How to Use AI in HVAC Design: Options, Duct Checks and Proposals

Use AI for the work around the design: organising survey notes, listing the questions the duct layout still needs answered, drafting good-better-best options and checking the proposal against the calculation report. Keep sizing in Manual J, S and D software run by the qualified person, and never accept a tonnage, airflow or duct size from a chat answer.

A proposal goes wrong in two places. Before the calculation, the survey notes are thin and nobody notices. After it, the written options drift away from the numbers. An assistant helps at both ends and is unreliable in the middle, so the job is to put it where it is useful. The house used throughout is illustrative: two storeys, about 2,100 sq ft (195 m²), an early-1990s build with an ageing system to replace.

Follow me on Instagram@sagnikteaches

What surrounds the calculation, and who does each step

Manual J is the residential load calculation (the publisher's page lists the 8th edition). Manual S covers selecting and sizing equipment from the loads, the manufacturer's performance data and the design conditions, and it sets size limits so that properly installed equipment can deliver comfort. Manual D covers residential duct systems, including the interaction between a given blower and a given duct system. Wherever your authority accepts a different method, the same split applies. The publisher lists approved software for Manual J (including Wrightsoft Right-J8, Elite RHVAC and Cool Calc Manual J) and for Manual D (including Wrightsoft Right-D, Elite Ductsize and Cool Calc). Its list names no software for Manual S, so look at what your own design tool covers.

Connect on LinkedInSagnik Bhattacharya

Around that core sit the steps a contractor controls. The table shows who does each one:

Subscribe on YouTube@sagnikteaches

Only step 4 produces the numbers a proposal relies on. The assistant never appears in that row, and every other row uses it on words, not on engineering.

Step two: turn survey notes into a design input sheet

Technicians write notes the way they talk. The assistant's first job is to sort them into the inputs the calculation needs and to mark what is missing. A set of notes from the house (illustrative):

Two-storey, about 2,100 sq ft, built early 1990s. Existing 3-ton split
AC and 80% gas furnace in the basement. Metal trunk in basement, flex
runs to upstairs rooms through the attic. Owner says main bedroom
upstairs is hot in summer, bedroom 3 cold in winter. One return in the
hall, one in the living room. Filter is a 1-inch slot. Attic
insulation looks thin, not measured. Windows mostly original double
glazing, one big west-facing one in the family room. 8 ft ceilings
both floors. Total external static at the blower on the old system
0.82 in. w.c., taken at supply plenum and return drop. Owner wants it
quiet and lower bills.

The prompt: sort these notes into a table of calculation inputs with three columns, input, what the notes say, and status. Use only the notes. Write NOT STATED for anything missing. Do not estimate any value. The reply (illustrative) is short on conclusions and long on gaps, which is the point:

The table shows what the software will ask for and the notes cannot supply: wall construction, window schedule, attic depth, air tightness and duct details. Those are a short return visit or a phone call to the owner, which costs far less than a corrected proposal. Note the last-but-one row. The existing 3-ton unit is what someone chose years ago, not the house's load, and the sheet labels it "for reference only" so nobody drifts into sizing from it.

What to photograph and measure at the first visit

A better survey makes every later step cheaper, and photographs cost nothing. A short list the technician can keep on the phone:

• The rating plates on the existing indoor and outdoor units, with model and serial numbers readable.
• Every supply register and return grille, with the room name written on a piece of paper in the shot.
• The filter rack, with the filter out and in, and a tape measure beside the slot.
• The ductwork where it can be reached, at the trunk, at each branch takeoff and wherever flex is visible.
• The attic insulation with a tape measure standing in it, and the same for any other accessible cavity.
• The thermostat, the equipment space with its clearances, and the condensate route.

Type the readable details into the notes rather than leaving them in the photos, because an assistant's reading of a photo is not a measurement. Anthropic's image documentation lists counting and spatial reasoning as approximate and asks users to verify. A photo of a tape measure in insulation supports a number you type; it does not replace one.

