View Full Version : Zoning Bypass - let me set the record straight.
01-24-2007, 02:29 PM
I have been doing HVAC for 2 years now. Proud owner of a Universal EPA license and Nate certifications. I admit, I am not sure if my zoning/zoning by pass skills are correct. They are probably incorrect, because there is no proper documentation written on the subject by anybody including ACCA. Every other HVAC pro I know has a different opinion.
Let me state the scenarios that make me wonder on this subject. In addition, I will describe how I approach the matter. You tell me if I am incorrect in my zoning/bypass questioning.
1. Variable vs. static speed furnace and their influence on the zoning bypass design.
a. ACCA recommends not installing zoning bypasses if the equipment’s speed is automatically adjustable.
b. According to pros, the only equipment that can get away without bypasses is Carrier and Trane. Those companies can manage air movement with the help of zone dampers.
c. I myself believe like this: a VAV furnace can reduce the output of the air by some percentage, but there is a limit to it. If I have a zone that takes 50% of all the air and that zone is closed the VAV furnace will not be able to reduce itself to 50% output. Am I correct here?
2. 80% Rule.
ACCA says: If you have two zones (floor zoning) in your system and they are almost identical in size, when one of the zones is closed you need to bypass 80% of the air from that zone. That assumes 20% of the extra air goes into the other zone. How should one design the zones? Should I design it so that it takes 20% of air extra when activated or zoning simply means closing and opening of zones without any extra air delivered to active zones? Completely confused here. If we should allow running 20% of air extra to the active zone than the first issues with the VAV furnace may go away. Can some expert collaborate here?
3. It is also worth mentioning that there are no best practices when it comes to zoning and returns. No one talks about specific returns for a zone. That is the only way to keep the preserves of the air correct.
01-24-2007, 02:40 PM
lennox does not want any bypass damper on their harmony systems.
if it is done with multi-stage v drive equipment, airflow can be dropped quite a bit for smaller zones and still move plenty of air for heat or a/c running on 1st stage. multiple or larger zones open and blower ramps up and lets unit operate on 2nd stage as needed.
bypass dampers are a caveman fix for not having the modern controls and equipment. :D :D :D
01-24-2007, 03:30 PM
You have a good question and I can’t wait for some of the more knowledgeable technicians to the answer. I’m not sure if I know the question. I think you are asking if the variable speed fan on the unit can drop the delivery 50% while the damper strokes 50 % of the zone closed. Are you saying you would like to use a by pass in order to maintain your existing zone delivery? You could call the unit distributor to see how they like there equipment set up and if they have a controls package they would recommend for there system.
01-24-2007, 03:56 PM
I haven't ever looked at the Lennox harmony zone system, so I have no idea how it operates.
Does it modulate damper positions to manage the airflow like the Carrier Infinity and Trane Integrated zone systems do?
Does the zone control have direct control of the blower motor, similar to the Carrier and Trane systems?
As for the 3rd party zone controls, If you have a system that has close to a 50%/100% capacity split between 1st and 2nd stage, and only 2 zones that are close to the same size, you can still zone without using any bypass and without oversizing any ductwork. If one zone is a bit bigger than the other, you could still get by without bypass or dump zones, but you would have to oversize the ductwork to the smaller zone, or set the damper for the larger zone so it never fully closes.
I have done a number of zoning retrofits on single stage systems in 2 story houses where it was not practical to add bypass because of the equipment location in an upstairs closet. I set a minimum open position on each damper so the equipment still has the minimum required airflow when only one zone is calling. I have had to set dampers to as little as 15% open, and to as much as 50% open. Its not perfect, but everyone I have done it for sure was happy to finally have relatively independent temperature control of the upstairs and downstairs of the house. The one where I had to set the dampers to 50% minimum open is funny. In the summer, the downstairs thermostat almost never calls for cooling, and in the winter, the upstairs almost never calls for heat, yet the temperature differential is never more than 2 or 3 degrees. Before adding the zone system, he would typically see an 8-10 degree differential between upstairs and downstairs, lol.
