Air-to-Water Heat Pumps for Low Energy & Net Zero Houses
Air-to-Water Heat Pumps for Low Energy & Net Zero Houses
Air-to-Water Heat Pumps for Low Energy & Net Zero Houses © Copyright 2018, J. Siegenthaler, all rights reserved. The contents of this file shall not be copied or transmitted in any form without written permission of the author. All diagrams shown in this file on conceptual and not intended as fully detailed installation drawings. No warranty is made as the the suitability of any drawings or data for a particular application. presented at: John Siegenthaler, P.E. Appropriate Designs Holland Patent, NY www.hydronicpros.com presented by: February 14, 2018 8:30-10:00
• Most North America heating professionals are familiar with ductless mini-split heat pumps.
www.johnstone.com Ductless mini-split heat pump airconditioning-repair-nashville.com geothermal heat pump • Most are also familiar with geothermal heat pumps air-to-water heat pump ? • Very few are currently familiar with air-to-water heat pumps.
2014 global market: 1,745,000 air to water heat pumps sold Japanese manufacturers [Daikin, Mitsubishi, Fujitsu, Hitachi, Samsung, LG, Toshiba] German manufacturers [Dimplex, Wolf, Viessmanm, Bosch,Vaillant] Canadian manufacturers [ThermAtlantic, Nordic] 2014 China market : 987,000 units (12% increase over 2013) 2014 European market: 232,000 units (5% increase over 2013) #1 France, #2 Germany, #3 UK Still only about 2% of heat sources sold in Europe Due to low gas and oil prices, AWHP are subsided in Europe based on CO2 reduction targets, rather than energy efficiency.
Many current models use inverter drive variable speed compressors for capacity control.
Some use EVI (enhanced vapor injection) compressors. Global air-to-water heat pump market: According to JARN (Aug 2015) This is not the case in other global markets…
So what is an air-to-water heat pump? evaporator TXV comp. RV condenser fan cool outside air cold outside air air-to-water heat pump (in heating mode) warm fluid hot fluid heat to building OUTSIDE INSIDE circulator liquid refrigerant changes to vapor absorbing heat liquid refrigerant liquid & gaseous refrigerant hot gas cool gas hot gas condenses to liquid releasing heat In heating mode: The heat pump extracts low temperature heat from outside air, and transfers it to a fluid stream (water or water & antifreeze) to be used by a hydronic distribution system. evaporator TXV comp. RV condenser fan warm outside air hot outside air condensate drain heat from building cold fluid cool fluid OUTSIDE INSIDE circulator air-to-water heat pump (in cooling mode) hot gas cool gas hot gaseous refrigerant condenses to liquid releasing heat liquid & gaseous refrigerant liquid refrigerant vaporizes absorbing heat hot gas liquid refrigerant In cooling mode: The heat pump extracts low temperature heat from a fluid stream (chilling it), and dissipates that heat to outside air.
14" x 8" duct this cut would destroy the load-carrying ability of the floor joists 2 x 12 joist 3/4" tube Water is vastly superior to air for conveying heat A given volume of water can absorb almost 3500 times as much heat as the same volume of air, when both undergo the same temperature change Why hydronics vs. forced air?
Self-contained air-to-water heat pumps warmer climate application (water in outside unit) colder climate application (antifreeze in outside unit) image courtesy of SpacePak OUTSIDE INSIDE heat! exchanger to / from! load antifreeze! protected! circuit OUTSIDE INSIDE • Heating + cooling + DHW • Pre-charged refrigeration system • some are 2-stage for better load matching • No interior space required • No interior noise
Split system air-to-water heat pump Indoor unit Heating mode: 1. condenser 2. circulator 3. expansion tank 4. aux element 5. controls Cooling mode: 1. evaporator 2. circulator 3. expansion tank 4. controls Outdoor unit Heating mode: 1. compressor 2. evaporator 3. expansion device Cooling mode: 1. compressor 2. condenser 3. expansion device indoor unit INSIDE OUTSIDE outdoor unit refrigerant lineset
Split system air-to-water heat pump European split system air-to-water heat pump supplying heating and domestic hot water -4 ºF outside, 113 ºF leaving water temperature www.NIBE.eu
It’s not just about matching BTU delivery to load… Ductless mini-split heat pumps rely on forced air delivery. While generally acceptable for cooling, forced air delivery doesn’t provide optimal comfort for heating. • There will be some temperature stratification from floor to ceiling. • Mini-splits blow cool air into spaces while defrosting outdoor unit. • Cold floors are a common complaint with forced air heating. • High wall cassettes do little to counteract natural downdraft from large window surfaces.
