If you are having trouble getting sufficient hot water, then your hot water boiler may need purging. Purging a boiler is a simple way of saying that you are going to get the air out of it so the water can run freely. Air bubbles stop water flow and need to be purged, or the boiler will not function.
Turn on the main water feed valve into the boiler. This will allow water to flow into the boiler.
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Clear the air out of the boiler. At the top or near the top of the boiler, there will be either a spring-loaded pressure valve or a wrench-adjusted nut to bleed out the pressure. Slowly open that upper valve, either using your fingers if it's a spring-loaded valve or a wrench if it is nut. As you open this valve, you will notice a hissing sound. That is the air being purged from your boiler.
Position your bucket beneath the valve. When the hissing stops, water will begin pouring out of the valve. Catch it in the bucket while turning it off with the wrench or by releasing the spring load. Once you have turned off the water, your boiler will be air free.
Turn off the main water supply. Now that your boiler is purged of the air bubbles that were blocking it, you will be good to go.