If anything I would worry about it stealing too much heat and wasting propane. The first few days after I installed my HX, I had the circ just running continuously. We went out of town just for a couple days in very cold weather and it kept the boiler pipes in my pole barn about 70. My electric HP puts out much colder air than your propane furnace would too. If you think about it the HX should perform this well. If you are away from home frequently, it might be worth having and aquastat cycle the pump only as needed to maintain 55 or so but it's propably not worth the effort. I already have an digital aquastat on my UG lines for another reason, that I can temporarily re-wire/re-program to maintain the loop temp from freezing if take a long vacation in the winter. My boiler is 350ft away too and yours is 5. One thing to check if the pellet boiler has a thermic bypass loading valve (Danfoss, Thermovar, etc). Not sure if your Maxim would have one of these but gassers use them to keep the return temp the boiler sees above 140*. With the boiler not running, these would be closed to where the water is only short cycling through the boiler and not back to the house. Others folks here have installed bypass piping with valves around the thermic valve for vacation purposes that allow the water to flow correctly. I would recommend you do a test to make sure you water is flowing through the house when the boiler is cold. As a precaution, I also wrap heat tape around the Danfoss valve and where the water does not flow since this valve is about $250.

It doesn't apply to your situation directly, but my gas boiler (basement) and wood boiler (in an outbuilding) share the same water, so all I do when I leave town is turn the gas boiler on and close the valves leading to the wood boiler cracked just enough to get enough thermosiphon going to keep the lines and boiler above freezing. I also have a circulator on the wood boiler for mixing to prevent thermal shock, and I leave it on 24/7. I don't need to crack those valves very much to keep everything about 60 degrees.