Only thing I might suggest you try, if it is plugged with lime/scale, would be CLR, or some other type of product like that. Remove the core from the heater and try filling it with the above. Let it stand for a while then try flushing it through with a garden hose. Might work, might not. I think for a price they are reproducing new cores for the recirculating heaters. You buy them and reuse your tank ends.
