Heating output capacity does not match Heating Delivered

asked 2024-11-19 09:56:25 -0600

muhl
173 ●6

I configured a single family home with garage/semi-conditioned space under living space in CT. The living space has 25,000 BTU/hr of heating capacity from a minisplit. Yet, when reviewing the DataViewer outputs, only ~12-13,000 BTU/hr of heating is delivered and results in Unmet Hours. While the specified MSHP product has appropriate HSPF, SEER, and maximum capacities, I cannot identify the actual sizing specified. The Option Description uses the Sizing Methodology for ACCA, which states "...while taking into account the heat pump's reduced capacity at the design temperature." Many of the products on the market, rated as EnergySTAR Cold Climate heat pumps have enhanced low-temperature performance that could certainly exceed this existing ACCA calculation.

Is there a method to 1) observe the calculation limits, 2) modify these limits per Option, and/or 3) modify these limits consistently across Options within a BEOpt library?

I am running BEOpt 3.0.1 on Windows 11, using this file and these outputs.

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answered 2024-11-21 11:13:22 -0600

shorowit

11931 ●203 ●9 https://www.nrel.gov/b...

There is a lot packed into your question.

First of all, the mini-split capacity was set at 25 kBtuh for the existing home. I don't know how that was chosen, but when you specify the size of the equipment, then the sizing methodology (ACCA or otherwise) will not come into play. If you want to use ACCA sizing (or MaxLoad sizing, which is intended to prevent any unmet load), you need to autosize the equipment. The text you quoted "...while taking into account the heat pump's reduced capacity at the design temperature." pertains to the MaxLoad sizing methodology, not ACCA.

Second, the 25 kBtuh heating capacity is the rated capacity, so it occurs at an outdoor temperature of 47F. BEopt currently assumes a heating capacity retention (heating capacity at cold temperature compared to heating capacity at 47F) that is representative of models on the market. This obviously doesn't help you specify a unit that is higher than average, but in the next version of BEopt, we will be offering the option of specifying the heating capacity retention. That said, note that if you were to increase the heating capacity retention, the heat pump COP must get worse in order to achieve the selected rated performance (SEER/HSPF); that is a confusing concept for many users.

I don't know if I've fully addressed your questions, but hopefully it helps clear some confusion.

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Yes, we may not seek autosizing, due to other factors:

Resident may have already installed or own a specific capacity. We want to reflect the actual known data in the model. We still want to know the Manual J-based calculations of the heating/cooling demands on design days using ACCA methods.
Resident in our geography is likely specifying cold climate heat pumps (See NEEP reference AHRI Cert #⁺: 212925867 for this model) The heating output is maintained from 47F to 5F.

Can these performance specs be the model?

muhl ( 2024-11-22 07:56:27 -0600 )edit

I don't know of any residential user interface that allows detailed inputs for a specific HP. (Though OpenStudio-HPXML does.) Also: 1) RESNET recently looked at the NEEP database and found a huge amount of bad data. It was disheartening. The vast majority of models had at least one egregious issue. I don't know if it's been improved yet. 2) Heating capacity will always drop at colder temperatures. If a HP doesn't show that trend, it's because they are gaming the rating and artificially reducing their rated capacity to make the cold climate performance look better. Modeling HPs is hard.