Danfoss DHP-R 7Ua User Manual page 36

settings are selected to be a starting point, and to work in most plants, but dif-
ferences between different plants makes it impossible to have factory defaults
that are optimal.
To fine tune controllers that have a P-area, I-time, or in the case of a WCS, a
D-factor, the system must be up and running. The database plots found under
Databases / Information are good tools to see how current settings are wor-
king for the controller. P-areas and I-times that are too small make the control
signal overshoot or, in a worst case, oscillate uncontrollably. Too large a P-area
and too long an I-time make the controller slow and it takes too long to reach
the set point value. The D-factor dampens overshoot and makes a controller
more stable, allowing a smaller P-area. Too large a D-factor makes the control-
ler slow and jittery, and might cause the controller to oscillate.
Selecting suitable settings for the integral control for heating and cooling is a
balance between comfort and economy. Fewer starts and stops are better for
a heat pump, and larger values on Start hp1 and Delta start hp give this result.
This does, however, also result in larger swings in the primary supply tempera-
ture. Having a larger Start extra heater ensures that the extra heater is not star-
ted unless really necessary, but also allows the temperature to drop further.
The slope of the room temperature compensation curve is a kind of P-factor
for the controller. Incorrectly set room temperature compensation can cause
the primary supply temperature to oscillate, and due to the high inertia of a
building heating system this can be a very low frequency oscillation. Too weak
compensation on the other hand does not do a very good job at all at keeping
the room temperature constant.
The main factor to get a comfortable room temperature is to adjust the outdo-
or compensated set point curve. This is a job that has to be done continuously
until all outdoor temperatures have been tested. Temperatures that are too
high are not only bad for comfort, but also bad economically. Room tempe-
rature compensation should not be used as a substitute for adjusting the set
point curve. It should be used to take care of factors other than the outdoor
temperature, such as sunshine and wind, and other internal unintentional heat
sources.
Adjusting the heat stop and cooling stop limits requires knowledge about the
building and how it works in different weather situations. The main purpose of
these functions is to save money by avoiding heating or cooling the building
when it is not really necessary. What you typically want to avoid is heating the
building during the night and cooling it during the day, or worse, heating and
cooling at the same time.
In summary, there is some fine tuning to do. Much can be done when com-
missioning, but some things that are weather dependent cannot be optimised
until the weather is there. Fine tuning will get you better comfort and smaller
energy bills.
36 – Danfoss