Insufficient Supply Air on Second Floor
Just wrapping up building a high performance house in Zone 7 Northern Wisconsin. I have a Mitsubishi 2.5 ton heat pump and air handler ducted through the house along with a Renewaire EVL Premium with dedicated exhaust and supply via the ductwork in the house.
The HVAC contractor did a pre-drywall test back in December and most of the ductwork was completed, though he hasn’t followed the plan we had designed exactly. At that time, he was mainly checking to see if there was enough supply to the 2nd floor since that was the only ductwork that would be inaccessible after drywall. He said there was more than enough supply so we closed it in.
The months passed and we finished the house and he finished the ductwork the rest of the way, including adding the zone damper that is controlled by the 2nd floor thermostat. He started his final test and the whole house was okay as far as CFM supply except the 2nd floor is very low. He wants to pull the zone damper back out since there is enough supply without it.
What should I do?
– Should I let him pull the zone damper out and have no control over the upstairs other than the floor registers and the balancing dampers (which I can access through light holes)?
– Should I open up drywall and redo some of the ductwork?
– Can we dial back the rest of the balancing dampers to allow more air to get to the 2nd floor?
I attached a drawing of the 2nd floor ductwork and then drew in red how the ductwork was actually done.
Lastly, and secondary to my other question, the whole system seems way too loud. It sounds like the bathroom exhaust fan is on from anywhere in the house. The system was supposed to be very quiet and I am wondering what could be causing this? Ductwork with too many elbows and turns? It does seem that they had a lot more funny ductwork than there should’ve been.
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Just to be 100% clear the contractor has a manual J calculated number of CFM for each room and the duct work he designed to deliver those CFMs is failing to meet that number when he measures the CFMs in each room? Or was this system designed on the back of a napkin without the use arithmetic and now when he puts his hand over the vent, he feels there is not enough air movement?
Seems to me pulling the damper will have a very small affect on air flow as most dampers fully open don’t restrict the flow much. I have to ask is this a way to lower his costs, if he can return the damper not buy the zoning controls and second thermostat?
Walta
Yes, a Manual J was done. The system was designed by a mechanical design firm where the ductwork was very specific and the CFM of each room also specific. I have been pushing them to follow the design religiously (technically he bid the design as planned by design firm) and there has been some pushback on that (that's overkill, that's not how we do things around here, for example evidently no one uses balancing dampers around here). I have found that our rural area is fraught with contractors that haven't done anything different in decades. I've tried to be both accommodating but also keep them as close to the design as possible.
He did a pre-sheetrock test of each of the registers with a flow hood and confirmed that he had enough CFM to the 2nd floor so we could proceed with sheetrock (but they had more work to do). Everything else was still accessible in the basement. The only change to the 2nd floor trunk was the addition of the zone damper. There have been a bunch of other changes throughout the rest of the house though. They didn't follow the design perfectly, but they also aren't super far off.
They also were having trouble getting enough airflow in other places when I keep the Merv 11-13 2" filter on the main return in.
I wouldn't expect that it is an effort to lower his cost. He is trying hard to follow through on the design as it was bid and designed. I think moreso he's just thinking that the zone damper is the only thing that changed for the 2nd floor trunk so why would we now not be getting enough airflow.
I can upload images of the rest of the design or the Manual J if that would be helpful.
Thank you very much!
A zone damper in open position might add 15' to 20' of equivalent length. This amount of pressure is noise on most HVAC systems. About the only time it can create a lot of restriction is if it is placed closed to elbows or T where there is a lot of turbulence.
It could also be the damper is not adjusted properly in the open position, should be easy to check.
Noise could be from a number of things.
First I would try pulling the registers and see if the noise goes away. Lot of decorative grills tend to be fairly restrictive, if this is the case, selecting a low loss ones would help.
Ducts and fittings can add noise especially if the velocity is high. Hard to see in the picture what size the ducts, generally anything under 500FPM is pretty quiet.
The installers might have cranked the air handler up to the max to get proper flow. There is significant sound difference between the air handler at low and high. Turning it to the lowest setting that will still provide enough cooling and heating can help.
It might be worth while to do a couple of pressure check through the entire ducting and see where your losses are. I have seen where a single fitting was responsible for about 1/2 the pressure loss in the entire system.
