We have a newer oil burning furnace that is used for our domestic hot water as well. our problem is that the temperature of temp. starts plenty hot for about 2 mins, then dips for a while then comes back up again for a few mins and then nose dives unless you turn it off. We've had our oil company out a few times and people keep saying "that's just how these types of systems work" but I talk to others that have them, and they dont' have this issue. Last time out they put a "tempering" ball valve on our cold water line feeding into the furnace. They told my wife we could use this to adjust the flow of cold water into the furnace to help maintain better hot water....not being home and being able to ask the guy questions about it, has me questioning if this is a good soloution to the problem. I don't want to restrict the cold water in if it's going to affect the furnace poorly. Any advice would be great! My wife and I have taken to showeing at the gym instead because at least they have hot water consistently...