Residential Heat Pump Water HeatersHeating System Interaction Factor

RTF HPWH SubcommitteeNovember 6, 2014

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Presentation Outline

• Brief Recap from Last Meeting• Heating System Interaction Factor – Problem Statement– Sensitivity– Definitions– Options– Recommendation

3 Recap from Last Meeting (10/21)• Presentation and notes from meeting can be found here• Summary of topics and direction from subcommittee:

– Model calibration• The subcommittee agreed that the HPWH model developed within SEEM was

sufficiently calibrated for the purposes of estimating a UES.– How to treat the impending federal water heater standard

• The subcommittee agreed to assume the federal standard goes into effect now (instead of April 2015). They thought that the staff resources to develop multiple measure sets was not worth the effort and could be cumbersome for programs.

– Whether to have an "any size" tank measure or separate small/large tank measures• The subcommittee agreed that an "any size" measure based on the weighting of

small and large tanks in the region was appropriate. A relatively short sunset date (~1 year) should be used to monitor market shifts in average tank size.

– Valuing self-install labor at zero cost• The subcommittee agreed to defer this discussion to the broader RTF, since it relates

to the Guidelines and many measures in addition to HPWH.

– HVAC interaction• The subcommittee ran out of time and did not reach a recommendation.

Subcommittee agreed to meet again to discuss this item.

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Staff Highlighted Areas• HVAC interaction factor (interior installs only)– Provisional measure assumed full HVAC interaction

(100%); provisional research was inconclusive on this– Analysts believe average value likely falls between 50 and

100%– Should we use an assumption in this range OR pursue

additional research?• If we use an assumption, analysts suggest 75%

RTF Judgment Required Ahead

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Heating System Interaction• Applies only to interior ducted or non-ducted installations (i.e. garage and

unconditioned basement installs are excluded from this discussion)• Heat pump water heaters extract heat from the space where they are

installed. Some of that heat energy is replaced by the heating system.• We have observed that not every unit of energy removed from the air by

the HPWH is replaced by the heating system

Electric Storage Tank Water HeatersLocation Overall nBasement 29% 240Main House 44% 361Garage 19% 155Crawl 4% 31Other 3% 27

Is the WH Space Location Conditioned?Location No YesBasement 16% 84%Main House 5% 95%Garage 92% 8%Crawl 86% 14%Other 75% 25%Overall 30% 70%

Source: RBSA

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• HVAC System Interaction = m*cp*ΔT + QUA

• Typical air Δ T at 68F entering air: ~15F

Warm House Air

Cool HPWH Exhaust Air

Tank Heat Losses

Conditioned Space Installation Heat Flows

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• HVAC System Interaction = m*cp*ΔT + QUA

• Δ T now depends on outside air T• m now depends on added infiltration load

Warm House Air

Cool HPWH Exhaust Air

Tank Heat Losses

Ducted to Outside, Conditioned Space Heat Flows

Added Infiltration Load

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Localized Cooling – Temperature Depression

• “Interior Ducted to Outside” (left) case exhausts air outside – no (or much less?) localized cooling so use as reference case

• “Interior Non-Ducted” (right) case shows lower average air temperatures– Approximately 8F colder across our study set

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Sensitivity of Total Savings to Heating Interaction Factor (%)

HZ1 HZ2 HZ3 HZ1 HZ2 HZ3 HZ1 HZ2 HZ3Zonal Electric Resistance Electric Furnace Heat Pump

-

200

400

600

800

1,000

1,200

1,400

1,600

Tier 1, Interior

50%

75%

100%

Tota

l Ene

rgy

Savi

ngs (

Hot W

ater

+ H

eatin

g), k

Wh

Interaction Factor

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Sensitivity of Total Savings to Heating Interaction Factor (%)

HZ1 HZ2 HZ3 HZ1 HZ2 HZ3 HZ1 HZ2 HZ3Zonal Electric Resistance Electric Furnace Heat Pump

-

200

400

600

800

1,000

1,200

1,400

1,600 Tier 2, Interior (Non-Ducted)

50%

75%

100%

Tota

l Ene

rgy

Savi

ngs (

Hot W

ater

+ H

eatin

g), k

Wh

Interaction Factor

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Definition: Thermal Utility• Thermal Utility

– The ratio of useful internal heat gains that offset a heating requirement to total gains– Example:

• Heating is required in a house for 6 months of the year • Internal gains are 2,000 kWh/yr Useful gains are 1,000 kWh/yr. Thermal utility is 0.5.

