Furnaces and boilers, the gas side. Episode fourteen of the Gas Service Animated series.

How close can a furnace sit to something that can burn? For a listed furnace or boiler, the nameplate and the manufacturer's instructions govern the clearances. The code adds a second tool: a table of specified forms of protection that reduce those distances by specific amounts. One example from the table: a required thirty six inch clearance drops to eighteen inches above, and twelve inches at the sides, when the combustible surface is covered by twenty four gauge sheet metal spaced off the wall on a ventilated air gap. Two rules never bend. The reduction can never interfere with combustion air, draft hood relief, or access for servicing. And the distance is still measured to the combustible surface itself, ignoring the shield in front of it.

Here is the iron rule of return air. When supply ducts carry heated air to rooms outside the furnace space, the return air must come back through ducts sealed to the furnace casing, ending outside the space that contains the furnace. Return air must never be taken from the mechanical room or closet where the furnace sits. Watch why. A blower pulling from the appliance space puts that room under negative pressure. It starves the burners for combustion air, and it can drag flue gases back down through a draft hood and into the very airstream that feeds the house.

Every furnace carries a high limit: a control that watches temperature and opens the gas valve circuit if the appliance overheats. The fan control has a different job — it runs the blower that moves the heat. Watch the limit work. Airflow drops, temperature climbs past the set point, the switch snaps open, and the burner shuts down while the blower keeps running to carry the heat away. The code is blunt about these devices: safety limit controls shall not be used as operating controls. The field rule is even blunter. Never bypass, jumper, or stretch a safety. It is the last line of defense between a fault and a fire.

On a boiler, the water side protects the gas side. Every hot water boiler and every steam boiler must have a low water cutoff — an automatic means that shuts off the burner when the water level drops below the safe point, because a boiler fired dry is a catastrophic failure. Every boiler also carries its safety or pressure relief valve, and the code is specific. The discharge piping must be at least the same size as the valve outlet, it must be piped to discharge near the floor, and no shutoff valve is ever placed between the relief valve and the boiler, or in the discharge pipe. Cap or throttle a relief valve and you have removed the last safety the boiler has.

The supply plenum is the hottest sheet metal in the system, and its clearance to combustibles follows the listing, just like the appliance itself. Ducts connected to a listed furnace keep the plenum's clearance for the first stretch of their run before it can taper. Nothing combustible belongs pressed against a hot plenum — no framing, no storage, no paper-faced insulation. And a flexible appliance connector or gas piping never runs through the plenum or the ductwork. Gas lines stay outside the airstream, where they can be seen, reached, and serviced.

Now the gallery of bad ideas. A jumpered limit keeps the burner firing straight through an overheat, cooking the heat exchanger the control was built to protect. A blocked return grille starves the airflow, so the furnace rides its limit and short cycles on overheat instead of the thermostat. Storage crowding the burner compartment feeds a fire and chokes the combustion air at the same time. And a relief valve that is capped, throttled, or piped uphill traps the very pressure it was installed to release. Every one of these looks like a shortcut. Every one removes a layer of protection.

So on the furnace or boiler call, the checklist writes itself. Measure clearances against the nameplate, and confirm any reduction uses a listed protection method. Trace the return air, and confirm none of it comes from the space that holds the appliance. Prove the limits open and close on their own, with no jumpers anywhere in the circuit. Follow the relief discharge from the valve to a safe, open, downward termination. And after any remodel, confirm the room still delivers the combustion air the appliance was installed with.

Furnaces and boilers, the gas side. Clearances come from the listing, and only the specified forms of protection can shrink them. Return air never comes from the room that contains the furnace. Limit controls and the low water cutoff are the last line of defense — never jumper a safety. And the relief valve discharges full size, downward, and never capped. Next episode: vent termination rules — where the flue gases are finally allowed to leave the building.