Steven Winter HPWHs User manual

Heat Pump Water Heaters

Selecon and Quality Installaon Guide

SPONSERED BY:

PREPARED BY:

June 7, 2012

Cape Light

Compact

Cape Light

Compact

Contents

1Background...........................................................................................................................................1

2What Are Heat Pump Water Heaters? ................................................................................................2

3HPWH Performance..............................................................................................................................3

3.1 Performance Metrics...........................................................................................................3

3.2 Performance and Potential Savings .....................................................................................3

3.3 What Affects Performance?.................................................................................................5

3.3.1 Hot Water Usage......................................................................................................5

3.3.2 Ambient Air Temperature........................................................................................5

3.3.3 Inadequate Space .....................................................................................................6

3.3.4 Tank Set Point Temperature ....................................................................................6

3.4 Dehumidification .................................................................................................................6

3.5 Interaction with Space Conditioning Systems.....................................................................6

3.6 Reliability and Safety...........................................................................................................7

4Selecting Locations for HPWHs..........................................................................................................7

4.1 Space Requirements.............................................................................................................7

4.2 Heating and Cooling Considerations...................................................................................8

4.3 Noise…..............................................................................................................................10

5Selecting a HPWH Model ...................................................................................................................10

6HPWH Installation...............................................................................................................................11

6.1 Size, Weight, and Space.....................................................................................................11

6.2 Managing Condensate........................................................................................................11

6.3 Drain Pans and Blocks.......................................................................................................12

6.4 Heat traps...........................................................................................................................12

6.5 Tempering Valves and Set Point Temperature..................................................................13

7Operation and Maintenance ..............................................................................................................14

7.1 Filter Maintenance.............................................................................................................14

7.2 Inspect Condensate Drains.................................................................................................15

7.3 Typical Water Heater Maintenance ...................................................................................15

References.................................................................................................................................................16

1 Background

Heat pump water heaters (HPWHs) promise to significantly reduce energy consumption for domestic

hot water (DHW) over standard electric resistance water heaters (ERWHs). Water heater efficiency

is usually described by the energy factor (EF). Typical electric resistance water heaters have EFs of

0.9 or above, which means that the heaters are over 90% efficient in converting electricity into

thermal energy in the water heater. New HPWHs have EFs of 2.0 to 2.5, meaning that HPWHs can

use less than half the energy that a standard electric tank would require. High energy factors in

HPWHs are achieved by combining a vapor compression system, which extracts heat from the

surrounding air at high efficiencies, with electric resistance element(s), which are able to help meet

larger hot water demands.

In New England homes, water heating is responsible for 17% of total energy consumption – the

largest energy use in homes after space heating. In the Northeast, 1.4 million households (26%) use

electricity as their primary water heating fuel1and consume – on average – over 2,600 kWh each

year to heat water.2In many cases, switching to a HPWH can cut this consumption in half, saving

several hundred dollars per year in the “average” home. Although HPWHs cannot be installed in all

locations, they do offer significant energy saving potential.

Although HPWHs were first commercialized in the 1980s, they were typically add-ons to existing

water heaters (i.e. a heat pump would pump water from a tank water heater, heat it outside the tank,

and pump it back into the tank). These systems required specialized knowledge for installation and

sometimes required both an HVAC contractor and a plumber to install the system. The development

of integrated or “drop-in” HPWHs (where the heat pump and tank are integrated within a single

appliance) allowed for easy installation by a single trade (Tomlinson 2002). While integrated

HPWHs are not entirely new, earlier residential products (available around 2000) achieved minimal

market penetration and experienced reliability issues

Recently several manufacturers, including General Electric (GE),

Rheem, AO Smith, Stiebel Eltron, and Air Generate, have

introduced integrated HPWHs (Table 1). Integrated refers to the

fact that the units are packaged replacements for conventional

ERWH tanks; they include the hot water tank, heat pump, backup

resistance heating elements, and all controls.

