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There is nothing wrong with Honeywell TPI. You need to set the stat cycle rate to once every 20 minutes (Cr=3).

A flow temp of 75°C will only give you a one minute cool down period which is insufficient to prevent cycling because you're looking for a 20°C drop. And as you've already said, a lower flow temp doesn't heat the house enough, there isn't much more you can do.

Radiators are sized for higher temps, flow temp 65°C, trvs on all but two rads, auto bypass turned down as flow through non-trv rads is sufficient. Cylinder is blue jacketed about 5m from the boiler, so nothing special. Hw is timed and cylinder stat turned down to prevent boiler cycling near...

Well, I have a 415 which works 100% and doesn't micro fire. The important thing is to match the flow rate with the first minute's firing (about 75% of max boiler output) and subsequently d0. However, you're stuffed from the start if your boiler is significantly over sized. A 28Kw boiler...