IMPLEMENTATION OF AIR - SUCCESSFUL TIGHTNESS REQUIREMENTS FOR RESIDENTIAL BUILDINGS
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SUCCESSFUL
IMPLEMENTATION OF AIR
TIGHTNESS REQUIREMENTS
FOR RESIDENTIAL BUILDINGS
Bruce Nelson, P.E.
Senior Engineer
Minnesota Office of
Energy Security
Best2 - Air Tightness of Building – Session WB6-5Residential Air Tightness is both Advocated and Required • In the early ’80s building scientists were saying: “A better wall is a tighter wall before it is a thicker wall.” • Weatherization of existing homes • International Energy Conservation Code air sealing checklist
Stack Effect
Both New and Existing Houses are Tighter than We Might Think • Energy codes • Buyers demand for comfort • Builder concern for durability – Attic bypasses sealed to reduce ice-dams
Eight 1990 Minnesota Homes • Nelson, et al, 1993 • The estimated annual infiltration rates averaged 0.27 ACH • ASHRAE recommendation is 0.35 ACH • 3 of 5 homes in the sample with natural gas water heaters failed a worst cast draft test
Metro Airport Commission
Weatherized Homes
• Bohac and Cheple (2002)
• where the maximum depressurization for proper
venting is 3 Pa:
– the percentage of homes where appliances would fail
to vent properly doubled after tightening
• where the maximum depressurization for proper
venting is 5 Pa
– the percentage of homes where appliances would fail
to vent properly doubled after tighteningAir sealing to improve residential
energy efficiency
1. Air leakage is no longer a reliable source
of fresh air for ventilation
• Need mechanical ventilation
2. Air leakage is no longer a reliable source
of Make up air
• Need another source, or
• Need to stop using house air for make upMinnesota’s approach Residential ventilation task force (RVTF) 1. How tight are new homes? 2. How much mechanical exhaust would it take to create a dangerous negative pressure? 3. How much negative pressure can vented appliances tolerate?
Minnesota Code Basic
Assumptions
• Three variables that affect the safe operation
of installed vented combustion appliances:
– 1. house air tightness,
– 2. exhaust rate of installed fans, and
– 3. depressurization tolerance of combustion
appliances.
• A house will become depressurized when air
is mechanically exhausted and there is no
mechanically supplied make up air1. How Tight Will Houses Be? • Maximum likely tightness for a house built under the new energy code. • The RVTF chose the value of: – 1.5 ACH50, or for a 2500 sq. ft. house: – 236 L/s @ 50Pa – 500 cfm @ 50Pa • Basis: unpublished measurements of new homes by several RVTF members and home builders
How Tight Were They? • Shelter Source, 2002 study of new homes • Some built during energy code transition in 1994 & 1998 • Others built in 2000 under the new energy code
How Tight Were They? (Shelter Source, 2002)
9
8
Number of homes in range
7
6
2000
5
1998
4
1994
3
2
1
0
le ss 614 755 897 r 897
or to to to
Ov
e
47 2 47 3 61 5 75 6
liters per second @ 50 Pascals2. Exhaust Rate of Installed Fans • Clothes dryers, bath fans, range hoods, and central vacuum equipment • Flow rate would be derated at high negative pressures • But not significantly at 5 to 10 Pa
3. How Much Depressurization
can be tolerated?
• RVTF definition:
– the negative pressure environment that a
vented combustion appliance can fire from a
cold start and within three minutes will
establish normal draft, or
– the negative pressure that, when imposed
on a firing appliance, will not cause the
appliance to back draftMinnesota Energy Code
Assumptions
Depressurization
Path Appliance tolerance
0 Direct vented appliance 50 Pascal
1 Closed controlled combustion 7 Pascal
wood burning appliances
2 Atmospherically vented furnaces 5 Pascal
or boilers and decorative
wood-burning appliance
3 Atmospherically vented water 2 Pascal
heaterHouse Depressurization as a Function of Exhaust Rate for Tight, Medium & Loose Homes Air Ach L/s Cfm tightness @50Pa @ 50Pa @50Pa Tight 1.5 236 500 Medium 3.6 566 1200 Loose 7.2 1133 2400 Note: Table 1 is incorrect on the paper – corrected version is online
House Depressurization as a Function of
Exhaust Rate for Tight, Medium & Loose Homes
50
45
40 Tight, 1.5 ach 50
Meduim, 3.6 ach 50
Depressurization, Pa
35
Loose, 7.2 ach 50
30
148 L/s
25
20
15
10
5
0
0 100 200 300 400 500 600 700 800 900 1000 1100 1200
Net exhaust rate, liter per sesond (L/s)
Note: Figure 1 is incorrect on the paper – corrected version is onlineWriting the Code • The National Appliance Energy Conservation Act of 1987 (amended) preempts states from adopting regulations regarding the energy use of specified appliances • The basis of the Minnesota code was safety • To comply with the Minnesota code most builders installed sealed combustion
But Make Up Air is Already
Addressed in Building Codes!
• National Fuel Gas Code
(NFPA54/ANSI Z223.1) says:
– 5.3.1 (g) Air requirements for the operation of
exhaust fans, kitchen ventilation systems,
clothes dryers, and fireplaces shall be
considered in determining the adequacy of a
space to provide combustion air
requirements.
• And has been since at least 1992Make Up Air is Already Addressed in Building Codes • Q: Why aren’t builders complying with this requirement? • A1: They don’t know how • A2: The code does not give any guidance • A3: ASHRAE Handbooks give no guidance • A4: There are no how-to books that give any guidance
The Minnesota Code
• What the Minnesota code did
differently was to:
– explicitly denote when make up air is
needed,
– specify how much make up air is
required, and
– stipulate how the make up air is to be
provided.Secondary Affects
of the Minnesota Code
• Nearly 100% of natural gas furnaces in
Minn. are sealed combustion.
– Meaning high efficiency
• Discouraged natural gas water heaters.
• Homes are likely to be much more efficient
than would be expected by increasing air
tightness alone.Minn. New Homes are Very Efficient
(Linner, 2008)Conclusions
• It is possible to implement a code
requiring tight construction. But
provisions must be included for:
– Ventilation for air quality, and
– Make up air to counter the
depressurization created by exhaust
appliances.National Observations • The supply of make up air for vented combustion appliances appears to be getting relatively little attention. • The “30 Percent Solution” proposal for the IECC addressed neither ventilation nor make up air.
Implications for
U.S. Housing Stock
• American Recovery and Reinvestment Act
requires states to enforce national model
codes.
• National model codes require air tightness
for new homes.
• Although the national model codes do
have crude provisions make up air for tight
homes, generally they are not enforced.Bruce Nelson, P.E.
Senior Engineer
Minnesota Office of Energy Security
651-297-2313
bruce.nelson@state.mn.us
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