Hayward Heater Low Flow . These hayward pool heater troubleshooting codes and explanations will help you fix the problem. If the low speed is too low on the pump you should make the automatic low speed higher so even if the timer is supposed to be in low it won’t be too low for the pressure switch on the heater. Hayward H150FDN HSeries Low NOx Heater Natural Gas 150K BTU from www.wholesalepoolequipment.com “hp” code on a pool heater signifies “restricted water flow or low water flow.” in some cases, the code is displayed because of the high pool water temperature. Because low water flow to the heater will often trigger the hs code, you’ll need to verify that your heater has adequate water flow above the minimum required (20 gpm for h150fd and h200fd, 25 gpm for h250fd, and h300fd, 30 gpm for h350fd and h400fd). Heater runs for a while then shuts off and shows ‘hi or hp’ on display.