• Forced air heating may aggravate allergies or other respiratory symptoms. • There will be some sound from forced air terminal units.
Properly designed radiant floor, wall, and ceiling panels can operate with virtually no detectible sound. www.amazon.com It’s about providing COMFORT
historicshed.com Training programs for “net zero” houses often promote mini-split heat pumps as the only necessary heating & cooling system. They often discourage the “complication” and cost of hydronic systems. Why is the “net zero” housing market defaulting to mini-split heat pumps rather than hydronics? source: Revision Energy
Based on this - who can blame them ??
historicshed.com Common suggestion for net zero houses…. Install a ductless mini-split air-to-air heat pump, with 1 or 2 indoor wall cassettes, and leave the interior doors open for heat distribution.
from a green building website blog… “Leave bedroom doors open during the day If you want to heat your house with a ductless minisplit located in a living room or hallway, you’ll need to leave your bedroom doors open during the day. When the bedroom doors are closed at night, bedroom temperatures may drop 5 F° between bedtime and morning.” “If family members don’t want to abide by this approach, or don’t want to accept occasional low bedroom temperatures during the winter, then supplemental electric resistance heaters should be installed in the bedrooms.” The COPs of cold climate ductless mini- split heat pumps at sub-0ºF ambient conditions is seldom discussed.
source: Revision Energy Maintaining heating capacity at sub-0ºF conditions doesn’t imply that COP is being maintained.
What happens to the COP of ductless mini splits at low ambient air temperatures? Site 1 : COP = 1.1 at 0 ºF Site 4 : COP = 1.8 at 0 ºF
Low ambient air-to-water heat pump yields good performance at low outdoor temperatures: At ambient = 0 ºF, leaving fluid = 120 ºF, COP =2.04
Low ambient air-to-water heat pump performance COP = 2.04 at 0 ºF ambient and 120 ºF leaving water temperature. Heat pump power: 5817 watts Distribution circulator power: 25 watts Heat pump circulator: power: 200 watts Total power to system: 6042 watts Heat pump heat output: 40,500 Btu/hr Higher than the measured COP of several ductless mini split heat pumps @ 0 ºF ambient.
COPHP+circulator = 40,500 Btu hr 6.042kw ( ) 3413 Btu hr kw ⎛ ⎝ ⎜ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ ⎟ = 1.96
historicshed.com Ductless mini-split heat pumps provide heating & cooling • Room-by-room zoning An air-to-water heat pump has the potential to provide: • Zoned cooling (air & radiant delivery) • Domestic water heating • Pool heating in summer • Higher distribution efficiency • Fewer (if any) interior refrigerant piping connections • Radiant & convective heat delivery
1. Growing interest in Net Zero houses: The typical net zero house has a very low loss thermal envelop, and a sizable solar photovoltaic array on the roof. Net metering laws - where they exist - allow owners of photovoltaic systems to sell surplus electrical power back to the utility at full retail rate.
Thus, surplus kilowatt hours produced on a sunny summer day could conceivably be “parked” on the electrical grid, and reclaimed to run a heat pump on a cold winter night with no technical or economic penalty.
Space heat + DHW loads are so small it doesn't pay to put a gas meter on these houses. AWHP could provide heating, cooling, & DHW Source: Zerohomes.org Several trends suggest that a growing market will emerge for air-to-water heat pumps. Here are some key indicators:
Net Zero house in Seattle SPECS • 2,426 Square feet • 4 Bedrooms • 2.5 Baths • Radiant heat • Air-to-water heat pump • SIP construction • 9.5 KW PV system Source: www.tclegendhomes.com
• Superior comfort: Radiant panel heating is better match to human physiological comfort needs.