Step three: duct-layout questions for the return visit

Duct problems rarely show in survey notes because nobody asked the right question while standing in the attic. Give the assistant the filled-in input sheet and ask for questions, each with a reason, grouped by what they affect. A reply (illustrative) for this house:

• Which upstairs rooms have no return path with the door closed? Reason: pressure imbalance can explain a hot main bedroom regardless of the equipment.
• What are the trunk dimensions and the run length, including flex, to each upstairs room? Reason: the duct design needs them.
• Are any flex runs kinked, compressed or draped over framing? Reason: restrictions raise resistance and cut airflow.
• What is the filter rack size and the filter's pressure drop at the design airflow? Reason: it counts against the blower's available pressure.
• Where exactly was 0.82 in. w.c. measured, and was the filter clean? Reason: a repeat reading on a clean filter shows how much of it is filter.
• What does the new blower's table give for external static pressure at the design airflow? Reason: compare it with the measured figure before choosing equipment.
• How leaky are the ducts, and is a leakage test planned? Reason: leakage in an attic duct changes both comfort and the load calculation's duct losses.
• Is the attic ductwork insulated, and to what level? Reason: it is a calculation input.
• Can a return be added upstairs, and where would it run? Reason: a layout change belongs in the duct design, not in an afterthought.

A technician takes the list back and fills it in on site. The assistant did not diagnose anything. It listed what is unknown, and the answers go into the calculation. For wider reading on pulling answers out of manuals and spec sheets, see AI for HVAC contractors on manuals and spec sheets.

One sanity sum belongs on the same sheet. Manufacturers' installation manuals commonly call for about 350 to 450 CFM per ton of cooling, with 400 CFM as the nominal figure. For a 2.5-ton system that is about 1,000 CFM (roughly 470 L/s), within a range of 875 to 1,125. Some manuals also warn against running below about 350 CFM per 12,000 Btu/h of rated cooling, because the evaporator coil can frost, and manuals point you to the rating plate and the blower's performance table for the external static pressure the unit can work against. Your equipment's manual and rating plate govern. The sum is a quick test of whether a layout is in the right territory, not a design method. If a sketch of the duct system seems built for 700 CFM, ask why before anything is installed.

Step four stays outside the chat

The qualified person runs the load calculation, selects equipment and designs the ducts in software that implements the procedures. The report that comes out is the single source for every number in the proposal. For a closer look at why a chat answer cannot stand in for it, read can AI do an HVAC load calculation.

What the assistant may do with the report: list missing inputs, explain it in plain English for the customer, flag any line that disagrees with the survey notes (a report that says insulation is R-38 when the notes say three inches of loose fill is worth a second look), and draft questions. What it may not do: choose a tonnage, adjust a load "for safety", pick duct sizes or compute a friction rate. The line is easy to hold when the report is on screen, and hard to hold when someone asks the chat "so what size do we need?" with nothing else in the window.

Here is the mistake that shows why. Given the old system's size and an instruction to "recommend equipment", an assistant (illustrative) wrote: "Most houses of this age need 3 tons, so go up to 3 tons to be safe." The report for this house selected a 2.5-ton nominal unit at design conditions. The audit step below caught the mismatch because the proposal said 3-ton and the selection printout said 2.5. Manual S sets size limits precisely so that "bigger to be safe" is not the rule, and the proposal had no basis for the larger unit.

One more use sits just before the calculation is trusted. Give the assistant the survey notes and the input page the software printed, and ask it to compare them line by line:

Compare these survey notes with the input page of the load report.
List every disagreement as: item, what the notes say, what the report
says. Do not decide which is right. If an item appears in only one
document, list it as MISSING and name the document.

A reply (illustrative) for this house might list three disagreements: the report's attic insulation is a high value while the notes say "looks thin", the report counts three returns while the notes record two, and the report gives a 9 ft ceiling on the first floor while the notes say 8 ft. Each is a question for the person who ran the calculation. The assistant cannot say which document is right, and it should not try. What it does is make sure a mismatch is noticed before a number built on a wrong input reaches a proposal.

