If a tankless heater in Reston is showing codes, losing flow or going cold mid-shower, scale from moderately hard Fairfax Water and an undersized gas supply are the usual suspects. New installs need a Fairfax County permit, and a Residential Plumbing permit when new gas pipe goes in. Call the number on this page to be connected with a local plumber in Reston.
Reston in brief
Reston's planned neighborhoods, townhouse clusters and single-family streets were largely built from the 1970s through the 1990s, with a median home year of 1983. Water comes from Fairfax Water and sewer from Fairfax County. Many tankless units here were added during renovations, often on gas pipe that was never resized. Fairfax County requires a permit for water heater work, and a tankless unit that goes in on undersized pipe tends to complain on the coldest mornings, when it has to work hardest.
What to do right now
- Write down any error code shown on the unit's display; it helps the plumber arrive with the right parts.
- Check that the outlet powering the unit has power and that its breaker has not tripped.
- Open a single hot tap fully; some units will not fire at very low flow.
- Clean the cold-water inlet screen if your manual shows how, since scale and debris collect there.
- If you smell gas, stop. Leave the house, then call 911 and Washington Gas at 844-WASHGAS (844-927-4427), option 1, from outside.
Symptoms and likely causes
| Symptom | Likely cause |
|---|---|
| Flow tapers off over months | Scale in the heat exchanger or a clogged inlet screen |
| Hot, then cold, then hot again | Cold water sandwich after a short pause, or a unit at its limit |
| Shuts down with two showers running | Undersized unit or gas line |
| Ignition failure code | Gas supply, igniter or flame sensor fault |
| Venting or airflow code | Blocked intake or exhaust, often leaves or nests outside |
What affects the cost
- Whether a descaling flush and inlet cleaning solve it, or a part such as a flow sensor or igniter is needed
- Gas pipe upsizing from the meter, which Fairfax County treats as new gas piping needing a Residential Plumbing permit
- Vent repairs or rerouting through a townhouse wall
- Adding service valves if the original install left none, which makes every future flush quicker
- Scale control: Fairfax Water's Potomac plant tests show about 70 to 185 mg/L of hardness, averaging roughly 7.5 grains per gallon
When a unit is old and needs a major part like a heat exchanger, the plumber can price repair against replacement so you can compare. Units mounted on an exterior wall near the lakes or wooded common areas also get checked for blocked intake screens, since leaves and insects can trigger airflow codes that look like a bigger fault.
Tankless Water Heaters questions in Reston
My tankless unit keeps showing error codes. Repair it or replace it?
What causes a blast of cold water partway through a shower?
How hard is the water here, and how often should the unit be flushed?
Where does the vent go in a townhouse or a house with a basement?
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