It’s not just about pushing Btus into a space to match heat loss. What are the advantages of using hydronic heating in these houses? • Easily adapted to renewable heat sources) (solar thermal, hydronic heat pump, biomass) source:Wagner Zaun Architecture • Simple room-by-room (“wireless”) zoning is possible with many heat emitter options. Don’t have to leave all doors open for internal heat balancing. A limitation of single point heat/cool delivery such as wall cassette.
• Very high distribution efficiency (A single ECM circulator operating on 10 to 40 watts supplies all heating distribution) • Non-invasive installation of small tubing (3/8” & 1/2” PEX, PERT, or PEX-AL-PEX) (Installing this tubing is like pulling electrical cable) • In many cases a single heat source can supply heating and DHW (fewer burners, less vents, less fuel piping) • Water-based thermal storage is easily adapted to “time-of-use” Electric rate structures.
What is distribution DISTRIBUTION EFFICIENCY? Efficiency = desired OUTPUT quantity necessary INPUT quantity Distribution efficiency for a space heating system.
Consider a system that delivers 120,000 Btu/hr at design load conditions using four circulators operating at 85 watts each. The distribution efficiency of that system is: distribution efficiency= 120,000 Btu/hr 340 watts = 353 Btu/hr watt distribution efficiency= rate of heat delivery rate of energy use by distribution equipment
The electrical input power for a circulator can be estimated: A typical wet-rotor circulator with PSC motor has a maximum wire- to-water efficiency of about 25 percent. (ECM circulators will have significantly higher wire-to-water efficiencies) we = 0.4344 × f × ∆ P nw/w = 0.4344 × 5× 3.83 0.25 = 33.2watts Consider a 200 ft long circuit of 3/4” copper tubing operating at 5 gpm with 180 ºF supply and 160 ºF return water temperature. It would have a pressure loss of 3.83 psi The required electrical input power to operate this circuit is: we = 0.4344 × f × ∆ P nw/w We = electrical input power (watts) f = flow rate (gpm) ∆P = pressure drop of circuit (psi) nw/w = wire-to-water efficiency of circulator
Compare this to a 4-ton rated geothermal water-to-air heat pump delivering 48,000 Btu/ hr using a blower operating on 1080 watts. The distribution efficiency of this delivery system is: nd = Q we = 50,000Btu / hr 33.2watt =1506 Btu / hr watt nd = Q we = 48,000Btu / hr 1080watt = 44.4 Btu / hr watt These numbers mean that the hydronic system delivers heat to the building using only 2.9 percent (e.g. 44.4/1506) of the electrical power required by the forced air delivery system. With good design it’s possible to achieve distribution efficiencies > 3000 Btu/hr/watt This will become increasingly important in low energy and net zero buildings...
A flow of 5 gpm in a circuit with a 20 ºF temperature drop is moving about 50,000 Btu/hr. The electrical input to a standard PSC circulator operating at 25% wire-to-water efficiency is 33.2 watts. The distribution efficiency of such a circuit is:
Consider a design heating load of 30,000 Btu/hr • Assuming a common ∆T of 20 ºF across the heat emitters f = Q 500(∆ T ) = 30,000 500(20) = 3gpm • Each panel rad is 24” x 72” x 4” operating w/ average water temperature of 110 ºF, (120 ºF supply & 100 ºF return) yielding output of 3,850 Btu/hr each, total system heat output of 30,800 Btu/hr • Flow rate per panel radiator is 3/8 = 0.38 gpm • Head loss of each panel radiator (balance valve partially closed) at this flow rate is 3.38 ft. • Assume each homerun circuit is 120 ft of 1/2” PEX at 0.38 gpm, head loss = 0.8 ft. • Add 10% safety factor to head loss for a total of 4.6 ft.
