The domestic hot water heat exchanger probably won't be a worry -- if it's empty there's room for expansion inside, and if it's full the mass will take longer to freeze. If that's damaged it will be the fittings where hoses connect, as likely as any other fittings in your fresh water system. So long as everything is accessible, repair shouldn't be too difficult or costly. Sounds like you've got the real high value stuff covered e.g. watermaker.