I've got a 100K Btu condensing boiler for heating and domestic HW, both controlled by zone valves.
Added another zone valve for swimming pool, feeding 100k Btu stainless heat exchanger set in return from sandfilter to pool.
My above ground pool is 16ft diameter x 4ft deep, with the boiler set at max temperature and running flat out it takes just over 2 hours to raise pool temp 1deg C. Which is about right if you do the sums.
I would highly recommend keeping the pool covered when not in use to reduce evaporative cooling and if the pool is outside use a bubble wrap type solar cover to maximise heat input from the sun. On a good sunny day the top 4 inches of my pool can reach over 40deg C with the solar cover.
