Honeywell RLV3120A1005 vs Honeywell RTH21B: Line-Volt Heat vs Programmable Comfort

The Honeywell RLV3120A1005 and the Honeywell RTH21B look like two versions of the same errand, but they belong to different electrical worlds. The RLV3120A1005 is a digital non-programmable thermostat built for electric heat only, the kind of control that lives on a wall next to a baseboard heater or a wall panel. The RTH21B is a programmable single-stage thermostat for a conventional heating and cooling system, with a flexible schedule and 1 heat/1 cool compatibility.

That split decides most of the purchase before either model’s features get a hearing. A buyer with electric resistance heat needs the line-voltage unit; a buyer with a furnace and an air conditioner needs the low-voltage programmable one. The comparison below works through the differences that matter, then names which buyer belongs with which thermostat.

Honeywell RLV3120A1005 Digital Non-Programmable Thermostat for Electric Heat Only

Type: Digital non-programmable thermostat
Intended Use: Electric heat only
Brand: Honeywell Home
Model: RLV3120A1005
Control Style: Digital, manual setpoint
Schedule: None

The RLV3120A1005 is a single-purpose device. It controls electric resistance heating — baseboard units, wall panels, and similar line-voltage loads — and it does so with a digital readout and a manual setpoint rather than a weekly schedule. There is no programming to learn, no app, and no setback routine to configure; the occupant dials a temperature and the thermostat holds it.

That simplicity is the whole argument for this model. Electric heat zones are often small and separately controlled, and a homeowner with a few baseboard heaters may want each one set independently without wiring a network or programming four schedules. The digital display makes the setpoint easier to read than a mechanical dial, and the non-programmable design removes the most common source of confusion in thermostat ownership.

Trade-off: With no schedule and no remote access, the RLV3120A1005 cannot save energy automatically — every setback has to be done by hand, which means an empty house often stays heated.

Check the current price on Amazon

Honeywell RTH21B X2P Programmable Thermostat

Type: Programmable thermostat
System Compatibility: 1 heat / 1 cool
Stages: Single-stage
Schedule: Flexible
Brand: Honeywell Home
Color: White

The RTH21B is the conventional-system counterpart. It is designed for a single-stage heating and cooling setup — one heating stage, one cooling stage — and it adds a flexible programming schedule on top of that control. The white housing is meant to sit on an interior wall and blend in, and the programmable interface is the feature that separates it from a basic manual thermostat.

For a household with central air and a furnace, the schedule is where the value sits. A programmable thermostat lets the occupant set warmer and cooler periods around the household’s routine, so the system is not conditioning an empty house at full demand. The 1 heat/1 cool compatibility covers the most common residential configuration in the country, which is exactly why this model exists at an accessible price tier.

Trade-off: Single-stage compatibility and a flexible schedule are the whole feature set — this is not a smart thermostat, so there is no phone control, no learning behavior, and no remote adjustment from outside the house.

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Side-by-Side Specifications

Specification	Honeywell RLV3120A1005	Honeywell RTH21B
Type	Digital non-programmable	Programmable
Intended Use	Electric heat only	1 heat / 1 cool conventional system
System Stages	Electric resistance heating	Single-stage
Schedule	None	Flexible schedule
Control Interface	Digital, manual setpoint	Programmable interface
Color	Not stated in name	White
Brand	Honeywell Home	Honeywell Home
Price Tier	Budget	Budget to mid

Reading the Differences

The first row of that table is the one that ends the debate for most buyers. These two thermostats do not connect to the same kind of wire. The RLV3120A1005 is a line-voltage control for electric heat, and the RTH21B is a low-voltage control for a central system with a furnace and an air conditioner. Putting either one on the other’s wiring is not a preference question; it is an incompatibility.

The second real difference is scheduling. The RTH21B carries a flexible schedule, and the RLV3120A1005 carries none. For a central system running through a whole house, that schedule is the main cost-saving mechanism the device offers. For a single baseboard heater in a room that is occupied on an irregular basis, a schedule is often less useful than a simple manual setpoint, which is why Honeywell built the electric-heat model without one.

The third difference is scope. The RTH21B is a whole-home device: one thermostat, one system, one schedule covering the house. The RLV3120A1005 is a zone device, and a home with electric baseboard heat typically needs one per zone. That changes the math — a buyer comparing sticker prices is not comparing equivalent purchases, because the electric-heat path may require several units.

The fourth difference is the feature ceiling. Neither thermostat is smart. The RTH21B’s programmability is its top feature, and the RLV3120A1005’s digital display is its. A buyer who wants remote access, geofencing, or usage reports is shopping in the wrong category entirely, and should look at Honeywell’s connected models instead of either of these.

Which Should You Buy?

Choose the Honeywell RLV3120A1005 if the heating is electric resistance — baseboard, wall panel, or similar line-voltage equipment — and the goal is straightforward local control. It suits a homeowner who wants a readable digital setpoint, does not want to program anything, and is comfortable adjusting temperatures manually. It is also the right pick for a single room or zone where a schedule would never match the actual occupancy pattern.

Choose the Honeywell RTH21B if the home has a conventional single-stage furnace and air conditioner and the priority is a set-and-forget weekly schedule. It suits a household with a predictable routine that wants the system to ease off during work hours and overnight without anyone remembering to turn it down. The 1 heat/1 cool compatibility covers the standard residential setup, and the white housing keeps the wall installation unobtrusive.

Choose neither if the goal is app control, voice commands, or learning behavior. Both of these are conventional thermostats, and neither connects to a phone.

FAQ

1. Can the Honeywell RLV3120A1005 replace the Honeywell RTH21B?
   No. The RLV3120A1005 is a line-voltage thermostat for electric heat only, and the RTH21B is a low-voltage thermostat for a 1 heat/1 cool system. They control different electrical loads and are not interchangeable.

2. Can the RTH21B control electric baseboard heating?
   The RTH21B is specified for 1 heat/1 cool single-stage systems, not for electric resistance heating. A baseboard heater needs a line-voltage thermostat like the RLV3120A1005.

3. Does the RLV3120A1005 have a programmable schedule?
   No. It is a digital non-programmable thermostat, so the setpoint is adjusted manually and there is no weekly schedule.

4. Does the RTH21B work with a two-stage or heat-pump system?
   The RTH21B is a single-stage thermostat with 1 heat/1 cool compatibility. Multi-stage and heat-pump systems need a thermostat rated for those configurations.

5. Which one costs less to buy?
   Both sit in the budget range for their category, and the RTH21B’s listed price is modest for a programmable model. Buyers should confirm current pricing on Amazon, since both fluctuate.

6. How many RLV3120A1005 units does a house need?
   One per electric heat zone. Baseboard and wall-panel heating is typically zoned room by room, so a whole-house installation usually requires several units.

7. Do either of these thermostats offer phone control?
   No. Neither the RLV3120A1005 nor the RTH21B is a smart thermostat, so there is no app, no remote access, and no voice assistant integration.

Bottom Line

The Honeywell RLV3120A1005 vs Honeywell RTH21B decision is settled by the wiring, not the feature list: electric heat takes the RLV3120A1005, and a single-stage furnace-and-AC system takes the RTH21B. For most buyers with a conventional central system, the Honeywell RTH21B is the better purchase — its flexible schedule delivers the energy savings that a non-programmable thermostat cannot, at a budget-friendly price. Buyers heating with electric baseboard should go the other way and pick up the RLV3120A1005, one per zone.