Ok went back to the church Friday afternoon to see if I could fix it before their Sunday service. And I finally resolved the issue. I checked the pump torque (as suggested) and that seemed no different to another 15-50 pump head I had. I swapped them just be be on the safe side and no improvement.
So with no blockage in the boiler isolator valves, the AAV ok, the pump rotor free, the pump upto torque and pressure, the bypass valve ok, the flexi hoses clear and clear flow from flow to return (via a short or my pressure wash machine) the only thing left is the heat exchanger.
But this showed no clear evidence of blockage. At just 600 mbar pressure it was passing way more than the 14L/min flow, well within spec. So why overheat.
After removing the exchanger and a bit of investigation consulting with vokera we found the problem.
The heat exchanger has an inlet on the rear left and let’s the water pass into a manifold where the flow splits into two flow tubes which pass through the flame fins from left to right. Within these flow tubes, coaxial style, is the hot water circuit. So the flow through the flame section is restricted to being between the outer and inner pipe, quite a narrow cross section compared to the overall pipe diameter. On the right hand side the two flow tubes pass into another manifold where the water moves to the front of the heat exchanger and then flows into another two coaxial flow pipes from right to left through the exchanger. On the left the two pipes combine the flow into a manifold (similar to the inlet manifold that spilt the flow). From the manifold the water exits via the outlet on the front left.
This works fine if there is no sediment in the flow water. But a relatively small amount of silt can accumulate in the front of the right manifold. This starts to inhibit flow through the front tube of the pair but the rearward tube easily carries the flow. The reduced flow in the front tube means more silt accumulates, and so on. Eventually the front tube becomes complete blocked but good flow continues in the rearward tube.
With the front tube blocked it kettles and cooks the scale onto the narrow walls. So under flame the front quarter of the exchanger gets very hot even though the rearward tube is taking through the hot water. However the overheat thermostat is mounted on the exchange at the front left of the left front manifold and senses the overheat. That front tube hotspot will always trigger the thermostat
That is the fault condition I have, good apparent flow overall but an overheat and flameout.
It’s hard to flush out even with the power flusher as the water just passed through the rearward tube and no descaler touches the scale in the front tube. And once it’s completely blocked and baked no flushing will touch it. Maybe a long term pickle might but basically the exchanger is dud.
So a quick visit to Wolsey for a new one, 230 pounds to the customer (plus Labour) and all is well.
But it shows the importance of keeping sludge and lime down.
One can see why heat exchangers are better without coaxial piping and using full diameter pipes and a plate exchanger for hot water instead.
