Frozen Condensate Line on a High-Efficiency Furnace: Signs, Fixes and Prevention

Quick answer: On a high-efficiency (condensing) furnace, a small drain line carries away the water the furnace produces. If any part of that line runs outside, through a garage, attic or crawl space, it can freeze during a cold snap. Water then backs up, a pressure switch or float switch shuts the furnace off, and you get no heat, often with gurgling, water around the furnace or an error code. Turn the furnace off, warm the exposed section of line gently (never with an open flame) and call for furnace repair if it will not restart. The permanent fix is to reroute or protect the drain so it cannot freeze again.

When temperatures drop into the teens, we see a wave of no-heat calls from homes with newer furnaces. Many of them have the same cause: a frozen condensate line. Here is why it happens, how to recognize it and how to stop it from happening every winter.

Why high-efficiency furnaces make water

A high-efficiency furnace squeezes extra heat out of its exhaust. The exhaust cools so much that the water vapor in it condenses into liquid. This condensate collects in the furnace and drains away through a trap and a small plastic line, either to a floor drain, a laundry sink, a condensate pump or, sometimes, to the outside. On a cold day, a furnace can produce a surprising amount of water. Read more in furnace leaking water.

Where condensate lines freeze

• Lines that end outdoors, especially on the north side of the house
• Lines running through unheated garages, attics or crawl spaces
• Lines that sag, so water sits in a low spot and freezes
• Condensate pumps in unheated areas
• Discharge lines from a pump that run outside
• Furnaces in attics with drain lines passing near soffits

Sometimes the exhaust and intake vent pipes themselves can be blocked by ice or snow buildup at the termination outside, which causes similar shutdowns.

Signs of a frozen condensate line

• No heat during a cold snap, often in the early morning
• The furnace starts, then shuts down, or never lights
• Gurgling or bubbling sounds from the furnace or drain
• Water around the base of the furnace or dripping from the cabinet
• An error code pointing to the pressure switch or a condensate problem (check the chart on the furnace door)
• Ice visible on the end of a drain line outside
• The problem goes away when it warms up, then returns

What to do right now

1. Turn the furnace off at the thermostat
2. Find the drain line: follow the small plastic tube from the furnace to where it ends
3. Look for the frozen section: usually where it passes through a cold space or exits the house
4. Warm it gently: warm towels, warm (not boiling) water poured over the outside of the pipe, or a hair dryer on low from a safe distance
5. Never use a torch, heat gun on high or any open flame, and never thaw near the furnace burner area
6. Protect the furnace and electrical parts from any water you use
7. Clear snow or ice from outside vent pipes if they are blocked
8. Reset the furnace once by turning power off for 30 seconds and back on
9. Call for service if it will not run, if water is near wiring, or if it freezes again

Protect your pipes while the heat is off: open cabinet doors under sinks on outside walls and let faucets drip. See no heat in freezing weather.

Why not just keep thawing it?

Thawing gets the heat back on, but the line will freeze again in the next cold snap. Each freeze can also crack the line or the trap, causing leaks inside the house. And repeated shutdowns while a home is empty, such as a shore house or while you are away, can lead to frozen water pipes and serious damage.

Permanent fixes

A technician can choose the right fix for your home:

• Reroute the drain indoors to a floor drain, laundry sink or other approved drain
• Add or relocate a condensate pump to a heated space, discharging to an indoor drain
• Shorten and re-slope the line so water never sits in it
• Insulate exposed sections in cold spaces
• Add heat tape rated for this use on sections that must pass through cold areas, installed according to the manufacturer's instructions
• Check the trap and drain connections on the furnace
• Adjust vent terminations if snow or ice routinely blocks them

Check the vent pipes too

High-efficiency furnaces vent through two plastic pipes (exhaust and intake) on a side wall or roof. After snowstorms and freezing rain, make sure both are clear. Exhaust vents can also build up frost in extreme cold. Never block or cover them. If they repeatedly ice up, the termination may need to be raised or relocated.

Furnaces in attics and garages

Some homes, especially newer construction and additions, have the furnace in an attic or garage. These installations are more likely to have drain lines in cold spaces. Ask your technician to check the full drain route during your fall tune-up.

Heat pumps and condensate

Heat pumps produce water too, mainly from the outdoor unit during defrost. Ice buildup around the base of a heat pump is a related problem; make sure water drains away from the unit and that gutters do not drip onto it. See heat pump not heating.

Mistakes to avoid

• Using a torch or open flame to thaw plastic lines
• Pouring boiling water on brittle plastic pipes
• Bypassing the pressure or float switch to get the furnace running
• Repeatedly resetting a furnace that keeps shutting down
• Ignoring water inside the furnace cabinet, which can damage the control board

Prevention checklist before winter

• Trace the condensate line from furnace to drain
• Make sure no part runs outdoors or through unheated space without protection
• Confirm the condensate pump works and sits in a heated area
• Check slope; no sags where water can sit
• Clear outside vent pipes of leaves and nests
• Book a heating tune-up that includes the condensate system

See the seasonal maintenance checklist.

Older homes and newer furnaces

Many homes in our area replaced an old chimney-vented furnace with a high-efficiency model. When that happens, the drain has to go somewhere, and in homes without a convenient floor drain it sometimes ends up routed outside. If that describes your home, it is worth having the drain route checked before the coldest weeks of the year.

What a service visit includes

When we come out for a frozen or backed-up condensate line, we thaw and clear the line safely, flush the trap, test the condensate pump and its safety switch, check the pressure switch and its hose, inspect the inducer housing for water, verify the vent and intake terminations are clear and run the furnace through several heating cycles. Then we recommend the simplest permanent fix for your home and give you an upfront price before any rerouting work.

Same day help when the heat stops

We clear frozen and clogged condensate lines, fix pressure switch problems and reroute drains so they do not freeze again, with same day service and a 24/7 emergency line across Ocean, Monmouth, Burlington and Middlesex counties, including Jackson, Tinton Falls, Medford and Toms River. Our $89 heating tune-up checks the whole condensate system (terms apply).