Use case
Replacement AC sizing — no blueprint, no Carrier HAP, just describe the job.
The most common HVAC job is a single-aggregate residential replacement — one home, one condensing unit, swap it out. Manual J sizes for the actual building, not for whatever the rule of thumb said 20 years ago. BuildSolver runs that calc in about 90 seconds from a conversational description.
Example replacement
1,200 sq.ft single-story · Phoenix, AZ 85003 · replacing a 3-ton Carrier condensing unit installed in 2008. Existing ductwork stays.
FOR ESTIMATION ONLY · ACCA MJ8 §6.4
Free · No signup · Daily limit applies
01 · The problem
Brand tools size for their brand. Rule of thumb oversizes.
- Carrier HAP is free — but only sizes Carrier equipment. Same for Trane TOPSS, Daikin sizing tools. You commit to a brand before the homeowner has agreed to a price.
- Rule-of-thumb (1 ton per 500–600 sq.ft) oversizes most replacements by 20–40%. Short-cycling, humidity issues, warranty callbacks.
- Manual J takes 30 minutes in Excel per revision. By the time you have a number, the homeowner has called two competitors.
02 · How it works
Describe the home; get a sized result with citations.
- Describe the home. Square footage, location, wall construction, window count, insulation level. The chat asks follow-up questions when it needs them.
- BuildSolver runs Manual J. Deterministic code, not LLM inference. ACCA Manual J 8th Edition §6 (sensible) + §8 (latent), with climate-zone defaults from IECC for the supplied ZIP.
- Get the result and a client PDF. Block load, sensible/latent split, recommended capacity. Pro and Team tiers add company branding.
03 · Why BuildSolver fits replacement
Brand-agnostic, conversational, mobile-first.
The output is a Manual J load number, not a recommended SKU. Use it to compare quotes across brands, present a defensible number to the homeowner, and only commit to specific equipment once the job is yours. Manual S equipment selection runs separately when you need to match a specific coil.
04 · BuildSolver vs Carrier HAP
Same Manual J procedure, different ergonomics.
Carrier HAP runs Manual J too — but the inputs are form-based, it’s a desktop install, and the output sizes Carrier equipment. BuildSolver runs the same procedures in a conversation that finishes on your phone.
| Feature | BuildSolver | Carrier HAP |
|---|---|---|
| Inputs | Conversational | Form-based, desktop |
| Brand lock | None | Carrier equipment |
| Platform | Web, mobile-first | Windows desktop install |
| Time to first number | ~90 seconds | 5–10 minutes form-filling |
| Output transparency | Every row cites ACCA section | Detailed but desktop-only |
05 · FAQ
Common questions about replacement sizing.
+How do I size a replacement AC without blueprints?
You don't need them. Describe the home from your site visit — square footage, location, wall construction, window count, insulation level — and BuildSolver runs ACCA Manual J on those inputs. Manual J was designed before residential CAD was common; the procedure works fine from a description.
+What's the rule of thumb vs Manual J for replacement?
Rule of thumb (1 ton per 500–600 sq.ft) oversizes most replacements by 20–40%, which causes short-cycling, humidity problems, and warranty claims. Manual J accounts for envelope, infiltration, internal gain, and climate. BuildSolver runs the full Manual J in about the same time it takes to do a rule-of-thumb estimate.
+Can I size a heat pump replacement the same way?
Yes. BuildSolver runs both cooling (ACCA MJ8 §6) and heating (ACCA MJ8 §5) loads. For dual-fuel or cold-climate heat pumps, the heating load drives the selection — the chat will surface both numbers and the balance point.
+Is this for residential only?
BuildSolver is tuned for residential and light-commercial single-aggregate replacements. For multi-zone commercial systems with VAV boxes or complex ventilation, run the preliminary on BuildSolver and have a mechanical engineer review before installation.
+Do I need to know the current tonnage?
No — and you shouldn’t use it as a proxy. The existing system was likely sized using a rule of thumb 15–25 years ago. Manual J sizes for the actual building, which is usually smaller than what’s installed. Right-sizing the replacement is one of the easiest ways to improve homeowner comfort.
Try it on your next replacement quote.
Free tier covers 10 calculations per day. No signup required for the first three. Pro tier adds your company branding to the result PDF.
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