SylvanLMP
Call a local Licensed Master plumber (ask to see the license)
They will drain your water lines and use inert gas like nitrogen to blow out the domestic water lines.
They will check your drip legs on your gas piping to make SURE there is no moisture build up that can freeze.
They will add non polluting anti freeze Or completely drain all traps and cap all opens to prevent very dangerous sewer gases from entering this structure.
Also besides asking for the license you want a copy listing YOU as co insured with a certificate of insurance in case you do have a freeze up your plumber is accountable.
if the contractor has any objections to guaranteeing his/her job find one who knows their business.
The license protects YOU not the plumber so demand to see it.
Before they even go near your heating system ask them what FORMAL education did they go through for heating applications.
Get it in writing.