The demand for these integrated HPWHs has been fueled by higher

electricity rates and the availability of various incentives (state,

federal, local, utility, etc.) that promote the installation of more

efficient equipment. Furthermore, the new Federal water heater

standard, which takes effect in 2015, mandates EFs around 2.0 for

all new electric storage water heaters with capacities greater than 55

gallons (Federal Register 2010). This regulation will effectively

mandate the use of HPWHs in applications with large hot water

demands and where electricity will be used to heat water.

1http://38.96.246.204/emeu/recs/recs2005/c&e/waterheating/pdf/tablewh2.pdf

2http://www.eia.gov/consumption/residential/data/2005/c&e/waterheating/pdf/tablewh6.pdf

Figure 1. An integrated HPWH

installed in a basement.

Table 1. Key Specifications of some integrated HPWHs currently available in the US market.

Model

Capacity

(gal)

Energy

Factor

First Hour

Rating

(gal)

GE GeoSpring™

2.35

63.0

AO Smith Voltex®

60 / 80

2.33

68.0 / 84.0

Stiebel Eltron Accelera®300

2.51

78.6

AirGenerate AirTap™

Integrated

50 / 66 2.39 / 2.40 60.0 / 75.0

2 What Are Heat Pump Water Heaters?

A “heat pump” is a device that moves heat from one place to another; it works much like an air

conditioner or a refrigerator. While a refrigerator moves heat from inside the fridge into your

kitchen, a heat pump water heater (HPWH) moves heat from your basement or mechanical room

into the hot water tank.

HPWHs are primarily designed as replacements for standard, electric resistance water heater tanks

(ERWHs). HPWHs are able to achieve higher efficiencies (and energy factors) through use of the

vapor compression heat pump cycle. Auxiliary electric resistance elements are also installed for

reliability and quicker hot water recovery. Most heat pumps operate as “hybrid” devices – i.e. they

use the heat pump whenever possible, but built-in controls switch to conventional resistance heating

when there are large hot water needs.

Figure 2. Simple schematic of HPWH operation.

The heat pumps in these hybrid water heaters can heat water at high efficiencies, but the heat pumps

have heating capacities lower than those of traditional electric resistance elements. Typical 4.5-kW

electric resistance elements, for example, can reliably heat over 20 gallons of water per hour. The

heat pump has a lower heating rate; GE, for example, publishes a rate of 8 gallons per hour at 68°F

air temperature. This gap illustrates the benefit of a “hybrid” configuration – heat pumps are used

when possible for highest efficiency, but resistance heat is used when there’s high demand for hot

water. Heaters with larger storage tanks generally revert to backup heating less often than lower-

volume heaters.

Most HPWHs have several operating modes (manually adjustable by the users):

•“Hybrid mode” is typically the default; it uses both the electric resistance element(s) and

the heat pump to meet demand, but it uses the heat pump whenever possible to maximize

efficiency.

•“Heat pump mode” uses only the heat pump. This can improve efficiency, but it

dramatically reduces the recovery capacity of the water heater. If occupants use a great

deal of hot water in a short period of time, they may run out of hot water sooner and it

will take longer to heat more water.

•“Electric resistance mode” works like a traditional ERWH. This mode can be used when

cooling effects from the heat pump aren’t desired or if there is a problem with the heat

pump.

The names of these modes differ by manufacturer, but most models include some combination of the

above operating modes.

3 HPWH Performance

3.1 Performance Metrics

The efficiency of residential water heaters in the United States is measured and reported using the

energy factor (EF) value. The energy factor represents the efficiency of the electric element and tank

losses under a specifc 24-hour test procedure (Burch and Erickson 2004; Federal Register 1998).

In the real world, people use water quite differently than in this rating test procedure. When talking

about measured, real-world performance in this document the term “coefficient of performance” or

COP is used to describe efficiency. COP is the ratio typically used to rate performances of heat

pumps, air conditioners, etc. Like EF, COP is the unit-less ratio of useful energy output to energy

input. In the standard test procedure described above, the COP would be equal to the EF. In the real

world, COP can differ substantially from EF (for reasons discussed below).

