Boiler Not Firing when controlled on Heating side option of Timer/Control Box

this could possibly be the room thermostat at fault, with the programer set at heating only and the room thermostat turned up you should have 240v power on the white wire from the 3 port valve, if you haven't then the room thermostat is at fault. this is assuming you have power from the programmer to the room thermostat which you should have if wired correctly and new

It sounds like you've already taken some steps to address the issue with your boiler not firing when controlled through the heating side option of the timer/control box. Since you've replaced the 3-port valve, actuator, and the control box itself without resolving the problem, it could indeed be an electrical or boiler control issue. Here are a few additional troubleshooting steps you might consider: 1. **Check Wiring:** Ensure all wiring connections are secure and correctly connected according to the manufacturer's specifications. Loose or faulty wiring can disrupt the communication between the control box and the boiler. 2. **Check Sensors and Thermostats:** Faulty temperature sensors or thermostats could prevent the boiler from receiving the correct signals to fire up for heating. Inspect these components to ensure they are functioning correctly. 3. **Boiler Control Settings:** Review the settings and programming on the boiler itself. There might be settings within the boiler's control panel that need adjustment or reset to allow it to respond to the heating signals from the control box. 4. **System Flush and Maintenance:** Over time, central heating systems can accumulate debris or airlocks, affecting proper circulation and boiler firing. Consider performing a system flush or bleeding the radiators to ensure there are no blockages impeding the flow of water. 5. **Professional Inspection:** If these steps don't resolve the issue, it might be beneficial to have a qualified heating engineer or electrician inspect the system. They have the expertise and tools to diagnose and identify any underlying electrical or control faults within the boiler system. In complex heating systems, diagnosing the exact cause of a control fault can be challenging. A professional inspection might be the most efficient way to pinpoint the issue accurately and ensure that the boiler operates as intended for both hot water and heating functions.