Heats up quick and shuts off. Hot water won't take priority..

It sounds to me like you have a dirty flame sensor. In the most simple sense the flame sensor's job is to shut off the supply of gas to the burners in the case when the fuel does not ignite.

If your heater is coming on and firing up for about 15 seconds (and you see the flame ignited etc) you have a problem with your flame sensor. This usually means that the flame sensor is dirty and needs to be cleaned. Trust me when I say that when I called a repairman out and he spent about 12 minutes in the house diagnosing the problem and cleaning the sensor I felt foolish paying the guy about $100. So now I do it myself.. . And it is dead simple.

All you need is a screw driver and or a nut driver and a scotch brite pad (I use a dark red one). Turn off the furnace.. . Either by switching off the breaker (or pulling the fuse) or by using the switch on the side of the furnace. Open up the top half of the furnace (usually just a single panel). Take a look at where the gas jets point at the heat exchanger there should be a metal L shaped rod in the path of the flame.. . That is the flame sensor. It will have at one end some kind of electrical connection and it will be attached to a piece of metal somewhere near the gas jets usually with a single screw. Unscrew that screw and remove the flame sensor.. . If you need more room you can remove the electrical connection.. . Then polish the tip of the sucker with the scotch brite pad until it is as shiny as the rest of the sensor and screw it back in and presto you are done!

If your problem is something other than it functions normally but only for about 15 seconds then this is likely not your problem. If this is not your problem we are going to need more details about your problem in order to help you!

Good Luck!

Gary

When you say "hot water wont take priority" it makes me think you are talking about a boiler? More info please.