RH After Heating Calculator
What heating (or cooling) air does to its relative humidity at constant moisture.
Why winter indoor air is desert-dry: 5 °C/80% outdoor air heated to 22 °C lands at ~26% RH. Same physics, reversed, is why AC supply air can fog.
Formula
Disclaimer: This tool is for general informational and estimation purposes only and is not professional financial, tax, accounting or legal advice. All figures are estimates — verify with a qualified professional before making decisions. Read the full disclaimer.
RH After Heating Calculator is a free rh after heating for HVAC designers, technicians and energy auditors — instant, accurate and 100% client-side, with the governing formula and reference shown next to the result so the number can be defended, not just quoted.
About RH After Heating Calculator
What heating (or cooling) air does to its relative humidity at constant moisture. The calculation implements RH₂ = RH₁ × p_sat(T₁)/p_sat(T₂) (w constant) (ASHRAE Fundamentals psychrometrics). Why winter indoor air is desert-dry: 5 °C/80% outdoor air heated to 22 °C lands at ~26% RH. Same physics, reversed, is why AC supply air can fog.
How to use RH After Heating Calculator
- 1Enter Initial air temperature in °C.
- 2Enter Initial RH in %.
- 3Enter Final temperature in °C.
- 4Read RH after temperature change instantly — no submit button needed.
Why use RH After Heating Calculator?
- ✓Implements the standard formula — RH₂ = RH₁ × p_sat(T₁)/p_sat(T₂) (w constant)
- ✓Reference cited on-page: ASHRAE Fundamentals psychrometrics
- ✓Live worked example: the substitution recomputes from your numbers
- ✓Runs entirely in your browser — nothing uploaded, free forever
Frequently asked questions
What formula does the RH After Heating Calculator use?+
It computes RH₂ = RH₁ × p_sat(T₁)/p_sat(T₂) (w constant), per ASHRAE Fundamentals psychrometrics. The formula is displayed under the result along with a worked example substituted with your own inputs.
What should I keep in mind when using this calculator?+
Why winter indoor air is desert-dry: 5 °C/80% outdoor air heated to 22 °C lands at ~26% RH. Same physics, reversed, is why AC supply air can fog.
Is this based on ASHRAE methods?+
The calculations follow the standard ASHRAE/ISHRAE formulations cited under the result, simplified where industry practice does the same. For permit-grade design, confirm with the full code procedure.
Is the RH After Heating Calculator free to use?+
Yes — completely free, no sign-up, no limits. It runs client-side in your browser, so inputs stay private and results are instant even on slow connections.
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