OT Domestic Hot Water Control

[timsim]

Hi,

I have problem with domestic hot water (DHW) production from a Remeha Avanta Plus 24s and I am hoping that this forum may have some useful guidance.

After nearly two years of investigation I am now nearly certain that it is a control problem and that purchase/construction of an OTGW and monitor will confirm this. However, before going to this trouble I'll describe the problem more fully because the forum may be able to also say what is going on in the Remeha.

The Remeha is running under OpenTherm with a Honeywell ST9520 acting as the controller.
This controller is part of Honeywell's Sundial pack 5 which also contains two DTS92E thermostats as I have two zone system. DHW production is controlled by a tank thermistor and a timer. Honeywell's installation guide makes clear that the ST9520 does not provide DHW control.

The problem is that after any power interruption the flow temperature during DHW is limited to about 25º below the maximum flow temp set by P1 (85º,in my case). This means that I have maximum flow temp of about 60º which is not hot enough to heat the water well.
My set point for DHW (P2) is 65º.

I overcome this problem by resetting the boiler to its factory defaults and then changing the user parameters to my desired values. It then runs without problems until the next power interruption (planned or unplanned).

I believe that this is an OT problem because the ST9520 controller offers an alternative TPI control. If I select TPI the problem does not occur. Also, a post in the HVAC forum on Jan 1, 2014 describes a similar (but not identical) problem with a different boiler. The OP used OTGW and showed that the ST9520 is sending zeroes for DHW setpoint despite the fact that the controller is not supposed to 'support' DHW control.

In my case I wonder whether zeroes sent by the controller during the initialisation phase of the boiler after power is restored are pushing the DHW set point to its minimum value fixed by the boiler (40º). This is 25º degrees below my set point of 65º and, for reasons I do not understand, it lowers the flow temperature (t1) to 60º, (85 minus 25).

I cannot explain why this supposed stream of zeroes does not cause problems after I have 'corrected' the problem as explained above.

A secondary question is why reducing the DHW set point (P2) reduces the maximum flow temperature set point (P1) by an identical amount. This surely lengthens the DHW recovery time.

I will be very grateful for any help offered.