Navien NFC-H Combi Boiler Pressure Problems Explained

Need a licensed gas fitter for your gas boiler? Call CANRO at (604) 359-1081 — residential natural-gas heating and hot water across Metro Vancouver since 1999.

What the pressure gauge is telling you

A hydronic boiler is a closed loop full of water, and that water has to be kept under a modest pressure for the system to work. Too little, and air can enter, the pump can struggle and the boiler may shut down to protect itself. Too much, and safety devices may open. The gauge on or near the NFC-H shows the loop's pressure, and the normal range is marked on the gauge or set by the installer for your house. We never quote a number, because the right value depends on the height of the system, the unit and the installation, and the manufacturer's manual for your appliance is what sets it.

Two habits make the gauge useful. First, read it when the system is cold and record the value. Second, read it again when the heating has run for a while. A modest rise as the water warms is normal in a closed system, and the technician can tell you what to expect for your unit. What is not normal is a gauge that keeps sliding down over days, or one that climbs well past the marked range.

Pressure that keeps dropping

This is the most common pressure complaint. Water is going somewhere, or air is coming out of solution and collecting. The causes, roughly from most to least common, are these.

1. A small leak in the loop: a radiator valve, a fitting, a union, a pipe joint or a leaking connection at the boiler. Old houses with mixed piping have many candidates.
2. A relief valve that is weeping or discharging, often because pressure has been rising too high, or because the valve seat is worn.
3. An expansion problem that lets pressure swing and then makes the relief valve open.
4. Air that is being bled out of the system, by a homeowner or by an automatic vent, and not replaced.
5. A component inside the appliance that is leaking, which is more serious and needs a technician promptly.

Look for damp patches, rust stains, drips at valves and moisture under radiators, and check the relief valve's outlet pipe if you can see it. Write down what you find. Do not keep topping up the system to make the gauge look right. Every time fresh water enters, it brings dissolved air and minerals, and a leak that is being topped up hides itself until it becomes a much bigger problem.

Pressure that rises too high

When pressure climbs sharply as the boiler runs, the loop has nowhere to put the expanded water. In a closed hydronic system, an expansion component absorbs that volume, and when it loses its air charge or fails, the pressure spikes and the relief valve may discharge. Overfilling, for example by leaving a fill valve open or topping up too far, produces the same picture. So does a fault in an automatic fill arrangement, where one is fitted.

Because a relief valve is a protective device, a homeowner should never block, cap or tamper with it. If it is discharging, note when, how much and what the gauge showed, and call a licensed gas fitter.

Pressure that looks fine but the boiler locks out

Not every pressure message means the loop has a pressure problem. A boiler measures pressure with a sensor, and that sensor can drift or fail. A unit that reports low pressure while the gauge reads normal, or the reverse, may have a sensor or wiring issue. A technician compares the gauge, the unit's own reading and the behaviour of the system before deciding. The unit's display reports categories of faults, and the manual explains what each category means. We treat a message as a lead and confirm it at the appliance.

Pressure on the hot-water side is a different thing

On a combi, there are two separate water systems. The heating loop is closed and has the gauge we have been discussing. The domestic hot-water side is fed from the household cold supply, so its pressure is the house's water pressure. Weak flow at the taps is a hot-water-side complaint, and its causes are different: a clogged inlet strainer, blocked aerators, a partly closed valve or scale. If the taps have poor pressure but the heating gauge is normal, the problem is on the domestic side, and a technician will look there.

Pressure patterns at a glance

Pressure and older Lower Mainland houses

Pressure problems are more common in houses with older systems, and the reasons are practical. Cast-iron radiators have valves and packing nuts that can weep as they age. Steel pipe corrodes at threaded joints, and a pinhole can open where nothing was visible the year before. Houses that have had renovations may contain patched sections of copper joined to older steel, and those transitions are places where water finds its way out. In a basement where the boiler and pipes sit near a floor drain, a slow leak can disappear without leaving any puddle, so the only evidence is the gauge.

When we investigate, we look along every visible run of pipe, at every radiator valve and at the joints near the boiler, using a light and a dry paper towel. A slow weep leaves a tide-mark or a faint crust of mineral. We also look at floors and ceilings below the pipework. If the leak is hidden inside a wall or a slab, in the case of in-floor heating, we discuss how to isolate and locate it, and what the options are.

What the technician does

We start by watching the gauge through a cold state and a heated state, then look for leaks by eye, by touch and by pressure testing where needed. We inspect the relief valve, check the expansion component's behaviour, look at the fill arrangement and check the water condition. In houses with old radiators, we check valves and packing. When we find the cause, we fix it, restore the pressure properly, purge air and confirm the gauge holds over the visit and, if necessary, over the following days. If the leak is in a component inside the appliance, we explain what that means for repair or replacement.

CANRO Heating & Cooling has served Metro Vancouver from Burnaby since 1999. Our technicians are Technical Safety BC licensed gas fitters, and the company holds gas licence LGA0043870 and boiler licence LBP0046450. We service Navien boilers as an independent contractor and do not claim to be a factory or authorized dealer.

Frequently asked questions