01-24-2007, 04:17 PM
As for bypass for equipment with a standard blower or a VS blower, I really like motorized bypass dampers that have a control that modulates the damper position based on the air pressure in the supply plenum, but I only use them if the system has a standard blower.
What I have seen with a motorized bypass damper on a VS system is that the VS blower and the bypass fight each other.
As an example, lets say the system is single stage cooling and has a VS blower. The system has 2 zones, and both zones are calling.
One zone gets satisfied and its zone damper begins closing.
As the zone damper closes, the static pressure in the supply goes up, resulting in a higher ESP through the furnace. In response, the VS blower starts to ramp up, and at the same time the motorized bypass damper starts to open.
Sounds good so far, but there is a hitch.
The VS blower responds to changes in the static pressure MUCH faster than the motorized bypass damper, so you end up with the two reacting to pressure changes out of sync with each other, so they sit there constantly over correcting. The damper continuously modulates from opening to closing, and the VS blower continuously ramps up and down, so you end up with VS module and damper motor failures.
If there are only 2 zones, you can get by with slightly undersizing the motorized bypass damper, but with 3 or more zones, you can't do that because you need a bypass that is big enough for when only one zone is calling, which makes it oversized when 2 zones are calling.
With VS equipment, just use a weighted arm type bypass. If it is set up properly, it responds at the same rate that the VS motor ramps up and down, so the VS motor does little if any hunting before it stabilizes.
As for sizing bypass ducts/dampers, use the zone control manufacturers instructions. Do not use normal duct sizing methods to size bypass ducts! Doing so will result in a massively over sized bypass.
I use EWC controls when I use a 3rd party zone system. They have some easy duct and bypass sizing instructions.
01-24-2007, 10:59 PM
What you guys are saying makes sense. This thread already has more information regarding the subject than ACCA books.
Seems like Carrier and Trane figured it out with the help of zoning dampers that control the blower.
Currently I'm working with Goodman for this project and it's a VS system. The 2 zones are split something like 40% and 60%. Looks like I will install a damper that never fully closes on the 60% zone. Let's say I set it at 20% open. The smaller zone will carry about 5% more air, because of the higher pressure. The VS system will drop about 20% from its total output and will operate at 80%. My duct work with the zones can handle at this point: 40% + 20% + 5% = 65%. I am short, but I can either increase the minimum open position on the bigger zone from 20% to 35% or I can install a weighted arm type bypass that will handle about 20% of air and I'm even.
Looks like having all those controls installed helps, because all you have to do is to balance the system at the end.
For my next zoning projects it's possible I will just buy Trane systems.
01-24-2007, 11:19 PM
Wow, I just read the info on the link (http://www.ewccontrols.com/ultrazone/duct_sizing.htm) you gave me and something came up:
"Some Zoning systems call for bypassing the air into a zoned area, this will NOT allow for maximum comfort. This method allows the bypassed air to over shoot the thermostats temperature set point. What this type of system does is it will leave the zone dampers partially open when they should be fully closed, allowing for the bypass air to flow into an already satisfied zone. This will negate the effect of zoning and drastically reduce the comfort level, and savings potential of the home."
This means you can not leave the bigger (closed) zone minimum open, becuase that means you are actually not zoning and you're throwing everything off. In this case I prefer the weighted arm type bypass solution better.
01-24-2007, 11:38 PM
Yeah, I only do the thing with the minimum damper positions when installing a bypass duct or or dump duct isn't practical, and I'm retrofitting zoning or replacing a failed zone system on an existing single stage system. I see a good sized number of 2 story homes with 1 system that is in an upstairs closet with nowhere to run a bypass, and no good place to run a dump zone.
The effect they talk about isn't that big of a deal most of the time in a 2 story house with the zoning split between upstairs and downstairs. Its not even really a problem if both zones are the same size.
