Annual Tankless Water Heater Service

Purpose

Define the annual service protocol for residential tankless (on-demand) water heaters. Tankless units in hard-water areas calcify quickly - scale on the heat exchanger drops efficiency 10 - 20 % per year if neglected, and eventually overheats the exchanger to the point of failure. A annual descale prevents a heat-exchanger replacement.

Scope

Applies to:

• Indoor + outdoor residential tankless gas units (natural gas + LP)
• Indoor + outdoor residential tankless electric units (limited descale scope)
• Whole-home + point-of-use units

Excludes commercial / multi-unit applications (separate scope) and units already exhibiting active failure (service call, not maintenance).

Responsibilities

• Dispatcher confirms unit make/model + indoor vs outdoor at booking; flags any soft-water systems (less aggressive descale required)
• Lead technician runs the protocol + records the descale duration on the field report
• Customer witnesses the visible scale flush + understands the recommended cadence

Procedure

1. Customer briefing. "I'm going to flush the heat exchanger with descaling solution. About 90 minutes total. Hot water will be unavailable during the service."
2. Cut power. Electrical disconnect for the unit; for gas units, also close the gas supply valve.
3. Close water supply. Both inlet (cold) and outlet (hot) shutoffs on the service valve set.
4. Verify the customer has service valves installed. If not (older installs, lower-end units), the descale requires installing temporary connections. Quote the service-valve install as a follow-on - every tankless customer should have these.
5. Connect a submersible pump + bucket circulation loop:

• Pump in bucket → hose to cold service-valve port
• Hose from hot service-valve port → returns to bucket

1. Fill bucket with 3 gallons of food-grade descaler (white vinegar OR a tankless-specific descaler like CLR PRO). Vinegar is gentler and good for annual maintenance; chemical descaler is required for heavy scale.
2. Run the pump for 45 - 60 minutes. Cycle the descaler through the exchanger. The bucket will turn cloudy white as scale dissolves - that's the visible proof of work.
3. Flush with clean water for 10 minutes after the descale. Continue until the discharge runs clear.
4. Clean the inlet water filter screen. Many units have a small mesh filter at the cold inlet - debris accumulates here annually.
5. For gas units: combustion analysis. CO at the flue, O2, stack temp. Document on field report.
6. For gas units: inspect the vent termination + flue path. Birds, ice damage, debris.
7. Restore power + gas + water. Open valves slowly, watch for leaks at the service-valve connections.
8. Cycle the unit. Open a hot tap and verify the unit fires, modulates, and delivers hot water at customer-expected temperature.

Acceptance Criteria

• Descale cycle ran the full duration; bucket showed visible scale
• Inlet filter cleaned (photographed before/after)
• For gas units: combustion analysis within manufacturer spec
• Hot water restored to customer-expected temperature + flow
• No leaks at any service-valve connection after 30 minutes
• Customer signs field report; cadence recommendation given

References

• Manufacturer service literature for the specific unit (Rinnai, Navien, Rheem, Bosch, Takagi, Noritz)
• IAPMO Uniform Plumbing Code, Chapter 5 (Water Heaters)
• Manuall internal: Annual Water Heater Service