CT87K ROUND HEAT THERMOSTAT (HONEYWELL) | This model has been designed to be especially user-friendly and features a simple and precise round that's been celebrated for its ease of use and precision. Mercury-free and featuring a decorative covering ring to cover surrounding wall marks, the round heat only manual thermostat is a modern classic that has earned its distinction as a top end thermostat. The CT87K Honeywell thermostat is compatible with heating only systems including: forced air (gas, oil and electric), hot water and steam, heat pumps without backup.