I have a hydrotherm boiler with a mil...

Make sure your thermostat can be used with a millivolt system.Most thermostats are for 24 volt systems and don't work for millivolt systems. Also your thermostat wiring cannot be too long and it also needs to be the correct gauge. Normally the thermostat wire size should be 18 AWG up to 50 feet in distance. Also all your wiring connection have to be clean and tight(solid connections). If your connections are the slightest bit loose your millivolts will drop.