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Solid fuel heating systems include wood, wood pellets, coal, and biomass (corn cobs, etc.). Most coal systems have been replaced. Wood and biomass systems serve as supplemental heating or sole heat sources in some houses. Central solid fuel furnaces, free-standing wood stoves, fireplace inserts, and masonry/factory-built fireplaces all fall under solid fuel heating. Dual-fuel systems combine solid fuel with gas or oil, and add-on solid fuel furnaces supplement existing gas/oil systems.

The episode acknowledges that solid fuel central furnaces and dual-fuel systems are uncommon in most markets. Wood stoves, inserts, and fireplaces receive coverage in a separate Fireplaces and Chimneys chapter. Biomass systems should be disclaimed with specialist evaluation recommended due to proprietary designs and limited inspector familiarity.

Related heating system components begin with condensate disposal systems required for all condensing appliances (Category IV gas furnaces/boilers, condensing oil furnaces/boilers). The Cooling System chapter covers primary condensate disposal in detail since air conditioning creates similar drainage needs. Understanding condensate acidity from Category IV gas appliances affects disposal location recommendations to prevent property damage.

Thermostats receive comprehensive treatment despite also appearing in the Cooling System chapter. Both heating and cooling systems depend on proper thermostat operation, location, and configuration. Digital versus analog thermostats, programmable features, multi-stage system controls, and heat pump-specific emergency heat settings all impact system operation. Thermostat location importance—avoiding heat sources, drafts, and poor circulation areas—affects comfort and efficiency.

Humidifiers add moisture to forced-air heating systems, particularly important with gas and oil furnaces that produce very dry heated air. Heat pumps produce less dry air, making humidifiers less critical. Installation requirements specify supply plenum mounting (preferred over return plenum to prevent moisture damage to fans and heat exchangers), level installation to prevent leaks, and avoidance of unconditioned freezing-prone areas.

Bypass ducts on some humidifiers require proper damper operation—open during heating season, closed during cooling season. Metal bypass ducts from manufacturers are required; flexible plastic HVAC duct is unacceptable. Drain pipes from some humidifiers must terminate at appropriate waste receptors.

Maintenance neglect is common with humidifiers. Evaporator panels (pads) require periodic replacement, and units need regular cleaning. Many inspected humidifiers are non-functional. Recommending evaluation of most humidifiers is appropriate given widespread maintenance deficiencies.

Fuel distribution systems provide the final topic. Heating systems require fuel delivery from sources to appliances. Natural gas systems pipe from meters; propane systems pipe from above-ground or underground property tanks; oil systems pipe from property tanks; electricity receives coverage in the Electrical System chapter.

Fuel distribution systems are often installed and maintained by plumbers, though HVAC contractors and fuel suppliers also perform this work. The natural connection between plumbing and fuel distribution means this topic receives detailed coverage in the Plumbing System chapter's fuel storage and distribution section. Cross-referencing prevents redundant content while ensuring inspectors know where to find comprehensive fuel system information.

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