Boiler "on" times

[Jabba355]

Hi

I have just had my oil certikin boiler serviced and begun to heat my freezing 15 degree cold water up for the first time (in my ownership)...what I don't get is the boiler only stays on for 5 minutes then cuts out for about 7 minutes...then comes back on again for another 5 minutes...the boiler guy tells me this is normal as it is a gradual process that will take up to 4 days to heat the pool....I cant understand why the boiler cant stay on all the time though until it gets to the required temp ??? he mentioned something about the plastic pipe work not being able to take high heat and this is how swimming pools work ??? anyone know if this is correct or just BS ?..