I need to fix some leaky return shutoff valves on a hydronic heating system. Is there such a thing as an inflatable device that can be used to plug the pipe to prevent the system from draining while the washer is being replaced?
There are all kinds of temporary pipe plugging options, inflatable, mechanical, etc. Here is a link to one supplier (not an endorsement of products) Pipe Plugs and Stoppers
If you can stop the flow of water in the system you can freeze the water in the pipe, then fix whatever you have to. This works on steel and copper pipe. I have frozen pipes up to 2 inches. A pipe freezing kit is great but you can also use dry ice. Find a place that sells dry ice and buy the crushed type. Make sure you use heavy gloves to protect your hands. Get a face towel or equivalent and use it to hold the dry ice as you wrap it around the pipe. Use a lot- 2to3 inches of ice. Then duct tape the cloth to keep it in place. Wrap this whole thing with paper towels or newspaper for insulation and wait for the pipe to frost up just past the dry ice wrap. Test for water stoppage downstream. When frozen you can complete your repairs. When done remove everything, allow to thaw and you are done. And by the way where is the washer in a hydronic heating valve? all the ones I have seen have brass to brass seats.