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BEopt electric furnace strange behavior

asked 2019-03-02 15:07:15 -0500

duerstad's avatar

updated 2019-03-03 11:53:48 -0500

Two identical BEopt residential designs, one with a gas furnace and the other with an electric furnace, behave the same with one exception. Regardless of whether the electric furnace is sized or autosized, there are a few times, even days, when its output goes to zero. There are never any unmet hours. In DView the delivered energy is exactly the same except for these few periods when delivered energy from the electric furnace drops to zero. These are not periods of extreme cold although they are during periods of low temperatures. Delivered energy from the gas furnace, on the other hand, seems plausible--no such "dropouts." As a result, annual delivered energy for heating is always 65.1 for the electric furnace, but 74.8 MMBtu for the gas furnace. Again--no unmet hours ever show up and the living space remains at the setpoint despite delivered energy temporarily dropping to zero.

Any thoughts on this?

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Graphs of the anomalies would help

Eric Wilson's avatar Eric Wilson  ( 2019-03-03 10:40:17 -0500 )edit

Hopefully this provides the chart suggested. If it does, then blue is the electric furnace, usually hiding the identical delivered energy of the gas furnace. Instead of continuing to cover up the orange, it drops to zero. I've also noticed that site energy of the electric furnace during these "episodes" does not drop to zero, but continues just as does the output of the gas furnace.

duerstad's avatar duerstad  ( 2019-03-04 12:45:26 -0500 )edit

There must be another heating source if the zone temp remains unchanged. Or a bug where the electric heater sill provides hot supply air while energy use goes to 0. I think there is another heater.

rraustad's avatar rraustad  ( 2019-03-04 12:52:07 -0500 )edit

Could you please share your model? I developed a simple model in BEopt but I don't have such dropouts. It could be because of the 'space conditioning schedules'. I can help you further if you share the model.

Ehsank's avatar Ehsank  ( 2019-03-12 09:16:44 -0500 )edit

Thanks for your interest and offer.

duerstad's avatar duerstad  ( 2019-03-12 11:40:47 -0500 )edit

2 Answers

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answered 2019-03-11 08:56:57 -0500

duerstad's avatar

Fortunately, the "dropouts" are virtually the exception that proves the rule that the heat coming from each furnace is identical. Except for these rare dropouts the results are identical in the greatest detail. There is never any other evidence of the presence of an additional heat source. In either case, I believe that the results are usable. They are also very consistent with results for the same building in OpenStudio. Unless someone else has this strange experience, I don't think it deserves further attention. Thanks for the ideas.

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answered 2019-03-12 12:32:43 -0500

updated 2019-03-13 13:30:58 -0500

No problem. Are you sure you are looking at the right data? Because for me it looks different. Attached you can see the heating loads (kWh & Btu) for the electric and gas furnace based on the site energy use. The only load pattern that is significantly different between these two cases is the Heating Fan/Pump activity, which totally makes sense. Attached you can also see the corresponding graph. As you can see, it never stops working during this period. You can also double check to see if a proper simulation engine is selected under the 'settings' just to make sure the E+ you are using is not a modified version.

Please double check what boxes you have checked in DView and let me know if you are still experiencing the same issue.

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Update: If you look at the 'Delivered Energy', as you also pointed out, the dropouts occur when there is an anomaly in 'Heating Fan/Pump' energy use. However, based on the outputs shown below, the 'Total Supply Air Flow Rate' and 'Supply Air Temperature' is identical for both cases. It means that the fan is adjusting its power to keep the flow rate under a certain level (about 1000 cfm in this case). Basically, these dropouts occur when the supply air temperature drops and supply air flow reach its maximum. Then, the electric furnace stops delivering heat (delivered energy is the heating energy delivered to space by HVAC). Something that I couldn't figure out is the difference between electric and gas furnace in terms of systems' properties. I checked the IDF files generated by BEopt and both systems look identical, which doesn't help with explaining the difference in their behavior.

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Asked: 2019-03-02 15:07:15 -0500

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Last updated: Mar 13 '19