3.2 Performance and Potential Savings

Traditional electric resistance water heaters (ERWHs) provide hot water at rather high costs. Table 2

shows some example costs for several conventional water heating systems, and the electric resistance

tank is among the highest-cost systems. Table 3 compares water heating costs for ERWH and a

HPWH.

Table 2. Approximate water heating costs for a home consuming 55 gal/day of hot water. Costs

are only examples; fuel cost, water use, and EF can vary considerably.

WaterHeater Type Fuel Cost EF* Annual Cost

Electric Resistance Tank $0.17 /kWh 0.90 $650

Oil - Tankless Coil $3.60 /gal 0.50 $610

Oil - Indirect Tank $3.60 /gal 0.75 $410

NaturalGas Tank $1.50 /therm 0.59 $300

NaturalGas Tankless $1.50 /therm 0.80 $220

Condensing NG Tankless $1.50 /therm 0.92 $190

Propane Tank $3.50 /gal 0.59 $760

Propane Tankless $3.50 /gal 0.80 $560

* For appliances that aren't given EF ratings, such as boilers and

indirect water heaters, this is a typical, effective value.

Table 3. Example water heating costs, ERWH vs. HPWH. Water use and energy costs can vary

substantially.

Avg. Hot

No.

People

WaterUse

[Gal/day]

Standard Tank

(EF 0.9)

2 35

$410 $140 to $370 $40 to $270

4 55

$650 $220 to $580 $70 to $430

6 75

$880 $310 to $800 $80 to $570

Annual Hot Water Cost

HPWH

Savings

When replacing an ERWH, a HPWH usually results in substantial savings. Substantial savings are

also possible when replacing other types of water heaters (such as boilers with tankless coils and

propane heaters where propane prices are quite high). Despite the possibility of large savings, there is

also potential for a wide range in HPWH operating costs (as Table 3 shows).

The performance ranges in Table 3 come from an evaluation project of several HPWHs in New

England homes. In 2010, several New England utilities (National Grid, NSTAR, and Cape Light

Compact) sponsored the installation and evaluation of heat pump water heaters in 14 homes across

the region. Steven Winter Associates, Inc. (SWA), with support from the U.S. DOE Building

America Program, monitored the performance of these systems in detail for over a year.

The results of the evaluation were encouraging. Most importantly, all customers were satisfied with

the supply of hot water. With respect to efficiency, the measured COPs were largely in-line with

manufacturer EF ratings.

Table 4. Measured performance of 14 HPWHs installed in Massachusetts and Rhode Island

homes.

Min. Average Max

1.0 1.9 2.6

MeasuredCOP

The average, measured COP was more than twice the EF of typical electric resistance water heaters;

this basically means that – on average – water heating costs were half what they would have been

with ERWHs. The wide range in COPs shows that performance of HPWH’s can vary tremendously.

The key factors that cause these variations are discussed in some detail below.

3.3 What Affects Performance?

Energy used by HPWHs varies much more than for conventional ERWHs. The four main reasons

for these variations are discussed below.

3.3.1 Hot Water Usage

The primary driver of the efficiency and energy usage of a HPWH is hot water usage. Electricity

consumption increases – and HPWH efficiency decreases – with both overall water consumption (i.e.

average gallons used per day) and intensity of water consumption (i.e. gallons consumed over a short

time period).

During intense, high-volume hot water draws – such as several long, consecutive showers – a hybrid

HPWH will often automatically switch to electric resistance heating. Electric resistance can provide

more hot water faster, but it also consumes at least twice the electricity when compared to heat pump

mode. Heaters with larger storage tanks can generally provide more hot water without resorting to

backup heating.

In short, homes where occupants use a moderate amount of hot water throughout the day will

experience the highest efficiency from a HPWH. Homes that use large volumes of water – or use a

great deal of water in short periods of time – will see lower efficiencies. Even in rather high-volume

applications, however, energy costs will usually be substantially lower than with a standard electric

resistance heater.

