Boilers include pressure gauges to help you monitor their operation and to sidestep any potential safety issues. Many people tend to notice their boilers losing pressure over hours, days, or weeks, but they can't find any evidence of a leak in the system. In these cases, it's natural to wonder whether your boiler can lose pressure without leaking.

The answer is no. If your boiler is losing pressure, it's leaking air or water from somewhere in the system. Your heating system is effectively a closed loop, which means that the system pressure should only really change with temperature. A noticeable drop in pressure or a need to routinely top up the system with more water is a sure sign of a leak.

Why Isn't There Evidence of a Leak?

To understand why you might not see a leak, you need to know how leaks affect the pressure in your heating system. In general, large leaks will cause rapid and noticeable pressure drops. If hot water is visibly spilling from a pipe, relief valve, bleed valve, and so on, you'll be able to stand in front of your boiler and watch the pressure gauge fall rapidly.

On the other hand, small leaks can still cause the pressure to drop surprisingly quickly. A weeping valve on a radiator might result in a substantial pressure drop overnight. Likewise, the relatively low pressure from an inactive boiler might not be enough to push a significant amount of water through a leak. If you're only losing water while the system is pressurized, it might take a while to notice the problem.

How Can You Find Small Leaks?

There's no silver bullet for locating small leaks. You should start by inspecting all exposed plumbing, including near your radiators. Remember that you aren't looking for a gushing torrent of hot water. The evidence of a leak may be subtle, so you'll need to check closely. Valves are typical sources of trouble, so inspect the area around them as carefully as you can.

Professionals will usually simplify this process by using an infrared camera. This camera can detect unusual heat sources behind walls or in hidden locations, allowing them to spot relatively small amounts of hot water exiting the system. If you have access to one of these cameras, you can use it while the system is running to assist in your hunt for leaks.

Finally, it's worth considering the pressure gauge itself. A faulty gauge can produce inaccurate readings, but this problem should be relatively obvious. If adding water to the system doesn't cause the needle to budge, it's a good bet the gauge itself is faulty. Avoid filling the system in these cases since you can damage the boiler or your pipes by overfilling them.

Remember that topping up a hydronic heating system is never normal. If you're experiencing routine pressure drops and can't find the cause, contact an HVAC professional as soon as you can to help you locate your hidden leak and perform boiler repair.