Download Ventilation - City of Berkeley

Ventilation
Residential Low Rise
As of January 1, 2010, the 2008 California Building Efficiency Standards requires mechanical
ventilation for Residential units. All new low rise dwelling units1 and existing low rise residential
buildings with additions over 1000sft2 now have to meet the requirements of ANSI/ASHRAE
Standard 62.2 2007 Ventilation And Acceptable Indoor Air Quality In Low Rise Residential Buildings
(ASHRAE 62.2 ).
ASHRAE 62.2 allows for window operation in certain climate zones3 to meet this standard, however
that exception does not apply in California. The California Energy Commission specifically states
“Window operation is not a permissible method of providing whole building ventilation required in
section 4 of that standard” 4
Bath fans are required in all baths even if there is a window. A kitchen fan is also required. For
smaller homes bath and kitchen fans can meet all the whole house ventilation requirements. Larger
homes will need a combination of local exhaust and whole house ventilation or your existing forced
air system can be modified to meet the requirements. The ventilation requirements can be met either
using intermittent or continuous fans. The advantage of using continuous ventilation is that smaller
equipment is needed.
To determine the required continuous ventilation rate you can use the following table:
Ventilation requirements in Cubic feet per minute
Floor area
Bedrooms
(ft²)
0-1
2-3
4-5
6-7
>7
≤1500
30
45
60
75
90
1501-3000
45
60
75
90
105
3001-4500
60
75
90
105
120
4501-6000
75
90
105
120
135
6001-7500
90
105
120
135
150
>7500
105
120
135
150
165
Example: What ventilation rate would you need for a 1200 sft house with 2 bedrooms?
Using the table above, you would need 45 cubic feet per minute (cfm).
Or you can use the following formula for a more precise value:
7.5 1
100
=1200/100+7.5(2+1)
=12+22.5
=34.5 cfm
1
Subchapter 7 Section150(o) 2008 Building Efficiency Standards for Residential and Nonresidential Buildings
Subchapter 9 Section152(a)Exception 5 2008 Building Efficiency Standards for Residential and Nonresidential Buildings
3
4.1 Exception .a ANSI/ASHRAE Standard 62.2-2007
4
Subchapter 7 Section150(o) 2008 Building Efficiency Standards for Residential and Nonresidential Buildings
2
Rounding up, you would need fan rated for continuous duty capable of providing 35 cfm. By using the
formula rather than the table, you can meet the ventilation requirements by putting in a smaller fan, so
don’t be afraid of doing the math.
How much intermittent ventilation would you need?
You can also provide intermittent ventilation controlled by a timer that operates the ventilation system
no less than one hour in twelve, in other words, twice a day. You can calculate the intermittent
ventilation rate by using the following formula and table5:
= intermittent fan flow rate
= required continuous ventilation rate
=ventilation effectiveness (from table)
=functional on time (%)
Ventilation effectiveness for intermittent fans
Daily functional on time- Ventilation effectiveness- 0.33
≤ 35%
0.50
35% ≤ < 60%
0.75
60% ≤ < 80%
1.00
80% ≤ For the same example used before, that is a 1200 sqft house with 2 bedrooms, we already
determined that for continuous ventilation 35 cfm (
would be needed.
So if we wanted to operate the fan 30% ( ) of the time which would be 72 minutes every 4 hours,
using the table we come up with .33 ( ) ventilation effectiveness.
!"#$
= 353 cfm
%.!!&%.!%
An intermittent fan for the same house running 30% of the time would need to be capable of
ventilating at 353 cfm.
Infiltration credit: For older homes, if you have had a blower door test showing that you have a
leakage rate over 2 cfm per 100sft of living space, you can reduce the amount of required ventilation.
The amount of reduction would be half the amount of infiltration measured above and beyond the
2cfm per 100sft default level.
What equipment do you need?
Negative pressure systems, exhaust only: As mentioned, bath fans are required. The minimum size
5
4.4 and table 4.2ANSI/ASHRAE Standard 62.2-2007
required for a bathroom is 25 cfm6 if run continuously or 50 cfm if it is operated by a switch.
