Frequently Asked Questions

Answers to common questions about Supply & Return and the HVAC calculations behind it.

Is Supply & Return free?

Yes. Every calculator and guide is free to use with no account. The site is supported by advertising.

Are the values I enter stored?

No. All calculations run in your browser, and your measurements never reach our servers. See the Privacy Policy.

Can I size a central air conditioner with the room BTU calculator?

Only as a rough check. The ENERGY STAR table is meant for room air conditioners. Whole-house systems should be sized with a room-by-room load calculation such as ACCA Manual J.

Why do you warn against oversizing?

An oversized air conditioner cools the air quickly and stops before it has removed humidity, so rooms feel cold and damp. Oversized furnaces short-cycle. Both waste energy and wear out sooner.

What is the difference between BTU, BTU/h and tons?

A BTU is a quantity of heat; BTU/h is a rate. Equipment is rated in BTU/h, though often written as BTU. One ton of cooling is 12,000 BTU/h, about 3.5 kW.

Where do I find R-values and U-factors?

Insulation is labelled with its R-value per thickness, and windows carry an NFRC label with the U-factor. For existing walls, typical values by construction type are a reasonable starting point; an energy audit can measure more precisely.

What design temperature should I use?

The 99% heating design temperature and 1% cooling design temperature published for your location by ASHRAE or your local building department, not record extremes.

Why is 1.08 used in the CFM formula?

It combines the density and specific heat of standard air with 60 minutes per hour. At high altitude a smaller factor applies.

Does the duct calculator work for flexible duct?

It uses the friction equation for smooth metal duct. Flexible duct has higher friction, especially when compressed or sagging, so size it at least one standard size larger and keep runs short and straight.

How accurate are the SEER2 savings?

The arithmetic is exact, but the result is only as good as the cooling hours and electricity price you enter. Your past bills are the best guide to cooling hours.

Which edition of ASHRAE 62.2 does the ventilation calculator use?

The 2016 edition, with the 2010 rate shown for comparison. Your local code determines which applies.

Do the calculators work for heat pumps?

Yes for heat loss, airflow, ducts, SEER2 and ventilation. When sizing a heat pump, compare the heat loss with its capacity at your design temperature, which is lower than its nominal rating in cold weather.

Can I use the results for a permit application?

No. Permit applications usually require a full calculation prepared to the standards your jurisdiction adopts. Use the tools to understand and check that work.

How do I report an error?

Email logan.luo@adsnova.cn with the calculator, your inputs and the result you expected. See the Contact page.