Water – What could cause a water pressure dip at the cut in pressure

First, the square footage of the house doesn't mean a thing in this case. The important factors are water pressure and supply line size and the height of the fixtures above the pressure tank and the pressure tank size. The well pump fills the tank, then the air bladder applies pressure to the water in the tank. a check valve stops water from going back towards the well. When you call for water, the internal tank pressure pushes it through the pipes.

A few different problems may exist here that would cause your problem.

The tank pressure membrane may be waterlogged. It will still show static pressure, but drops to a very low PSI when a faucet is turned on. Often, you will hear the pump cycle more that usual in this situation or you may see the water pulsate instead of a steady flow. What is your static vs open line pressure?

Another common problem is the backflow check valve is leaking. This causes water to be pushed back towards the well. the tank pressure will slowly decrease between pump runs.

If this problem has been long standing, then the pipe size going to the second level may be too small. Second floor fixtures or ones a longer distance away should be fed with at least 3/4 inch line, not 1/2 inch line.

The last factor may be that the well is not delivering enough gallons per minute. If the pump has to run for excessive time to fill the pressure tank, this could be the problem.

I'm sure there may be a few other items to check, but these are the most common problems.

Allowing for the possibility that it could be otherwise as diagnosis via internet is imperfect....

I see both a pump problem and a check valve problem here. While they may be located in the same place (if your only check valve is on the pump - which is the way mine is set up) they are not the same thing...though I also see another possibility that would do both.

• Pump problem - pressure not getting above 27 PSI, or 15 with a faucet open.
• Check valve problem - when you shut off the pump the pressure drops to zero - unless you are using water, the pressure should stay at 27 if it was pumped up to 27. When you only had 10 PSI in the tank, there should have been some water storage - when raised to 38, no water could possibly make it into the tank, since that's designed for 40/60 PSI operation and won't take any water until the the water is over 38 PSI.

The third possibility - you have a leak in the pipe above the pump and check valve - so the pressure is limited, and the water drains away from the leak without the check valve being to blame. If you can go and listen at the top of the well while someone else turns the pump on, you might actually hear this leak, if its in the well above waterline. Or you might have a recently soggy spot in the yard if it's after the pipe leaves the well.

While I don't think it's to blame in this problem, I'd also suggest rechecking your tank pre-charge to see if it has stayed at 38 PSI or started to fall again. Often when the precharge becomes low it's not simply a matter of needing to add air - it's a failure in the bladder or diaphragm.