• Circulator requirement is 3.0 gpm at 4.6 ft. • Even with an ECM circulator that’s 30% w/w efficient, this requires an input of about 8.6 watts we = 0.4344 × f ×∆ P ncirculator = 0.4344 × 3.0 × 4.6 0.43 ( ) 0.3 = 8.6watt distribution efficiency = 30,800 Btu hr 8.6watt = 3581 Btu / hr watt • Assume a homerun distribution system to 8 identical panel radiators heat source pressure regulated variable speed circulator buffer tank TRV manifold station 8, 24" x 72" x 4" panel radiators 120 ft x 1/2" pex homerun circuits 3-40 watts
Why is the NA hydronics industry leaving its “best cards” on the table? distribution efficiency = 30,800 Btu hr 8.6watt = 3581 Btu / hr watt pressure regulated variable speed circulator TRV manifold station 8, 24" x 72" x 4" panel radiators 120 ft x 1/2" pex homerun circuits 94 3581 = 2.6% In this comparison the hydronic system uses only 2.6% of the electrical energy required by the forced air system for equal heat transport (source to load). distribution efficiency= 80,000 Btu/hr 850 watts = 94 Btu/hr watt
2. The 30% federal tax credits on geothermal heat pump systems ended December 31 2016: That removed a significant purchasing incentive, and forces geothermal heat pump systems to compete against other types of heat pumps in an unsubsidized market.
Several trends suggest that a growing market will emerge for air-to-water heat pumps. There’s a possibility this tax credit could be reinstated. Do you want to build your business model on the assumption that subsidies will always be there? “The GHP industry experienced a 50% loss of residential sales during the year (2017), with hundreds of layoffs and thousands more jobs in jeopardy.” https://www.geoexchange.org/wp-content/uploads/GEO-Industry-News-January-20 18.pdf NYSERDA does have $1500 / ton (cooling capacity) GSHP rebate program at present.
https://www.nyserda.ny.gov/All-Programs/Programs/ Ground-Source-Heat-Pump-Rebate THIS CHANGED 2/9/18
3. Air-to-water heat pumps are significantly less expensive to install compared to geothermal heat pumps: Several trends suggest that a growing market will emerge for air-to-water heat pumps. air-to-water heat pump typical installed cost = $(30% to 50%)X geothermal heat pump typical installed cost = $X This is especially true if vertical boreholes are required for the earth loop. In my area, these holes cost about $3,000+ per ton for drilling, pipe insertion, and grouting.
Additional cost is incurred for connecting multiple vertical piping loops, and routing piping back to the location of the heat pump. Replacement of any affected pavements or landscaping also needs to be factored into the cost of installing a geothermal heat pump system.
4. Diminishing returns: As home heating loads decrease due to better thermal envelopes, the difference in annual heating cost between heat pumps operating at seasonal average COPs that vary by perhaps 1.0 or less, decreases. air-to-water heat pump typical installed cost = $(30% to 50%)X geothermal heat pump typical installed cost = $X Several trends suggest that a growing market will emerge for air-to-water heat pumps. Here are some key indicators: The incrementally lower operating cost of the higher performance heat pump may not amortize the higher installation cost within the expected life of the system.
You don’t pay for COP! (you pay for kilowatt•hours) The annual savings in heating energy between two heat pumps with different seasonal average COPs can be estimated using this formula: S = load 1 COPL − 1 COPH ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ Where: S = savings in seasonal heating energy (MMBtu*) load = total annual heating energy required for the building (MMBtu*) COPL = seasonal average COP of heat pump having the lower of the two COPs COPH = seasonal average COP of heat pump having the higher of the two COPs * 1 MMBtu = 1,000,000 Btu Example: A house has a design heating load of 36,000 Btu/hr when the outdoor temperature is 0 ºF, and the indoor temperature is 70 ºF.
The house is located in Syracuse, NY with 6,720 annual heating ºF•days. The estimated annual space heating energy use is 49.7 MMBtu. Assume that one heat pump option has a seasonal average COP of 3.28. The other heat pump has a seasonal COP of 2.8.
S = load 1 COPL − 1 COPH ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ = 49.7 1 2.8 − 1 3.28 ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ = 2.6MMBtu / year The cost savings associated with an energy savings of 2.6 MMBtu/hr depends on the cost of electricity. For example, if electricity sells at a flat rate of $0.13 / KWHR, the cost savings would be: Cost savings = 2.6MMBtu year 292.997KWHR 1MMBtu ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ $0.13 KWHR ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = $99 / year Can the added cost of the higher COP heat pump be recovered in a reasonable time?
Example house: 36,000 BTU/hr design load at 70ºF inside & 0 ºF outside Location: Syracuse, NY (6720 heating degree days) Total estimated heating energy required: 49.7 MMBTU / season Average cost of electricity: $0.13/kwhr Distribution system: radiant panels with design load supply temperature = 110ºF AIR-TO-WATER HEAT PUMP OPTION Based on simulation software, a nominal 4.5 ton split system air-to-water heat pump supplying this load has a seasonal COP = 2.8.
