Rapak above ground pool heater is leaking from the bottom

To find items like this Ken you need the proper alpha-numeric model numbers rather than descriptions like '100,00 BTU' or 'Titanium'. The search queries are simply

make model manual

and

make model parts

Less words is best.

https://www.google.com/search?q=Insta+Matik+100%2C000+BTU+Digital+Thermostat

https://www.google.com/search?q=Titanium+above+ground+pool+heater+parts

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there are mounting screws at bottom of burner tray

is this heater is been install for above the ground pool it will be better either way to use a hard schedulle 40 pvc to prevent any checking in the hoses for future leak ,is the heater connections are correct in and out , mean from the return the filter go to inlet side the heater them the outlet side of the heter (HOT WATER) GO TO RETURNS of the pool ? also check if the pump got full prime not air,is the filter is clean? also check for any brocken parts inside the flowvalve control, after years low PH AND TA will destroy internal parts in heater include leaking heater exchangers.

low water flow is making the heater knock, (PULSATE) clean filter or backwash, check pump and skimmer baskets and empty. if still low flow and knocking, check impeller on pool pump to make sure it isnt clogged.

If you want all your water going thru the solar heater before re-entering the pool, you'll need to plumb the end of your filter to the solar heater, then the water exiting from the solar heater would go where the filter hose did go back into the pool. Hope this helps.