Potterton Profile 80E Starts OK on HW but not on CH?

Boiler would start (fan running for 20 seconds) but would not complete the ignition cycle. Replaced the PCB. After replacing the PCB the boiler goes through full start sequence and ignites when programmer is set for HW only. When progammer is set to CH nothing happens; no start up at all. Replaced the programmer with new unit but same problem. Replaced room thermostat but still the boiler will not start when programmer on CH. Circulating pump is one year old.