ERV/HRV Duct Tubing Question

jb9

Hello,

I have been looking at Zehnder's ComfoTube duct tubing because it is only 3" in diameter. Are there other options for tubing that have a similar lower profile? I have limited wall cavity space and am curious as to what products are out there. Can other tubing be used with different brands? Fantech, etc...

Thanks.

Zman

It really has nothing to do with brands and everything to do with how much air you are trying to move over what length with what type of fan.
Many more details are needed.

Gordy

Inoflex. Pricey though.

jb9

Thanks Gordy and Zman. I agree that I need to get the duct run lengths and the cfm required... I will get that shortly and report back.

jb9

Ok. After running some numbers around my heat loss and natural infiltration of the building envelope, it looks like I am going to size my ERV at 130cfm capacity for the house and 38cfm for the 2nd floor (3 bedrooms). The lengths of 3" flex tubing I am probably going to need to get to my 2nd floor locations will be around 20-25 feet. Does this sound reasonable? I am not sure how the total cfm capacity of a unit gets reduced as it gets distributed to its branches. For example, if a unit has a capacity of 100cfm and it goes through a junction box with 2 flex tubes, does each tube push 50cfm?

Zman

The amount of air you can push through a duct that small will vary greatly depending on the design of the ERV. Some are designed to just inject air into an existing system, others are better suited for the kind of distribution you are attempting.
I might make sense to contact the manufacture of you choice and get some recommendations.

Gordy

To say 100 cfm to a distribution box, and then each tube getting 50 cfm would depend greatly on the tube lengths, and how many bends are in each run.

Jamie Hall

The air from the distribution box will distribute according to the flow resistance in each output side, and it varies more or less according to the square of the resistance -- so even a relatively small difference will result in a significant difference in flow.

My concern is velocity. Unless my algebra is lying -- which is quite possible -- you are looking at about 13 feet per second to get 38 cfm in a 3 inch duct. That's moving. You will need some blower power to do that, and you may have some noise problems.

jb9

Ok. I have now spoken with the Zehnder folks twice and I have a better understanding of how their system works and is properly commissioned. Each 3" Comfotube will drive about 12 cfm of air. For example, if a bedroom room needs 24 cfm, it will need two Comfotubes to achieve the 24 cfm. There are register boxes that accept 1 (12), 2 (24) or 3 (36) Comfotubes.

Here are my rooms and the strategy I am considering:

2nd Floor:
3 Bedrooms (2 Comfotubes each at (12 x 2) 24 cfm) SUPPLY
2 Bathrooms (2 Comfotubes each at (12 x 2) 24 cfm) RETURN

1st Floor (Open floor plan):
1 half Bathroom (2 Comfotubes at 24 cfm) RETURN
Kitchen (2 Comfotubes at (12 x 2) 24 cfm) RETURN
Living Room (4 Comfotubes distributed in 2 registers) SUPPLY
2 more Comfotubes at 24 cfm elsewhere.

Basically, I will use a 20 Tube Manifold that need to be distributed throughout the house. 10 on the supply side and 10 on the return side. Hopefully, I can find enough wall space.

hvacfreak2

Probably best to allow for some redundancy for this critical application and go with 40 tubes. 20 tubes of 3 inch flex good grief man , you taking this to the moon or something ?

jb9

That's interesting. Do you think the manufacturer is suggesting something excessive? I'm just trying to find an ERV that will work for my requirement of ~90cfm and this is what they told me. Do you have any suggestions on how to approach this based on my limited wall space for ductwork?

Gordy

Do you actually need to move 90cfm?

If you just asked how to move that much that's probably how you have to do it with their distribution.

Zman

Duct work is sized just like piping.
You determine how much resistance the fan is designed to overcome at the flow rate you desire. You then size the duct work based on this resistance and flow. How far you are going and how many turns you are making are huge factors.
The ERV manufacture publishes the fan specs.
The duct manufacture publishes the resistance info.

hvacfreak2

See attached , from ASHRAE site.

I would put 50 cfm ( 4 or 5 inch ) into that fan coil you mentioned in another post. 50 cfm of treated air isn't asking a lot for a small ducted unit ( looked like that was serving bedrooms upstairs from memory 018 or 024 ). You will need to construct a box for the return grille , make it taller to accept the oa connection ( it's now a mixing box ) . That leaves 45 cfm of the 95 total mentioned. Run a 5 or 6 inch round down to the basement for the first floor strategy ( 2 - 3 inch outlets ). I would control the eru based on demand ( co2 ) and have it scheduled for occupied times only. I would also have it control to off when outside conditions exceed design numbers.

Or a 20 or 40 3 inch round flexopus kraken lurking in the attic. Either way best of luck with your project.

jb9

Thanks hvacfreak. I actually like the idea of supplying outside air into the return of the ducted mini split. Most folks advise against it but it kinda makes more sense to me. I might try and fabricate some register boxes that can accept tubing from the ERV and the ducted mini split. I will try and make some drawings to show what I mean. Neither the Mini Split mfgs nor the ERV mfgs want their systems to be integrated. Most of the ERV mfgs don't sell duct work components either. I haven't decided what makes the most sense but I think I am getting closer. Also, what is the "20 or 40 3 inch round flexopus kraken" you are referring to? I tried to google that product and couldn't find anything.

Zman

"20 or 40 3 inch round flexopus kraken" I think this is a mythological creature .

If you are doing a ducted mini split anyway, you should absolutely tie the ERV into it.
Pretty much all commercial ERV's are done this way.

jb9

Thanks Zman. Another vote for integrating the 2 systems. To me, it seems like a intuitive approach. I may just tie into the endpoint register boxes...

jb9

Ok. I wanted to attach a photo and actually share something with the forum (since I am always asking questions of you kind folks). Anyhow, here are the register box offerings from Zehnder. You can see there's a triple, a double and a single. The heads can be simple grates or diffuser heads (used for commissioning). There are also in-line diffusers that can be put inside the Comfotubes (I saw them at a trade show). I thought this might help someone out who hasn't seen the Zehnder system. I do like the low profile of the tubing as I stated earlier. Also, now that I could display the register boxes, I actually think it might work to combine the supply registers with the MSPs. All I would need to do is make a register box with 3 knockouts (2 for the ERV and one for the MSP). I think this might actually be a good solution. Fabricating a register box is fairly standard operating procedure in the HVAC world, yes? Again, I hope this information is helpful for folks who don't know what the Zehnder components look like.