Continuous bath fans must be sound rated at 1 sone. Intermittent fans must be sound rated at no
more the 3 sones unless they are rated for more than 400cfm. The fan manufacturer will also have
sizing requirements that also must be followed. Kitchen fans whether a ceiling exhaust fan or a kitchen
range hood are also required. For continuous use, a minimum of 5 air changes per hour (ach) of the
kitchen area is needed. Say your kitchen is 12’ by 14’ with a 10’ ceiling. You would need an exhaust
fan capable of providing a continuous 140 cfm. (12’x14’x10’x 5 ach) /60. For intermittent use, a range
hood providing 100 cfm would be the minimum size fan needed.
If your 1200 sft 2 bedroom house had a bath fan that was 1 sone and rated for continuous duty at 35
cfm, and a 100 cfm intermittent kitchen rage hood, you would meet the requirement at least for the
mechanical side. Although this is the least expensive and easiest method to meet the requirement
equipment wise, it may not improve your air quality if the sources of replacement air are coming from
crawlspaces or garages or attics. When you use a negative pressure method of ventilation such as
this, it must be in combination with sealing off undesirable sources of replacement air.7 Most old
Berkeley homes have no air barriers between the crawl space and the living space. The floors are
predominantly 1x6 fir subfloor, covered by a finished oak floor. When there is a negative pressure in
the living space above this type of floor, as much as 60% of the replacement air can come from the
crawl space. If there is an attached garage, ducts terminating originating or passing through the
garage must be sealed. Drywall edges and walls in the garage must be sealed.
Positive pressure systems: If you would prefer to chose the source of the air coming back into the
house , you might want to pressurize the house. You can have a fresh air ventilation system installed
on the return side of your existing forced air furnace. The air coming into your furnace is required to
have a screened inlet ten feet away from any possible source of contamination such as other exhaust
ducts. The intake must also be filtered to at least MERV 6.
The watt draw of this type of system must be less than 0.58w/cfm. If a default value of 0.8w/cfm is
used as a design performance approach value, no HERS verification of watt draw is needed. The “Fan
On” switch may be used for continuous operation or a timer must be used for intermittent use.
It is important to note that by pressurizing the dwelling you could be increasing heat loss in the winter
by exfiltration through cracks and gaps in the building. This could also increase the amount of moisture
inside the wall surfaces.
Balanced System: A Heat Recovery Ventilator balances the air coming in with the amount of air
going out and recovers heat from the exiting air to warm the air coming in. Though this is a more
expensive system, however it creates fewer problems as air is neither forced out, nor pulled in through
the leaks in the building. Once again, the air coming into your HRV is required to have a screened inlet
away from any possible source of contamination such as other exhaust ducts and be filtered to at least
MERV 6.
6
7
ASHRAE 62.2 requires 20 cfm, California code requires 25 cfm minimum.
6.1 ANSI/ASHRAE Standard 62.2-2007
Switching: Continuous systems need to be installed to operate without occupant intervention, but the
override must be readily accessible. Any switches controlling a required ventilation system need to be
labeled as to function unless it is obvious such as a bath fan switch.
Owner’s manual: Installation and operation instructions shall be provided to the owner and occupant.
Ducting: 3” flex duct is not permitted. No more than 5 feet of 3” smooth duct for fans up to 50cfm is
allowable. 4” flex duct can be used for 70 feet for up to 50cfm fans, 35ft for 80cfm fans, not permitted
Additional Information:
• 2008 Building Efficiency Standards Compliance Manual and look at section 4.6.
Equipment Image
Means
Continuous
local fan
bath (neg
pressure)
Pros
Inexpensive
cons
Cannot identify
or control quality
of replacement
air, heat loss
due to infiltration
$
250-300
Continuous
local fan
kitchen
(negative
pressure)
Relatively
inexpensive
Cannot identify
or control quality
of replacement
air heat loss due
to infiltration
400-1000
Outside air
input to
heating
system
(positive
pressure)
Filtered air from
known source,
switching
measures
with existing
fans
( negative
pressure)
Low cost, when
using existing
appropriately rated
fans
More complex,
250 plus install
system must
meet energy
use, heat loss
increased due to
infiltration to out
of doors heat
loss due to
exfiltration $
Cannot identify
<100
or control quality
of replacement
air, heat loss
due to infiltration
Heat
recovery
ventilator
(HRV)
Balanced
Filtered air from
known source,
recovered heat
from out-going air,
balance system,
not relying on
infiltration
Energy recovery
though moisture
exchange. Useful
in areas with high
cooling loads.
Energy
recovery
ventilator
(ERV)
Whole house
fan
$$$$Expensive
1300 plus install
$$$$$Expensive 1500 plus install
not suited for
this climate zone