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OpenMEP duct sizing

OpenMEP duct sizing

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github.com/samarmstrongScience & Education0Updated 3w ago
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About OpenMEP duct sizing

OpenMEP duct sizing is an MCP server for agents and HVAC engineers that sizes duct sections and grades existing ones using fixed equal-friction or Manual D rates. It accepts airflow as an input and returns preliminary results for engineering review.

Key features

  1. 01Size individual duct sections
  2. 02Analyze connected duct networks
  3. 03Apply Manual D friction rates
  4. 04Grade existing ducts
  5. 05Flag network and sizing issues

Use cases

  1. 01Choose duct sizes from design airflow
  2. 02Review existing duct dimensions
  3. 03Check a network against fan static pressure

MCP support

Exposes `size_duct` and `size_duct_network` tools to MCP clients. The tools are read-only and return sizing recommendations, grades, and findings.

Configuration

Local server config is the same for every client. Copy it into your client’s MCP config file.

mcp.json · Universal config
{
  "mcpServers": {
    "samarmstrong-openmep": {
      "command": "npx",
      "args": [
        "-y",
        "@openmep/mcp"
      ]
    }
  }
}
Run commandnpx -y @openmep/mcp

Local servers need a runtime on your machine (e.g. Node.js 18+). Config file locations differ by client; see your client’s docs.

Packages
npm@openmep/mcp0.2.0

Tools

2

Saved tool list · updated 2w ago

ParameterTypeRequiredDescription
cfmnumberYesAirflow the run carries, CFM.
roleenumNomain (at equipment), branch (serves several terminals), or runout (one terminal). Default main.main | branch | runout
existinganyNoExisting section to grade: { diameterIn } for round, or { shape: rect|oval, widthIn, heightIn }.
airflowTypeenumNoSystem, which sets the velocity cap. Default supply.supply | return | exhaust | outside-air

Details

From the project README

@openmep/mcp

An MCP server that gives any agent OpenMEP's deterministic HVAC duct sizing. No editor, CAD host, account, or network access: the agent describes the ducts, the engine sizes them.

{
  "mcpServers": {
    "openmep": { "command": "npx", "args": ["-y", "-p", "@openmep/mcp", "openmep-mcp"] }
  }
}

Claude Code: claude mcp add openmep -- npx -y -p @openmep/mcp openmep-mcp. Requires Node.js 24+. Transport: stdio.

Tools

ToolInputReturns
size_ductcfm, optional role (main/branch/runout), airflowType (supply/return/exhaust/outside-air), existing section ({diameterIn} or {shape: rect|oval, widthIn, heightIn})Recommended standard round diameter, velocity, friction; for an existing section its circular equivalent and ok/undersized/oversized
size_duct_networkitems: terminals with requiredCfm, segments (optionally with existing and lengthFt), fittings (optionally equivalentLengthFt), equipment, linked by connectedItemRefs; optional fan (externalStaticInWg, componentLossesInWg)Every loaded segment's CFM, role, recommendation, and grade; with fan, the Manual D design (available static, total effective length, friction rate, governing paths); findings for missing CFM, disconnected terminals, dangling references, mixed systems, undersized and oversized runs, friction rate outside 0.06–0.18

Both tools are read-only and idempotent. Invalid input comes back as a tool error naming the field (NetworkInputError [invalid-item]: items[2].requiredCfm: ...) so the model can correct it.

Example prompt that works in a fresh Claude Code session with only this server:

I have a furnace feeding a supply trunk that tees into two runouts: bedroom 1 needs 120 CFM, bedroom 2 needs 90 CFM, and the living room is fed off the trunk end at 200 CFM. All drawn as 6 inch round. One return grille in the hall at 410 CFM on a 10 inch drop. Is anything undersized?

The agent builds the network in one size_duct_network call and reports 7, 6, and 8 in runouts, an 11 in trunk, and a return drop just under size at 10 in.

Engineering basis and limits

Equal friction at 0.08 in. w.g./100 ft, or at the Manual D rate derived from the blower's available static pressure when fan and lengths are given, with role-based velocity caps per system, rounded up to standard round diameters; see @openmep/hvac-domain for the formula, sources, and what is excluded (fitting losses, static pressure summation, fan selection). Airflow is a design input the server never estimates. Treat results as preliminary sizing for an engineer to review.

The server uses the official MCP TypeScript SDK and speaks the initialize-handshake protocol revisions (through 2025-11-25), which current clients support.

Library

import { createOpenMepServer } from "@openmep/mcp";
await createOpenMepServer().connect(transport); // any MCP server transport

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