AI Voice Receptionist for HVAC Businesses: Full Build From Scratch Using Retell AI
Brendan walks through building a complete AI voice receptionist for an HVAC company — from call flow diagram to production-ready agent that books appointments, handles emergencies, and transfers calls automatically
The Strategy
Brendan Jowett of Inflate Agency built a fully operational AI voice receptionist for an HVAC business using Retell AI and ElevenLabs. The system handles inbound calls autonomously — greeting customers, routing them to the right flow (new job booking, quote request, emergency, general questions, appointment check), collecting customer information, booking appointments via Cal.com, and doing live call transfers for emergencies. The build starts with a conversational flow diagram in Whimsical, uses ChatGPT to generate a structured markdown prompt from the diagram, and then deploys via Retell AI with ElevenLabs voices. The agent is stress-tested using Reliable AI — a simulated testing platform that runs AI-generated customer personas through the agent hundreds of times before going live.
How It Works
Map the conversational flow in Whimsical — diagram every pathway the agent needs to handle: new job booking, quote requests, emergencies, general questions, appointment status, cancellations and rescheduling.
Export the diagram and feed it into ChatGPT with a prompt framework (Role, Task, Flow, Examples, Notes) to generate a structured markdown prompt.
Create a Retell AI account and start a new single-prompt voice agent. Paste the generated markdown prompt into the agent builder.
Select ElevenLabs as the voice provider and choose an appropriate voice for the business persona.
Configure built-in functions — end call trigger, emergency live call transfer (warm transfer with AI debrief), and Cal.com calendar integration for appointment booking.
For CRM systems that need custom API calls, connect via Make.com or n8n as a middleware layer to format and send booking data.
Purchase or redirect a phone number through Retell AI using Twilio — or set up a redirect from the existing business number to the Retell AI number for after-hours handling.
Test manually by calling the agent and running through each flow to check for speech artifacts, missed questions, or out-of-order responses.
Connect the agent to Reliable AI — paste the Retell agent ID and API key — to auto-generate 26+ test cases covering every flow in the prompt.
Create customer personas (age, accent, demeanor) in Reliable and generate scenario-based simulated calls to stress test the agent at scale before going live.
Results
Brendan's agency Inflate AI builds and sells these systems to HVAC and trades businesses. The video demonstrates a working agent that correctly handles new customer intake, collects name, address, phone number, system type, and issue description, then routes to appointment booking. Simulated testing platform Reliable AI auto-generates test cases and runs AI personas against the agent to surface issues before production. Real production systems have been tested at 100-200 live calls per deployment.
Our Take
This is the most complete HVAC AI receptionist walkthrough available — Brendan covers every stage from scoping to stress testing, which most tutorials skip entirely. The Whimsical → ChatGPT → Retell pipeline is a replicable agency workflow that any builder can follow. The insight about hyphens causing speech artifacts in ElevenLabs is a specific, hard-won production detail that saves hours of debugging. The warm call transfer for emergencies — where the AI briefs the human before connecting the customer — is a genuinely smart UX decision. Reliable AI for simulated testing is the real differentiator here: most builders manually test 5-10 calls and ship. Testing at 200+ simulated calls with varied personas is how you build something that actually holds up in production. Best suited for automation agencies building AI receptionist services for local trades businesses.
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