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Simultaneous heating and cooling during summer months

asked 2018-12-06 06:21:10 -0600

Vishak gravatar image

updated 2018-12-06 07:04:00 -0600

Software:Open Studio DOE Prototype building: medium office Location:New York-POUGHKEEPSIE-DUTCHESS Issue: Initially I modeled this building which had lot of heating(natural gas consumption) during summer months.So,in order to curb heating load,I set thermostat to 50F for summer months so that it doesn't get heated.However,after setting thermostat,cooling was also shut down during summer months.It led me to conclude that cooling in the summer months was a function of over-heating in the building. I am using a plant loop HVAC to condition my building with a single reheat for each zone. The boiler and the chiller are auto-sized. I have attached before and after images of consumption and the images for the heating and cooling plant loops used. Please let me know how I can achieve significant independent cooling in summer months?!

Loops:

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Before:

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After:

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I referred to some past questions on the topic on the blog and some of them suggested adding a sequence number to the cooling and heating zone equipment.Sequencing doesn't seems like an option to me since there is only a heating equipment in the zone.

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answered 2018-12-06 07:06:12 -0600

What's the parameters of your Setpoint Manager (Outdoor Air Reset I guess? What temperatures?) on the supply outlet of the AirLoopHVAC?

ANyways, this is very much likely due to too much reheat happening. I suggest you read up on SetpointManager:Warmest if you are looking for something more efficient/smarter.

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Setpoint Manager is Outdoor Air Reset and the control variable is Temperature. As in the figure there is a single node (red node) in the plant loop for cooling and heating. However, correct me if wrong, if there was too much reheat, it should have cooled independently when reheats were switched off in summer artificially using the thermostat setpoints. My drop-down box for the control variable has options to set only to temperature. How do I change this scenario?

Vishak gravatar imageVishak ( 2018-12-06 10:53:42 -0600 )edit

You misunderstanding the principle of a VAV Reheat system. I don't know the actual settings of your SPM OA Reset (the low/high temps and setpoints), but let's assume you're using a standard Appendix G reset. So you take the outside air temperature, and compute a given temperature setpoint for your air supply, which is going to be in between 55°F and 60°F. Now let's assume that you also have a lot of outside air flow requirements, which is driving the supply airflow a lot. You'll end up providing too much cooling in all of the zones, and will need reheat to not overcool.

Julien Marrec gravatar imageJulien Marrec ( 2018-12-07 03:46:29 -0600 )edit

In the middle of the summer, all of the reheat and the cooling that you see has nothing to do with your heating setpoint temperature schedule.

Anyways, A SPM:Warmest will instead check all of your zone loads, and decide what is the highest temperature it can supply while appropriately cooling your WORST zone, thus eliminating reheat in that zone, and mimizing reheat in the others. This is similar to a BMS "polling" the temps (and flows) of all of your VAV boxes and determining the temperature like this.

Julien Marrec gravatar imageJulien Marrec ( 2018-12-07 03:47:43 -0600 )edit

Read carefully the I/O reference guide for the different types of SPMs, and make sure you understand what they do.

Julien Marrec gravatar imageJulien Marrec ( 2018-12-07 03:49:00 -0600 )edit

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Asked: 2018-12-06 06:21:10 -0600

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Last updated: Dec 06