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Hydronic Unit Heaters in Plant Loops

asked 2018-11-04 00:08:09 -0500

updated 2018-11-04 00:29:00 -0500

What's the best way to model these in Open Studio?

I have a plant loop with a boiler, that part I have figured out. There are a number of hydronic unit heaters (water, not steam) similar to the following link, that I am trying to enter into the model:

  1. So far, I have found "Unit Vent Heat Coil" which seems to fit, although there are no inputs for fan speed/volume. Also, I can't figure out what "U-factor Times Area Volume" means within his context.

  2. In addition, how do I account for pumping energy? The unit heaters are fed through "close coupled tees" where the pumps are running all the time off of the main loop, but the unit heater fans are controlled through thermostats.

  3. Furthermore, how can I account for piping heat losses in uninsulated pipes?

I have several of these unit heaters within a plant loop of an existing 50+ year old commercial building I am modelling.

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answered 2018-11-05 07:19:06 -0500

updated 2018-11-05 07:29:54 -0500

I would model it with the four pipe fancoil zone terminal object and connect it to your hot water plant loop. You should also create and connect them to a fake cooling plant loop (set the chiller power to hard size 0 Watts).

The pumping energy will be accounted because you already have a pump in your hot water loop. The power consumed will the volumetric flow multiplied by the increase in total pressure and divide by the performance.

If you need to take account of the distribution pipes energy losses, you should change the adiabatic pipes (at least one) in your hot water plant loop by outdoor pipes.

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answered 2018-11-05 07:53:41 -0500

  1. There is a better option calledZoneHVAC:UnitHeater in EnergyPlus - and also OpenStudio - that would serve this purpose. You go to the Thermal Zones tab, and drag and drop from "Unit Heater", in your case either the "CV Unit Heater HW" or "VAV Unit Heater HW". You can then attach the coil to the plant loop of your choosing. You can enter the W for the heating coil (instead of UA), and the maximum air flow rate in the ZoneHVAC component.

  2. Model a secondary plant loop, connect it to the boiler plant loop with a HeatExchanger:FluidToFluid.Add your unit heaters to the demand side of the secondary plant loop, and on this loop make your pump run all the time.

  3. Pipe - Indoor and Pipe - Outdoor would serve here. Note that I wouldn't encourage you to model indoor pipes that are in conditioned zones especially if you have a lot of them: it'll get too complicated for little reason. Stick to modeling indoor pipes in unconditioned spaces and outdoor pipes. You can also preprocess your stuff to calculate an equivalent pipe instead of entering dozens of them.

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I took your option of the primary and secondary loops. What about the fact that the pumps are running constantly? The order of operations is that during the start of the heating season, all the pumps (primary and secondary) are turned on. At the end of the heating season, they are turned off. Full operation of roughly 6 months.

Is there a way I can schedule the pumps within the plant loops?

re-measure's avatar re-measure  ( 2018-11-19 21:34:38 -0500 )edit

Yeah, head over to the "Control" subtab of your plant loop, and drag an "Availability Manager Scheduled". Go read the difference, in the E+ I/O reference guide for AvailabilityManager:Scheduled, ScheduledOn and ScheduledOff. Do not mind the fact that some of the description for field 'Schedule Name' mention that it affects the Fan. In fact it affects the primary mover of the loop, a Fan for an AirloopHVAC, a Pump for a PlantLoop.

Julien Marrec's avatar Julien Marrec  ( 2018-11-20 02:44:45 -0500 )edit

I think this is heading in the right direction. However, when I insert this in as "scheduled AVM" with a priority run profile of every day between oct to apr, my electricity and NG use goes down. I would expect my electricity use to go up. I have a feeling this has to do with how my loop is set up maybe? Alternatively, if I try this with one of my airloops (RTU), I get no change in electricity use. video explainer: model:

re-measure's avatar re-measure  ( 2018-11-20 19:37:25 -0500 )edit

@Julien Marrec I'm having a hard time with loop and/or air loop control/operation and the use of setpoint/availability managers, there aren't any concise tutorials out there. I can see now that my air loops ARE actually running with the fans FULLY ON all the time. This is probably due to me setting the operation schedule to "Always On" (set by default when creating new air loop). But how do I get the fans to follow the zone thermostat that I have set in "thermal Zones" -> "Heating Thermostat Schedule"? . I assume normal fan operation for an RTU would follow a thermostat and not constant on.

re-measure's avatar re-measure  ( 2018-11-23 12:23:07 -0500 )edit

Do read the difference between AVM:Scheduled, AVM:ScheduledOn, and AVM:ScheduledOff like I said. the control statuses each send are quite different.

Julien Marrec's avatar Julien Marrec  ( 2018-11-26 04:55:48 -0500 )edit

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Asked: 2018-11-04 00:08:09 -0500

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Last updated: Nov 05 '18