Sorry for any confusion. I wrote my initial post when I was a little tired, and so perhaps it wasn’t as clear as I intended. The guest who said their room was “icy cold” at 28 degrees and the guest who left a review stating that the house took days to warm up are not the same. I wanted to start off with a humorous anecdote but possibly failed in the execution.
As others have noted, guests can sometimes be a little strange, and whether it’s requesting heating in the middle of summer (which has happened to me, too!), or thinking that the heating needs to be turned up to 30 degrees at minimum, we all have a story that has left us perplexed. I’m fortunate that this is my first review noting an issue with the heating, and my concern was how to reply to it. However, I’ve received 3 new bookings since it was posted, and so I think my worries may have been unfounded.
I’ve definitely had a few guests who have expressed an issue with the heating during their stay. Generally, it’s something that falls into the category of unreasonable or uninformed. I just hosted a guest who requested a “portable fan heater” for one of the bedrooms. I discovered they didn’t know that there were temperature and output settings on the heater they could adjust. @GutHend’s advice was very informative (thank you - if I could give you a badge for effort, I would). However, as @Inna has noted not all complaints raised can be addressed by making improvements to your listing, or tactful hosting.
I completely agree that my expectations of what is warm may not be the same as that of my guest, that I need to try to accommodate their level comfort (where reasonable), and that any instructions on how to use the heating need to be easy to understand. As I’m certain they are, I’ll try to think of ways to make this information more prominent than it already is. Guests don’t comment on the difficulty of the instructions, they just tend not to look at them at all.
I think this may be my last post, however please do feel free to keep the discussion going. Stories about how guests would like to sleep in a furnace are always comforting to read (and amusing). 
Thank you again to everyone who contributed!
(A lot of people don’t seem to know what a heat pump is, which was a surprise. To offer some clarity, it’s this! The remote control is exactly like ours.)