Wood furnace relief valves

I have a standard (infrequent duty) temp/pressure relief valve on a standard (electrically gutted) water heater piped into my wood furnace. The tank regularly overheats when little use is made of the abundant hot water, the relief being vented to drain. After a year or so these valves start leaking, otherwise the system is a godsend.
Does any one know of an industrial regular duty pressure modulating relief valve that's designed for more or less continuous or long-term duty (3/4" pipethread)? It should preferably be similar, with a manual test/vent handle, and be settable to whatever pressure 0-60psi.
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