high pressure spa

[santa rosa]

I have a 5-year old plaster in-ground pool with built in spa. When the pump is running in normal mode, and the flow is split 75/25 between the pool and the spa, the filter pressure is 24 psi. As soon as I turn on the spa heater and all the flow is directed to the spa, the pressure jumps to 40 psi. I just cleaned the cartridge filters. The four jets in the spa appear to flow well and evenly. Any though as to why the pressure would jump up? Everything is plumbed in 2 1/2 PVC. The pump is 1.5 HP.

Thanks!