Low Temperature Hydro-air

Larry_10

Our new Vitodens 8-32 has been installed. For 5 days now our downstairs has been served by a hydro-air handler (Ecologix CP60), but the weather has been mild before last night when it went down to 11F, so we had a real test. We set a reset curve of 0.6 with an 18C shift. At 11F this yields a heating circuit temp of 133F. Did a lot of research to find an AHer that would work well at low supply temps, and in general the early test shows that practice is matching the theory. The air has been plenty warm, relatively slow velocity and much more comfortable than the oversized equipment it replaced. The AHer uses a variable speed PSC motor so it keeps the air temps warm with lower supply temps by slowing down the blower and going for long runtimes.

Have noticed only one sort of weird Vitodens behavior at this point. When the AHer starts up it waits for warm air, but often the Vitodens temp needs to drop about 20 - 25F below the calculated boiler temp before the burner turns on. I sort of understand what it's doing, but needing to see such a large drop seems a bit excessive. From what I know this differential cannot be changed. What is the reason for this behavior?

The other Vitodens/AHer anomaly is with DHW priority mode. Right now the AHer doesn't handle this mode, but it sounds like they will address this issue with a future firmware update to the AHer control board. In the near term, my initial thoughts on a workaround was to sense when the Vitodens went into DHW priority, and use a relay to break the "W" circuit between the thermostat and the AHer as a way of making the AHer think that setpoint had been reached. After observing the AHer in operation, I no longer believe this is a viable solution. The AHer achieves long runtimes via it's normal variable speed operation of it's blower and integral circulator, but also by continuing to run for about 15 minutes after setpoint has been reached. The heat output during this "off-cycle" is reduced compared to the "on-cycle". During mild weather, the "off-cycle" is turned way down and you barely percieve operation. However as the weather gets colder, the "off-cycle" seems more aggressive (but still reduced from the "on-cycle"). The only proper solution is being able to signal the AHer when DHW priority occurs, and have the unit take proper action.

The Vitodens 8-32 provides no direct way of telling another piece of equipment when it goes into DHW priority. Anyone have any suggestions on the best way to do this? Certainly a flow switch on the DHW circuit would be one approach. The other approach I've thought of is to use a setpoint controller like the Johnson Controls A419 in cooling/cut-in mode with the temp probe in the DHW circuit. I'm guessing you could figure out a cut-in setpoint to trigger the control as the Vitodens ramps up DHW production and then select a differential that would cut-out the control as the Vitodens ramps down at the end of DHW production. Any advice on this issue would be appreciated.

Mike T., Swampeast MO

Glad your system is working Larry! Those Ecologix AHs did seem a good choice with a Vitodens.

The cut-in differential may well change over time--remember the control does learn about your system. If it finds that a sudden and quick drop in temp at the LLH immediately preceeds a nice, long burn it may well decide to begin firing more quickly. Will probably take operation in a good range of outdoor conditions before it decides to change. The differential point may even become variable--becoming less in colder weather. Really hard to speculate operational details are rather skanty.

My system seems to have learned to allow even more variance as time has passed. (Of course it's nothing but smooth sailing once loss exceeds minimum modulation.)