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boiler sizing conundrum in OS

asked 2015-12-01 18:55:05 -0500

SVVitale's avatar

updated 2015-12-01 19:30:39 -0500

I am looking for some guidance. I have set up a model for a Packaged Rooftop VAV with Reheat and I have set all of my sizing to Autosized. My boiler supplies hot water to a heating coil inside my AHU and to two Reheat coils, one at each VAV box. My simulation comes up with a boiler that I estimate to be about 1/4 of the proper capacity AND I also have many unmet hours in heating.

I've been trying to troubleshoot my model and am using a measure that provides good flow and temperature data at select nodes. My AHU has a setpoint manager on the supply air downstream of my heating coil which controls my heating coil (and DX cooling coil in sequence) to maintain a fixed supply air temperature. My node diagnostics seem to indicate that the supply air temperature downstream of my setpoint manager is colder than my setpoint during the design winter day. I also have a setpoint manager on the downstream side of my hot water boiler which also does not seem to be hot enough (setpoint=180 degrees F and actual temperature is around 125 degrees F).

All of this tells me that my OS inputs are making E+ undersize my boiler. Furthermore, just for fun I input a hard boiler capacity that is closer to my hand-estimated boiler size and my unmet heating hours went down significantly and my node diagnostics also indicated improvement.

Are there some key areas of my model that spring to mind that could be causing E+ to undersize my boiler in Autosize mode?

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answered 2015-12-01 19:12:02 -0500

updated 2015-12-01 19:12:17 -0500

My first guess is that the Sizing:Plant object for the boiler loop is set to 140F (default). When you tell the boiler to operate at 180F, it doesn't have enough capacity. I'd first run through the checks listed in this Answer. Or use the Measure that's linked to that Measure to do it automatically.

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Thanks. This is a big help. Your guess was not the reason, exactly. What I found was a problem with my setpoint schedule and also I had input 55 degrees F as the sizing discharge temperature for my central heating coil. The link you provided was also very helpful.

SVVitale's avatar SVVitale  ( 2015-12-02 12:04:46 -0500 )edit

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Asked: 2015-12-01 18:55:05 -0500

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Last updated: Dec 01 '15