Boiler losing pressure: the field diagnostic tree

In plenty of boiler rooms, topping up the water has quietly become a maintenance task: walk in, glance at the gauge, crack the fill valve for thirty seconds, write “pressure restored,” leave. It is the most expensive habit in hydronics. A boiler losing pressure does not consume water the way a truck consumes fuel — a closed loop has no physical reason to lose any. Every litre added is a litre that went somewhere else, replaced by a fresh litre carrying dissolved oxygen and minerals.

So the diagnosis does not start at the gauge. It starts with a two-branch question, and the answer changes everything that follows.

Boiler losing pressure: water loss, or pressure loss?

Two completely different failures produce the same reading. In the first, the loop really is losing water and pressure follows it out. In the second, not a drop leaves: the nitrogen cushion in the expansion tank has bled away, so cold pressure collapses while hot pressure spikes. Until that question is settled, everything else is guesswork.

Tools required

• Water meter with a totalizer, residential size, fitted temporarily on the make-up line.
• Calibrated test gauge (0–60 psi) teed in beside the boiler’s own gauge — the original lies more often than anyone expects.
• Air pressure gauge with Schrader fitting, plus a small compressor or nitrogen bottle for the pre-charge.
• Infrared thermometer to spot an abnormally warm return heading toward a drain.
• Light and inspection mirror for connections in pits and behind tanks.

⚠ Safety: three conditions before opening anything

Loop water can exceed 80 °C and it is under pressure; a drain opened too quickly throws boiling water. Kill the heat demand, let the loop fall below 50 °C and lock out the burner supply. Never plug a weeping relief valve to “buy time” — a valve that spits is a symptom, not a defect to silence. On equipment covered by Quebec’s pressure installations regulation, replacing a safety component goes into the logbook the operator is required to keep.

Step 1 — Isolate the make-up and start counting

This is the step almost nobody takes, and it is the one that answers the question. Close the automatic fill valve, install a water meter on the make-up line downstream of the backflow preventer, then reopen. Read the totalizer once a week, along with cold pressure in the morning and hot pressure in the afternoon.

Without a meter, the automatic fill hides everything: it holds the displayed pressure steady while the loop drains into a floor drain, and the gauge stays reassuring. While you are there, check the backflow preventer that is mandatory on that line — its annual test is a separate regulatory obligation, covered in our article on backflow preventers in Quebec.

Step 2 — Branch A: it loses pressure, not water

Meter frozen, pressure falling: the water is still there, but the cushion that absorbed its expansion is gone. A bladder tank loses nitrogen by diffusion through the membrane, slowly, over years. Once the pre-charge drops below static pressure, the tank sits permanently full of water and absorbs nothing: the relief valve spits on every temperature rise, and cold pressure ends up below setpoint on the next cycle.

The rule is simple and well documented by manufacturers: pre-charge must equal the loop’s cold fill pressure, measured with the tank isolated and drained — never read off the system gauge. A tank shipped at the factory default of 12 psi suits a system with a high point around 20 feet, which describes a house, not a building. On a six-storey Montreal property with the high point some sixty feet above a basement boiler room, roughly 26 psi is needed just to push water to the top of the riser, plus a few psi of margin — which leaves a 30 psi relief valve almost no room to work with. The full measurement procedure is in our guide to checking expansion tank pre-charge.

Step 3 — Branch B: the water leaves, and it shows

The meter is moving and the boiler room is talking. Work the evidence in this order, most common first:

1. The relief valve, usually piped straight to a floor drain: a thin permanent trickle goes unnoticed and empties hundreds of litres a month.
2. Circulator seals, which leave whitish mineral halos on the flanges long before they drip outright.
3. Automatic air vents at high points, whose float eventually sticks open — almost always in a room nobody visits.
4. Coil connections inside air handlers: a fitting weeping into a condensate pan looks exactly like normal condensation.

Repair, then re-meter for a week: a loop still losing water after an obvious repair had two leaks.

Step 4 — Branch C: the water leaves and nothing shows

This branch rewards method over luck. Section the system loop by loop using the isolation valves, make-up still closed, and compare how each section holds pressure over a few hours. Three suspects dominate in Montreal’s building stock: piping buried under a slab or embedded in a concrete floor, a line crossing a trench between two pavilions on the same site, and a circuit partly abandoned during a renovation — operationally forgotten, still hydraulically connected, quietly draining away.

An intermediate heat exchanger deserves the same scrutiny: if it feeds a lower-pressure circuit, heating water transfers across without ever showing up in the room.

How much make-up water is acceptable?

No standard sets a number, but a reference exists: your own. The only useful benchmark is what your loop draws when it behaves, measured once and written into the log. Any marked deviation from that baseline then becomes an alarm, with nothing to argue about — and make-up water stops being an invisible consumable and becomes a maintenance indicator.

Step 5 — Repressurize and set a threshold

Once the repair is done, do not restore “the pressure it had before.” Restore the cold fill pressure calculated for the building’s actual height, set the tank pre-charge to that same value, purge, then read again hot. Log four numbers: cold pressure, hot pressure, pre-charge, meter index. They take thirty seconds and, a year later, they are worth more than anyone’s memory.

Field case: Saint-Laurent, 380 litres a week and two valves

Industrial building in Saint-Laurent, hot water boiler feeding shop unit heaters and two preheat coils. Two low-water cutoff lockouts in January, each cleared by a manual top-up. No water on the floor, no complaints, maintenance file up to date.

Meter installed on a Friday, read the following Friday: 380 litres. The loop was not losing a little — it was emptying, and the automatic fill was keeping up. Sectioning during a weekend shutdown: the shop loop held pressure, the ventilation loop did not. Two causes, stacked. First the relief valve, piped to a drain, passing a trickle on every temperature rise — a direct consequence of an expansion tank whose pre-charge had fallen to 4 psi. Second, a drain valve left cracked open on a section isolated during a past renovation, inside a locked room, above a drain.

The fix: valve replaced, pre-charge reset to the calculated value, abandoned section closed and capped. Make-up measured the following month: under 20 litres. No lockouts the next winter.

The next move doesn’t cost a part

Before the first real heating demand, fit a meter on the make-up line of every boiler room in the portfolio and read it monthly. A few dozen dollars, twenty minutes of work, and it is the only instrument that tells a healthy loop apart from one continuously refilling itself with fresh water.

For the building manager, that monthly line of litres changes the conversation entirely: it replaces “the boiler keeps losing pressure,” which cannot be scheduled, with a trend readable month over month — and it makes visible a corrosion problem that otherwise only introduces itself the day the heat exchanger needs replacing. It is the first number Montréal Combustion looks for in a file before proposing anything on the boiler service side, and the one that is almost never there.