Step five: draft good-better-best options from the report

You can sketch the tier skeleton from the survey notes before the calculation runs, since what each tier would change in the ducts and controls is already visible on site, and leave equipment and capacity blank. Three options mean three equipment selections. Each tier uses a different model, so each needs its own Manual S selection from that model's performance data. An option written before its selection exists is the usual source of a mismatch. Run the selection for every tier first. Then give the assistant the report excerpt, the model numbers, the capacities from the manufacturer's data at design conditions and the warranty terms from the manufacturer's document, pasted in as text. Ask for plain-English option descriptions that use only those facts. A report excerpt for this house (illustrative):

Report excerpt (illustrative):
Design cooling load 28,400 Btu/h (about 8.3 kW)
Design heating load 41,200 Btu/h (about 12.1 kW)
Selected: 2.5-ton nominal, two-stage, per manufacturer data at design
conditions. Design airflow 1,000 CFM.
Duct design: existing trunk reused; new return added upstairs;
two upstairs branches replaced.

Task: write the BETTER option for a homeowner in under 120 words,
using only the facts above and the warranty text I paste below.
Mark any claim you cannot trace as [SOURCE?]. No savings figures.

A reply (illustrative) names the equipment type from the selection, mentions the new upstairs return and the two replaced branches, and states the warranty from the pasted text, including its registration condition. It would add a sentence about quiet running and the assistant would mark it [SOURCE?], because the manufacturer's text did not say that. You either find the sentence in the brochure or delete it. The skeleton of the three tiers, with the price lines left to your price book, looks like this:

The price column is deliberately empty. The assistant has no idea what your parts and labour cost, and an invented number in a proposal is an error you carry into a customer's decision. For the pricing and margin side, see the good-better-best pricing tutorial, which works through a replacement quote in detail.

Step six: the explanation a homeowner actually needs

The customer's first question about this house will be why the new unit is not the same size as the old one. The report answers it, and the assistant can turn the answer into plain words if you give it the report and forbid additions. A draft (illustrative) that survives the audit reads: "The calculation for your house as it stands today selected a 2.5-ton unit. We did not copy the size of the old system, because a replacement is sized from a calculation of the house as it is now, not from the unit it replaces. If you add attic insulation or replace the west-facing windows, the numbers change, and we can run the calculation again."

Every claim in that paragraph traces to something: the 2.5-ton selection to the report, the reason for not copying the old size to the sizing method itself, and the effect of insulation and windows to the fact that both are inputs to the calculation. It makes no promise about bills or comfort, and it invites a conversation about improvements instead of pressing the customer towards the largest option. Put it above the three options, then add the assumptions the proposal relies on: ducts reused where the report says so, a static pressure test after installation, and the owner's agreement to the return added upstairs.

Step seven: trace every number in the proposal

Before anything goes to the customer, run an audit. Give the assistant the proposal and the three sources it is allowed to rely on: the report, the manufacturer documents and the price book. Ask for a table of every number and every technical claim in the proposal, the place it appears in a source, and a status of FOUND, PARTIAL, NOT FOUND or CONFLICT. Do not let it fix anything. A reply (illustrative) for the first draft of this proposal:

Four of the seven rows need action: the tonnage conflict, the invented savings claim, the missing warranty condition and the unsourced noise claim. The tonnage conflict is the important one, and the invented savings claim is the most typical. Models like to add a persuasive percentage, and a customer remembers it. The audit makes the rule mechanical: a number or claim with no source leaves the document.

Then a person signs the reconciliation. The qualified person confirms the proposal matches the selection for each tier, the technician confirms the duct changes are the ones the layout questions called for, and the file keeps the table next to the report. If a customer or an inspector asks later where a number came from, the answer is a document, not a recollection. The tutorial on AI estimating software for HVAC looks at where tools for replacement quotes and duct takeoffs help and where they miss.

After installation: measure, compare, record

The design is only a prediction until someone measures the result. After installation the technician measures total external static pressure, airflow and the temperature drop across the coil, and records them next to the design figures. The assistant can turn the technician's dictated readings into a tidy sheet and flag readings outside the design range for review. It must not decide whether they are acceptable.

A quick sum shows how the check works. The design airflow is 1,000 CFM for the 2.5-ton unit. A reading of 850 CFM is 85% of design, and 850 ÷ 2.5 = 340 CFM per ton, below the 350 to 450 range that manufacturers' manuals commonly give. The flag goes to the technician, who looks for the usual suspects: a clogged or too-restrictive filter, a damper left closed, a kinked run, a blower speed setting. The assistant can list candidates to check, and the technician diagnoses. The tutorial on service reports from technician notes covers turning readings into a customer-facing report.

