Harold Kestenholz
These systems were like side-arm heaters at the turn of the century. A 1-inch pipe ran around inside the fire box of the stove, then out to a riveted tank sitting nearby. Water from the tank entered the bottom of the coil to continue back to the tank by the power of gravity (difference in weight due to temperature) so it was called a thermosyphon action.
The tank-in-the-wood (or coal) stove is more dangerous today as the water pressures are higher. Your feeling that another relief valve would be safer is intuition giving a warning. Place another relief valve 20 pounds higher than your normal water pressure in the line going to the waterheater downstairs and check or replace the T&P valve once per year. The solid fuel heater can turn all the water in the stove tank to steam, then overheat and burn the metal. When the fire starts to die out, cold water can enter rapidly to cause a tank failure in the stove.
The old pipe idea in the stove was safer, as the pipes had a smaller diameter than a tank, so could stand higher pressures. They also contained less water, so the steam propelled less metal. Maintaining a small fire in the stove sent a small amount of water through the pipe to heat the larger tank all day. As the average electric bill for a family is $800 per year. Saving half that by high risk is not worthwhile.
