Here are some of the best free or low-cost venue automation systems and platforms that facility managers can use when planning technical retrofits on a tight budget. I’ll focus on tools that help with lighting, HVAC, access control, room scheduling, energy monitoring, and basic building automation planning without requiring a large upfront software cost.

1. Home Assistant

Best for: Flexible building/venue automation on a budget
Cost: Free, open source

Why it’s useful

• Works with a huge range of devices and protocols
• Can integrate lights, sensors, HVAC, occupancy, power monitoring, and more
• Good for prototyping retrofit concepts before buying enterprise systems
• Supports MQTT, Modbus, Zigbee, Z-Wave, BACnet via add-ons or bridges

Good for facility managers

• Creating a centralized dashboard
• Testing automation rules for occupancy-based lighting or HVAC
• Integrating mixed legacy and modern equipment

Limitations

• Not an enterprise BMS out of the box
• Requires some technical setup and maintenance

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2. OpenHAB

Best for: Open-source building automation with strong protocol support
Cost: Free, open source

Why it’s useful

• Mature automation platform
• Broad device compatibility
• Useful for multi-system integration in older venues
• Good rule engine and dashboard support

Good for

• Lighting schedules
• Sensor-based automation
• Combining multiple subsystems into one interface

Limitations

• Can be more technical to configure than commercial tools
• Smaller community than Home Assistant in some areas

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3. Node-RED

Best for: Workflow-based automation logic and retrofitting control layers
Cost: Free, open source

Why it’s useful

• Visual programming makes automation logic easier to design
• Great for integrating APIs, sensors, controllers, and data streams
• Often used alongside Home Assistant or MQTT brokers

Good for

• Event-driven workflows
• Occupancy-triggered actions
• Alarm and notification routing
• Proof-of-concept automation

Limitations

• Not a complete venue management system by itself
• Usually needs to be paired with other tools

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4. Grafana + InfluxDB

Best for: Monitoring energy, environmental data, and retrofit performance
Cost: Free tiers available; open-source versions available

Why it’s useful

• Excellent for visualizing trends from utility meters, temperature sensors, CO2 sensors, and occupancy data
• Helps justify retrofit investments with data
• Can track before-and-after performance

Good for

• Energy benchmarking
• HVAC optimization analysis
• Reporting for stakeholders

Limitations

• Monitoring/analytics, not direct automation
• Needs data sources and some configuration

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5. ThingsBoard Community Edition

Best for: IoT-based facility monitoring and control
Cost: Free community edition

Why it’s useful

• Designed for connected devices, telemetry, dashboards, and rule chains
• Useful for distributed venues or multi-site setups
• Can manage sensor data and simple control logic

Good for

• Asset monitoring
• Environmental sensors
• Alerting and dashboards

Limitations

• More IoT-platform oriented than classic building automation
• May require a bit of platform learning

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6. Blynk IoT

Best for: Simple remote monitoring and control prototypes
Cost: Free plan available

Why it’s useful

• Fast to deploy for small retrofits or pilot areas
• Mobile-friendly dashboards
• Works well for custom controller projects

Good for

• Pilot automation zones
• Quick proof-of-concept work
• Simple sensor dashboards

Limitations

• Free tier limits can be restrictive
• Not ideal for large complex venues

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7. MQTT + Mosquitto

Best for: Low-cost communication backbone for retrofit systems
Cost: Free, open source

Why it’s useful

• MQTT is a lightweight standard for device messaging
• Mosquitto is a reliable open-source broker
• Often forms the backbone of DIY and semi-professional automation stacks

Good for

• Connecting sensors, controllers, and dashboards
• Decoupling old and new systems during retrofits
• Integrating multiple platforms

Limitations

• Not a full UI or management layer
• Requires other tools on top for scheduling and visualization

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8. BACnet Tools / Open-source BACnet Integrations

Best for: Facilities with existing HVAC/building control infrastructure
Cost: Free tools and libraries available

Why it’s useful

• BACnet is common in commercial HVAC and BMS systems
• Open-source BACnet stacks and utilities can help with retrofits, diagnostics, and integration planning

Good for

• Reading legacy building controller data
• Identifying integration opportunities
• Testing interoperability before vendor selection

Limitations

• Mostly technical tools, not polished facility software
• Usually needs someone comfortable with controls/networking

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9. OpenRemote

Best for: Open-source smart building and facility integration
Cost: Open-source/community options available

Why it’s useful

• Built for asset management, monitoring, and automation
• Good for integrating multiple building systems
• More “facility/industrial” oriented than hobby platforms

Good for

• Central dashboards
• Multi-system orchestration
• Retrofit planning and phased deployment

Limitations

• Less beginner-friendly than some alternatives
• Deployment can be more involved

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10. SpinalCom / other open smart-building platforms

Best for: Larger technical retrofits needing flexible integration
Cost: Varies; some community/open components available

Why it’s useful

• Aimed at smart building integration
• Can support complex retrofits and heterogeneous assets

Limitations

• Availability, support, and licensing vary a lot
• Not always truly free end-to-end

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Best choices by use case

If you want the easiest free starting point:

• Home Assistant
• Node-RED
• Grafana + InfluxDB

If you need strong protocol integration:

• OpenHAB
• MQTT + Mosquitto
• BACnet tools

If you want IoT-style dashboards and telemetry:

• ThingsBoard Community Edition
• OpenRemote

If you are doing a pilot retrofit zone:

• Blynk
• Home Assistant + Node-RED

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What facility managers should look for in a retrofit tool

For budget-limited venue retrofits, prioritize systems that support:

• Open standards: BACnet, Modbus, MQTT, Zigbee, Z-Wave
• Legacy integration: older HVAC and lighting controllers
• Occupancy-based control
• Energy monitoring
• Scalable dashboards
• Alarm/notification handling
• Easy phased deployment

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Practical recommendation

For most budget-conscious venue retrofits, a strong free stack is:

• Home Assistant for device/control integration
• Node-RED for automation logic
• MQTT/Mosquitto as the message backbone
• Grafana + InfluxDB for performance tracking

That combination gives you:

• Low software cost
• Good flexibility
• Useful retrofit planning data
• Room to grow into a bigger BMS later

If you want, I can also give you:

1. a comparison table,
2. a recommended stack by venue type (theater, stadium, convention center, school, etc.), or
3. a free tools shortlist specifically for HVAC/lighting retrofits.