Boiler priority control
Simulate a relay box that gives a wood boiler priority over a heat pump
Screenshot of the tool as it opens; it is a real capture, not a mockup.
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What it makes
A working model of one boiler-priority relay box: a live circuit that lights each wire carrying 24 V for the current thermostat call and boiler water temperature, a table of what runs for every call with hot and cool water (highlighting any case that breaks the priority rules), a power view with worst-case VA on the box and equipment transformers and voltage at the end of each wire run, and a Markdown commissioning checklist.
How to use it
- Start from a quick start: Recommended, Gate in the call path, Two relays, B-type equipment, or Spring-return valve.
- Pick a thermostat call (heat, heat + aux, emergency, cool, fan) and drag the boiler water temperature; the aquastat closes and opens with its dial and differential.
- Watch the Live circuit, then open What runs when to see every call at once. Use the Fault menu to see what a stuck aquastat, unbonded commons or a burned relay coil does.
- Set your transformers, coil loads, valve and wire runs, and check the Power & wiring view for headroom and voltage drop.
- Work through the commissioning checks and download the checklist; save settings to reopen later.
Build notes
- Built around one real install (a thermostat, heat pump and air handler, a wood boiler with an aquastat, and a 3-wire zone valve). Your equipment's terminals and conventions may differ; confirm against its wiring diagram.
- This is a planning and troubleshooting model, not a substitute for the equipment manuals or a licensed installer.
- 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.