PTAC with hot water Baseboards Openstudio

asked 2021-11-19 16:47:15 -0600

Jacob gravatar image

updated 2021-11-20 07:53:14 -0600

I have a "Baseboard convective water" system sized to fully meet space loads (from a previous sizing run with no unmet hours) as primary on my Zone Equipment list and "PTAC Dx clg elec htg" second on my zone equipment list. the baseboards are hooked up to a hot water loop and boiler. They appear to be using energy as expected. After running this set up I had about ~5,000 heating hours unmet (building in zone 7) and no reduction in fan energy vs when I had the PTAC with hot water heating alone. I hard sized the PTAC w elec heat capacity to 0 with no effect, I set the supply airflow while heating to 0 with no effect. I also tried autosizing the baseboard and increasing the baseboard sizing well over what it should take with no effect.

1) any ideas on why annual fan energy didn't change by even 1 kWh per year vs just the PTAC with electric heat? I would think that the hot water baseboards would take the load and the PTAC would not run thereby saving fan energy.

2) why I am seeing ~5,000 unmet heating hours regardless of baseboard sizing?

Thank you!

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@Jacob is there a way that you can share the OSM using Dropbox, Google Drive, etc.?

Some thoughts without seeing the model:

1) The PTAC might have a constant flow fan (OS:Fan:ConstantVolume) or cycling fan (OS:Fan:OnOff) set up to run continuously contained within it. Click on the PTAC in the Thermal Zones tab, then scroll down on the right until you see the fan object type and inputs.

2) Check the availability schedule assigned to your HW baseboards, they may be forced off (schedule value of 0) when you want heating.

Aaron Boranian gravatar image Aaron Boranian  ( 2021-11-20 07:58:19 -0600 )edit

Thanks for the reply! The model is here:

1) my hope was that for all hours heating would be totally supplied by the baseboard so even CV fans should turn off. I don't want to schedule the fan off since it will be needed to provide cooling and I'm not sure how to set up a "during heating mode hours" schedule.

2) Baseboards show Always on discrete HVAC schedule so should be available.

Thank You!

Jacob gravatar image Jacob  ( 2021-11-21 10:43:34 -0600 )edit