I need to design a timer for furnaces

I have three furnaces in a large woodworking shop. Tenants and employees often leave loading doors and entry doors propped open to load and unload and don't bother to close them. Meanwhile, the furnaces are running full blast to keep up with the cold air blowing in. So....I need to design a circuit that will cut off the furnaces when doors are propped open for more than say 5 mins.
The furnaces have simple two wire 24v contol circuits. Open the circuit and they turn off, close the circuit and they come back on. You could operate them by simply twisting the two wires together.
I imagine security style door sensors would work to start the timer. Any ideas on who makes a timer like that? Would I need a seperate power source for the timer? I'm thinking I need something in the way of a three contact device so that the furnaces can still be controlled by the thermostats.

Thanks

I guess what you need is a off delay timer
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Gomez

I just saw those and they'll work. I also found another that doesn't need a power supply

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Also, ABB's Programa Cube is small enough to fit into a standard electrical 4" square box.

http://library.abb.com/global/scot/scot209.nsf/veritydisplay/6df5492b5c2c6a7e852573cc006b77b5/$File/1TRC111015B0201.pdf

Thanks for the input.

ed