Hi everyone,
I?m trying to model a ground source heat pump system in eQUEST using a
Water Loop Heat Pump system attached to a ground source heat exchanger.
The system seems to work fine during occupied hour but it just can?t meet
morning warm-up loads. I?m doing a systems energy analysis for our local
energy code compliance and there is a mandatory night setback of 55 ?F
from 70 ?F in heating. During winter months the system doesn?t seem to be
able to jump from 55 ?F to 70 ?F space temperature in less than 1-2 hours.
I?ve double checked the fan schedules and the system is allowed to come on
during this time.
Does anyone have any suggestions for getting the system to meet these
unmet load hours?
Robby Oylear
Maybe you can input or modify optimum start schedule.
Eric Yang
I?ve never used an optimum start schedule before. I tried inputting the
-999 flag in the schedule but without much luck. It reduced the number of
load hours from around 1100 total to 800, but did not eliminate the
problem. So far I?ve found that increasing max zone entering temperature
and increasing the supply cfm lowers the unmet load hours, but I?m afraid
of increasing the energy use unnecessarily. I?ve got the same building
using a different system to meet all of its unmet load hours with similar
CFM and btu-h heating/sf.
Robby Oylear
Robby:
Try putting in more heat exchanger capacity in the ground. Morning warm-ups are always a "bear" anyway.
Robby,
This is a typical, real world issue with heat pumps. We typically advise
owners not to use an aggressive night setback (more than 2 or 3 degrees)
because of this exact problem. We usually have a control sequence put in
place that limits the night setback if the outdoor air temperature falls
below a certain level (say 30 degF). Can you apply for a variance under the
local building code? If this is how the building will be operated in real
life with a 15 degree setback, you will probably get too cold complaints
from the occupants.
One way around the modeling problem would be to use perimeter baseboard in
the model. I would set it to electric, and schedule the baseboard to only
to be available during the morning warm-up period. It shouldn?t add too
much energy into you model and will eliminate most of your unmet load hours.
Good luck!
Jon Evans, PE