Started having problems a week ago when the gas furnace was working but the radiators weren't. Here's how to fix that - in case anyone needs to know - an wants to save a ton of coin in the process.

At first glance - or rather 'listen' - the furnace itself was making slight hammering noises it never made before. (It's a 15-20 yr old gas boiler.) That should have been the first clue that the water was low. But what I zeroed in on was the fact that the circulator wasn't working.

Purchased a new replacement at Grainger's, then found the same one on-line for half price. (Note to self: never research a price after you've made the purchase.) But a pump in hand is worth twice the price, I guess.... Installation wasn't difficult, but still no heat even with the circulator now running.

Now I had to drain the furnace to install the pump, and both system supply and return valves were turned off for the install. Still the hammering noises when fired up, even with the auto- and manual-fill valves open. Time for Plan B....

I figured that since I drained the furnace, I could refill it at the same place. Made up a short length of hose with double female fittings and connected the furnace drain hose to a another to the closest sill cock. Cautiously, I opened the drain valve - and the furnace audibly began to fill. Previously, I researched this on line and no source mentioned using the drain to fill the furnace; indeed, one HVAC tech said you shouldn't/couldn't do it then tried to sell be some rather expensive - and unnecessary - work.)

Now it's important to get rid of the air that has been blocking the system: open the bleed valve on the expansion tank next to the furnace as well as the one at the piping's highest point. (You may have a galvy expansion tank in the attic.)

Now the auto fill valve is supposed to add water until the system pressure is 10-20#. City water is more than this, so once the air is bled, turn off the water and then bleed off the excess pressure via the same drain until the tri-indicator on the furnace reads 15# or so. Fire it off....

Shazzam! - all is good in the heat department. But that doesn't explain why the auto fill device didn't do it's job or where the water went in the first place. There aren't any leaks....

Cheers