I lived in the third floor of a building set up like this.
If you are the landlord and paying for the heat, you want the thermostat in the top floor apartment, at a high-ish temperature (maybe 72*F) and let the tenants on the 1st and 2nd floors deal with it.
If you are a tenant, and the landlord is paying for the heat, you want the thermostat on the first ...
This is a common problem in NYC. There's no perfect solution.
What I've done is put in an electronic thermostat with remote sensors on each floor.
Then, using the data I've collected, I tell the thermostat which devices to obey at each time interval.
In general, each floor is a degree or two off from the other ones, but the top floor can be much hotter/...
Generally you want to locate the thermostat where the inhabitants spend the most time. For example the living room.
But it's always going to be a compromise when trying to use a single thermostat to control an entire structure.
There may be another issue with that installation. Red pex is typically used for domestic hot water, and does not have an oxygen barrier.
Now it is possible that all of those red pipes are domestic hot water pipes, but without more context it is difficult to guess what is for what.
Yes, you can replace parts of a hydronic system with pex. Just make sure you use the right kind of pex pipe. There is special pex for hydronic heating systems. In addition to that the wall sizes for pex pipes may or may not line up with the wall size of the various kinds of pipes in your system, so when converting you would need to make sure that you kept ...