The static pressure reading closes the loop with step three. If the old system read 0.82 in. w.c. with a dirty filter and the new one reads well under the rating plate's limit, the duct work and the filter swap have done their job. If it reads high again, the duct design was incomplete, and the data tells you where to look before the customer calls.

Time and cost of the AI steps

These are planning figures for this house, with checking time included:

That is under an hour of assistant-supported work per proposal, on a plan of about $20 a month for one personal seat or $20 to $25 per user for business seats. The software for the load calculation, selection and duct design is a separate cost, and prices vary by product and licence, so check the vendors. The value of the assistant is not speed alone. Each of the four steps removes a way for a thin survey or a drifting proposal to reach a customer.

Write the rules down once so every proposal follows them. The assistant drafts and checks. The qualified person calculates and signs. The proposal contains no number and no claim that cannot be traced to the report, a manufacturer's document or the price book.

Questions HVAC contractors ask about AI and design

Can ChatGPT size an HVAC system?

No. Sizing needs a load calculation run in software that implements the published procedure, then equipment selection from the manufacturer's performance data. A chat answer rests on rules of thumb and on whatever you typed, and it cannot see the house. An assistant can explain a finished report in plain English and point out gaps in your inputs, which is a different job.

What if my authority accepts a different calculation method?

Use the method your authority accepts and keep the same split: the calculation runs in approved software operated by the qualified person, and the assistant works around it on notes, questions, wording and checks. The audit step described here works with any report, because it only compares the proposal with whatever the calculation produced.

Should customers see the load calculation?

Offer the report and give a one-page plain-English summary with it. An assistant can draft that summary from the report if you tell it to use only figures that appear there and to write NOT STATED for anything missing. Read the summary against the report before sending, because a friendly sentence can quietly change a number.

Which AI plan do I need for this?

One personal plan at about $20 a month is enough to start, as long as customer names and addresses stay out of it. Business seats cost around $25 per user monthly or $20 on annual billing, with two seats minimum, and do not train on your content by default. Choose them when an office person shares the proposal work.

Further reads

• Follow Up After Giving a Quote: An HVAC Replacement Sequence With AI — Follow up a proposal without nagging the customer.
• HVAC Markup and Margin: A Worked Check on Past Jobs With AI — Check markup and margin against your past jobs.
• Using AI in an HVAC Business: A Week-by-Week Setup Plan — A week-by-week plan for AI across the whole business.
• HVAC Warranty Claims: Document the Failed Part and File With AI — Document a failed part so a warranty claim holds.
• Best AI for HVAC Business Owners: Find Ageing Systems to Replace — Find ageing systems that are due for replacement.
• How to Price HVAC Jobs: Test Flat Rate Against Hourly With AI — Test flat-rate pricing against hourly with your own data.
• Heat Pump Installer Software: Surveys, Heat Loss and Quotes, AI Compared — What a heat pump installer's software must do at survey, heat loss and quote stage, which tools cover which job, where AI helps and what vendors charge.
• Heat Pump Quote Template: Explain Options With AI, No Savings Promises — Write a heat pump quote with AI that explains options, assumptions and exclusions, leaves design to the designer and never promises savings or grants.
• AI for Mechanical Contractors: Can It Read Specs and Drawings? — What AI reads reliably in a mechanical bid set, where it fails on drawings, and a 12-point hand check plus a 45-minute test to run on a past bid.
• Can AI Do a Heat Pump Heat Loss Calculation? What MCS Expects — Where AI helps around a heat pump heat loss calculation, and why the calculation, design and sign-off stay with approved software and the MCS installer.
• AI Tools and AI Development: The Complete 2026 Guide — the AI hub, including every tutorial in the AI-for-business series.

Sources: the standards publisher's pages for Manual J, Manual S and Manual D and its approved-software list, manufacturer installation-manual excerpts on airflow per ton and rated static pressure, and OpenAI and Anthropic plan pages; the house, report figures and prices are illustrative.