Estimated installed cost = $10,600 (not including distribution system) GEOTHERMAL WATER-TO-WATER HEAT PUMP OPTION: Based on simulation using simulation software, a nominal 3 ton water to water heat pump supplying this load from a vertical earth loop has a seasonal COP = 3.28.
Estimated installed cost = $11,800 (earth loop) + $8750 (balance of system) = $20,550 (not including distribution system) Deduct for 30% federal tax credit - 6165) Net installed cost: $14,385 (not including distribution system) Annual space heating cost: AIR-TO-WATER HEAT PUMP (COPave= 2.8) = $676 / yr GEOTHERMAL HEAT PUMP (COPave = 3.28) = $577 / yr Difference in annual heating cost: $99 / year Difference in net installed cost: $3,785 Simple payback on higher cost of geothermal HP: 3785 / 99 ≈ 38 years Diminishing returns of higher COPs Without subsidies the estimated difference in installed cost is $9,950 With reinstated 30% federal tax credit the difference in installed cost is $3785 Initial difference in annual heating cost: $98 / year Draw your own conclusions….
5. Air-to-water heat pumps are significantly less disruptive to install compared to geothermal heat pumps: Several trends suggest that a growing market will emerge for air-to-water heat pumps. www.thegeoecchange.org In my area, vertical earth loops cost about $3,000+ per ton for drilling, pipe insertion, and grouting. Additional cost is incurred for connecting multiple vertical piping loops, and routing piping back to the location of the heat pump. The drill “tailings” usually have to be removed from the site.
Replacement of any affected pavements or landscaping also needs to be factored into the cost of installing a geothermal heat pump system.
Horizontal earth loops require large land areas and major excavation.
6. As home space heating loads get smaller, the domestic water heating load becomes an increasingly higher percentage of the total annual heating energy requirement. A standard electric water heater providing domestic water heating in a situation where the heat pump can not, delivers heat at a COP of 1.0. If that energy was instead attained through an air-to-water heat pump, it could be delivered at a COP averaging perhaps 2.5 over the year. For a family of 4, needing 60 gallons per day of water heated from 50 to 120 ºF, and assuming electrical energy priced at $0.12 per KWHR, the difference in annual domestic water heating cost between these scenarios is $270.
Water heating COP = 2.5+ Water heating COP = 1.0 Several trends suggest that a growing market will emerge for air-to-water heat pumps. Some estimates put the DHW load at 25-30 percent of the total annual energy requirement in a well insulated modern home. Most ductless mini-split heat pumps cannot provide domestic water heating, but a properly configured air-to- water heat pump can.
7. The high COP cited for some geothermal heat pumps doesn’t include the power required to move flow through the earth loop. Example: A specific water-to-water geothermal heat pump has the follow listed performance information: Earth loop entering temperature = 30ºF Entering load water temperature = 100 ºF Flow rate (both evaporator and condenser) = 9 gpm Heating capacity = 27,700 Btu/hr Electrical power input = 2370 watts Several trends suggest that a growing market will emerge for air-to-water heat pumps.
Example of a commercially available earth loop flow center. 4, UP26-150 circulators (370 watts each) = 1,480 watts pumping power input.
Based on a typical earth loop, the pumping requirement is 10.5 gpm at 35.5 feet of head. This equates to an estimated pump input of 287 watts. COPHP only = 27700 Btu hr 2.37kw ( ) 3413 Btu hr kw ⎛ ⎝ ⎜ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ ⎟ = 3.42 The high flow and head required in some geothermal earth loops requires substantial circulator power. COPHP +loop pump = 27700 Btu hr 2.37kw + 0.287kw ( ) 3413 Btu hr kw ⎛ ⎝ ⎜ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ ⎟ = 3.05 Nominal 11% drop in “net” COP The ANSI 13256-2 standard for geo heat pump COP includes an estimate for the power required to move flow through the heat pump - BUT DOESN’T INCLUDE ANY ALLOWANCE FOR THE EARTH LOOP PUMPING POWER.