Gas water heater installation, and the part you cannot see

Start at the exhaust, not the appliance

A competent installer looks at where the products of combustion are meant to go before deciding what unit can be installed. That means inspecting the existing flue or chimney for size, condition, slope and blockage, and establishing whether anything else vents into it. The answer determines the unit type, so it comes first.

Watch for: A quote produced without anyone looking at the flue. Venting is the constraint on a gas water heater install, and a price that has not accounted for it is a placeholder.

Check whether the chimney has been orphaned

This catches a lot of people. When a furnace is upgraded to a high-efficiency model, it stops venting into the shared masonry chimney and vents out the side wall instead. The water heater is then alone on a flue that was sized for two appliances. Too much cross-section for too little exhaust means the flue runs cold, draft becomes unreliable, and moisture condenses inside the chimney and attacks it.

Watch for: A recent furnace replacement that nobody connected to the water heater. Relining the chimney to a suitable size is the usual remedy, and it is a real cost that tends to surface as a surprise.

Confirm there is enough combustion air

An atmospheric unit needs air from the room to burn, and it needs the room to be able to replace that air. Tight mechanical closets, sealed basements and modern air-tight construction all reduce what is available, and so does every other appliance competing for it — a dryer or a range hood pulling air out of the house can starve a water heater outright.

Watch for: A heater that was fine before a renovation and now backdrafts. Finishing a basement, replacing a louvred door with a solid one, or adding a powerful exhaust fan can each break a combustion air supply that worked for twenty years.

Choose the venting strategy the space actually supports

Atmospheric venting relies on hot exhaust rising naturally, which only works where a suitable flue exists. Power venting uses a fan to push exhaust horizontally out a side wall, which frees you from the chimney but needs an electrical supply and makes an audible noise. Direct vent draws combustion air from outside and exhausts outside through a sealed assembly, which is what makes it viable in tight or confined spaces.

Watch for: Power vent chosen for convenience without anyone mentioning the noise or the need for a nearby receptacle. Both are fine when you know about them and both cause complaints when you do not.

Gas supply, sediment trap and shut-off

The supply is connected with a shut-off valve within reach, a drip leg or sediment trap where required, and an approved connector rather than an improvised one. Every joint is leak-tested with a proper method before the appliance is fired.

Watch for: Leak testing done by smell, or by a squirt of soapy water on only the joint that was just made. All disturbed joints get tested, and a soap test on a threaded joint under a tank is not always conclusive on its own.

Set the unit, connect water and fit the relief discharge

The tank is set level with the manufacturer's clearances to combustibles maintained, water lines connected, and the temperature and pressure relief valve given a full-size discharge line terminating where code allows it to terminate. Where the unit is in a garage, elevation and impact-protection rules may apply locally.

Watch for: A relief discharge reduced in diameter, threaded with a cap, or run uphill. It is the appliance's last-resort safety device and it has to be able to dump freely.

Fire it and prove the draft

The unit is filled and purged before firing, then lit and observed. The installer should verify that the flue is actually drawing while the burner runs — not assume it. On an atmospheric unit this is commonly checked at the draft hood shortly after ignition, and ideally with the house in its worst case: exhaust fans and dryer running, doors closed.

Watch for: A draft test performed with every window open. That is a test of a house that does not exist. The realistic worst case is a closed-up house with the dryer and range hood running, and that is when backdrafting actually happens.

Inspection, CO alarms and handover

The permit gets closed by an inspector who looks at the vent, the gas work and the relief discharge. Before the installer leaves, you should know where the gas shut-off is, what the burner should look like, and that working carbon monoxide alarms are in place.

Watch for: No conversation about CO alarms at all. An installer who has just commissioned a combustion appliance in your home and never mentions carbon monoxide detection has told you something about how they work.