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FAQs

Why is it so important to have my ducts sealed?
What is the difference between supply side leakage, and return side leakage?
How much air leakage in CFM can my change-out or alteration job have?
How do you calculate the amount of air that is leaking from my duct system?
How much air should leak from my duct work?
What is the difference between 100% testing & sampling?
What is used to test the ductwork?
How long does a tight duct test usually take?
Are there any alternatives to avoid the need of a HERS rater?
Is it my option as to whether or not I want to be sampled or 100% tested?
What is the benefit of having 100% testing?
What is considered 'typical leakage' compared to other homes?

Why is it so important to have my ducts sealed?
The most energy consuming appliance in a home, by far, is the Heating/Ventilation/Air Conditioning (HVAC) system.  Ductwork, whether under the floor, or above the ceiling, is designed to deliver conditioned air to the rooms through the ceiling or floor vents which are referred to as supplies.  HVAC systems also have returns which suck the air out of the rooms at the same time the system is supplying the air.  All of this positive and negative airflow is contained and insulated in ducts.  If those ducts have leaks in them, you are literally blowing your money outdoors.

What is the difference between supply side leakage, and return side leakage?
As explained in the previous faq, an HVAC system has a center.  The center would be where the air is handled, and changed from a negative air flow (suction) to a positive airflow (blowing).  When there is a leak on the supply side of a duct system, the air you are paying to be conditioned is leaking to spaces you don't occupy- like the attic.  Should the leak be on the return side of the HVAC system, air from the unconditioned space where the return ducts are located, is being vacuumed in and mixing with the air you are breathing.  This scenario puts your health at risk and costs you money!

How much air leakage in CFM can my change-out or alteration job have?
In short answer, there are 4 ways to obtain passing results- 3 ways that deal with actual calculated air flow and leakage in CFM.  Those 3 ways are A) 15% of calculated airflow can leak using the 400 CFM per ton formula as explained in the the following question.  B) 60% reduction achieved in comparison to the initial leakage test performed before the change-out. C) Measure the amount of leakage to the outside, and make sure it's less than 10%.  Measuring leakage to the outside requires the use of a blower door, and ducts supplies still have to be taped off.  D) Finally, if 1 of the above 3 passing methods can't be achieved, passing results can be obtained so long as the contractor seals all accessible visual leaks and that has been visually verified with smoke by a HERS rater.

How do you calculate the amount of air that is leaking from my duct system?
In order to calculate the allotted amount of air your ducts can leak, you must go to 1 of 2 places- the furnace or the condenser.  When looking on the condenser, you'll generally find a model number.  Within the model number will be a number that when divided by 12 will yield the size of the unit it tons.  Calculations are performed with 400 CFM per ton.  So a 4.5 ton unit has 1800 CFM.  Another way to calculate the air load, is from the output BTU's of a furnace.  If the output BTU's were specified on the model sticker as 89,000, we would multiply 89 x 21.7 and get 1931.  Since we want the house to be guaranteed in the passing zone, we will use the smaller of the two numbers to calculate the target leakage in CFM.  To calculate the target leakage, multiply 1800 (total air load) by 15% (allotted leakage for change-outs) and we get 270.  We seal all of the supply registers in every room, pressurize the duct system with a duct blaster.  Sensors are attached to the duct system, and duct blaster and then channeled into a manometer (measures static pressure).  The system is pressurized to 25 Pascal (the typical amount of air pressure in a fully functioning duct system), the manometer reads the amount of air leaking in CFM. 

How much air should leak from my duct work?
For brand newly installed duct systems in which the entire system is new, the allotted leakage rate is 6%.  For all new construction, the allotted leakage rate is 4% if tested at rough-in, and 6% if tested at final.  For alterations and change-outs, there are 4 options for a system to pass which are outlined in question 3 of this page- How much air leakage in CFM can my change-out or alteration job have?

What is the difference between 100% testing & sampling?
The building codes state that the building owner or the agent of the building owner, has the option to have the 3rd party verification performed on their house/building (100% testing), or they can choose to be part of a sample, in which the installing contractor makes groups of no more than 7 jobs in which duct sealing was required, the HERS rater selects 1 job at random, and tests its.   Should it pass the first test attempt, every other house in the sample group will pass.  Should it fail the first test, a second test must be administered to another random job within the sample group.  Should the second house pass the first attempt, the first house will be revisited, re-tested till pass.  Should the second job fail the first attempt, the entire sample group must receive 100% testing.

What is used to test the ductwork?
A calibrated manometer that works with a duct blaster.  We use the Minneapolis Duct Blaster and DG-700 Manometer.

How long does a tight duct test usually take?
Depending on the size of your building, a test can take anywhere from 30 minutes - 2 hours.

Are there any alternatives to avoid the need of a HERS rater?
Yes there are, regardless of what climate zone you are in, you have the option to install an EER 12 with either a refrigerant charge measurement or TXV, plus 0.82 or 0.92 AFUE plus increased duct insulation.

Is it my option as to whether or not I want to be sampled or 100% tested?
Yes...this option rightfully so, lays at the option of the building owner.  If they want to be included in a sample, they must be notified of this, and they have to be informed of their option to have the 100% testing performed.

What is the benefit of having 100% testing?

Having your building tested is the only true way to guarantee compliance and minimal leakage.  A properly functioning HVAC system benefits the homeowner in many ways- saves money, provides a more healthy living environment, conserves fossil fuels, and further protects our atmosphere.

What is considered 'typical leakage' compared to other homes?
According to studies, the typical home in California has more than 30% of air flow leaking from the ducts alone.



Copyright © HERS Rater Services, Inc. 2013


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