I have an old Sears boiler (30 years or so) in a house that we just bought in July. I have one HVAC guy who installed a 25 volt aquastat yesterday. This morning, I got a 2nd opinion, and he said it needs a millivolt. It is now working with the millivolt, but both guys want $500 for their time. I have no idea who is right. Is there any way to find out?
If the 24 volt didn’t work and the millivolt did then I would think the millivolt was the right one. Ask sears what the right part is for that model. A millivolt creates it’s own electricity for when you don’t have a transformer to reduce the 110 down. Millivolt thermostats are different also.