3.3.2 Ambient Air Temperature

The efficiency of a HPWH increases substantially with increased ambient temperature (i.e. the air

temperature of the space where the HPWH is located). Heat pumps become much more efficient

when the heat source (in this case the ambient air) becomes warmer, and standby losses are lower

when the surroundings are warmer. A higher air temperature also increases the heating capacity of

the heat pump; this helps to minimize the use of back-up, electric resistance heating.

According to manufacturer literature, most HPWHs will only operate in heat pump mode when

ambient temperature is between approximately 45°F and 110°F. When the temperature of the

incoming air drops below the minimum temperature, the HPWH will switch into electric resistance

mode, reducing the efficiency of the unit. HPWHs also cool a space (they move heat from the

ambient air into the water); tests and literature show that a HPWH will reduce ambient air

temperatures by 2-6°F. For this reason, it’s recommended that HPWH be installed in spaces where

the ambient air temperature – before the HPWH is installed – is at least 50-55°F for most of the year.

Among the HPWHs evaluated in the study mentioned above (see summary in Table 4), one of the

systems had an overall COP of 1.0; this is not much better than an electric resistance heater. The

main reason that this COP is so far below the listed energy factor (2.3) was that the heater was

installed in a remote, cold section of the basement. Air temperatures in this space were between 40°F

and 55°F, and the HPWH often needed to operate in electric resistance mode. Even when in heat

pump mode, the efficiency at such low air temperatures was very poor.

3.3.3 Inadequate Space

To operate efficiently, a HPWH must be able to extract sufficient energy from the surrounding air,

and the energy available in the air is a function of:

•Temperature of the air

•Volume of air available – i.e. the size of the space.

HPWHs should be installed in rooms with adequate air volume to ensure efficient operation. See

manufacturer literature for exact space requirements; a minimum volume of 750 ft3is typical. In

addition, adequate clearances must be provided to allow for proper airflow and maintenance (see

Section 4.1).

Among the systems evaluated in the study mentioned above (see Table 4), the system with the lowest

overall COP (1.0) was installed in a very cold space. The second lowest COP (1.4) was observed in

a home where the HPWH was installed in a small closet. Even with louvered doors, the closet did

not have enough air for the HPWH to operate efficiently much of the time. As a result, the heater

operated in resistance mode quite often, and water heating costs were higher than in most other

homes.

3.3.4 Tank Set Point Temperature

Unlike resistance water heaters, the efficiency of HPWHs depends largely on the hot water set point

temperature. If a heater is programmed to provide hotter water, both the efficiency and the heating

capacity of the heat pump are reduced. A heat pump will operate more efficiently heating water to

120°F than to 140°F.

On the flip side of this, a tank of 140°F water can go further than a tank of 120°F water (providing

another 10 minutes or so of a hot shower). Sometimes keeping a tank at 140°F will reduce the use of

backup heating elements (which are much less efficient than the heat pump). It’s generally

recommended, however, not to raise the tank temperature much above 120°F unless this results in a

shortage of hot water or frequent use of the backup resistance heaters.

3.4 Dehumidification

Because HPWHs remove heat from the ambient air using a heat pump refrigeration cycle, HPWHs

also remove moisture from the air (much like an air conditioner or dehumidifier). The water vapor in

the air condenses as it passes across the HPWH’s evaporator coils and, as a result, the HPWH will

provide some dehumidification. When installed in a damp space (such as a basement) where a

dehumidifier is used, a HPWH can sometimes cut down on the time that the dehumidifier needs to

operate.

3.5 Interaction with Space Conditioning Systems

While all discussion of “efficiency” and “performance” has so far focused on the HPWH alone,

HPWHs can have an impact on the space conditioning systems in a house. As mentioned above,

HPWHs transfer heat from the ambient air to water, thereby cooling the ambient air. This effect is

usually a welcome benefit during summer months, but during the heating season, the home’s heating

equipment will need to make up for some of this energy.

