/ Field knowledge · IMC 804.3.8
Fuel is still burning when the draft fan stops.
When a manually fired appliance depends on mechanical draft, the question is what happens while fuel is still burning and the draft device stops. A fan symbol and an electrical circuit do not explain that failure. UL 378 listing, visible and audible warning, battery backup on the warning, and a smoke detector in the room — each with its own backup if building-wired — have to work together.
Educational 2024 International Mechanical Code (IMC) 804.3.8 from Daily Code Talk. Confirm the adopted edition. Distinct from mechanical draft and interlocks (804.3). The detailed failure-response procedure belongs to the selected system. This is not a controls design.
- The mechanical draft device must be listed and labeled to UL 378 and installed as its manufacturer directs.
- A warning device must give both a visible indication and an audible signal when the draft device fails or electrical power is lost while it is turned on. That warning needs battery backup if it receives power from building wiring.
- A smoke detector is required in the room containing the appliance or fireplace; it likewise needs battery backup when building-wired. The backup requirement applies to each device individually.
What this does not establish: battery backup for the warning is not backup power for the draft fan. A smoke detector is not a carbon-monoxide detector and does not replace other detection. A fan-running light, by itself, is not the required failure warning.
Can you say the sequence without looking at the drawings?
How draft-device failure and power loss are detected, how both warnings operate, and which components remain functional on their specified backup. If that explanation depends on an undocumented assumption, resolve it before closeout — not after finishes are complete. Do not create an uncontrolled live-fire or power-loss test.
Source: Daily Code Talk — IMC 804.3.8 · 804.3: mechanical draft · termination: 804.3.3–804.3.4 · solid-fuel: 801.11.
What to send
Appliance type (manually fired?), the draft-device cut sheet, and whether a control diagram names failure, power loss, visible/audible warning, and backup.
If a fireplace or wood stove has a draft fan
Send the sequence. MasterBuild will say whether 804.3.8 still has no UL 378 path — or the warning is only a running light.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
MasterBuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. UL 378 and the warning / smoke-detector / battery-backup controls are 2024 IMC 804.3.8 education from Daily Code Talk — not a controls specification. This page is not legal advice.