Water heater swaps: venting, strapping and discharge in plain English

A water heater swap is one of the few jobs where everything can look right and still be wrong. The old one comes out, the new one goes in, it makes hot water, the homeowner is happy, and you are back down the driveway in two hours. Nothing tells you that the discharge pipe now ends four feet off the floor with a threaded cap on it, because nothing will tell you until the day the valve opens.

Here are the checks, in the order you actually meet them. Most of them take seconds. None of them are about doing extra work — they are about not leaving the job in a state you would not want to stand next to.

Start with the document that outranks the code

Before anything else: the manufacturer’s installation instructions for the unit in the box. Not the one you put in last month, not the brand you have fitted for fifteen years. This one.

Clearances, the vent category, connector sizes, what the relief valve is rated for, what voids the warranty — the manual is the controlling document, and where the adopted code and the manual disagree, the more restrictive of the two is what the job has to meet. It is also the document an inspector will ask you about first, and it is the one you can always produce.

Why it mattersAlmost every argument about a heater install ends with somebody reading the manual. Be the one who read it first.

The pan, and the swap exception worth knowing

Where a leak would damage anything — a finished floor, a ceiling below, a closet with drywall — the heater goes in a pan. The pan has to be at least an inch and a half deep, big enough to catch what drips, and drained by an indirect waste pipe of at least three quarters of an inch (2018 IRC P2801.6 and P2801.6.1).

That drain runs full size and ends over a suitably located indirect waste receptor, or it runs outside and ends between six and twenty-four inches above the ground (2018 IRC P2801.6.2).

Now the part specific to swaps, and the one people get slightly wrong when they repeat it. The exception is about the drain, not the pan: where a pan drain was not previously installed, a pan drain is not required for a replacement water heater (2018 IRC P2801.6.2). The pan itself is still judged the same way as on any other install — if a leak there would cause damage, it goes in a pan. And an attic heater over a finished ceiling with a pan and no drain is a defensible install and a bad idea at the same time. The day it lets go, the customer will not care which one it was.

Why it mattersThe pan is the cheapest part of the job and the only one that limits how expensive the failure is.

The relief valve discharge, which is where most of the sins are

This is the single most commonly botched part of a swap, because the new heater’s valve is rarely in exactly the same place as the old one’s, and the old pipe almost fits. Written in plain terms, here is what the discharge pipe has to do (2018 IRC P2804.6.1):

• Be no smaller than the valve’s outlet, and run full size all the way.
• Serve that one relief device only — never tied into piping serving anything else.
• Discharge to the floor, to the pan, to a waste receptor, or outdoors, through an air gap in the same room as the heater — never connected directly to the drainage system.
• Run downhill the whole way, draining by gravity, with no traps, dips or sags.
• End at a point that is readily observable, and discharge in a way that will not scald somebody or damage the building.
• End not more than six inches above the floor or the flood level rim of the receptor, and not less than two pipe diameters above it.
• Have no threaded connection at the end of the pipe.
• Have no valve and no tee anywhere in it.
• Be made of approved material with a pressure rating of not less than 100 psi at 180°F.

Read that list against the last one you saw in a crawlspace. The two that catch people are the threads and the height. Threads on the end invite somebody to cap it, and a capped relief discharge turns a safety device into a sealed pressure vessel. The height is about nobody standing in front of it when it opens.

Why it mattersA relief valve that cannot discharge safely is not a relief valve. It is a tank with an expectation.

The gas side

Two things here, and both take a minute. A sediment trap goes downstream of the appliance shutoff valve, as close to the appliance inlet as practical — typically a tee fitting with a capped nipple installed vertically in the bottommost opening, or another device approved as an effective sediment trap (2015 IFGC 408.4). That section excepts several appliances, ranges and clothes dryers among them; a storage water heater is not one of them. Its whole job is to catch what comes down the line before it reaches the control valve.

And the connector: use a new one, rated and listed for the application, per the manufacturer’s instructions. Reusing the connector off the old heater is the classic ninety-second saving that nobody can defend afterward.

Then leak-test using a method the manufacturer’s instructions approve — leak detection solution or an approved instrument. Never with a flame, ever, and not once you have decided it is probably fine because you cannot smell anything.

Why it mattersNobody has ever been asked to explain a leak test they performed. Plenty have been asked about one they skipped.

The vent, and the check almost nobody does

On an atmospheric, draft-hood-equipped heater, the connector has to be installed with no dips or sags, sloping upward toward the vent or chimney at not less than a quarter of an inch per foot (2018 IFGC 503.10.7). A sag is where condensate collects, and a level run is where a flue quietly stops working.

Combustion air matters just as much and is easier to get wrong on a swap than on a new install, because the room has often changed since the last heater went in. Somebody finished the basement. Somebody added a bath fan, or a range hood, or a dryer. Any of those can pull hard enough to reverse a natural-draft flue.

So do the check that almost nobody does. Fire the heater, give it a couple of minutes, and hold the back of your hand or a light smoke source at the edge of the draft hood. Flue gases should be drawn up and in. If you feel them come out at you, that is spillage, and you do not leave.

Then do it again with the house in its worst case: bath fans on, range hood on, dryer running, doors to the space closed the way the family actually leaves them. That is the condition the heater will live in, and it is not the condition you tested in.

Why it mattersSpillage from a natural-draft appliance is combustion products entering the house. It is the one item on this list where the failure is not water damage.

Strapping, where it applies

Whether a tank has to be braced depends on where you are working. In California it is statute: any new or replacement water heater sold in the state since 1 July 1991 has to be braced, anchored or strapped when installed, to resist falling or horizontal movement in an earthquake (California Health & Safety Code sections 19210 to 19217).

The state’s own guideline, prepared by the Division of the State Architect, sets out how. At least two sets of bracing straps. The guideline covers tanks up to 52 gallons, and may be used up to 75 gallons if you add a third set at mid height. Above 75 gallons, the local building department approves the detail before installation.

The layout is simple: mark about nine inches down from the top of the tank for the upper set, and about four inches above the top of the controls for the lower one. The guideline’s methods use three-quarter-inch, 24-gauge perforated plumber’s tape, or half-inch EMT conduit used with that tape, anchored into studs — and it strongly recommends code-compliant flexible connectors between the heater and the water, gas and electrical lines.

If you are not in California, check what your state and local jurisdiction require. Seismic requirements are one of the most locally variable things in this trade, and the answer where you worked last year is not automatically the answer here.

Why it mattersA tank that walks in a quake takes the gas line with it. That is the reason for the rule, and it is why the lower strap has to clear the control valve rather than sit on it.

Before you put your tools in the truck

Five minutes, standing still, looking at what you built.

• Run the temperature down and back, and confirm hot water at a fixture — not at the tank.
• Look at every joint you made, dry, after it has been under pressure for a few minutes.
• Follow the discharge pipe with your eyes from the valve to its end. All downhill, no threads, no valve, ending where it should.
• Check the draft hood again, with the house exhausting.
• Photograph the connections, the vent, the pan and drain, the strapping and the data plate.

The photographs are not paperwork. Months from now, when somebody says the vent was like that when you got there, you either have the picture or you have a conversation you cannot win.

None of this makes a swap a long job. It makes it a finished one — built to the manual, built to what is adopted where you are working, and ready for whoever looks at it next.

This guide is general information to help you check your work. It is not legal advice or an official inspection, and it does not replace the requirements that apply to you, the manufacturer’s instructions, or a determination by an inspector or the authority having jurisdiction. Requirements vary by location and change over time; check the current rules where you work.