Chattanooga Hydrocollator M-4 Heating UnitChattanooga Hydrocollator M 4 An oversized drain and extra large tank to fit 24 standard size hotpacs Requires just 20" x 32" of floor space. Provides a thermostatically controlled constant temperature to keep HotPacs at the ideal temperature for maximum therapeutic benefit. The Chattanooga M 4 Hydrocollator Full fiberglass insulation on mobile units provides energy efficiency and prevents heat loss. Simple to fill and drain. No plumbing required. 3"