Hayward H400FDN HS Code - Rattling Noise


New member
Jun 23, 2020
West Islip
I recently had to change my Filter pump and once I did that i notice my Hayward Heater (2 years old) is throwing an HS Code but before throwing it there is a loud rattling sound coming from the pipes.

My pool is not salt water.

I noticed that the pressure is building up quicker once I got the new pump and it seems to happen more - once backwashed it works for a while then same problem. This NEVER happened before I changed the filter pump.

Could it be that the pressure rising is causing the issue? Not very familiar with this so any help or guidance is appreciated
Thank you


Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
Welcome to TFP.

HS code is telling you that the water output temp is too high. The code does not necessarily mean the sensor is bad but, is instead telling you there is a problem in the operation of the heater.

The rattling or kettling is from the water boiling in the heater.

You have a flow problem that is probably algae clogging your filter causing the HS fault, high filter pressure and low flow.

What test kit do you have?

We confirm algae with the Overnight Chlorine Loss Test. Then rid the algae with the SLAM Process

I suggest you read ABCs of Pool Water Chemistry