Have a 9-year old radiant heat system in Colorado Mtns. Weekend cabin. 5-zones of RH in Slab, 2-zones of baseboard, domestic hot water tank. Noticed for the last two weeks that the maximum and minimum pressure is down by about a pound per week. Have had same symptoms in the past. One was a valve packing slowly leaking, the second was a clogged air valve dripping. This time I can see no visible leaks. Looks like I need to get a pro in, but would like to start troubleshooting. I can pressurize with auto fill, which is usually off. Any suggestions on best way to Isolate which zone(s) might have a leak? I was thinking of leaving autofill on when we leave for the week. Would hate to flood the cabin if leak opens, but also burn up boiler if pressure drops to zero. Would welcome recommendations. Fraser Jim

The best way to isolate the leak is to use an air pressure kit to first individually test each zone and then to individually test each circuit of the zone exhibiting the leakage. I would first verify that the leak is not associated with the usual suspects (e.g., manifold circuit fittings, valves, etc). It doesn't take much of a hydronic fluid loss to create a significant pressure drop. A small pressure drop is more indicative of a small leak. So presumably you are not dealing with a slab assembly that was recently exposed to freezing temps without antifreeze protection and where the PEX was merely stapled onto the insulation and not fully embedded in the concrete.