Enthalpy

Supported Applications

The Enthalpy Series from Automation Components, Inc. (ACI) is a family of enthalpy transmitters and changeover controllers for HVAC economizer control. By measuring both temperature and relative humidity and combining them into a single enthalpy value, these devices help air-handling systems decide when outdoor air is actually cooler and drier to use than return air, improving free-cooling efficiency.

The series serves indoor and outdoor air monitoring in commercial and institutional HVAC systems: the transmitter reports enthalpy as a linear analog signal to a building controller, while the changeover controllers switch a relay directly at field-set trip points to command an economizer damper. A differential model compares indoor against outdoor conditions so the system always selects the lower-enthalpy air source.

Operating Conditions & Performance

The Enthalpy Series measures enthalpy over a range of 0 to 50 BTU with an accuracy of ±1 BTU at 77°F (25°C) and a repeatability of 0.5% RH. Long-term stability is rated at less than 2% RH drift over five years, and the relay changeover models use a deadband of 1 BTU or 0.5°F (0.27°C).

The transmitter provides a 4 to 20 mA loop-powered output, while the changeover controllers switch a Form 1C relay rated 250 VAC for dry-contact economizer control. Controllers accept 24 to 36 VDC or 24 VAC ±10% power and draw up to 70 mA, whereas the loop-powered transmitter runs on 24 to 36 VDC and draws up to 25 mA.

All models operate over an ambient range of -40°F to 140°F (-40°C to 60°C) and 0 to 95% RH non-condensing, and share a common housing measuring 85.22mm high by 107.95mm by 34.47mm deep. The enthalpy calculation uses a standard elevation of 1000 ft above sea level, and the products are RoHS compliant.

Configuration Options

The Enthalpy Series divides into a transmitter and a set of changeover controllers that share the same temperature and humidity sensing but differ in how they act on the reading:

• Enthalpy transmitter — converts the sensor into a linear 2-wire, 4 to 20 mA loop-powered output for a building controller, powered from 24 to 36 VDC.
• Changeover controllers — compare the measured enthalpy against internal trip points and switch a Form 1C relay rated 250 VAC directly, powered from 24 to 36 VDC or 24 VAC.
• Differential control — the differential model compares indoor against outdoor conditions and is ordered together with the transmitter, which supplies the 4 to 20 mA enthalpy input it acts on.

Beyond the base model, controllers offer field-selectable set-up: an enthalpy/temperature changeover curve can be chosen with temperature set points such as 75°F, 70°F, 67°F, and 63°F, along with elevation and slope selections made in the field to suit the site.

Key Product Differences

The standard lineup follows the ACI ordering structure and consists of four models:

• A/ENT (Item 122439) — Enthalpy Transmitter, 0 to 50 BTU, 4 to 20 mA loop powered.
• A/ENT-CTRL (Item 122440) — Outdoor Enthalpy/Temperature Changeover Control with a 24 VAC Form 1C relay contact.
• A/ENT-CTRL-F1C (Item 122441) — Outdoor Enthalpy/Temperature Changeover Control with a Form 1C dry-contact relay.
• A/DIFF-ENT (Item 122426) — Differential Enthalpy Changeover Control, indoor vs outdoor, with a 24 VAC Form 1C relay contact, ordered together with the A/ENT.

For custom orders, the model is built up as A/ - <Sensor Series> - <Accuracy> - <Elevation> - <Curve> - <Slope>, where the sensor series selects the transmitter or one of the changeover controls and the elevation, curve, and slope are configured to match the application.