Our radiant heat system was down for 7 days due to a faulty pressure regulator - for those not familiar that is the upside down cone shaped device on the incoming water supply that keeps the system replenished. The pressure regulator was original to the house or at least 40-50 years old. It had a huge build up of deposits, sediment, and all kind of gunk from a supposedly clean city water. The system was unable to replenish itself as water evaporated and burned off. Symptoms were the system was making more noise than usual and the water pressure gauge read zero. We originally thought the worse when I read (on this forum) that usually indicates a leak. Replacing the regulator fixed everything except a small drip at the pump, but we are replacing the pump anyway with a newer, more energy efficient model.
I think it's a good idea to have the device physically removed and visually inspected buy the repair person when you have a system checkup, especially if it's old or original, or just replace it. Replacing it was just under $200.