I tuned a piano for the third year in a row for a little country church. It is a little one room frame church built over 100 years ago. In this church they have an older Everett. This church has no running water. The heat/ac is only turned on for Sunday morning services. The rest of the time the heat/ac is turned completely off. During the week what ever the temperature is outside, it is the same inside the church. I tuned this piano about one year ago. I always come expecting to have to do a large pitch raise, however that is never the case. This year, no key was more than 8 cents off. The entire bass section was dead on. The elderly gentleman who let me in, sat patiently and watched me tune the piano. After I was finished, he asked "Do extreme changes in temperature and humidity affect pianos all that much." I kind of chuckled and said, "Yes, normally, however this piano doesn't seem to mind." To the pianos defense, it does have a dehumidfier rod that a tech installed years ago. It probably needs to be replaced with a newer one along with a humidistat, however I may just leave well enough alone.

Nice pictures!

It does not sound so surprising, I dont know your climate but moisture is raising the pitch, then in dryer confitions (not likely to occur for long) it lowers.

Then no heat during cold extra dry winter days is way better than heating.

2 hz is quite something yet , may be they pound may be the strings will break, probably you tuned at another season.
I have seen similar old pianos keeping 440 <41 for 2 years and with very long travels in trucks, unheated stockeage then heated, playing by an automat so I know the piano was not left unplayed.

I admit i was even more surprized than you, probably, because originally the piano have been PR abd tuned tge same day.(plus pins where somewhat slippy on some notes).

Conclusions
Old pianos can have an extreme stability, particularely once you can obtain an extra firm (active) pin setting.

To be honest the high treble have fallen, not unison, but the pinblick had unglue from the back there, long threaded bolts took care of that aspect before
tuning.... May be next time I will find it really off, but I doubt of thst.

BTW It is a 1904 or 14 old American player... Very well build. (warm tone, if only we change hammers it will be even better but as they are, juiced and voiced fast , a real pianist can make music on it without being embarrased.

I once tuned an old, completely shot, Ivers & Pond tall upright that hadn't been tuned in who knows how long. It was five cents flat... I was shocked. Compare that to the Yamaha C7 I tuned at one of my churches over the weekend: the bass was 10 cents flat, slowly tapering up to six-ish in the treble.... and I tuned it exactly one month ago! Hopefully, my speech about filling the Dampp-Chaser system (that I re-installed last month) sank in so that doesn't happen again next month!

Because old soundboards are rarely very reactive to moisture changes, the instruments tend to stay put. (someting about inner stress of the panel and resiliency of old wood cells)

Ryan you must make a written notice to the elder of the church. Here you need to elaborate on the negative events that could lead to a complete loss of the piano. A servicers of church need to install electric oil cooler in the winter time. Otherwise parishioners would be left without Sunday Mass...

Many years ago I visited the beautiful Sharon Temple in Ontario, Canada. It's an unheated frame building which a breakaway sect of Quakers built for their charismatic leader to use as a study and retreat. One of the issues that led these Quakers to break away was their leader's love of music, of which other Quakers of the time did not approve. (The many modern Quakers I know love music, but this happened back in the early 19th century).

Anyway, the temple had an ancient Steinway grand -- maybe a square grand, I can't remember now. I sat down to play it and was amazed that it was in quite acceptable condition. Now I know that relative humidity changes, and in the southern Ontario climate especially very low RH in heated buildings in the winter, are the really deadly ones for pianos. (I guess constant very high RH like the US Gulf coast experiences would also be terrible.) This piano had never been in a heated space.

Leave it like it is, non-regulated dampchaser and all. Those Everett Studios are pretty stable, almost as stabel as the Hamiltons.

That stability is surely due to the lack of winter heating for long enough to dry the piano. I have seen big uprights which were just that stable, as well.

Just wait, however. One day someone will come up with the bright idea that they need to run some heat all the time in the winter, and ruin the long term equilibrium of that beautiful old Everett. Then you would not only need the humidistat, but the whole Lift Saver System.