We have a deep water well with a new submersible pump. I was getting ready to install a new pressure tank and noticed that the old one is placed after several fixtures, including the hot water heater. Everything I see online indicates that the pressure tank should be the first thing in sequence after the water line comes into the house, with everything else following after. A friend says that it doesn't matter, but I can't wrap my head around that answer. Should the pressure tank be first in line or not?
The pressure tank is just a reservoir of water to ensure stable pressure as the pump is not a constant pressure pump.
The situation without a pressure tank.
Let's say you open a tap at .5 gallons per minute, pressure will immediately drop and when the pump comes on at the low event let's say 60 psi pumping 5 gallons per minute. 10 times the demand, the pump will verify quickly reach the high event, we'll say 80psi and shut off. Back down to 60 on back up to 80 off. So on and off continuously and frequently. Maybe 15 times in a minute.
With a pressure tank
All things being equal, at 60psi the pump cuts in but the excess flow gets bufferd into the pressure tank. Once it reaches 80 psi and cuts out there is a reserve of water stored in the tank. The pump can remain of longer and once the low pressure event is reached and it cuts in it can stay on longer. Longer cycles are better.
All that being said, the actual volume of water transferring in and out of the tank isn't all that much. It's the water compressing air that gives you the buffer. But water does transfer in and out. If it's piped in far downstream where there is a pipe reduction or sidewall restriction due to length there will be a lag in the flow causing fluctuations in pressure. Depending on the specifics of how it's piped in, it's possible to have hot and cold water stacking due to cold water being pumped into the hot side of the system.
So in reality it not absolutely critical to have the tank first but it is conventional and more sensible. Even if you had a leak it's problematic. Let's say you have a bad leak in your 1st fixture before its shutoff. Which valve downstairs do you close? The one just upstream of the pump? Where is the pressure device? Can the pressure drop from the leak now cause the pump to dead head into a closed isolation valve? You still need to isolation the tank from the other side of fixture number 1. Are these things you want to be thinking about when you have a big leak happening?
TL;DR: that's really, really weird.
It makes no sense for the pressure tank to be after the hot water heater. That way, if you don't draw hot water for an hour anything hot in the pressure tank will have cooled to ambient temperature. You won't get any hot water until the tank has exhausted enough to start the pump, and even then your incoming hot water will be diluted by the remaining cold water, which means the water from your faucet will continue to slowly heat up.
It also means that water only moves through the heater while the pump is on, so that it will rush in and out, probably stirring the hot and cold together, instead of introducing the cold at the bottom and taking the hot from the top.
Even better, if you draw cold water, you'll actually be pulling cold water from the pressure tank into the top of the hot water heater, and then lukewarm or hot water from the bottom of the heater into the cold water line (at least until the pump turns on). More temperature confusion.
Having other fixtures between the pump and the pressure tank is less of a problem; it just means that the water flow may reverse depending on whether the pump is on.
IN actual performance it does not matter where the pressure tank is installed in the well water supply line. Normally, it is piped first but if it 2nd or 3rd it won't make much difference. The Normal piping arrangement of the storage/pressure tanks is for a "TEE" fitting to be installed in the tank inlet/outlet which is the same fitting, so if the pressure tank is not the first device it really doesn't matter. However it is just not the recommended or accepted way to pipe the tank.