Heat load calculator
Size the heat loss of a garage, shop or room and check a hydronic heater
Screenshot of the tool as it opens; it is a real capture, not a mockup.
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What it makes
A design-night heat loss in BTU/hr broken down by walls, ceiling, slab edge or floor, each door and window, and air leakage; a chart of the load against outdoor temperature with the heater's output at low, planned and high water temperatures; the flow, velocity and pump head the loop needs; and a Markdown summary you can keep with the job.
How to use it
- Start from a quick start (3-car garage insulated or bare, Workshop, Room or addition) or enter your own.
- Set the design night outside temperature and the temperature you want to hold, then the space's size, walls facing outside, and the wall, ceiling and floor assemblies.
- Add each door and window with its size and type, and set the air changes per hour.
- Pick a heater (or enter a rated output and its rating temperatures) and your supply water temperature; read the stats bar for output and the outdoor temperature it keeps up to.
- Check the Water side view for flow and head, then download the summary or save settings.
Build notes
- U-values and air-change rates are typical assembly values; results are estimates, not a Manual J.
- Heater output is scaled from its rating point by entering temperature difference, a standard approximation; confirm with the manufacturer's selection data.
- Works fully offline once opened; nothing is fetched.
Versions
The site runs the newest. Every earlier version stays downloadable, unchanged, for builds already under way.