Can I shut off return valves (water) on my furnace? Honeywell valve failed...

Can I shut off the (water) return valves on my furnace heating system? The Honeywell valve failed on the supply side and I am getting heat from 1 zone via my hot-water radiators. The house is new to us, but it is 50 years old and has lots of little issues (not surprisingly). I would like to shut off the heat for the rest of the summer and hope that shutting off the return-side will help matters.

thanks in advance

PS this is an oil furnace and it needs to remain on to heat the hot water tank.

I'm trying to figure out what you really mean- what I'm interpreting is that a zone-valve on your hydronic (hot water) heating system has failed open, and you want to turn off the valve on a return line to stop a convective flow from bringing some heat into the zone even when the heating system's circulation pump isn't on. In most cases that would be safe, but more info would be useful. (To be sure replacing the defective valve before the heating season is advisable.)

BTW: Most people refer to the heat source of hydronic heating systems as a "boiler" rather than a "furnace" (as distinct from hot-air furnaces), even though the water isn't boiled to steam as in steam-heating systems. (The water actually DOES micro-boil at the heat exchanger plate surfaces, but the vast majority of the water is always liquid.)