The times when having dampers partly open all the time would cause problems is mostly with 3 or more zones. It would be real easy to overheat or overcool one of the zones if if the other 2 are calling a lot.
The Trane and Carrier systems, and possibly the Lennox system, get around that problem by constantly monitoring the actual and set temperatures in each zone and modulating the damper positions to redirect airflow, adjusting airflow, and with multistage equipment, adjusting staging. None of the 3rd party systems even come close.
I like EWC controls, but a lot of their marketing hype is just BS, but that is true with most products of any kind. ;)
01-24-2007, 11:47 PM
Looks like I'm getting somewhere. How do you know what position to set the weight driven bypass damper? Do you actually measure the CFM in the open zone when the other zone is closed and you keep adjusting until you get it or you visually watch the damper until it open full giving you a known CFM value at that point? I have never done this so I'm speculating here.
01-25-2007, 12:38 AM
I aree with Marks reply concerning carrier, lennox, tranes zoning systems. When i adjsut a bypass duct or barometric relief duct, usually i will run all the zones open and fan on high and adjsut the weights on the damper until its teetering or until when you barely touch it will want to open. This should ensure when you close a zone the added static pressure should open the damper. Sometimes you have to play with the weights a little bit before it is adjsuted correctly. Hope this helps.
01-25-2007, 01:43 AM
When I adjust a bypass, I set my starting position like partonorder mentioned. I'll mark the the damper arm at that point. If I sized the damper correctly, and the duct system is sized correctly, I shouldn't need to move the weight any closer to the pivot point on the next step.
Once the starting position is set, I'll adjust zone calls and equipment staging for the available control scenario that causes the system to have the most excess air to deal with. On a single stage system, this would be in the cooling mode with only the smallest zone calling, but it can vary depending on the system. Like if I have a 2 stage cooling system serving 3 zones, and have the controls configured so that 2nd stage can only come on if at least 2 zones are calling, the mode where I have the most excess air to deal with may be in 2nd stage cooling with the 2 smallest zones calling. I rarely have 2 systems set up exactly the same way because I tailor the operation of each system to the particular situation rather than having one particular setup that I try to apply to everything like some of the companies I often am cleaning up after do. :)
I'll then check the ESP of the indoor unit to make sure I'm still inside the fan curve and have a minimum of 350 CFM/ton.
If I'm enough above the minimum airflow + some cushion to allow for the air filters loading up, I'll go listen to all the supply registers for that zone. If I can't hear any air noise, I'll go back and move the weight on the bypass damper out from the pivot point a little to increase the supply static pressure. and recheck the ESP and noise level.
My goals are to move the highest amount of air into the zone that is calling, maintain at least 350 CFM/ton of airflow, leave enough of a cushion in the fan curve to allow for the filter to get loaded without dropping the air flow below 350 CFM/ton, and not create objectionable air noise from the supply registers.
Once that is set, I'll cycle the system through every possible combination of zones calling and staging, while watching the ESP to make sure at least 350 CFM/ton is maintained throughout.
This is all assuming the duct system and bypass are reasonably correctly sized. It is a real bear adjusting an oversized bypass, extra weights are needed. If the ductwork and/or bypass are undersized, it gets more complicated because you have to start messing with combinations of weight position and minimum zone damper settings to get it all working as well as possible. In cases like that, you have to insure the equipment always has at least 350 CFM/ton moving through it, even if you end up with a little air noise, or more air going to zones that are not calling than you would like.
Just try to avoid air noise in a master bedroom, or moma is gona make everyone unhappy!
If you hadn't guessed already, when you price the installation of the zone system, you need to include a few extra hours for adjusting the system on startup, and 1 or 2 return visits to make minor adjustment and/or reeducate the customer.
If the customer is an engineer, plan on at least 5 return visits and/or long phone conversations. Maybe 10, plus extra time going over things again during every PM visit, if they are a retired engineer! :D
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