Boiler Heater leaking

I have a huge mess. I have a wells-mcclain boiler heater--it is ancient (from the early 50's?). The unit is in great working order, however I have a huge leak. I think the expansion tank is okay. Hubby keeps telling me he thinks it's a domestic coil, but he doesn't know a thing about home repair or heaters. I have a pipe coming out that has a release valve attached to it. This is leaking hot water. I've had the valve replaced by a plumber thinking it was that. The pressure to the system climbs really high, and I have to release the one valve so the pressure will come down (I also have this funny looking valve that sticks up with a clip on it, but think that's for air). That pipe that is leaking has the pressure relief valve on it (I think that's what it is) and the pipe leaks scalding hot water It filled up a blue tupperware storage container and overflowed in eight to nine hours today. I even had the water to the house turned off along with one of the water valves turned off to the heater and had the main switch to the heater turned off. Any ideas of what it could be? I'm loosing tons of water, and this heater is really a good old heater. It's fueled by oil and is a baseboard unit.

 

Some pictures (more are always better) would help but if you are describing the safety valve on the boiler proper it is almost surely an expansion tank problem.

The other possibility is a leaking (through) make-up water valve, sometimes called an automatic fill valve or pressure reducing valve (PRV) on the incoming cold water piping to the boiler.

 

It's possible he is right. In addition to the things Furd mentioned, a domestic coil leaking into the boiler will also cause the relief valve to open.