32 years in the trade has taught me a thing or two. Your furnace is fed with a pressure reducing valve that allows 12 pounds of pressure into your system. It is possible for this valve to malfunction and allow house pressure to enter your system and pop the relief valve. Shut your furnace off. If you see the pressure going up then it is the pressure reducing valve; if you do NOT have a hot water coil.... If you have a coil in your furnace then it could be either the feedwater valve or coil. Turn the water off feeding your coil and if the pressure goes up then it is the feed valve. If this is the problem then it is a matter of replacing the valve and you should be all set. Always remember to turn the power and water supply off on your furnace when working on it. If you are not sure of what you are doing ask some questions or get help....When taking pressure off your furnace always close the valve to your expansion tank. It will only take a few seconds to drop the pressure if this valve and you feed water valve are closed....Please subscribe and have a good day.