Homeowners with natural gas water heaters have difficulty justifying the expense of a more efficient condensing heater. Combination space and domestic hot water systems bundle together the two loads, which saves energy and makes them more cost-effective. These systems also help eliminate combustion safety concerns. Historically, mechanical contractors have custom engineered and pieced together combi systems in the field, paying little attention to efficiency and optimization. But condensing heating plants will only reach their energy saving potential when all components are designed and installed correctly.

1. Performance and Optimization of
Residential Condensing Combination
Space and Water Heating Systems
Ben Schoenbauer, Center for Energy and Environment
April 23, 2013

2. Sponsors and Partners
 NorthernSTAR - A DOE Building America Research Team
 Sustainable Energy Resources for Consumers Grants
 Center for Energy and Environment
 Sustainable Resources Center
 University of Minnesota
 The Energy Conservatory

3. Overview
 Background: Project and Equipment
 System Design
 Field Results: Efficiency, Savings, Comfort
 Cost
 What’s next

4. How this project came about
− Weatherization is able to seal homes tighter and tighter
− Leads to combustion safety issues
− Requires sealed combustion
− Requirements on Savings vs installed cost rule out high
efficiency water heaters
− Forced to use safety budget to install 60% direct vent tanks with
very little energy savings
− SRC got a SERC grant to look at using a DIA

5. What is a Dual Integrated Appliance?
A. A mechanical system that uses one heating plant (natural gas
burner) to provide both space heating and hot water
B. Space heating side can be either hydronic or forced air
C. Systems can use a closed or open heating loop
This project will look at natural gas forced air DIA systems. Boiler
based systems will be closed loop and water heater systems
have an open loop.

10. Page 10
PROBLEM
+ Some contractor’s had little experience
+ System schematics often developed on site
+ Little or no sizing information provided
+ System components came from several
manufacturers
+ Manufacturer’s settings typically do not lead to best
performance
+ Decided to design and optimize systems in a
laboratory
+ Could then provide contractors with more detail
installation guidelines
Installation and Sizing

11. Page 11 Full report at:
http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/labtests_combi_spacewh.pdf

12. Page 12
Steady-state heating plant efficiency

13. Page 13
Air handler performance
+ Hydronic coil transfers heat
from water to air
+ Goals:
+ Return water <105 °F
+ Delivered air >110 °F
+ Goals must be balanced with
capacity needs
+ Installation parameter charts
were developed for each air
handler

14. Page 14
Tset =
140 °F CFM =
1500
GPM = 5
100°F
60,000 Btu/hr
120°F
Poor install
(for a 40,000 Btu/hr design load)

15. Page 15
Tset =
140 °F CFM =
900
GPM = 2.5
112°F
45,000 Btu/hr
105°F
Better install
(for a 40,000 Btu/hr design load)

16. Page 16
Tset =
125 °F CFM =
900
GPM = 2.5
106°F
43,000 Btu/hr
90°F
Best install
(for a 40,000 Btu/hr design load)

17. Page 17
Tset =
120 -
140 °F
CFM =
800 -
1000
GPM =
2 - 3
100 - 115°F
20 – 50 kBtu/hr
80 - 105°F
Ideal future install
(for a 40,000 Btu/hr design load)

18. Field Implementation and Monitoring
o 250 installs in Minnesota
• Lessons learned from implementation
• Detailed pre/post monitoring on 20 sites

19. Page 19
Minneapolis
+ Heating Degree Days: 7565
+ Heating Design Temp: -13.4 F
Project Averages
+ Space Heating Design Condition: 25,000 btu/hr
+ DHW Daily Usage: 41 gallons/day (830 Btu/hr)
+ Combined Gas Consumption (AFUE~80 and EF~58):
900 therms/year
Typical Installation
+ Unfinished basements or mechanical rooms in finished basements
House Characteristics

20. Page 20
Venting
Installation
B-Vent
Direct

21. Page 21
+ Condensation Management
Installation:

22. Page 22
Mixing Valve
Installation:

23. Page 23
+ Locations and equipment size may change
+ Tankless water heaters and combi boilers are
often wall mounted
+ Location of the large gas line may change
+ Additional power lines
Installation:

