Tankless Coil Water Heater Problems
On this page (6 sections)
The Short Answer: A tankless coil is a copper coil inside a boiler that heats household water as it flows through. When the hot water is lukewarm, runs out, or swings between scalding and cool, the usual causes are a boiler that is not kept hot enough, mineral scale lining the coil, or a failing mixing valve. A leaking coil can also let boiler water into your plumbing, which needs prompt service.
How a tankless coil works
Unlike a wall-mounted tankless water heater, a tankless coil has no burner of its own. It sits in the boiler’s water, and the boiler’s control (often an aquastat) keeps that water hot enough to heat the coil on demand. Because the coil’s output depends on boiler temperature and flow, it is common for these systems to deliver a modest, steady flow of hot water rather than enough for two showers at once. A mixing valve on the outlet blends in cold water so very hot coil water does not reach faucets.
If there is a separate storage tank next to the boiler, you have an indirect water heater, not a coil. See SuperStor indirect problems or Amtrol indirect problems.
Symptoms and likely causes
| What you notice | Likely cause | Urgency |
|---|---|---|
| Lukewarm water, especially in summer | Boiler temperature lowered, or the boiler not firing for hot water | Service soon |
| Fine for a minute, then cooler | Demand exceeds the coil’s capacity, or scale reducing heat transfer | Service; reduce simultaneous use |
| Hot water flow weak at every tap, cold flow normal | Scale or debris restricting the coil or its strainer | Service |
| Scalding at times, cool at others | Mixing valve sticking or set incorrectly | Plumber |
| Rusty or discolored water only from hot taps | Possible coil leak letting boiler water into the domestic side, or rust elsewhere in the plumbing | Prompt service |
| Boiler pressure rising, relief valve on the boiler discharging | Leaking coil pushing house water pressure into the boiler | Prompt service |
What you can check
- Boiler status. Is the boiler running and showing normal operation? If heating and hot water both stopped, the boiler is the starting point.
- Hot versus cold. Compare flow and temperature at several taps. If one fixture is poor, the problem is at that fixture.
- Mixing valve. Note whether the valve on the coil outlet has a setting dial and whether it was recently changed. Do not take it apart.
- Boiler pressure gauge. If your boiler has a gauge, note the reading and whether it has been climbing. A rising reading alongside discolored hot water is important information for the technician.
- Timing. Write down when the water is worst: after the heat has been off, during the first shower of the day, or with two taps running.
What the heating technician will check
A boiler technician will confirm the boiler maintains the temperature required for coil operation and that its controls give hot water priority if designed to. They will check the mixing valve, measure temperatures at the coil inlet and outlet, and look for scale. Coils can sometimes be descaled with the domestic side isolated; that requires shut-off valves on the coil piping and a cleaner approved for potable water. They will also pressure-test the coil if a leak between the boiler and the domestic water is suspected. Boiler water often contains treatment chemicals and is not meant to be drunk, so a leaking coil is replaced, not sealed.
Repair, replace, or convert
Mixing valves, strainers, and aquastats are routine repairs. A scaled coil can sometimes be cleaned; a leaking coil is replaced if the boiler model still supports it. Many owners of older boilers with coils instead add an indirect tank heated by the boiler, which stores hot water and lets the boiler stay off when there is no demand. A heating contractor can tell you whether your boiler is suited to that change.
Frequently Asked Questions
Q: Why is my hot water worse in summer? A: In summer the boiler is not running for heat, so it may cycle cooler, or someone may have lowered its temperature to save fuel. The coil can only heat water when the boiler is hot enough.
Q: Is discolored hot water from a coil dangerous? A: It can mean boiler water is leaking into your household water. Use cold water for drinking and cooking until a technician has checked the coil.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- A. O. Smith technical bulletin 51: puddles around a heater (opens in a new tab)Common water-leak sources and the distinction between fittings and a failed tank.
- CPSC: carbon monoxide fact sheet (opens in a new tab)Carbon monoxide hazards and emergency response.
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