Short cycling boiler. I'm stumped.

Do you see that gray tank up by the ceiling"

Tap on the side of the tank near the bottom with a hard mallet or a soft hammer. You are listening for whether the tank is full of water. If it is full of water, you will get a very dull thud. If it is not full of water you'll get a very soft tapping sound. It is a bit difficult unless you are skilled.

The problem is most likely the tank has filled with water and provides no "cushion" for expansion.

When you start at 15 psi, it should NOT rise more than 20 psi if the tank is working properly.

I am making the assumption that you purged all zones properly and there is no residual air.

Lower the pressure to 20 psi when the boiler is fully hot (190°). Then when there is no demand for heat and it cools all the way back to room temperature, read the pressure and advise.

@LRCCBJ Has done a great job getting you this far. I noticed that you have a Extrol expansion tank. This tank is supposed to keep the water pressure in your boiler form going too high when the water heats up. I see that you got close to 30 PSI when the water got above 180°. That could be a problem with the expansion tank. You are going to learn about expansion tanks today.

So to be clear with your system, when you fill the boiler to 15 PSI, the expansion tank should also have 15 PSI air pressure in it.  That means that there is no water in the tank and both sides of the membrane that separate the air from the water are at 15 PSI.  Your boiler may have about 20 gallons of water and the radiators and pipes may have another 10 to 20 gallons of water in there. So let's just call it 40 gallons in the system at 60° cold fill pressure.  When you get the water to 190° and you add almost 4% more water by volume because the water expanded there is now 41.6 gallons of water in the closed system.  

The only place that water can go is in the expansion tank since the system is closed and the backflow preventer (Watts 9D) won’t let any water out.  So when the 1.6 gallons goes into the tank that is full of air pressure, that water will compress that air as it moves the air out of the way to make room for the water.  The pressure will go up about 2 to 4 PSI. That is normal.   When the water cools down after the heating cycle is over, then that pressure will drop down to 16 PSI or 15 PSI as the air pressure in the tank pushes the shrinking volume of water back into the system.   

I hope you have followed me up to this point.

Now what if the tank only had 6 PSI of air pressure in it?  Whe you put 15 PSI water pressure in the system, the water pressure was higher than 6 PSI and started to compress the air as more and more cold water enters the tank.  You may have added 2 to 3 gallons of cold water to the system that ended up in that tank in order to compress that air pressure in the tank from 6 PSI to 15 PSI.  That tank is only about 4.5 gallons total.  

With the tank more than half full of water when the boiler water is cold, there may only be about 1.5 gallons of air space left in that tank. So as the water expands to 1.6 more gallons of water, there is less room for the water to expand and that air cousin will increase in pressure until the relief valve pop off pressure is reached and you end up with a puddle of water on the floor.  

One point:

Before you go through the effort to refill the tank with air as Ed mentioned above, take the plastic cap off the Schrader valve (on the very bottom of the tank) and push in the pin slightly. Either you will get air or you will get water.

If you get air, follow the procedure as Ed mentioned above.

If you get water, the game is over and you must replace the tank. No need to attempt to add air……….will not work.

As @LRCCBJ said, water coming out of valve is solid indicator of failed tank. You should be able to pick up a new one at local big box.
https://www.homedepot.com/pep/Amtrol-No-30-Expansion-Tank-for-Hydronic-Boiler-EX-30/202268962

@PC7060 said water coming out of valve is solid indicator of failed tank. You should be able to pick up a new one at local big box.

Ouch. Water from the air side of the tank is a definite sign that the tank must be replaced.

There are two types of expansion tanks You want the one for a closed system that comes pre charged to 12 PSI. They are usually Gray or Green. The other type is for an open system like your hot water heater and comes pre charged with over 45 PSI Pressure. They are often White

When you get your replacement tank, you want to fill the air side of the tank with 15 PSI because the Watts 1156 will make the water side 15 PSI. Check that air pressure before you install the replacement tank. That way you will be sure that you have it set correctly.

For now you can still operate the boiler but it will discharge water thru the relief valve if the pressure gets above 30 PSI. Don't worry, it is only water. You can close off the manual feed valve for tonight so you don't keep adding water after every overpressure event. The operating LWCO will protect the boiler in the event your water level gets too low.

I would have used a brass nipple between the PRV and the tee but you already have it on so no big deal.

It's recommended by the boiler and expansion tank manufacturers to check the expansion tank air pressure annually. I do this on all of my boiler maintenance jobs and I have found that the tanks normally lose a few PSI every year. I'm surprised if I find one that still has the same pressure in it as it did the year before.

Does the pressure switch have an adjustment on the side of it? That may need to be changed to get this to operate properly.

Looking at the instructions, that pressure switch is not adjustable

  ALSO. Jumping P1 and P2 is only a temporary test. it is not a fix.   This test will prove the pressure switch is the cause of the short cycling or it will eliminate the pressure switch as the source of the short cycling.