In the Northeast, placing HPWHs within conditioned space will require the heating systems to

consume more energy. Even when installed in basements – which may not be fully “conditioned” –

HPWHs may require the space heating systems to generate more heat overall.

These interactions are certainly something to consider, but they are very difficult to quantify. There

will always, however, be overall savings when compared to a standard electric resistance water

heater. In a worst-case scenario, the HPWH would run in electric resistance mode all winter. During

the warmer months, however, there will be benefit from the higher heat pump COP along with some

free cooling and dehumidification.

A HPWH may also alter comfort in areas of a home: in the summer, the cooling and

dehumidification can be beneficial, but the cooling may provide comfort problems during the winter.

Carefully consider the HPWH location within a home, and if seasonal comfort problems are a real

concern, most HPWHs can be manually adjusted to resistance mode during the coldest spells.

3.6 Reliability and Safety

The integrated HPWH appliances currently available have only been on the market since

approximately 2009-2010. While it is logical that the higher complexity of HPWHs over standard

ERWHs could lead to greater failure rates and higher maintenance costs, the HPWH models

currently available do not seem to be experiencing significant reliability issues. As these systems

have been installed for only 2-3 years (at the writing of this document), it’s premature to predict

longer-term reliability. Initial evaluations, however, do not seem to hint at any longer-term

performance concerns.

4 Selecting Locations for HPWHs

4.1 Space Requirements

HPWHs require more space than traditional ERWHs because of their additional height, weight,

required air volume, and clearance requirements. HPWHs are generally taller and heavier than

traditional ERWHs. Measures to manage condensate, such as placing the unit on blocks (see Section

6.2), may further increase the height requirements of HPWHs. If installed on a frame floor, the

additional weight of the units, particularly larger capacity models, may require structural

reinforcement of the floor.

Table 5. Weight, volume, clearances, and operating temperatures for various HPWH models. The

list is not exhaustive; refer to manufacturer literature for the most accurate up-to-date information.

Model

Dry

Weight

(lbs)

Wet

Weight

(lbs)

Minimum

Room

Volume

(ft

)

Minimum Clearances

HP Inlet Air

Operating

Temperature

(°F)

Air

Inlet

Air

Outlet

Front

Rear

Top

Smith

332*/

410*

827/

1,069

750 3' 5' 2' 6" None 45-109

190

602

700

5.5"

14"

45-120

Stiebel

Eltron

287 952 500 16" 15.75" 8" 8" 16" 42-108

* shipping weight

Because HPWHs extract energy from their surrounding environment, enough air volume and

adequate clearances must be provided to allow for proper operation of the unit. Improperly placed

HPWHs are significantly less efficient than properly installed units (see Section 3.3.3). Required air

volume of a room in which a HPWH is installed ranges from 500-1000 ft3. A volume of 750 ft3 – a

typical minimum value – corresponds to a 10-ft by 10-ft room with 7’6” ceilings (see literature for

exact requirements).

In addition, HPWHs require larger clearances for proper operation. Air entry and discharge must be

free of obstructions to provide proper air circulation. Figure 3 shows a properly installed HPWH with

adequate clearances at the air entry and discharge. Added clearances are also required for removal

and cleaning of the air filter and for servicing of the unit. Piping installation must be carefully

considered to prevent the pipes from blocking the air filter or maintenance panels.

Figure 3. Heat pump water heater with appropriate clearance for air movement.

4.2 Heating and Cooling Considerations

When selecting an appropriate location for a HPWH, the interaction with the home’s heating system

must be closely considered. As mentioned above, HPWHs extract heat from the surrounding air and

cool the air where they are installed. In the Northeast, there are three potential drawbacks from this:

•The cooling results in a comfort problem near the heater.

•The home’s space heating system must run more frequently to make up for the heat

removed by the HPWH.

•The HPWH cools the space so much that it can no longer operate in heat pump mode (at

around 45°F, most units switch to standard electric resistance mode).

The first item – comfort – is perhaps the simplest to address during planning. HPWHs should not be

installed in spaces that are normally occupied and where the cooling (and noise) will result in

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