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Outside air reset for a boiler with a primary-secondary loop

asked 2018-02-27 15:51:36 -0500

updated 2018-03-21 19:01:28 -0500

What is the recommended method to implement outside air reset for a boiler with a primary-secondary loop, and a storage tank (i.e. un-fired Water Heater (WH)) connecting the loops? This is more complicated than a primary-only loop since there are two setpoint managers (SPM).

The below scenario worked best for me. I couldn't get the idle boiler return water to track the tank temperature as close as I think it should (it is proportional but offset by ~5°F), but perhaps that is moot since my BoilerEfficiencyCurve is flat.

  1. Primary loop pump is intermittent constant speed, and flow rate is auto-sized.
  2. Secondary loop pump is continuous constant speed at a selected flow rate of 40 gpm.
  3. The boiler is constant flow, `BoilerEfficiencyCurve' is constant, minimum part load ratio (PLR) is 1, the water delta is 20°F, and the design outlet water temperature is 150°F.
  4. Un-fired WH has a source side effectiveness of 1.
  5. The secondary loop serves domestic hot water and heating hot water coils.
  6. Primary loop SPM: An SPM:Scheduled at ~>10°F than the highest secondary loop supply water temperature during the year. I used 145°F per the screenshot below. In reality, my 20°F-delta intermittent-constant-flow boiler return water temperature should track the tank temperature when idle (it is proportional but offset by ~5°F), and yield a 20°F delta when firing (it does the latter). A dual setpoint SPM in the primary loop didn't help.
  7. Secondary loop setpoint control: A custom outside air (OA) reset OS:Schedule:FixedInterval, since OA reset SPMs can't be applied to WH objects. My maximum setpoint is 135°F, my minimum setpoint is 120°F, and my WH differential is 5°F.

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answered 2018-03-02 07:49:00 -0500

The default Setpoint Manager (SPM) is schedule-based, but OpenStudio allows you to apply many different types of SPMs, including one specifically for applying outdoor air reset. In the HVAC Systems tab, while viewing your boiler loop, scroll through the Library list of HVAC components on the right. You should find a line item for "Setpoint Manager Outdoor Air Reset" (see below).

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Drag and drop this item onto the last node of your supply side, replacing the schedule SPM. Then, you can click on the new orange tag icon representing the OA reset SPM to edit parameters (see below).

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For a primary-secondary loop, you may need to apply the same OA reset SPM to the supply side of the secondary loop. In your case, the loop where the "water heater" serves the service hot water draws.

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I put an OA reset SPM in the primary loop, and left the secondary loop SPM as constant. However, this gives secondary loop supply water temperatures with more variation than expected. Even with constant setpoints everywhere, the variation is high. I'd prefer to have only one SPM in the secondary loop, at its current location or inside the tank. However, OpenStudio requires two SPMs, and a setpoint schedule in the water heater object. Is there any way to get around this? If not, how do all three of these setpoint schedules interact?

Bo White gravatar image Bo White  ( 2018-03-02 18:17:43 -0500 )edit

Yes, OpenStudio requires a SPM on the final supply node of each loop as a bare minimum. It looks like your original schedule SPMs are applying "Boiler Setpoint Temperature" for the primary loop and "Water Heater Setpoint Temperature" for the secondary loop. This "Water Heater Setpoint Temperature" should also be applied to the Water Heater object, so that it provides the correct temperature for the secondary loop.

Double-check loop settings (min / max temp, min / max flow, etc.) and loop sizing settings for both primary and secondary loops by clicking on the Supply / Demand Equipment dashline

Aaron Boranian gravatar image Aaron Boranian  ( 2018-03-03 15:44:44 -0500 )edit

I found that an OS:Schedule:FixedInterval was needed in the secondary loop (OA reset SPMs can't be applied to WH objects), and that the primary loop needed an SPM:Scheduled at ~>5°F than the highest secondary loop supply water temperature during the year. In reality, my 20°F-delta intermittent-constant-flow boiler return water temperature should track the tank temperature when idle, and yield a 20°F delta when firing. A dual setpoint SPM in the primary loop didn't help. How can I make the return water realistic? If BoilerEfficiencyCurve isn't dependent on return water, is this moot?

Bo White gravatar image Bo White  ( 2018-03-21 18:06:10 -0500 )edit

The "Efficiency Curve Temperature Evaluation Variable" can be set to either LeavingBoiler or EnteringBoiler. So, return water temperature can be important if you're using the second option.

Maybe I missed it, but what about your return water temperature in this scenario is not realistic? Could you post an image of boiler return water temperatures in your initial question post?

Aaron Boranian gravatar image Aaron Boranian  ( 2018-03-21 18:23:54 -0500 )edit

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Asked: 2018-02-27 15:51:36 -0500

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Last updated: Mar 21 '18