Bypass duct
Bypass duct
I have a 4 zone system plus a "dump zone" The "dump zone" is totally unregulated (it's just three ducts that come off the main plenum with standard dampers. I also have a bypass duct from the end of the mail duct to the return intake. I noticed that your instructions say that the bypass should enter the return more than 6 feet from the unit. Mine inters about 2 1/2 feet from the air handler. I also noticed mention of an "SAS" (sensor?). I do not have one of these. The bypass is regulated by a weighted damper. Are there any possible problems with the bypass duct / SAS as I have it, or is it OK like this?
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Pcyrana
Re: Bypass duct
The Supply Air Sensor (SAS) is included with every zone panel except the ST series panels. It is recommended but not necessary to install.
However, if you are using a bypass damper a temperature sensor is mandatory.
The SAS will prevent damage to your equipment (coil or heat exchanger) if you are air that is to hot or to cold through it.
As far as the distance of the bypass off the plenum is concerned, 6 feet is recommended room permitting.
With 2 1/2 feet it would be advisable for you to include a hand damper in the bypass run.
I will include an updated illustration showing how the proper bypass loop should be installed.
However, if you are using a bypass damper a temperature sensor is mandatory.
The SAS will prevent damage to your equipment (coil or heat exchanger) if you are air that is to hot or to cold through it.
As far as the distance of the bypass off the plenum is concerned, 6 feet is recommended room permitting.
With 2 1/2 feet it would be advisable for you to include a hand damper in the bypass run.
I will include an updated illustration showing how the proper bypass loop should be installed.
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Re: Bypass duct
Thanks for the fast reply. There is plenty of room to have 6 feet or more of return air plenum if this is advisable. Also what do you think of a "dump zone" (air comes out of main trunk before any zone dampers and dumps to 3 or 4 registers in the home) but has no method of regulation via damper, barometric or electronic. Will this decrease airflow to the farther reaches of the system?
Just like the bypass entering 2 1/2 feet from the intake without an SAS, the contractor is saying this setup is fine. We have burned up 2 compressors and 1 coil with this setup perviously.
Just like the bypass entering 2 1/2 feet from the intake without an SAS, the contractor is saying this setup is fine. We have burned up 2 compressors and 1 coil with this setup perviously.
Re: Bypass duct
Patrick,
Help me understand (educate me) why you put a hand damper and either a motorized or barometric damper in the same bypass loop.
Help me understand (educate me) why you put a hand damper and either a motorized or barometric damper in the same bypass loop.
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Pcyrana
Re: Bypass duct
A hand damper installed in the bypass run prevents the bypass run from becoming the path of least resistance.
This is a benefit to the HVAC System for several reasons.
- Having a hand damper on the bypass run reduces short cycling due to bypass air mixing to fast due to excessive bypass volume.
- Hand dampers also reduce hunting on ECM motors. When a modulating bypass damper lets a varying volume of air back through the ECM motor it will begin to slow down and then ramp up as the bypass air volume changes. A hand damper reduces this hunting effect.
This is a benefit to the HVAC System for several reasons.
- Having a hand damper on the bypass run reduces short cycling due to bypass air mixing to fast due to excessive bypass volume.
- Hand dampers also reduce hunting on ECM motors. When a modulating bypass damper lets a varying volume of air back through the ECM motor it will begin to slow down and then ramp up as the bypass air volume changes. A hand damper reduces this hunting effect.
Re: Bypass duct
Patrick,
Many thanks for your response.
I have always installed barometric type dampers in bypass loops and carefully set them with good luck. I will try using a EWC bypass motorized damper connected to the zone board, plus install a hand damper on the next job.
Many thanks for your response.
I have always installed barometric type dampers in bypass loops and carefully set them with good luck. I will try using a EWC bypass motorized damper connected to the zone board, plus install a hand damper on the next job.
Re: Bypass duct
My contractor wants to know what happens to the excess air that is blocked by the hand damper in the bypass? How should the hand damper be set since the barometric damper is set to control the airflow.
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Pcyrana
Re: Bypass duct
The use of a hand damper to adjust bypass pressure drop - as recommended by Manual Zr, due out by the end of the year.
The excess air is not blocked by the hand damper it is simply throttled down to allow more time for the bypass air to mix with the supply air.
The Bypass Hand Damper should be set so that the pressure drop after the Hand Damper is equal to the pressure drop at the end of the longest branch run.
To do that:
Determine the Longest Run Pressure Drop (using a manometer)
- Measure static pressure at the main plenum. - ex. .5" wc
- Measure static pressure at the end of the longest run - ex. .28" wc
- Subtract longest run pressure drop from the main plenum pressure - .5 - .28 = .22" wc
- .22" wc is the pressure you want to read after the hand damper,
Now simply adjust the hand damper until the pressure after it is equal to or near .22" wc
You measure the pressure drop after the bypass damper with only the smallest zone calling and the system running on high speed. Thy bypass damper should be fully open.
The excess air is not blocked by the hand damper it is simply throttled down to allow more time for the bypass air to mix with the supply air.
The Bypass Hand Damper should be set so that the pressure drop after the Hand Damper is equal to the pressure drop at the end of the longest branch run.
To do that:
Determine the Longest Run Pressure Drop (using a manometer)
- Measure static pressure at the main plenum. - ex. .5" wc
- Measure static pressure at the end of the longest run - ex. .28" wc
- Subtract longest run pressure drop from the main plenum pressure - .5 - .28 = .22" wc
- .22" wc is the pressure you want to read after the hand damper,
Now simply adjust the hand damper until the pressure after it is equal to or near .22" wc
You measure the pressure drop after the bypass damper with only the smallest zone calling and the system running on high speed. Thy bypass damper should be fully open.