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Design Evolution for HVAC Systems
• Ductless Heat Pumps
• Reliable
• Efficient
• Energy saving
• Can be aesthetically pleasing
• Give homeowners high levels of
satisfaction
What We Know Having my
DHP installed
was as easy
as falling off a
log!
DHP Cold Weather Performance
This looks scary, lets focus first on the scale on the left and legend at the bottom
90-100° F
Well below 0° F
How Hot Would You Like Your Supply Air?
Warm air produced at low outdoor air temps – we’ll take it!
• Important Considerations
• Sizing and selection
• Placement
• Homeowner education
• Sizing and selection
• There are BIG differences between models
with same nominal size
What We Have Learned
Heating cooling
loads must be
calculated
Detailed capacity
must be known
• Stringent energy codes and
legislated code cycles following in
the wake of above-code programs
• Growing market demand for more
energy-efficient and sustainable
homes
• Heating and cooling loads dropping
sharply in new homes (Yes, I said
cooling)
• Existing technology limitations are
giving way to highly efficient
emerging technologies and practices
The Present and Future of New Construction
• The goal of sizing HVAC equipment is to find the best match
between the house and the equipment
• Optimal size is the best match, or balance, between the load of
the house and the capacity of the HVAC equipment
• Modern, efficient homes are creating a dichotomy in which the
loads don’t match well with traditional, ducted equipment
The Nature of Equipment Selection
Square Footage and R-U Values
Duct Multiplier
ACH
Limited range in new
construction
It’s an educated guess
This is the old Greek
formula: UAΔΤ
40K 60K 80K 100K 120K
Selection
Process
5 choices
for gas
Almost all new homes are
best matched to 40k
systems
Heating Load Inputs: It’s Pretty Simple
Square Footage and R-U Values
Duct Multiplier
ACH
Limited range in new
construction
It’s an educated guess
This is the old Greek
formula: UAΔΤ
24K 30K 36K 42K 48K 60K
Selection
Process A few more
choices for
HPs This range is better,
but still not good for
lower load homes
Heating Load Inputs: It’s Pretty Simple
• ~40 pilot homes across two phases, distributed
across the region
• Full verification and commissioning
• 13 months of continuous metering data with
extensive metering array
• Environmentals (temperature, RH, VOCs)
• HRV performance
• Hot water consumption and performance
• Heating/cooling systems performance
• Detailed cost data and interviews identify
challenges and barriers for builders
NEEA Performance Home Pilot
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
45,000
R-0 R-7 R-11 R-19 R-27 R-30 R-38
BT
Us P
er
Ho
ur
of
Heat
Lo
ss
Heat loss through 2,000 Sq. Feet of Attic - Boise, ID
House size does matter, but not nearly as much as whether or not it’s insulated
Heat Loss: It’s All About R-values
Loads Getting Lower
• What kind of loads are we looking at for new above-code homes?
• <8,000 – 36,000 BTU/hr. design heat load
• Upper end of range characterized by large homes in cold
climates
• Homes in NEEA’s pilot range from <8,000 to 18,000 BTU/hr.
Square Footage and R-U Values
Duct Multiplier
ACH
Limited range in new
construction
It’s an educated guess
This is the old Greek
formula: UAΔΤ
Selection
Process
Need more
appropriate
options 24K 30K 36K 42K 48K 60K
Lowering UA,
Removing duct multiplier,
lowering ACH
Equipment Selection: Where We’re Headed
ACH Reduced Infiltration
No Duct Losses
Improved R-values
Selection
Process
Need more
appropriate
options
Surfaces (UA)
24K 30K 36K 42K 48K 60K ~0-36K
Great! Simpler inputs into load calcs, but DARN, now we need
new selections of heat pump capacities
Equipment Selection: Where We’re Headed
• Considerations
• How to heat remote rooms
• Backup heat in bedrooms may be a good
idea
• Home geometry and floor plan influence
heat distribution
DHP: Point-Source
• First Question:
• Will humans be thermally comfortable in the absence of a central heating system?
• Second Question:
• Are humans thermally comfortable in homes with central heat?
Central Questions About the Lack of Central Heating
If Central Systems are Better, Please
Explain the Following….
