Monday, October 15, 2007

Heat Pump Parametrics

Topic: Calculating heat pump parametrics using DOE/ORNL's nifty online tool to justify modeling assumptions

Note that residential heat pump EER and SEER ratings are obtained from a certification test performed with the supply blower fan working against a 0.10, 0.15 or 0.20 inches of external static pressure, for 2 ton and less, 2-1/2 to 3-1/2 ton, and 4 ton and larger units respectively, per Table 2 on page 59149 of this DOE test procedure document.

One interesting thing to note is that the test tolerance is 0.05 inches of static, so a manufacturer wishing to maximize the performance rating of a given heat pump may utilize external static conditions of 0.05, 0.10 and a 0.15 respectively for the given size ranges in the test procedure.

Even more interesting (
see footnotes below) is that commercial heat pumps connected to ducts aren't allowed to count supply fan energy in their EER/COP/SEER/HSPF calculations, according to ANSI/ARI/ASHRAE ISO Standard 13256-1:1998.

Thus for most low-cfm heat pump applications, the impact of the test-condition supply fan energy can be conservatively ignored. That is, not subtracted-out from the relevant performance equation in order to calculate a new EER, SEER, COP or HSPF.

Which is not to say that supply fan energy should be ignored, only that it needs to be modeled separately
at actual design conditions. Supply fans for commercial heat pumps usually run continuously, and can easily be modeled as such in eQuest -- just be sure to select 'Indoor Fan Mode' as 'Continuous' on the 'Flow Parameters' panel of the system 'Fans' tab.

(click on the image to see a larger version)

If challenged to subtract-out supply fan energy by LEED or energy rebate reviewers, refer to ANSI/ARI/ASHRAE ISO Standard 13256-1:1998 Water-Source Heat Pumps - Testing and Rating for Performance - Part 1: Water-to-Air and Brine-to-Air Heat Pump. Minor excerpts reprinted below.

Footnotes courtesy of Xiaobing Liu:

1. The fan power included in the EER/COP calculation may not be the same as the fan actual demand when the heat pump runs. For water-source heat pumps, ANSI/ARI/ASHRAE ISO Standard 13256-1:1998 (Water-source heat pumps - testing and rating for performance - Part 1: Water-to-air and brine-to-air heat pump) states following requirements for fan power input in COP/EER calculations

4.1.2 Power input of fans for heat pumps without duct connection

In the case of heat pumps which are not designed for duct connection and which are equipped with an integral fan, all power consumed by the fans shall be included in the effective power input to the heat pump.

4.1.3 Power input of fans for heat pumps with duct connection If a fan is an integral part of a heat pump, only the portion of the fan power required to overcome the internal resistance shall be included in the effective power input to the heat pump. The fraction which is to be excluded from the total power consumed by the fan shall be calculated using the following formula: φfa = q x Δp / η


φfa is the fan power adjustment, in watts;

η = 0.3 × 103 by convention;

Δp is the measured external static pressure difference, in pascals;

q is the nominal airflow rate, in litres per second.

This value shall be subtracted from the heating capacity and added to the cooling capacity.

2. The reason for this is that, in a particular heat pump installation, depending on the air flow and external static pressure of the duct work, the fan power is different, which is unknown when the heat pump is tested in the lab. So, to isolate from the unknown effect of the external duct work, only the portion of the fan power required to overcome the internal resistance shall be included in the effective power input to the heat pump if the heat pump is designed for duct connection. Therefore, the fan power in the real operating condition should be specified in the simulation to more accurately predict the fan energy consumption.


Mikes said...

All heat pump types operate using similar principles - by harvesting energy from the environment and 'compressing' it to a temperature that can be used for a home's hot water and heating needs.

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