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Boiler Hourly Loads Energy Plus

asked 2021-06-11 09:13:55 -0500

njharvey1994's avatar

Hi all, working on a boiler with radiant floor heat model. I have the system set to auto size the boiler and all thermal zones. I have energy bills from the building I am trying to model so know what to expect for an electricity and gas consumption as well as peak loads. When I run the model, electricity and peak loads are a good match, but my gas consumption is more than double what it should be. I spent 2 days trying to figure out the issue and I have determined that something seems off with the sizing. My peak heating load is 34 kW (116 MBH) for a design temperature of -27 C (-18 F) which is great, very reasonable. But when I look at the hourly boiler heating rate. I am getting peak loads every hour of the day from January until March, even when the outdoor air temperature is well above design day temperatures. Does anyone know how the loads are being calculated? Why are my loads so constant, they should fluctuate significantly through out the year.

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I'm a little confused where you say you think the problem is sizing, but the peak load is very reasonable. Is that 34 kW the design load that is being calculated for the boiler in your EIO file? If the sizing in the EIO looks ok, then it sounds more like an operational problem. Maybe start by checking how reasonable the return/supply temps to the boiler are, making sure the loop is making setpoint and considering your boiler performance assumptions (the Curve:Biquadratic).

codybond's avatar codybond  ( 2021-06-14 09:13:07 -0500 )edit

It may also be the case that what you see as a "peak load" is the startup load as the boiler is trying to raise the zone temp from an unoccupied setpoint to a higher occupied setpoint. This would occur only for a short time each day...

Jim Dirkes's avatar Jim Dirkes  ( 2021-06-14 11:17:12 -0500 )edit

Thanks Cody, the supply and return temps to the boiler look good. After further investigation however, I think something is up with the radiant portion or the controls of my design. Supply temperatures to each radiant zone are reasonable, but a few of the zones have too high or too low of return temperatures. Also, when I check mean zone air temperature on cold days, it is dropping to well below setpoint temperature. Since I have the boiler and radiant loops set to autosize, I'm not sure why temperature isn't being maintained. I'm pretty new to Energy Plus and really appreciate the help.

njharvey1994's avatar njharvey1994  ( 2021-06-14 13:00:59 -0500 )edit

Below is a link to my model if anyone has time to a look.

njharvey1994's avatar njharvey1994  ( 2021-06-14 13:04:10 -0500 )edit

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answered 2021-06-16 10:26:41 -0500

updated 2021-06-16 10:43:26 -0500

Hi Nicole, it seems like there are two main problems: 1) Sizing is not as expected, and 2) Boiler consumption is too high.

For 1), you might need to dig into the sizing routine for the low temperature radiant object. Here is a decent starting spot in the Engineering Reference. I did not see anything glaringly wrong with your inputs. In every case I looked at, your design day schedule parameters were identical to your operational schedule parameters, so conditions are being completely influenced by outdoor and zone conditions. The zone conditions are a bit loose just because of the overall model set-up. You're not using any air loops, so once the radiant heating exceeds your single setpoints there is nothing controlling the zone temperature (this is why the zones are getting so hot). This could be having some effect on the sizing routine, but I'm not totally sure.

Ultimately the regular simulation is calling for more heating energy than the sizing calculation, which is what is causing your boiler to not be able to make its setpoint at times, which in turn means it is often maxing out at its autosized design capacity of ~15 kW. You could simply tweak your Heating Sizing Factor in Sizing:Parameters, but it may be an indication that your applied zone loads are different than what the real-life equipment was designed for. Another trick is to remove all the loads for your Winter Design Day schedule, which will increase the autosized capacity.

For 2), getting the capacity sized correctly is probably going to increase your consumption even more, which is really the main problem. You can come at this one from two different ways. Either 1) your zone loads are not high enough or 2) your real-life boiler is more efficient than the model boiler.

I'm using there term zone loads sort of broadly. This ends up being the interaction between direct zone loads (people, elec equipment and lights) and other factors like thermostat setpoint, windows and their properties and geometry surfaces and their properties. If the total interaction of those loads are low, then the radiant heating is going to work harder to make its setpoint (which is not actually controlled by the zone thermostat).

The other side is maybe the energy balance is good, but the boiler is using too much propane to make that energy. You've already got it at a nominal 95% efficiency, but that leaves you at the mercy of the DOE model in terms of varying conditions. You may want to try out some of the efficiency curves in the Boilers.idf file that is located in the DataSets folder within your energyplus installation.

You could also consider adjusting the availability of the boiler using a scheduled or HighTemperatureTurnOff availability manager on the loop.

Another strategy could be to see if a particular zone is blowing up your consumption. I noticed the kitchen and garage zones alone are using ~58% of the total ... (more)

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Thanks so much Cody, I will dig into this a little more with your suggestions and see what I can get figured out. I really appreciate the help.

njharvey1994's avatar njharvey1994  ( 2021-06-16 10:31:15 -0500 )edit

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Asked: 2021-06-11 09:13:55 -0500

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Last updated: Jun 16 '21