Heat delivery looks pretty awesome up to 15 feet and still useful up to 30 feet
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Te
mp
F
Master Guest Kitchen Office Master Closet Bathroom Living Room OAT
House has low UA. Very tight. 12K ductless heat pump and low-efficiency HRV. About
900 kWh/year for space heat.
1-ton DHP with low-efficiency HRV maintains ~70° F
On ~900 kWh/year
Indoor temps
are 70-75° F
Room Temps in Low-Load House: Single
Head, Distribution via HRV into Bedroom
• When sizing variable capacity heat pumps, the minimum capacity is nearly as
important as the maximum capacity
• When the house load drops below minimum capacity, performance
drops from severe short-cycling
• A look at the max/min capacity at 47° F can be an indicator of how the
unit will perform in these conditions
• We call this the unit’s “turn-down ratio”. It varies greatly by model.
• Look for a unit with at least a 4:1 ratio between its maximum and minimum
capacity at 47° F
Sizing Variable Capacity Units
-
5,000
10,000
15,000
20,000
25,000
Man
ufa
ctu
rer'
s St
ate
d H
eat
ing
Cap
acit
y
at 4
7 d
eg
F (B
tu/h
r)Comparison of Nominal 1-ton DHP Models
"Rated" Capacity
If this house had part load needs of around 3,000 Btu/hr for much of
the year, which of these systems would meet that without short cycling?
Sizing Variable Capacity Units
-
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
Man
ufa
ctu
rer'
s St
ate
d H
eat
ing
Cap
acit
y
at 4
7 d
eg
F (B
tu/h
r)
Comparison of Nominal 2-ton DHP Models"Rated" Capacity
Same question, would a larger system solve that problem?
Sizing Variable Capacity Units
Check out the turn down ratios at 47°F, 30°F, and 15°F. It’s over 4:1 at 47°F
Detailed Capacity Chart
© 2014 Powerhouse Dynamics
Don’t panic, look at outdoor temps (around 50°F). Look at power
usage in gray and green spikes. Lots of cycling, 400 watts when on.
Sizing Variable Capacity Units
© 2014 Powerhouse Dynamics
New control algorithms via an updated control board. See the peak
watts and length of runs. Far less cycling and only using 15-20 watts.
Sizing Variable Capacity Units
0
200
400
600
800
1000
1200
1400
1600
1800
0
5000
10000
15000
20000
25000
30000
<0 0 - 9 10 to 19 20 to 24 25 to 29 30 to 35 35 to 39 40 to 44 45 to 49 50 to 54 55 to 59
H
o
u
r
s
i
n
T
e
m
p
B
i
n
DHP Sizing
BTU/Hr Hours in bin Boise Hours in bin Seattle
Design load:24 K@9F
Nom Size 24K
10 K low output
4K low output
3300 hrs.
1529 hrs.
Check out total hours where this system will short cycle.
With a 2.5:1 turndown, its 3300Hours. Its cut in half for a 6:1 turndown ratio!
Boise DHP – Low Output Sizing
0
200
400
600
800
1000
1200
1400
1600
1800
0
5000
10000
15000
20000
25000
30000
<0 0 - 9 10 to 19 20 to 24 25 to 29 30 to 35 35 to 39 40 to 44 45 to 49 50 to 54 55 to 59
H
o
u
r
s
i
n
T
e
m
p
B
i
n
DHP Sizing
BTU/Hr Hours in bin Boise Hours in bin Seattle
Design load:15 K@27F:
Nom size 18K
7 K low output
3K low output
4375 hrs
2810 hrs
In Seattle, short cycling total hours can be cut again
by almost half by upgrading a 2.5:1 ratio to 6:1
Seattle DHP – Low Output Sizing
• Use an appropriate number of indoor heads
• In most homes, one head per floor is enough:
• An optimal system often consists of:
• 1 DHP in the main living area, +1 smaller unit in
the master suite
• 1 DHP in the main living area, +1 ducted mini-split
serving bedrooms
• 1 DHP in the main living area, plus small electric
resistance heaters in the bedrooms
• If using ER heaters, use smaller units (750w),
control with digital wall T-stats
DHP System Design
• Orient heads to take advantage of throw and mixing
• Place in largest, most open areas
• Orient to blow down central hallways
• In rooms with high ceilings, place DHP < 8’ off the floor to minimize
stratification effects
• Don’t leave units set in “Auto” mode
DHP System Design
• Master bedroom in single-
story home
• Five surfaces exposed to
exterior
• 65 sq. ft. of glazing
(26 percent of floor area)
• Design heat load around
2,700 BTU/hr. for this room
alone
Home Geometry and DHP Performance
POP QUIZ
Q: How much 70° F air must
you deliver to keep this room
comfortable?
