Resideo T827K1009/U Heat Only Thermostat - 50-90 F for Vdc and millvoltage systems

The T827K1009 is a non-programmable thermostat from Resideo (Honeywell Home), heat-only, 50–90 °F range.

A non-programmable thermostat holds one setpoint until someone changes it. That is the whole design, and it is the right design more often than the market admits: in a shop, a rental, a spare room or anywhere occupancy has no schedule, a programmable stat gets overridden permanently within a week and then behaves like this one with extra steps.

This is a millivolt thermostat. It is built for systems powered by a thermopile rather than by a transformer — standing-pilot fireplaces, wall heaters, unit heaters. A 24 V thermostat will not switch reliably on millivolt power, and a millivolt stat is not rated for a 24 V circuit. Check which system you have before ordering: the giveaway is whether the appliance works during a power cut.

Where it hangs decides what it reads. A thermostat measures the air touching it and nothing else, so an interior wall about five feet up, out of direct sun, away from supply registers and doorway drafts, and clear of the back of a wall carrying hot water or flue pipe. Most complaints that a stat is “off by three degrees” are placement, not calibration — the stat is reading the truth about a spot nobody lives in.

Confirm the system type before ordering — low voltage, line voltage and millivolt thermostats look similar and are not interchangeable.

The millivolt power for this stat comes from the thermopile in the appliance. If that is what failed, see the Q313A1188 and Q313A1139.

• Brand: Resideo (Honeywell Home)
• Manufacturer Part #: T827K1009/U
• Type: Non-programmable thermostat
• Stages: Heat-only
• Temperature range: 50–90 °F
• System: Millivolt
• UPC: 085267271356
• Category: Thermostats