Galv to copper
 

I need to attach an expansion tank to a water heater. How bad is it if I don't use dielectric fittings to go from the copper to the Galvanized? Yes I know you are not supposed to do it but will it rust out before the water heater?

Most likely won't have a problem. I've seem many installations without the dielectric fitting. Typically the water heater will need to be replaced before the connection. However, the fittings don't add that much to the cost or complexity of the installation.

If you no that you are supposed to do it then why not do it. It will rust before the water heater will. The expansion will not work wright. The you will be doing it then.