Why boiler water is hard to measure, how the controller samples it on a blowdown line, how to install and calibrate the probe, and what to check when the reading or the blowdown misbehaves.
The blowdown line carries boiler water at full pressure and temperature. Hot pressurized water flashes to steam when released and scalds instantly. Isolate, depressurize, and let the line cool before opening any fitting or removing the probe. See Pressurized Systems.
A cooling tower probe sits in a steady sample stream. A boiler probe usually sits on the surface blowdown line, which is closed most of the time — so the water around the probe goes stale and cools between blowdowns, and reads nothing like the boiler.
When the valve opens, a second problem appears: the water drops in pressure as it flows, and part of it flashes to steam. Steam bubbles around the electrodes make the conductivity read low and erratic.
The answer is to sample: open the valve on a schedule, flush fresh boiler water past the probe, and read only once the reading is representative.
| Mode | How it reads | Use when |
|---|---|---|
| Continuous | All the time | The probe sees a steady flow of boiler water — for example on a continuous blowdown line that never closes |
| Timed Sample | Every interval, opens the valve and flushes, then reads while flowing. If the reading is above the setpoint at the end of the flush, the valve stays open until it falls back by the differential | The probe is on a normally closed blowdown line — the usual boiler installation |
| Sample & Hold | As Timed Sample, then closes the valve and lets the trapped sample settle before reading. If it is still above the setpoint, blows down for one pulse, holds, and reads again, until it clears | Flashing makes the flowing reading unreliable |
See Setpoint Control — Sampling for the settings in detail.
The InteliHive’s Boiler setup profile creates the blowdown mode with Timed Sample and these starting values:
| Setting | Starting value | What it does |
|---|---|---|
| Interval | 10 min | Time between samples |
| Flush | 2 min | How long the valve opens to bring fresh boiler water to the probe |
| Settle | 3 s | Readings at the start of the flush are ignored |
| Hold | 90 s | Sample & Hold only — how long the trapped sample settles before it is read |
| Pulse | 30 s | Sample & Hold only — each blowdown burst between holds |
| Differential | 50 µS/cm | Blowdown stops once the reading falls this far below the setpoint |
| Limit alarm | 1 h | Alarm if one blowdown lasts longer — the valve, orifice, or drain may be blocked |
| Minimum on / re-entry gap | 5 s / 2 min | Stops short bursts and rapid re-opening on a bouncing reading |
These are the Boiler profile’s values. A controller set up with another profile starts sampling much faster — a 1 min interval and 30 s flush. If a boiler controller shows those shorter values, it was not set up with the Boiler profile: set the timings for the boiler by hand. See Factory Defaults.
Lengthen the Flush if the reading is still changing at the end of it: the line between boiler and probe is long or cold. Shorten the Interval on a boiler with a heavy load, where conductivity rises quickly between samples.
The Boiler profile also treats implausible readings as a fault. It stops the blowdown, raises Policy Blocked (200), and latches — the blowdown stays off even after the reading recovers — when:
A latched trip cannot yet be cleared from the screen or the app, and acknowledging the alarm does not clear it. Contact support. See Latching trips.
Calibrate at the setpoint, with the boiler in control.
See InteliHive Calibration for the steps on the controller.
| Boiler pressure | Saturation temperature |
|---|---|
| 0 psig (0 bar) | 212 °F (100 °C) |
| 15 psig (1.0 bar) | 250 °F (121 °C) |
| 50 psig (3.4 bar) | 298 °F (148 °C) |
| 100 psig (6.9 bar) | 338 °F (170 °C) |
| 125 psig (8.6 bar) | 353 °F (178 °C) |
| 150 psig (10.3 bar) | 366 °F (186 °C) |
| 200 psig (13.8 bar) | 388 °F (198 °C) |
| Symptom | Likely cause | What to do |
|---|---|---|
| Reading low or erratic | Steam flashing around the probe | Check the throttling valve or orifice is downstream of the probe and set; lengthen the flush, or use Sample & Hold |
| Blowdown valve stuck open, so the probe sees water that has already flashed | Clean or replace the valve | |
| Probe fouled or coated | Clean the probe — see Conductivity | |
| Reading high | Solids and debris packed around the probe | Clean the line and the probe housing |
| Blowdown not happening — valve faulty, orifice or drain blocked | Check the valve operates; clear the orifice and the drain | |
| Blowdown limit alarm | Limit time too short for the boiler | Lengthen the limit alarm |
| Valve, orifice, or drain blocked, so blowdown cannot bring the reading down | Clear the obstruction | |
| Probe temperature alarm | Sample too hot for the probe | Check the probe’s rating; cool the sample or relocate the probe |
| Reading does not match the handheld | Handheld sample not cooled, or the handheld not calibrated | Cool to 25 °C (77 °F); check the handheld against a standard |
| Calibrated far from the setpoint | Recalibrate with the boiler in control |