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How a Design Engineer Decodes Division 23 Specs for Electric Unit Heaters (EUH) – A Practical Controls Perspective

November 21, 2025

When reviewing Division 23 – HVAC specifications, you’ll often find short control statements like:

“Electric unit heater shall have a standalone wall-mounted thermostat that energizes the heater below 50°F.”

For many new engineers, this looks simply…
But translating this one line into a complete controls submittal requires real understanding.

Here’s how I decode it:

Identify the Control Intent
1. The key phrase: “Standalone thermostat” + “energize below 50°F.”
2. This tells you that:
3. The heater must not depend on BMS/BAS to operate.
4. Freeze protection is automatic and local.
5. The thermostat is mechanical or electronic, but not network-controlled.

Define the Control Strategy
1. Your submittal should clearly show:
2. Control Input: Wall thermostat (SPST).
3. Action: Energizes heater when space temp < 50°F.
4. Manual Reset?: Only required if spec mentions high-limit protection.
5. Power Source: Direct electrical feed (not through BAS relay).
6. BAS Monitoring: Optional ON/OFF status via dry contact (if spec allows).

 Prepare the Control Submittal Drawing Include:
1. Thermostat location
2. Heater power connection
3. Control wiring diagram (T-stat -> Heater)
4. High-limit safety (factory-installed)
5. Optional status point to BMS panel
6. Sequence of operation summary

Write the Sequence of Operation
Example wording for your submittal:
“The electric unit heater shall be controlled by a stand-alone wall-mounted thermostat. When sensed room temperature falls below 50°F, the thermostat closes the circuit and energizes the heater. The heater operates independently of the BMS to ensure reliable freeze protection. Optional status feedback may be provided to the BAS where required.”

🟢Why 50°F?
This setpoint is chosen as a freeze-protection margin to protect:
1. Pipes
2. Mechanical equipment
3. Critical control panels
4. Low-temperature spaces (mechanical rooms, pump rooms, electrical rooms)

Conclusion
A single line in Division 23 can expand into an entire control philosophy, wiring diagram, sequence, and submittal package - that’s the value a good controls design engineer brings.

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