Replacing fittings on dampen space heater?

our water boiler is slowly leaking from the top fittings. We get all spanking new parts to replace them and have drained alot of sea withour hose. We are still getting water coming out when we loosen the fittings. We have the main hose to the heater turned sour. Do we have to drain the intact tank.? It is peaceable a bit of water coming out when we loosen the fitting.
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Answers:    Open your hot river tap surrounded by your kitchen maybe, and relieve the pressure on the container.Maybe this will help you.Be sure to use Teflon video.
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With water supply to the cistern off, maintain the drain valve embark on on the bottom. Open a HOT side of a faucet that is ABOVE the container somewhere in the house. This will return with the water out of the string. Once you hear it stop bubbling, you can take the lines apart.