Honeywell TH2110DV1008 vs Honeywell RTH21B: Which Thermostat Fits Your Setup

The Honeywell TH2110DV1008 and the Honeywell RTH21B are both single-stage programmable thermostats aimed at homeowners replacing an old dial or basic digital unit. The decision comes down to wiring, power source, and how much scheduling flexibility the household actually needs.

This comparison covers both models on the specs that matter, names the trade-off for each, and ends with a clear "choose A if / choose B if" split. Buyers who already know their system type can jump straight to the side-by-side table.

Honeywell TH2110DV1008 PRO 2000 Overview

Model: TH2110DV1008
Series: PRO 2000
Stages: 1 heat / 1 cool
Orientation: Vertical
Display: Backlit
Power: Dual powered (battery plus system power)

This is a contractor-grade thermostat from Honeywell’s PRO line, the series sold through HVAC technicians rather than big-box shelves. The 1H/1C designation means it controls a single-stage furnace or boiler and a single-stage air conditioner — the most common residential setup in North America. The vertical format suits a wall where the existing thermostat runs tall, and the backlit display keeps setpoints readable in a dark hallway.

Dual power is the headline feature. The unit runs on system power when the wiring provides a common wire, and falls back to batteries when it does not. That combination gives installers flexibility on older homes where pulling a new conductor is impractical. The programmable schedule lets a household set separate wake, away, home, and sleep periods for weekdays and weekends.

Trade-off: The vertical orientation and PRO-series styling are built for a technician’s install, so a DIY buyer swapping a horizontal unit may find the footprint and menu structure less forgiving than a consumer-focused model.

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Honeywell RTH21B Overview

Model: RTH21B
Series: X2P
Stages: 1 heat / 1 cool
Schedule: Flexible
Display: Not specified in name; standard digital readout
Color: White

The RTH21B is Honeywell Home’s consumer-facing programmable thermostat, sold at a budget price point and built for straightforward replacement of an existing single-stage unit. The 1H/1C compatibility matches the same systems as the PRO 2000, so the two are interchangeable on most forced-air furnaces and central AC setups. The white housing blends into a standard drywall wall better than a dark contractor unit.

The "flexible schedule" label points to a programming scheme that adapts to irregular routines rather than forcing a rigid four-period weekday/weekend grid. That suits households with shift workers or people who are home at different hours day to day. The X2P platform is Honeywell’s value line, which means simpler menus and fewer installer-only settings than the PRO series.

Trade-off: At this price tier, the build is lighter and the feature set is thinner — buyers who want a common-wire install with battery backup or a backlit display in a dim hallway will find the RTH21B more limited than the PRO 2000.

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Side-by-Side Comparison

Feature	Honeywell TH2110DV1008	Honeywell RTH21B
Series	PRO 2000 (contractor line)	X2P (consumer line)
Stages	1 heat / 1 cool	1 heat / 1 cool
Orientation	Vertical	Standard horizontal
Power source	Dual powered (battery + system)	Battery powered
Display	Backlit	Standard digital
Schedule type	Programmable periods	Flexible schedule
Color	Dark PRO housing	White
Price tier	Mid-range	Budget
Best use	Technician install, common-wire systems, dim hallways	DIY swap, irregular routines, budget replacement

How the Two Thermostats Differ

The stage count is identical, so both units control the same class of equipment: a single-stage furnace or boiler paired with a single-stage air conditioner. Neither one handles heat pumps with auxiliary stages, two-stage furnaces, or multi-zone systems. Buyers with those setups need to look at a different tier entirely.

Power source is the first real divider. The TH2110DV1008’s dual power design means it can draw from the system’s common wire and keep batteries as a backup, which avoids the low-battery warning that interrupts a schedule. The RTH21B relies on batteries, so a household that ignores the low-battery chirp will eventually lose its programming.

Display and orientation come next. The PRO 2000 is vertical and backlit, which suits a wall where the old thermostat ran tall and a hallway that stays dark in the evening. The RTH21B is a standard horizontal white unit with a conventional readout — easier to match to a typical drywall patch, less readable in low light.

Scheduling philosophy is the subtlest difference. The PRO 2000 uses a fixed period structure that suits predictable routines: up at the same time, out the door at the same time, home at the same time. The RTH21B’s flexible schedule is aimed at households whose week does not repeat cleanly, and the simpler interface reflects that.

Installation difficulty rounds out the split. The PRO 2000 is designed for a technician who knows how to identify a common wire and configure installer settings. The RTH21B is built for a homeowner replacing a like-for-like unit with basic tools. Neither is a hard job, but the PRO unit assumes more background knowledge.

Which Should You Buy?

Choose the Honeywell TH2110DV1008 if: the wall has a vertical thermostat footprint, the system has a common wire available (or an installer is doing the work), the hallway is dim enough that a backlit display matters, and the household keeps a predictable weekly routine.

Choose the Honeywell RTH21B if: the goal is a fast, low-cost replacement of a standard horizontal unit, the schedule changes week to week, and the install is a DIY job with basic tools. The budget price tier makes it the lower-risk pick for a rental or a secondary property.

Buyers who are unsure about the common wire should check the existing thermostat’s terminal labels before ordering. A unit with a C terminal present makes the PRO 2000 the stronger choice; a two-wire or battery-only setup points toward the RTH21B.

FAQ

1. Are the Honeywell TH2110DV1008 and RTH21B interchangeable?
   Both are 1 heat / 1 cool single-stage thermostats, so they control the same equipment class. The wiring and power requirements differ, which is where the choice gets made.

2. Does the RTH21B need a common wire?
   The RTH21B is a battery-powered unit, so it does not depend on a common wire to operate. The TH2110DV1008 can use system power when a C wire is present and falls back to batteries when it is not.

3. Which thermostat is better for a heat pump?
   Neither. Both are single-stage 1H/1C models and do not support heat pump auxiliary stages. A heat pump household needs a different model.

4. Is the TH2110DV1008 a DIY install?
   It can be, but the PRO 2000 is built around installer configuration and assumes familiarity with thermostat wiring. A homeowner without that background will have an easier time with the RTH21B.

5. Which one has a backlit display?
   The TH2110DV1008 is the backlit model. The RTH21B uses a standard digital readout.

6. Can both units run a schedule?
   Yes. The PRO 2000 uses fixed programmable periods, while the RTH21B offers a flexible schedule aimed at irregular routines.

7. Which is the budget pick?
   The RTH21B sits at a lower price tier. The PRO 2000 costs more because of the dual power design, backlit display, and contractor-grade build.

Bottom Line

For a homeowner with a common wire, a vertical wall footprint, and a predictable weekly routine, the Honeywell TH2110DV1008 is the stronger thermostat — dual power and a backlit display are real advantages that the budget model cannot match. For a fast, low-cost DIY replacement where the schedule changes week to week, the Honeywell RTH21B is the sensible pick.