That’s not how it works. There is no flow regulator in the heater to make the water flow lower to match the heating rate. It is the other way around. The gas regulator adjusts to the flow rate. The problem is more likely that the heater has become limed after a few years of use. That is what happens to copper tube heaters with well water, they lime up fast just as a tankless coil in a boiler does. The gas regulator is probably stuck due to the liming. You’d ordinarily have to clean and rebuild that type of heater yearly for well use. Cold well water isn’t enough to bother the heater otherwise, it just should be big enough for the water flow and you don’t have much – a trickle is a problem.

http://www.heatpro.info heat/cool right-sizing

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