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The system in my house has an air trap that vents any air out of the system. It's mounted on top of the expansion tank. There's a vent device on top of the air trap that has a tiny float in it. If there's air in the trap, the vent opens. If you are going to replace your relief valve, MAKE SURE YOU GET THE RIGHT KIND. They are different than the ones used on water heaters. The valve will be rated at 30 psi max. To change out the valve, first turn off the water supply to the boiler. Then locate the boiler drain, connect a hose to it and drain the boiler. There should be some valves in the system loops that you can open to vent the boiler, open just one of them. After you replace the relief valve, close off the drain and fill the system. Try to vent as much air out as you can through the same vent valve you opened earlier. Once the system is full, fire up the boiler and start circulating water through the system. It will take a couple of cycles to get the air out. The system will be noisy until all the air is purged. Don't be surprised if you get new leaks after you replace the relief valve. Both the boiler drain and air vent are likely to leak; if this happens just drain the system again and replace them.
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