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How to reduce the number of hours heating not met in DesignBulder

asked 2021-06-30 12:44:52 -0500

Cipi's avatar

updated 2021-07-09 16:44:50 -0500

Hi, I have to model an office building served by an air-to water heat pump that provides heating cooling and domestic hot water to the building with the detailed HVAC. The purpose is to investigate LEED credit on energy performance. First, I had problems in finding a single system in DesignBuilder and I ended up modeling separate Heat Pump Heating, HeatbPump Cooling and DHW loop water heater. Anyway, the main problem is that I cannot manage to reduce the number of hours heating load not met. Even if I significantly increase the Heat Pump Heating power capacity, nothing changes. Can someone give an advice on how to handle this issue? Thanks in advance.

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answered 2021-06-30 17:02:22 -0500

sashadf1's avatar

I would try to have EnergyPlus autosize the rated capacities and airflows of your separate heating and cooling heat pumps to meet load. You can probably find a way to do this in DesignBuilder or in the IDF it generates.

If you are hard-sizing your heat pump capacities and still getting high unmet hours, look at the rated airflow rate(s). I have noticed in past models that if I increase the rated heating or cooling capacity of my heat pump coils and do not increase the rated airflows through those same coil objects as well, the desired effect of increasing HVAC output to match load and reduce unmet hours is not produced.

Another thing to investigate are the performance curves of your heat pump coil objects. The performance curves dictate how your capacity and EIR/COP degrade or otherwise change with ambient conditions. The curve values (function of temperature and flow fraction) combined with low ambient outdoor dry bulb temperatures can produce instantaneous capacities and COPs that are lower than rated, hence causing unmet hours.

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answered 2021-07-02 15:52:56 -0500

Luís Filipe S.'s avatar

The 1st thing to do here is to identify which zones have more unmet hours reported. This information is available at the Summary report "Time Set point Not met".

Then you will need to analise how the HVAC system is performing on these spaces and if the temperature setpoints are set up correctly and being met by the system.

Have also a look at this post:

Its not specific to DesignBuilder software but the approach is similar.

If you have a valid DesignBuilder license then you can always contact DesignBuilder support team for assistance.

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answered 2021-07-03 01:19:48 -0500

Cipi's avatar

Thanks for your feedback and indications. Unfortunately, it is not possible to give "Autosize" as input for the rated capacities and airflows. I applied your suggestion on increasing also the rated airflows, and it worked, but it just reduced a bit the number of not met hours, while the issue was not fully solved, since after a certain value they didn't change anymore. About the performance curves, I don't have them for the design heat pump I have to model, so I kept those from a default heat pump in the software database. Actually, it seems really like the system cannot manage to increase the temperature over a certain value, meaning cover the needed delta T. I also increased the number of hours of operation of the system and it helped a lot, but again it didn't fully solved the issue.

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@Cipi this isn't an answer to your original question, but rather comments in response to the solution offered by @sashadf1. In the future, please only add an "answer" when you are offering a solution to a question and add a "comment" for other situations.

Aaron Boranian's avatar Aaron Boranian  ( 2021-07-03 08:56:57 -0500 )edit

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Asked: 2021-06-30 12:44:52 -0500

Seen: 602 times

Last updated: Jul 03 '21