TheBigYahi
New Member
I recently had a Phoenix Light Duty 80 gallon unit installed and the modulation logic seems to be not very smart. When presented with any load it will always reach the set point without modulating over the full range then fire back up again as soon as the exhaust purge cycle ends.
For instance, for a single 2 GPM shower I calculate the load should be about 40,000 BTU. It will turn on at the specified droop (140F down to 132F) and will start coming back up. Once it reaches 135F the fan speed will drop and it will lose a degree and ramp back up. It oscillates like this a few times until it gets to 139F then the throttle doesn’t back off fast enough, it hits 140F and cycles off. Over the course of a 10 minute shower it cycles about 3 times.
This happens for all load sizes. One shower, two showers, the air handler using the hot water coil for Aux, etc. Per the specs it should be able to throttle down to 25,000 BTU but it’s just not smart enough to do so in practice.
I’ve tried normal mode, ECO mode, various max throttle settings, higher droop with small changes in the behavior but no fix.
For reference, max fire is 6300 RPM and min is 2150 RPM. My installers aligned the gas valve at high and low so it is working great in test mode. The valve really needed adjusting from the factory, it was really howling up a storm before getting it right. At an 80%/ECO setting it runs at about 5400 RPM and will modulate down to about 4500 RPM before prematurely reaching the set point. I’ve seen it down into the 3000’s occasionally when finishing up a no-load recovery.
For instance, for a single 2 GPM shower I calculate the load should be about 40,000 BTU. It will turn on at the specified droop (140F down to 132F) and will start coming back up. Once it reaches 135F the fan speed will drop and it will lose a degree and ramp back up. It oscillates like this a few times until it gets to 139F then the throttle doesn’t back off fast enough, it hits 140F and cycles off. Over the course of a 10 minute shower it cycles about 3 times.
This happens for all load sizes. One shower, two showers, the air handler using the hot water coil for Aux, etc. Per the specs it should be able to throttle down to 25,000 BTU but it’s just not smart enough to do so in practice.
I’ve tried normal mode, ECO mode, various max throttle settings, higher droop with small changes in the behavior but no fix.
For reference, max fire is 6300 RPM and min is 2150 RPM. My installers aligned the gas valve at high and low so it is working great in test mode. The valve really needed adjusting from the factory, it was really howling up a storm before getting it right. At an 80%/ECO setting it runs at about 5400 RPM and will modulate down to about 4500 RPM before prematurely reaching the set point. I’ve seen it down into the 3000’s occasionally when finishing up a no-load recovery.