A: Too much.
Avoid creating thermal
peninsulas unless you have an
active conditioning strategy to
meet the load. Room-by-room
loads still matter.
Home Geometry and DHP Performance
• Room-by-room loads matter and must be met
• In remote rooms, the heat gained through
internal surfaces and delivered via distribution
must outweigh the heat loss
• Delivering air at or near room temperature may
not be enough to keep the space conditioned
• Supplemental backup heat doesn’t transform a
low-load home into an energy hog
Using Auxiliary Distribution
SPECIFICATION
Volts: 120
Hertz: 60
Amps: 0.50 maximum
Watts: 25
CFM: 75
RPM: 1,800
Air has a low heat capacity.
Example 75F warm room
70F cool room
100 CFM
1.08 X 100 CFM X 5F= 540/BTUS/hr.
It’s Not a Ductless Purity Test. It’s OK to
Transfer Air by Other Means
• So, can I use an HRV or transfer fan to heat
rooms?
• What room-to-room temp difference are you
okay with?
• Without a high-efficiency HRV, you will be
cooling during the heating season
• Ventilation air typically can’t meet room loads
alone
• They only recovers and/or move heat. They
doesn’t make heat.
Using Auxiliary Distribution
• Select equipment with at least a 4:1 ratio between its
maximum capacity and minimum capacity at 47f
• Place and orient indoor heads to allow for future
maintenance/service
• Closets and dropped ceiling are good options
• Locate ducts and equipment in conditioned space
Ducted Mini-Split Design
• Good duct design is key
• Adhere to manufacturer’s guidance – some specify ducting limitations
for their units, others just provide static pressures for the fan
• Do a real duct design – follow Manual D guidance or use the “Easy
Ducts” process
• Design and install to minimize restriction
• Use grilles that will throw and mix well
• Use with a wall-mounted T-stat
• Locate T-stat appropriately
• Make sure controls reflect the necessary settings
Ducted Mini-Split Design
Ducted Mini-Split Design
• Setting the unit to sense
temperature at the T-stat:
• Function 42 has to be set to
“01” and the t-stat icon has to
appear on the screen
• “High Insulation” setting needs
to be activated
“FUNCTION 42 and Other Settings”
Ducted Mini-Split Design
Sensible Recovery Efficiency (SRE):
• The efficiency including the energy use of the
system.
• This value is used principally to predict and compare
energy performance.
Apparent Sensible Effectiveness (ASE):
• The efficiency not including the electrical
energy used by the system.
• This value is useful principally to predict
final delivered air temperature.
• HRV Ratings: Which Three-Letter Acronym Matters?
We recommend high-ASE HRVs as it relates to delivered air temperature.
People WILL shut off HRVs that blow cold air on them!
Using an HRV to Ventilate and Distribute
into Bedrooms
30
35
40
45
50
55
60
65
70
75
65 70 75 80 85 90 95
De
live
red
Air
Te
mp
Apparent Sensible Efficiency
Delivered Air Temp at Various Outdoor Temps and ASE
60F 40F 20F 0F
Outdoor Air Temp
Use HRVs with high
ASEs (over 85%,
ideally) to deliver
proper air temp.
About that ASE…
• Net Zero, low load, above code homes, are no longer a niche market in the
Northwest
• Low loads don’t make the home “comfort invincible”
• The best system in the world does nothing if it isn’t used properly—
• Homeowners must be educated on the operation of systems in their homes
Conclusions
• Leave interior doors open
• Clean filters
• Check drain pains
• NEVER use auto mode
• Don’t make big changes in
the set points
Advice to Homeowners
