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Impact of Ducting on Heat Pump Water Heater Space Conditioning Energy Use and Comfort

Created 8/6/2014 by Graham Parker
Updated 8/13/2014 by Graham Parker
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PNNL 23526. Final Report by Sarah Widder et. al.

This HPWH demonstration examines the overall performance of HPWHs installed in a conditioned space with a number of supply and/or exhaust ducting configurations, as well as the interactions between the HPWH and the home’s heating/cooling system. Specifically, this field evaluation of two HPWHs in the Pacific Northwest National Laboratory Lab Homes is designed to measure the performance and impact on the Lab Home heating, ventilating and air conditioning (HVAC) system during heating and cooling season periods of HPWHs in two pairs of configurations:1. a HPWH configured with exhaust ducting compared to an unducted HPWH2. a HPWH with both supply and exhaust air ducting as compared to an unducted HPWH.Important metrics evaluated in these experiments include water heater energy use, HVAC energy use, whole-house energy use, interior temperatures (as a proxy for thermal comfort), and cost impacts.
Posted By: Graham Parker 08/06/14 on 10:43 AM (Pacific Time)
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Comments (4)
Graham Parker on 08/06/14 on 10:43 AM (Pacific Time)
Field data from the PNNL Lab Homes demonstration of the GE HPWH in exhaut-ducted, fully-ducted and unducted configurations.
Hot Water Solutions on 08/12/14 on 09:13 AM (Pacific Time)
This is a great demonstration and report that will be especially valuable in understanding impacts of ducting heat pump water heaters and informing ducting recommendations and/or requirements.

One important clarification- the report states that the Northern Climate Specification requires exhaust ducting for Tier II products and full ducting for Tier III products. The Northern Climate Specification does not require ducting in any product tier, however it requires ducting capability for Tier II and Tier III products.
To read the most current version of the specification, please visit: http://neea.org/docs/default-source/northern-climate-heat-pump-water-heater-specification/northern-climate-specification.pdf?sfvrsn=8  
Dave Kresta on 08/12/14 on 02:27 PM (Pacific Time)
This is good to start having actual field data on this topic. As mentioned in the report, this particular experiment is quite limited in scope, and getting to a broader, region-wide practice to deal with the space heat-interaction question will take more work. The things I found unique in this experiment include:

1)     A very high water use rate of 130 gallons/day. This is more than twice what we see on average across the NW. It would tend to make the compressor run longer than normal to keep up, and potentially bias the numbers.
2)     Very cold outdoor temps during the experiment: average outdoor temps of 34 degrees. The experiment would likely have come up with the opposite conclusion if run in Portland or Seattle with outdoor temps of 50 to 55 degrees.
 3)     The configuration of the water closet effectively isolated it from the main living space (and I don’t believe it was actually conditioned, but not sure about that). May be different if the water heater is in a utility room that gets more “traffic” and is conditioned by the space conditioning system (mixing the air with other rooms more effectively).

I’m not disputing the methodology or findings at all, just wanted to provide a caution about not generalizing these findings too widely yet. 
Graham Parker on 08/13/14 on 08:56 AM (Pacific Time)

Thanks for the notes and clarification regarding Tier II and III ducting (capability). 

We agree that the scope was limited by the location of the testing and the climate (weather) at that location.

We specified the hot water draw to provide an 'upper bound' of what may be found in a residential setting.  We recognize that typical daily hot water draws are generally less and thus the performance of the water heater will be different for different draws.  Unfortunately we were limited in scope, budget and time to undertake a thorough parametric experiment.  

As for the issue of 'isolation' of the water heater, it was located in fully conditioned space with 2 large transfer grills in the exterior wall between the water heater closet and the bedroom closet and also 2 large transfer grills between the bedroom closet and hallway.  Thus, there was considerable 'potential' for exchange of air between the water heater closet and the living space. 

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