Want to add water to increase boiler system pressure but stumped

Pictures (from a bit back) to get the whole boiler and piping in. Or as much as possible. A diagram would help as well. Does the fast fill valve work? If so you can fill to the desired pressure about 20 and run the circulators. Depending on the system the air might come out on its own.

You don't need to do anything but lift the fast fill. If the PRV is not working on its own it may need to be replaced. Make sure any valves between the cold water supply , through the PRV and into the boiler are open. Pictures may help if you have a backflow preventer before the PRV that could be the issues.

What you can do in the meantime is get a garden hose and a female x female hose connector (or a washing machine hose) and add water to the boiler by connecting the hose to a cold water supply (outside faucet, washing machine valve etc) to the drain valve on the boiler and fill it that way.

The correct pressure would be 15psi should be enough for 2 stories.

This should get you running until you figure out why you are not able to add water.

Let the boiler cool to 120 or so. Then open the manual valve (counterclockwise) you should here water running into the boiler.

Sometimes the PRV valve may leak by. You want the pressure around 15 psi. If the PRV leaks by the pressure will go up. If it get to 20 shut the manual valve and have the prv replaced. You can run it , bleed it fine just control the water pressure manually until the prv gets changed.. Keep in mind that the water pressure will vary as the boiler heats and cools. When the water is heated it may rise to 22 psi or so. This is normal as long as it doesn't continue to rise

while the pressure is as low as it is,

check the expansion tank air pressure, and check no water comes from the schrader valve, set the air pressure to 12, 14-15, that you want your water fill pressure to be,

check the tank before you add water pressure to the system.

cold / cool boiler, not hot,

I concur with Mattmia. Get enough air in the tank before adding water and bleeding. The tank needs to have enough air so that when the water in the system heats up and expands the pressure won’t change much. That’s the tank’s job.

I have read this entire thread and have some observations:

• You have a classic design flaw that was causing a problem from the date of the original install.
• As a result of this design flaw, there may have been all kinds of attempts to resolve this problem.
• Since each tech that tried to resolve the problem didn't understand the problem, all the attempts have been unsuccessful.
• The final answer was to just close the manual boiler feed valve (black handle Yellow Arrow) and hope for the best.
• As a result of this "Fix" the auto feed valve no longer feeds water and over time may have failed, or may just need to be exercised to get it back working again.
• Set the auto feed pressure to 10 PSI and let the problem design add the pressure gradually.
• Then vent one radiator each day after the pump operates and the pressure will go back to 12 or 15 or whatever the pump differential pressure causes the pressure to increase.  

Common Mistakes explained:

If you look closely at the “Common Mistakes” slide I used in my one day Hydronics seminar, the Auto feed (pressure reducing valve) is connected to the inlet side of the circulator pump.  And the expansion tank is located on the other side of the boiler.  When the boiler was filled that very first time the pressure in the expansion tank was 12 PSI and the auto feed was set at 12 PSI. and all in the world was good.

But at some point the circulator pump comes on and then $#&T stuff happens.  The pump needs to have a pressure difference (∆T) to move the water thru the system.  Now let's say that ∆T is 6 PSI.   And since the expansion tank is the point of no pressure change (PONPC) Google it, When that pump runs and pushes water thru the boiler and it comes out at the other end where the expansion tank is connected the pressure must be 12 PSI there.  Now if we account for some pressure drop thru the boiler, (like 0.5 PSI) then the outlet pressure of the pump can only increase to 12.5 PSI.  Since the inlet side of the pump needs to be 6 PSI lower than the outlet of 12.5 PSI, the pressure on the inlet side of the pump MUST drop to 6.5 PSI to circulate the heated water thru the pipes from the boiler to the radiators.  And everything in the world is good. 

Except the pressure at the inlet of the pump is 6.5 PSI and the auto fill valve is set to 12 PSI and it is connected to a pipe with 6.5 PSI water pressure.  What do you think might happen?  You guessed it… the auto feed will make the inlet pressure to the pump 12 PSI and the discharge pressure from the pump will be 18 PSI.(6 PSI higher).  To do that the Auto feed added water.  Since the system is full of water and there is no place left to add water in the boiler or radiators, the water goes into the air space in the expansion tank.  This makes the cold start air pressure in the expansion tank 17.5 PSI.  This only happens once, the first time the pump starts, and the system will operate at this elevated pressure forever, (or until someone vents some air or water from the system).

Now that the expansion tank has extra water in it, there is less room for thermal expansion of the hot water in the system.  And since the cold start pressure is now almost 18 PSI when the water expands as a result of heating up, there is less room in the tank for expansion.  The hot operating pressure may increase above 20, above 25, maybe even above 30 PSI.  Oops that can't happen.  Above 30 PSI = relief valve discharge.  Now we need to replace the relief valve because it is leaking, right?  Wrong!!!

You need to reduce the water pressure in the boiler so the relief valve does not need to discharge. So just close the black handle valve (Yellow Arrow). The correct thing to do is to move the circulator pump to pump away from the expansion tank. But that is a subject for another book. https://www.heatinghelp.com/store/detail/pumping-away-and-other-really-cool-piping-options-for-hydronic-systems/

But for now, just set the boiler pressire at 12 PSI after you have all the radiators blead.

increasing the precharge of the expansion tank to the new cold fill pressure would also help a lot with that.

There are many ways to fix the problem.  Each one has its advantages.  The simple ones have the lowest cost. The more you do the higher the cost.

You have one major problem now.  Air in the second floor radiators.  Make the system air free now by first setting your expansion tank air pressure to 12 PSI (that must be done with no water pressure on the system side of the expansion tank). Pro Tip: you can charge and adjust the expansion tank pressure with your static pressure (pressure on the boiler gauge) lower than 10 PSI. Next, increase the pressure until all the radiators are vented (no air left). As a child I remember helping my mom do this by opening the radiators and letting the air out until water spilled into a tin can while she was in the basement watching the gauge so the pressure didn't get too high. With all the radiators air free, and the system at ? PSI, raise or lower the system pressure on the boiler gauge to 12 PSI.  Now the boiler pressure and the expansion tank pressure match.  

Now you have a system that is full of water and an expansion tank that can accept the full amount of water it is designed for, as it is heated.   Close the Yellow Arrow valve and you should have good heat for the season.

Next summer you can address the major problem and re-plumb the system for the best performance. Now you don't need expense of a plumbing redesign and all the discussion associates with it.

Sorry I didn’t see your question to me right away. “Get enough air in the tank” is my general answer meaning drain the old-fashioned expansion tank or pump up diaphragm tank to the correct system pressure, usually 12 psi. Here’s the method I used to use for what it’s worth.
Check the pressure of the tank. Release some water to bring the system pressure down below about 10. Pump up the tank to 12. If the system pressure on the boiler gauge increases then release some water. Check the tank again. If it’s below 12 then pump up the tank again and repeat subsequent steps. You’ll probably have to do this several times. When the tank pressure hasn’t dropped, then you can add water to 12. There are probably other ways to achieve this, but the whole point is to be sure that you have one tank full of air at the correct pressure with little or no water taking up space.
The title of your request caught my attention. How to add water? Make sure the expansion tank is right before adding water before increasing pressure!