I'm planning on using an electric tank water heater as a buffer tank in a radiant/DHW system. I have 110V wiring in my mechanical room and I've found a 240V 30 gallon water heater. I won't be using the heating elements, but I will be using the built in thermostat. So does the voltage matter? And if so, is there a simple and safe way to convert to 120V?

I am curious as to what you’re going to do with the thermostat. They are usually a simple mechanical switch and rated to handle whatever element is in there, so it's usually pretty beefy.

I'll be connecting the thermostat to a relay that will cycle hot water into the tank once it gets below a certain temperature. An example of what I'm talking about is here:
http://www.taunton.com/finehomebuilding/how-to/articles/add-a-tank-and-pump-to-a-tankless-water-heater-to-save-energy.aspx

Brock, I didn't understand at first that your reply contained the answer to my question--- that it's a mechanical switch. Thank you.

I get it, yes the switch even in a small 5 or 10 gallon tank will handle a circulation pump easy.

Do you already have it all setup with an existing tank and are just adding the circulation pump? Or are you adding the tank and pump right now?

I would be curious to know how much that helps with and without, sounds like it should help.

I don't have anything set up yet--- I probably won't do it in stages so I don't think I can report back on the difference it makes. sorry!

Oh ya, I just realised that from the other thread :)

yep--- i'm all over the place right now. trying my best at working with what i've got!