Vermont Energy Control Systems

Practical monitoring and control for the real world

Greenhouse Subsystem Details

The greenhouse subsystem has the four circulators, two zone valves, a heat exchanger, and four zones with differing properties:

  1. Office Radiant: Small heat load (<5000 BTU/hr), low temperature (<120℉). Controlled by standard thermostat.
  2. Mechanical Room Radiant: Small heat load (<5000 BTU/hr), low temperature (<120℉). Controlled by standard thermostat.
  3. Greenhouse Slab: Large heat load (>110,000 BTU/hr). Desired Glycol temperature dependent on outdoor temperature, wind speed, and sunlight. Controlled by a ‘dry contact’ in greenhouse management system.
  4. Benches: Moderate heat load (10,000-70,000 BTU/hr). Ideally operates at higher temperatures (120℉ - 150℉). Controlled by a ‘dry contact’ in greenhouse management system.
A diagram of the heat exchanger setup

Originally this subsystem was controlled with a collection of commercial zone controllers. Demand from any zone would turn on the appropriate circulator. If any zone was calling for heat, circulator P15 would also run to provide hot water to the heat exchanger. P18 is a Grundfos Alpha 15-55, configured to provide constant pressure. The other circulators are fixed-speed units. The mixing valve (V1) was controlled based on outdoor temperature in order to provide warmer water to the radiant zones when outdoor temperatures were cooler.