Quick Answer:

If a backflow device leaks only overnight, the most likely causes are temperature-related changes and pressure shifts when systems cycle down. Start by checking for thermal contraction and keep a simple temperature log to compare with leak times. If you can’t find a clear pattern or the leak worsens, call a professional.

Why This Happens

Nighttime conditions often change in ways that affect plumbing more than daytime conditions. Cooler air can cause metal and plastic parts to contract, creating tiny gaps in seals or seats. Pressure in the system can also fall or surge when irrigation, pumps, or heating systems stop running. Small debris or partially stuck valves can let the device occasionally give way, producing behavior that looks like it releases water randomly.

Sometimes a partial opening or vibration will create a faint sound before or during the leak; homeowners sometimes report a soft buzzing noise or clicking just before water is released. These signs point to intermittent movement or pressure pulses rather than a constant failure.

Step-by-Step What to Do

Step 1: Note the pattern

• Write down the nights and approximate times the leak appears. Include whether the leak starts after midnight, in the early morning, or only on colder nights.
• Check whether any irrigation, pump, or heating cycles run before the leak starts.

Step 2: Check thermal contraction

• Feel the device and nearby pipes (carefully) in the morning and in the evening. Cooler surfaces that match leak times suggest thermal contraction.
• Look for small gaps, loose clamps, or shift in the housing that could open when temperatures drop.

Step 3: Log temperature

• Use a simple thermometer or weather app to log nighttime low temperatures for several nights alongside your leak notes.
• Compare temperature drops with leak times to see if there’s a clear connection.

Step 4: Check system pressures and schedules

• Confirm whether irrigation timers, well pumps, or heating circulators run on a schedule that changes at night.
• Shut off those systems briefly (if safe) to see whether the leak still occurs; this helps isolate the cause.

Step 5: Inspect the device externally

• Look for mineral deposits, corrosion, or visible wear at seals and test cocks.
• Place a bucket or dry cloth under the device overnight to catch and measure dripping without having to be present.

Step 6: Try simple fixes first

• Tighten loose clamps or fittings gently; do not overtighten plastic parts.
• Clear nearby debris and make sure nothing is pressing on the housing or levers.
• If the device has replaceable seals and you’re comfortable doing basic repairs, follow the manufacturer’s guidance; otherwise, arrange for a pro.

What Not to Do

• Do not assume phantom leak — if water appears, treat it as real until proven otherwise.
• Do not force or heat fittings to make parts move; that can cause damage.
• Do not ignore repeated or worsening leaks; small nightly leaks can become larger or lead to freeze damage.
• Do not dismantle internal components unless you have the right tools and the manufacturer’s instructions.

When to Call a Professional

• Leak persists after basic checks, or you cannot identify a temperature/pressure pattern.
• Device shows steady leakage, major dripping, or fails pressure tests.
• You notice corrosion, cracked parts, or damage to the device body.
• Local codes require certified testing or repair for backflow prevention devices.
• Concerns about freezing, contamination risk, or complex irrigation/well systems.

Safety Notes

• Shut off the water supply before working on the device. Confirm valves are closed and system pressure relieved.
• Avoid contact with standing water near electrical equipment. Turn off nearby pumps or power if needed.
• Wear gloves and eye protection when inspecting or cleaning the device.
• If temperatures are below freezing, take extra care—do not rely on a leaking device to thaw ice.

Common Homeowner Questions

• Why does the leak only happen at night?
  Nighttime cooling and pressure changes commonly cause seals or seats to open slightly, leading to intermittent leaks.
• Can I wait until spring to fix it?
  Only if the leak is tiny and stable; otherwise, it may worsen or lead to freeze damage—monitor closely and record temperatures.
• Will replacing the device fix the issue?
  Often a worn internal part causes intermittent leaks, but confirm with a test or a professional before replacing the whole unit.