I wouldn't worry too much about filters if they were sized properly. For 3M filters, there is very little pressure difference between a merv13 and merv8 filter.
Hi Akos,
Thanks!
The damper is indeed in the open position
I attached new pictures so you can see ducts and CFM. They followed the plan as far as duct sizing goes on the second floor since that was going to be closed in by sheetrock. They did have some longer lengths though. I drew in red the route they ended up actually taking. Not sure that additional length would make that much of a difference.
I attached a picture of the zone damper since it is near some elbows are turns in the ductwork. Thoughts there?
On a related note, if the zone damper is closed because the second floor doesn't have any heating or cooling demand, how does the second floor get fresh air since it is through the same ductwork? Or does it not for those periods...
I will look into some of the sound issues. Step one is probably to understand if it is the air handler air or the ERV. It appears that the bathroom switches are stuck on "boost" and may be at a much higher volume permanently...
My thought is that from a supply balancing perspective, the easiest place to start would be scaling back anywhere that has above the needed supply. The contractor said they have "plenty" everywhere else but if they have more than the needed supply, they're going to have trouble getting enough supply to the distance places, if I am thinking of this correctly...
Extra length rarely matters unless through unconditioned space. Most of the restriction in ducts is generally from fittings and elbows, a couple extra feet of ducting barely budges the friction losses.
The zone damper is not in a great spot, there is a lot going on around it. Not terrible either though. Depending on which way the blades close, I would not be surprised if you get a bit more flow out of it if the blades are angled to match the offset fittings. It looks like there is a also manual damper just before it, that is redundant and can be take out.
If the noise is indeed the ERV and you can hear it on boost in the bedrooms, I would add either a length of flex between the ERV and the ducting or an inline duct silencer (ie Fantech LD6/8). Hard piping is great for air flow but can carry a lot of noise from a blower.
"have more than the needed supply, they're going to have trouble getting enough supply to the distance places, if I am thinking of this correctly"
Your absolutely correct. First step in balancing a system is to restrict the lower loss branches. In a low load home getting perfect balance is not critical, so as long as it is in the ballpark, I would not sweat chasing exact CFMs. Since an adjustment in one area changes the flow in other areas, the process is iterative and you can spend a lot of time chasing your tail for minimal gains.
Since the fresh air is supplied through your heating ducts, the zone damper closed position needs to be adjust so that some minimal airflow is still supplied. Each room should still get enough CFM for about 1 air change per hour (~20cfm for a bedroom). If this causes overheating in the winter time, you can look at partially blocking the return on the main floor so more air is pulled from the 2nd floor return. This would increase mixing in the house and reduce stratification a bit.
Excessive noise is usually due to excessive velocity, which can sometimes mean undersized ducts, too much restriction (like the damper), or at fittings/transitions. You may be able to improve things with turning vanes at transitions, depending on the design. You could potentially boost output in one zone by closing dampers in others, but you could mess up your balance doing that, and you might increase noise. It's probably the easiest/cheapest option if you already have dampers everywhere though.
The contractor should follow the design. If they think it's dumb, they can talk about how dumb it is with their crew as they build it to spec. As long as they bid the design, they should build the design. They get paid either way. You should hold them to the design they were contracted to build.
Bill
Hey Bill,
Thanks for your response.
The parts that are behind sheetrock, they did follow the design pretty closely because I made them. It was one of those things where I was gone for a couple days and they finished a lot of the ductwork and they didn't follow it to a T. I am trying to hold them to the design but only where it is clear we need to make adjustments.
One of the things they did was use 2x10 registers everywhere instead of the plethora of different sizes that were spec'd. Another would be that the design specs trunk reducing in size as they get further away and they have trunks remain large until the end branches.
It seems that the noise really is coming from the ERV exhaust. It may even be stuck on "boost mode" since all of the bathroom switches are showing as on all the time. So, that's the place I need to figure out first but I'll let the contractor figure out that one.
So, the supply issue to the 2nd floor bedrooms is my main concern. My thought is we start by balancing all the dampers in the basement and main to be no higher than the required CFM to make sure we're pushing all air possible to the more distant places and maybe that will get us closer.