24. Page 24
Increasing the water heater set point
Installation: Equipment Set Up

25. Page 25
Adjusting the water flow rate through the air handler
Installation: Equipment Set Up

26. Page 26
Adjusting the air flow rate through the air handler
Installation: Equipment Set Up
Most Common Methods:
• Dip switches
• Switching wires for various
speeds

27. Field Monitoring Insturmentation

28. Page 28
Existing Equipment
DHW Loads
Cold In from Mains
Hot Space
Heating Air
Runtime
Air Temp
Ambient T
Gas
ND Tank
Water
Heater
Electric Gas
Single
Stage
Furnace
Conditioned
Space

29. Page 29
DIA Installation
DHW Loads
Cold In from Mains
Supply to AH
Return from AH
Hydronic
Air
Handler
Hot Space
Heating Air
Water Temp
Water Flow
Consumption
Air Temp
Air Flow
Electric
Ambient T
Electric Gas
Water
Heater
Conditioned
Space

30. Page 30
EXISTING
COMBI

31. Page 31

32. Efficiency

33. Page 33
Installed Monthly Efficiencies

34. Page 34
+ Lab Testing
Why the low efficiencies in the summer?
Setpoint = 140 F
$1/therm
$0.12/kWhr

35. Page 35
+ Lab Tests – Idle Testing
TANK 2

36. Page 36
+ Lab Tests – Idle Testing
TWH 2 – Storage Tankless Hybrid

37. Page 37
TWH 1

38. Page 38
• DHW accounted for between 2% and 34% of
the total heating load. (13% on average)
Variance in Daily Water Usage

39. Page 39
Annual Efficiencies
25,000 btu/hr design heating load
and 40 gpd of hot water
Dashed lines represent possible
installed efficiency

40. Savings Potential

41. Page 41
1028 – possible 70% eff furn
1031 – Condensing Furn
1037 – Combi boiler annual eff 81%

42. Page 42
Power Consumption
+ During Space Heating
+ Existing Systems: 550 W
+ Combi System: 475 W
+ During Continuous Fan Operation:
+ Existing Systems: 425 W
+ Combi Systems: 50 W

43. Occupant Comfort

44. Page 44
+ Non-condensing furnaces: 130 - 140 ° F
+ Condensing furnaces: 115 – 130 ° F
+ Space heating heat pumps: 77 – 115 ° F
+ Combis this project :110 – 115 ° F
+ Could improve efficiency 3-5% allowing 105 °F air temps.
Supply Air Summary

45. Page 45
Delivered Water Temperature – Storage Water Heater

46. Page 46
Delivered Water Temperature – Tankless Water Heater

47. Page 47
Delivered Water Temperature – Hybrid Water Heater

48. Costs, Savings, and Payback

49. Page 49
Cost Comparisons

50. Page 50
Note:
• Typical loads were 25,000 Btu/hr design day space heating loads and 40 GPD hot water.
• Base Case: $725 per year gas bill (915 therms)
Savings and paybacks

51. Page 51
Savings and Paybacks
Note: Combi systems were cheaper to install that
a condensing water heater and condensing
furnace

52. Page 52
+ System Controls: Work to be completed in 2013
+ What cycle lengths are necessary to prevent short cycling for
efficiency? Room temperature?
+ How will fan and pump modulation improve efficiency and
comfort?
+ How much can temperature set backs (outdoor resets) improve
performance?
+ How will system performance change if we remove the air
temperature restrictions?
+ Other Equipment Needs
+ Improvements to air and water temperature delays
+ DHW priorities impact on comfort
+ Other Program Needs
+ Method for verifying savings for rebate programs
+ Consistent rating system for combi systems
+ Impacts on cost
What else do we need to know?

53. Page 53
Questions?
Ben Schoenbauer
bschoenbauer@mncee.org
More information available at:
www.mncee.org/dia