Getting "grok" with heat pumps as backup heating source

Good Day All,

We live in Central Maine (Winslow). Primary heat source is HWBB, secondary is wood stove on main floor. Open loop of lower floor and location of wood stove make it perfect for our needs. Heats the largely unused dining room up to uninhabitable temperature, kitchen to "a little warm but cozy", living room up to "just right". Then it keeps upstairs bedrooms about 63-65. So the oil fired boiler doesn't work very hard as long as we're diligent with the woodstove. We do have two mitsubishi mini spits, going on 10 years old. We use them primarily on "dry" in the summer months, then ocassionally we'll use the downstairs one to take the chill off for 20 minutes or so if the living room is cold. We really don't like the feel "forced hot air" for heat so we favor the wood stove/hwbb.

So, that all said we use about a tank and a half of oil per year (375-400 gallons). We were just away for a cruise, total 13 days away. I set heat on 60, switched off the indirect hot water boilermate, and (don't usually do this) switched the heat pumps on "minimum heat". We got an oil delivery while we were gone, (say 4 days into the 13 days), but when we arrived home 9 days later, oil guage read 3/4 full!! Now, we did have single digit temps ALMOST whole time away with at least 7 nights below zero. PLUS the woodstove was unused, of course. The "question mark" is this. I put that bottom floor heat pump on "minimum heat" thinking it was a good backup. The temp on the control said "50" so my layman's assumption was that this thing wouldn't kick in unless the machine's thermostat read that the room was below 50 degrees. The HWBB was keeping the room at 60. However, when I went to shut it off, after getting home yesterday, this son of a gun was billowing 50 degree air like it was trying to cool that room down as fast as it could. My layman's brain wonders if, this whole week, that thing was working its butt off to keep our home at 50 while the HWBB was working ITS butt off to keep the room at 60, thereby using a TON more oil and electricity than necessary?? I've NEVER felt competent in my understanding of how heat pumps "fit in" with backup heating. If the room is 60, and I've got it set to "minimum heat" and that means 50 degrees. I do NOT think it's intuitive to assume it will try to COOL the danged room down if another heat source has the room at 60. I would think, logically, it would just lay "dormant"…NOT blow out arctic air like its lie depended on it.

Thank you to any of you who can properly educate me!

Ken