Turn off all the water to the boiler and drain the lines. You need to do this even in the summer since the moisture from condensation may cause the paint to cure incorrectly.
Scrape the length of the pies you are going to paint to remove old paint and to rough the surface so that the paint will adhere to it. Old flaking paint will cause the new coat to become prone to flaking because of the heat.
Paint the lines with the metal and heat specific paint. A smaller paintbrush will make the project easier and cleaner since there will not be as much dripping. It is best to start the project early in the morning since this will give you a longer drying period in warmer hours.