If we do find that removing the zone damper gets us the supply needed, what are the concerns there? The 2nd floor overheating or not cooling enough? Would just tweaking the registers or the balancing damper on the 2nd floor trunk help to adjust that?
I am also wondering if we'll even have fresh air supply when the zone damper is closed since that is supplied through the same ductwork. That could be a concern during the winter since the 2nd floor will probably heat easier due to heat rising.
The reason to taper the main duct is to maintain ~constant velocity in the duct. If the duct is way too large for the flow in CFM, you get very low velocity and that can mess up the flow. This is actually a problem that comes up often in data centers that use raised floor, since the underfloor area acts as one massive "duct". The "massive duct" allows for lots of flexibility since all the airflow is available everywhere, but the efficiency of the system suffers. We run high static pressures on those systems to compensate, but you probably can't do that in a residential ducted system.
Removing the zone damper just turns your system into one big zone. I don't think that's a major problem, but it will mean you'll have to do more manual damper adjustments when changing between heating and cooling seasons. This isn't too big a deal though (I do it myself in my own home), it just becomes "one more thing" in the spring and fall that you have to remember to do.
Bill
The addition of a damper being the cause of flow restriction does not make sense.
Also, what kind of difference in CFM from pretest to finished system are we talking about here? Are you short something like minor like 10 CFM from the numbers on the plan or is there a major restriction?
If it's major, like there is a blockage in the duct somewhere, I suggest troubleshooting some more. For instance, is the damper installed to the motor properly? (ie just cause the indicator says open, does not necessarily mean its actually or 100% open). After that maybe get a cheap cellphone based camera scope and start scoping your ducts, you never know if rags or something got stuffed in there (maybe for drywall dust) and then got missed and left in.
Tangent Rant:
Welcome to wild world of HVAC. Its an amazingly complicated field and a lot for one person like a contractor to truly "know everything" because it's so broad. HVAC still drives me nuts because its 2022 and we still have hot messes in houses. HVAC is also very reliant on the framing. In my opinion, HVAC design and architecture should be done hand in hand, like literally in the same office or the same person! Moreover, we need smaller systems so we can have seperate systems on each level of a house vs getting into damper shenanigans to try to control comfort. minisplits suck for many small rooms, like bedrooms. Ducted systems are still the best for properly distributed airflow and filtering. Ducted minisplits still have a long way to go, and with their low pressure squirrel cage blowers, it gets impractical, give us a real blower in a compact solution. hrvs/ervs are expensive and are shit, they need to be cheaper and smaller because houses need multiples of them instead of one 100cfm unit with 4" ducts for an entire house trying to serve multiple functions.
I could go on. I'll stop. Thank you for letting me write this.
Jamie
It might be interesting to see the numbers with the zone damper removed It seems very unlikely to change the number much if at all.
Sound like someone is grasping at straws, to my ear.
Was the pre drywall test preformed with the same fan as the post drywall test?
Walta
Thinking about this zone damper made me think of one possibility: has anyone checked to see if it's actually opening all the way? There is always the possibility that the mechanism is messed up, and the damper is only partially open even though it's supposed to be (and might even indicate it being) in the full open position. A partially shut damper that erroneously claims to be open might explain the problem here, and it would could be contributing to the noise problem too.
Just a thought.
Bill
The HVAC crew came out today and pulled out the zone damper and, after seeing it, I can see how it would restrict flow. It takes an 8x10 duct and easily turns it into a 9x7 without even considering the louvres. They're going to size up that elbow and where it takes off and get a larger zone damper to ensure that they're allowing for the full airflow. Sounds like they'll be good for it. They're confident that they'll get enough airflow once that is done, I guess I'll follow their lead...
To be continued...
From a sound perspective, it seems that the ERV system is stuck on "boost mode," which is triggered by bathroom switches for exhaust. So, the ERV is running full tilt. They couldn't figure out why this would be and the green lights are stuck on on the boost switches too.
If you disconnect the switches, the ERV will turn down to low, then, if you reconnect a switch it stays on low until you press the button, once you hit the button once, it is stuck on boost permanently.
Any ideas on that one? They couldn't get ahold of proper support to troubleshoot that one.