J F M A M J J A S O N D

Heating Energy Required

Energy from Internal Gains Gains that just

make you hotUseful gains Useful

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Added Heating Load

Definition: Interaction Factor• HPWHs are a negative internal gain• HCƒ – heating and cooling interaction factor (0% - 100%)

– What fraction of the maximum possible interaction is “seen” by the HVAC system?– Primary hypothesis for the value being less than 100%:

• HPWH is located in a space that’s somewhat coupled to the outside• HPWH extracts heat and sometimes that heat comes directly from outside

• Note: HCƒ different from thermal utility which remains virtually unchanged

J F M A M J J A S O N D

HPWH Fractional Interaction

HPWH Full Interaction

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Available Studies

• Field study of HPWH interaction factors was inconclusive• What we know from PNNL Lab Homes Study1

– Interaction factor for interior HPWH installations ≈ 49%– Interaction factor for ducted HPWH installations ≈ 44%– Study looked at one installation of a HPWH in a closet next to an

exterior wall

1 PNNL. Impact of Ducting on Heat Pump Water Heater Space Conditioning Energy Use and Comfort. July 2014.

14 PNNL Study – Water Heater Location

PNNL. Impact of Ducting on Heat Pump Water Heater Space Conditioning Energy Use and Comfort. July 2014.

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Heating System Interaction Factors• Houses are not single zones (no surprise)• The HPWH install zone can thermally “decouple” to varying degrees from

thermostated part of house• What to do on interaction factor?

1. Consider more research to further explore this value. Options:I. PNNL Lab HomesII. More detailed assumptions, calculationsIII. Large scale metering and/or billing analyses

2. Assume a value. If a value is assumed:• Assume same value for ducted and unducted installs

– (When you don’t know something, make the simplest estimate)

• Assume same value for zonal and central heating systems– Across PNW, about half of interior, electric water heater installs are in zonal houses and half are in

centrally heated houses

• Analyst proposal is to use 75%.– The PNNL study looked at a particular installation (next to an exterior wall) which is likely to yield a

lower than average value.– The PNNL study did not consider occupant interaction (i.e. the possibility of raising the house’s t-stat

setting to overcome a cold zone caused by the HPWH).

RTF Judgment Required Ahead

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Additional Research Options (I)

• PNNL Lab Homes Study– Install 1kW space heaters in four different locations within lab homes.

• Main living area near thermostat• Master bathroom (a distant zone)• Kitchen ???• Bedroom ???

– Operate heaters on a schedule similar to HPWHs. – Observe change in space heating requirement

• Experimental period: 5-10 days per location

– Cost: < $100,000 (?)– Outcomes & Caveats

• Results from four locations could help bound the value for HCƒ

– What if HCƒ is not 1 for main living area experiment?

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Additional Research Options (IIa)• Temperature-depression correlation study

– Use existing data from both PNNL lab homes and HPWH field studies– Lab homes show a known temperature depression and a known

heating interaction• Associate an X degree temperature difference with a Y amount of interaction

for lab homes• Scale field study homes accordingly

– Required assumptions• Temperature difference is a surrogate for interaction• Zero temperature difference = 100% interaction• Both scale linearly

– Are the results generalizable?• We have only ~15 cases in the field study to use

– Are the required assumptions reasonable? • Analysts say no

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Additional Research Options (IIb)Caution: No option guaranteed to add more certainty than we have now

• Hand calculations– Balance heat flow in to HPWH zone from interior with heat flow to exterior and

heat extracted by HPWH– To solve, need to assume or know:

• Conductance & Area between house and HPWH zone• Air temperature in house and HPWH zone• Conductance & Area between HPWH zone and outside• Rate of heat removal from air in HPWH zone• Mass of air in HPWH zone

• Informed modeling– Use a multi-zone heat transfer model

• Requires making the exact same assumptions as for the hand calculations

– Potentially integrate with whole-house simulations to test against • Delphi panel

– Assemble national experts on HPWHs• Combinations of the above

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Additional Research Options (III)

• Large scale field study– Metering at 100-200 sites using a flip-flop protocol

• Random / representative sample• Half year in ER only mode• Half year in heat pump mode• Measure heating system, outdoor temperature, & DHW energy

on a daily basis• Cost: $500,000 to $1,000,000 ?

– Potentially applicable to other heating & cooling interaction measures (lighting, appliances, etc)

– Need caution in study design because signal here is relatively small

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Other Notes• Phase I SEEM Calibration implemented– Of minor importance (only impacts space heating

interaction)– For simplicity, used a full insulation retrofit package to set

the building U0

• Phase II SEEM Calibration implemented– Non-utility fuel accounted for

• Water heater scavenges some heat from the wood stove so we don’t see an electric grid penalty but we do need to buy more wood

• Cooling interaction ignored as of this time– ~30-50kWh/yr

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Interaction Factor: Subcommittee Recommendation to the RTF

Regarding interior installations and heating interaction factor:1. Recommend as a Proven UES with a heating system interaction factor

(HCƒ) of 75%;

2. Recommend as a Provisional UES with HCƒ = 75%, and direct staff to scope a PNNL lab homes study for further heating system interaction factor research;

3. Recommend as a Provisional UES with HCƒ = 75%, and direct staff to scope a large-scale metering study for further heating system interaction factor research; OR

4. Recommend ________________________________________

Christian, Adam, and Ben recommend option 2 above. Reason: Less costly research may bound the interaction factor to an acceptable degree, thereby not requiring further, more costly research. If this turns out not to be the case, further, more costly research may be pursued at that time.

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Overall Subcommittee Recommendation(To be filled out on the subcommittee call)

• Garage / unconditioned basement is Proven/ Active, as proposed.

• Interior _______________.• Sunset Date: 1 year