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I will say when a combi boiler fails to make DHW, or flow drops off, it can be a job to repair them. The boiler needs to be down to disassemble the DHW "module", so no heat either.
They are mounted in a tough space to work on. Inside a sharp sheetmetal boiler jacket Proprietary OEM repair parts are usually required to repair.

To build your own external DHW "combi" module you need a circulator, heat exchanger and flow switch with a single switching relay box.
Shop wisely and you could acquire all the parts for well under 500 bucks. E-bay is one thought.

When a faucet opens on the hot side, the flow switch in the cold supply turns on the P:2 and the boiler. Many boilers have a DHW priority connection, so heat loads drop while in DHW mode. Or the double throw relay in most relay boxes could also drop off the heat call.

This allows you to custom size the heat exchanger, add flush valves, and use off the shelf components not specific to the boiler brand. This HX could be removed in minutes taken to a radiator shop for an acid cook if ever needed.

For most homes you would want at least 100,000 BTU boiler. This is my shop and I have it connected to a 55,000 Lochinvar Knight. It covers a hand sink and hose to wash the truck.

Thanks for the thoughtful theories. In this case, the boiler was grossly oversized and stayed at low fire for most of the heating season. My suggestions were two fold. I had them increase the air at at low fire to provide more turbulence to better mis the air and fuel. This also pushed the flame away from the burner head. The second thing I did was to widen the differential. It was set at 5 degrees F and I changed it to 20 degrees. This allowed a longer run time and heated the flue more, increasing the draft. Not sure if these were the solution but it worked in this case. They didnt have to replace the burner head for the next five years when they removed the boiler and installed a packaged rooftop unit. Here is the link to the video

https://youtube.com/watch?v=vmn6d8QINac