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Getting the right pressure on my radiator system

I have old cast iron radiators that are in great shape. My house is about 118 years old and has three levels. The radiators were probably installed sometime in the 20's or 30's. A new boiler, a Raypac Type H residential boiler system, was installed in 1988. I'm pretty sure I know how to fill and bleed the system. In fact, I filled and bled the system over the summer after our construction project was completed. I couldn't get the pressure to reach the suggested 12psi. I got it to about 5psi but it was summer, so I didn't worry about it. Over the next several months, I checked the pressure from time to time to see if it has fluctuated, and the pressure remained the same.......until I turned it on a couple of months ago. The pressure went to zero, so I filled and bled the radiators again. Once again, I wasn't able to get the pressure high enough, and in a few days, the pressure went to 0 again. My questions are:

1) Am I filling the radiators the correct way? Once I fill and bleed the radiators, I turn the water off right away. Should I keep the water on until the pressure builds?

2) I read in a website that I need to pull the pressure relief on the boiler to make sure the boiler is full. Is this correct? If so, what does a pressure relief look like?

3) I went on the Raypac website, and it says "Open valves for normal system operation, fill system through feed pressure
regulator to minimum 12 PSI. Manually open air vent on
the compression tank until water appears, then close
vent." Huh?!! Compression tank? Can someone explain in layman terms?

4) I heard that it is normal to fill and bleed the system a couple of times to get all of the air bubbbles out. Is this true?

Thanks for your help!

Re: Getting the right pressure on my radiator system

Sounds like you may have a leak---from the boiler itself, or the associated piping.

At this stage in your learning how the boiler works, it may be best to have a boiler technician from a local service company come in to show you how the various components work with you standing right next to him so that you will be able to do it by yourself---in the meantime, the boiler should be shut down & not used if you are unable to maintain at least a few psi of water pressure on the gauge---to do so will crack the combustion chamber/heat exchanger, which may have happened already.

Once you learn the basics, it's an easy process to go thru the steps each time you have to service the system.

If the boiler pressure gauge keeps falling to zero, check all the piping, & check under the boiler, etc., for any leaks---if the piping is visible thru a crawl space or cellar access, check them at those points as well---take a flashlight & peer thru the observation portal of the boiler to see if you can detect any water dripping, or water inside the combustion chamber.

Some of the sites below have diagrams of system components & how they work; refer to the diagrams & make a note of where they are located on the internet.

At the Inspect-NY site, click onto the "parent directory", then onto "boilers,heating" in the left-hand column----a diagram of most of the components of a hot water heating system will appear---continue to scroll down this very long site to view more components & info on how boilers work.

Although a system as old as the one you have may have different components, it's best to start with the "house" water pressure---this is the water pressure you get from the taps when you open a faucet at the sink, etc.

If you have "city water" coming into your house, this may be anywhere from 30 pounds per square inch, up to 60 pounds per square inch (30 ppsi, 60 ppsi); if you have "well water" the pressure may be somewhat lower.

You will need to buy a low-cost water pressure gauge at HD/Lowe's, etc. to attach to your garden faucet to determine how much "house psi" you have.

The starting point of your system is this "house water pipe" that has probably 30-40 psi & ties into the near-boiler piping, then immediately into a component called a pressure reducing valve (PRV) this PRV reduces the water pressure to the standard 12 psi that all hot water boilers (and their piping) use to run the hot water heating system.

There is also a relief pressure valve (RPV) that will open & spill water onto the cellar floor in the event that water pressure in the boiler exceeds 30 psi---this is a safety device all HW boilers have & a little water on the floor won't hurt anything.

You should be able to look at the "water pressure gauge (dial)" on the front of the boiler to determine what the current water pressure is; under normal conditions (without the boiler running) it should be approx. 12-15 psi.

If the psi is lower than this, the PRV can be manually adjusted by turning the top screw until you get this basic reading on your boiler gauge.

Once you have 12-15 psi, you should be able to fill the radiators by simply opening the air vent on each one until water comes out of the vent valve.

If you have trouble filling the upper floor radiators, you will have to measure how many feet exist between the boiler, and the top floor radiators---if it is over 30 feet, you will have to slightly increase the psi by adjusting the PRV so that you have 17 psi on the boiler dial.

Please post back if you run into difficulties while attempting to fill the radiators.

The Expansion Tank (same as Compression Tank) is a component that must remain 1/2 full of water and 1/2 full of air---since water expands up to 5% when heated, the expansion tank acts as a coiled spring to absorb this extra water volume so excess pressure doesn't damage the system.


Re: Getting the right pressure on my radiator system

What Nashua Tech said plus more:

Assuming your gauge is okay, the pressure reducing valve should easily be able to reach 12 psi or more. There is a screw with a locknut on the valve to adjust the pressure. Loosen the locknut and turn the screw clockwise to raise the pressure. Give it 5 minutes at the most. If you accidentally add over 30 psi, the relief valve will open and let you know. If the pressure still won't raise above 5 psi, look for a large hex nut on the regulator about 1" across. If it has one, shut off the water supply and remove it. There will be a cylindrical sediment screen behind it which may need cleaning.

When working properly, with 12-18 psi, bleed the air from all radiators and open safety relief valve (pop off valve). If you don't have a relief valve, do not run the boiler. After you have removed the large amount of air, run the circulator pump and bleed everything again. Never run the pump with out water (if it is full of air for example). After you stop finding air, shut off the water supply and let the system operate for a day and check it again.

Although it is theoretically possible to leave the water make up on year 'round, it usually ends up with the safety relief valve having nuisance trips every week. You should not have a need for automatic make-up unless you have a leak. These should always be fixed ASAP to keep from adding more and more minerals to the system.

Widely fluctuating pressures on the system indicate an expansion tank problem. Ray Pack's instructions are for commercial expansion tanks with sight glasses. Your won't have one. If it is thin sheet metal, it is a diaphragm tank. If it old riveted steel, it is a standard old tank. The diaphragm types have a neoprene membrane that separates the air half from the water half. With no pressure on the system you can add air with a tire pump to 12 psi though the tire valve on the end. If water comes out the valve when you push in the stem the membrane failed. The diaphragms fail on a regular basis. A standard tank can become "water logged" and will have to be drained so it can act as an air cushion and allow for expansion of the water when it heats up.

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