Your Boiler Drains Water All Winter. Where Does Yours Go?

Oct 19

3 min read

Every January I take a call that sounds like a broken boiler and isn't. The house is cold, the boiler is locked out on a fault, and the appliance itself is fine. What stopped it was a half-inch plastic drain line that froze solid overnight.

If you have a condensing boiler, it makes liquid water every time it runs, and that water has to get out of the building. In Colorado, where that drain line ends up is the difference between a system that runs all winter and one that quits on the coldest night of the year.

Is this even your boiler?

Easy test — look at how it vents.

If the flue goes up a masonry chimney or a double-wall metal B-vent, this isn't about you. That boiler keeps its flue gas hot and dry on purpose, and the water vapor goes up the stack as heat.

If it vents through white or grey plastic pipe out a sidewall — PVC, CPVC or polypropylene — you have a condensing boiler. Those materials are only used there because the flue gas comes out cool and wet. Somewhere near the bottom of the appliance there is a small drain, and it runs whenever the burner does.

Variable-speed circulator, copper mains and plastic drain piping in a Denver-area boiler plant — Plastic vent and drain pipe in a Front Range plant. Plastic is only used where the flue gas is cool enough to condense.

Why does a condensing boiler make water at all?

That is the whole efficiency story in one sentence: a condensing boiler pulls its flue gas below the dew point on purpose.

A conventional boiler has to keep its flue gas hot enough to stay dry and rise up a chimney, and all the heat held in that water vapor leaves the building with it. A condensing boiler takes that heat back instead, and liquid water is what's left over. The condensate is a byproduct of the efficiency you paid for.

It is not rainwater. Condensate off a gas appliance runs about pH 3 to 5 — acidic enough to chew up metal drain piping. So the drain run gets condensate-rated material rather than bare metal, and it gets a neutralizer: a canister of limestone or marble chips the condensate trickles through on its way out. White chips when it's working. Gone, or packed down into a grey paste, and it isn't neutralizing anything.

Three-zone pump controller wired open on a mechanical-room wall in Littleton, Colorado — A zone controller, wired open at commissioning. The condensate side deserves the same attention and almost never gets it.

So why does it freeze?

Because it is a gravity drain carrying a trickle.

A condensate line doesn't move much water and it doesn't move it fast. There's no flushing action to keep it clear and no warm mass to carry it through a cold night. It is about the easiest pipe in a house to freeze.

Run to a drain inside the heated envelope — or to a small condensate pump that lifts it to one — that line is a non-issue for the life of the boiler. Run out through a rim joist, into a crawlspace, or across an unheated garage, it works fine in October.

Then the first hard freeze arrives and the line blocks. The boiler either sees the blockage and faults itself off, or the condensate backs up inside the appliance, which is the expensive version. Either way the heat stops on the night it was needed most, and the thing that failed was never the boiler.

Radiant manifold and PEX runs mounted under floor joists in an unconditioned crawlspace, Denver metro — A crawlspace: joists, a manifold, a duct. It is also the first place I look when a boiler faults on the coldest night.

What should I check before it gets cold?

Three things, and they take about five minutes.

Follow the drain line. Start at the boiler and trace it to where it ends. If any part of that run leaves heated space — a crawl, an attic, a garage, straight out through a wall — that is the piece to deal with now rather than in January.
Find the neutralizer. Usually a white plastic canister or tube sitting inline on the drain, sometimes on the floor beside the boiler. Take the cap off. If you can't see white chips in there, it's spent.
Look at your vent terminations outside. Both pipes, exhaust and intake, need to stay above the snow. A terminal buried in a drift stops a boiler exactly as effectively as a frozen drain does.

The honest version

Rerouting a condensate line to an interior drain, or adding a pump so it can reach one, is an afternoon's work in October. A no-heat call in the middle of a cold snap is a different kind of day for everybody.

If you want a second set of eyes on it, send me a photo of your boiler and where the drain goes. I can usually tell from the picture whether it is set up to get through the winter.

Ryan Van Inwegen · ByDesign Hydronics & Air · Littleton, Colorado · Jefferson County Class A Master Mechanical license 1044527