Combi Boiler Sequence of Operation: Where Each Stage Fails

A furnace tech sent to a no-hot-water call on a wall-hung combi already knows most of the sequence: safeties, a combustion fan, an igniter and flame proving. What changes is what starts it and what the boiler does once it is lit. Learn the order and every combi call becomes one question: which stage was the last one to complete?

The Hot Water Call, Stage by Stage

1. Demand. A combi detects hot water demand with either a flow switch or a flow meter. A flow switch only knows that water is moving. A flow meter also knows how much. NTI’s FTVN combis use a DHW (domestic hot water) flow meter.¹

2. A short start delay. Many combis wait briefly to confirm the demand is real before starting the burner. On NTI’s FTVN Series II it is a setting, adjustable from 0.5 to 20 seconds with a 0.5-second default.¹ Check that setting before calling a slow start a fault.

3. Hardwired safeties. If a limit or low-water cutoff is open, the sequence stops here and the boiler posts an error code.

4. Fan proving. The control starts the combustion fan and confirms it reached the commanded speed. A fan that spins but never proves speed stops the sequence here, the same as a stuck pressure switch on a furnace.

5. Trial for ignition. Spark and gas come on together for a few seconds. The burner either lights and proves flame, or the control retries and eventually locks out.

6. Modulation. The burner ramps on return water temperature (fire harder when it is cold, back off when it is hot), and a flow meter lets it start closer to the right rate. It should stabilize within a few seconds.

Where Each Stage Fails

Know which demand sensor the boiler has. Many flow switches use a magnetic piston that moves with the water and closes a reed contact. Solid particles, iron from old piping or a well included, can obstruct the piston, which is why these switches are meant for clean or filtered water.² A flow meter like NTI’s sits behind an inlet filter that clogs the same way. A combi that gives hot water “sometimes” deserves a look at the sensor and filter before anyone orders a board.

Ignition parts fail in their own ways. Hot surface igniters are brittle, and Weil-McLain warns that body oils from bare hands can make one fail.³ Many current boilers use spark ignition instead, but spark electrodes wear too, and some manufacturers replace the assembly at every annual service.⁴ On any intermittent ignition failure, check that the insulators are clean and the gap and position match the manual. The flame rod failure guide covers the flame-proving side of the same sequence.

The Heating Call and Why the Boiler Waits

The heating side starts when a thermostat, zone valve end switch or zone controller closes a dry contact. The ignition sequence is the same, but two things change.

First, domestic hot water wins. Wall-hung combis like NTI’s give hot water priority, and a three-way diverting valve sends boiler water either to the plate heat exchanger or out to the heating zones.¹ NTI’s control does hand priority back to heating if a hot water call runs past a timer, 45 minutes by default.¹

Second, the boiler may sit there after a heating call comes in. It may have just finished a hot water call at 180°F with a radiant zone asking for 120°F. It will not fire until the outlet temperature drops below the heating set point minus the on-differential, which NTI ships at 11°F.¹ Then the burner lights and ramps gently, where hot water calls ramp hard for comfort at the tap. A heating call with no flame for a minute after a shower is often a boiler doing its job.

Water Quality Is Part of the Sequence

Clean the DHW inlet filter at maintenance. NTI’s procedure has the flow meter turbine removed and the filter cleaned with a toothbrush and clean running water.¹ If the water carries sediment, add potable-rated filtration sized for the flow, and on hard water a polyphosphate cartridge slows scale.

NTI publishes limits for the DHW side of its FTVN combis: total hardness of 200 mg/L (12 grains per gallon) below 140°F and 120 mg/L (7 grains) above it, iron of 0.3 mg/L, and pH of 6.5 to 8.5. It also warns that operating above 135°F speeds up lime scale.¹

The same manual requires an ASSE 1017 or ASSE 1070 temperature-limiting or mixing valve on every combi installation, because the boiler can deliver scalding water. NTI’s table puts a scald at less than 5 seconds at 140°F.¹ ASSE 1070 devices limit the hot water temperature supplied to fixture fittings to reduce scalding risk.⁵ Check that it is there and set.

Can One Box Keep Up?

DOE’s sizing method adds up the fixtures used at the same time, then subtracts incoming water temperature from delivery temperature to get the rise. DOE suggests 120°F delivery and, without better data, 50°F incoming.⁶

Northern winters make the incoming number worse. Natural Resources Canada warns winter intake water can be as cold as 5°C (41°F).⁷ NTI’s own table for the FTVN150C (150,000 BTU/h input) shows 6.1 GPM at a 45°F rise but only 3.6 GPM at a 77°F rise, which is about 43°F water coming in and 120°F going out.¹ Federal rules cap showerheads at 2.5 GPM.⁸ Two showers at once is 5 GPM, and at 120°F delivery this boiler only gets there when the incoming water is in the mid-60s or warmer. The rough math for any combi is GPM = (input in BTU/h × thermal efficiency) ÷ (500 × temperature rise in °F), and it lands close to the manufacturers’ tables.

When the house needs more than that, a heating boiler with an indirect tank is often the better sale. The tank stores a buffer, and the boiler reheats it. The tank delivers its rated recovery only when the boiler’s capacity at least matches the tank’s rating.⁹ With a combi, one failure takes out both heat and hot water. Run the numbers with the customer. The hydro kit post covers the case where the boiler goes away entirely.

Before You Leave a New Combi

• Purge the air. Air pockets stop circulation in low-velocity radiant loops. Where the system allows, run it hot to drive out dissolved air.
• Run a combustion test at the firing rates the manual lists, high and low fire if it asks for both. Many residential analyzers measure O2 and calculate CO2, so CO is the reading that catches a wrong gas setting. As CO2 approaches the manual’s maximum, CO usually trends up with it. If they move opposite ways, look harder. The combustion analysis guide explains how the numbers connect.
• Treat the manual’s minimums as the pass line, not the target. Intake-to-exhaust separation and vent length limits are the worst a boiler tolerates, and recirculated exhaust dirties the burner and shortens part life. The vent joint post covers what makes plastic venting last.
• Check the condensate trap (the check ball should float), vent slope, every joint you made, and inlet gas pressure. Measure them. Do not assume.

A combi is a gas appliance with a plumbing problem attached. Learn the sequence, read the error code, find the last stage that completed, and most of these calls get short.

---

Additional Sources

1. “FTVN Series II Residential Condensing Gas Boiler: Installation, Start-up, Maintenance (420012001400, Rev. 10)”, NTI Boilers, Installation Manual, 2024.
2. “What Is a Flow Switch?”, Eicos, Manufacturer Technical Note, undated.
3. “GV90+ Gas-Fired Water Boiler: Boiler Manual”, Weil-McLain, Installation Manual, undated.
4. “Endura EXE 399-750 Installation, Operation and Maintenance Manual”, Fulton, Installation Manual, 2021.
5. “ASSE 1070-2020/ASME A112.1070-2020/CSA B125.70-20: Water Temperature Limiting Devices”, ASSE International, Standard, 2020.
6. “Sizing a New Water Heater”, U.S. Department of Energy, Energy Saver Guidance, 2017.
7. “Water Heater Guide”, Natural Resources Canada, Consumer Guide, 2012.
8. “Energy Conservation Program: Showerheads, 10 CFR 430.32(p)”, U.S. Department of Energy, Federal Standard, 2021.
9. “Installation, Operating, and Service Instructions: Heat-Flo Indirect-Fired Water Heaters”, Heat-flo LLC, Installation Manual, 2024.