I am not an expert, but just went through the painful process of trying to figure out a complicated ventilation system in my house. We are stuck using a downdraft for a variety of reasons. We have two 90 degree turns from under the island to out of the house, and are using a 1,000CFM in line-blower between the two turns in addition to the downdraft itself.

In your situation, was it feasible to run new ducts out of the house through the ceiling? That would be more expensive but would almost certainly provide much better ventilation. If you are going to stick with the existing ductwork, I would advise looking into the installation of an in-line blower to supplement. Mine will be operational next week.....

Weather the ducts go up, down or sideways, is not the issue. Size of duct, length, and bends all play a role in determining CFM capabilities. As long as your duct meets the parameters of your blower, you're fine, but I wouldn't just take your installers word for it. There are many charts graphs available to calculate your requirements, or you might call an HVAC contractor for some clarification.

How about calling the appliance mfg., who probably employed a design enginer at some point in time.
And tell them all the details, size, degrees of change in direction, distances to and between them as well as overhaul length.

Reading the subsequent posts, I should add that we paid for a technical consultation regarding the downdraft and blower requirements. It's so specialized. A good appliance store can probably recommend someone if you are unsure that your installer is steering you in the right direction.

You really should post this question on the Appliances Forum. They love to talk about ventilation there, and there are several experts who frequently post there.

I strongly disagree with Ctycdm that the direction of the duct is not an issue, though I strongly agree with him/her that size of duct, length and bends are very important. Grease, smoke and heat rise UP. You are working against physics by trying to force them to move in a downwards direction. That is one reason why downdrafts work so poorly in comparison with proper hoods that exhaust upwards.

I also do not understand why your hood's ductwork was installed without any input from you. Who are the "installers" you mentioned? Are they part of the construction crew for your kitchen reno? If so, was there any discussion in the Scope of Work regarding the hood installation?

Here is a link that might be useful: Gardenweb's Appliances Forum

This post was edited by shannonplus2 on Thu, Jan 17, 13 at 12:44

We have the exact same situation in our house: existing downdraft and the desire to install a traditional hood set up (mostly to free up our options for range replacement in the future). Originally we thought about doing just what you had done�hooking the vent up to the existing ductwork--but after reading what they had to say about that in the aforementioned appliances forum, we decided to nix that idea. And I agree with Shannon; from what I've read from the ventilation experts in the appliances forum, you will lose most of your ventilation ability by turning all those corners and running the vent down.

I fear though, that since the ventilation is already done, that your options may be limited to very expensive solutions at this point. Also, I'm thinking that 490cfm might be a little low for all those bends. We're going to do a minimum of a 600cfm vent to accommodate a necessary 90-degree bend to vent outside.

Wi-sailorgirl, re your point about a solution being expensive - I should have mentioned that the reason I asked my question in the last paragraph of my post is because if the ventilation was discussed in the Scope of Work, but the installers connected the hood to the downdraft against the OP's wishes, they'll have to correct it for free. The OP says "we wanted a hood that goes up and out of the house. However, our installers decided to install the pipes downward to connect to the existing downdraft pipes". It's unclear whether the OP stipulated what she wanted to the installers, but if she did, and the installers didn't do what they were supposed to, they must correct the installation for free. And of course the installers are going to say the downdraft connections are correct, cause they won't want to have to go back and do it right.

These installers are contractors for the big orange box (HD) we are all familiar with. We thought going with a big name and reputation would serve us better should something go wrong. But it has been one long headache after another.

During the design phase, we did tell the kitchen designer at HD that we want to change the existing downdraft to a venting system that goes up and out of the house. We even discussed it with the representative of the installation company that works with the kitchen designer to answer any installation questions and then provide the installation costs. However, when the group assigned to our house came, they installed a venting system that would just connect to the existing downdraft.

We now realize that they had no knowledge of the venting system that was supposed to be installed. That was a big communication failure on their part. So they are reluctantly changing the venting system to go up and out as it should have been, with the installer insisting the venting is installed correctly.

We could have avoided the unnecessary and unpleasant dispute that seems to follow with trying to get any mistake or wrong corrected throughout this painful kitchen remodeling. Everyone involved in this remodeling seems so reluctant to make any changes and deal with any issues that come up. Terrible customer service.

But thanks fo all the input. This forum is great!
Oh, thanks shannonplus2 for directing me to the appliance forum. I didn't know it existed.

It matters a whole lot if the duct is going up or down.

Which is why a lot of the old downdrafts actually have a nasty greasy pond of water in them, blocking airflow, when they are opened up.

Steam condenses and forms moisture. The longer the run, the more deviation from straight up, the less insulation it has, and the more that the duct travels through non conditioned space, the more chances of water vapor turning to liquid water, with assorted nasty grease and smoke additives. This is just reason #3451 why downdrafts are horrible.

you are missing the point. Downdraft venting is terrible. The fact that an updraft (hood) works better than a downdraft is because hot steam raises over the cooktop. We know that. Here we are talking hood. Once in the hood, it does not matter if the hood duct goes down or up. Only number of elbows and length matter. You need one elbow to go horizontal in the attic. One extra elbow to go down. If you limit it to two elbows you are fine. A hood duct that goes straight down to the crawl space and then straight and horizontally to the outside (2 elbows) works much better than a hood duct that does 4 elbows in a messy crawded attic before going outside. A straight attic duct (1 elbow) works slightly better than a crawl space duct that has 2 elbows from hood to outside. We were talking hood here, with duct that goes in the crawl space joining the downdraft duct. Forget about the downdraft venting. Vent is on the hood. We are talking hood.

The directionality, and route of the duct IS important. And yes, we’re talking a hood. Going up and down is terrible for venting. Adding any 90 degree bends is terrible. You are not understanding turbulence and velocity loss that occurs when things like that occur. More fan cant make up for a terrible duct route. More fan cant make an up down higgledy piggledy terrible duct route be as efficient as a straight 0 bend through the roof.

https://www.yumpu.com/en/document/read/46472115/kitchen-ventilation-systems-application-design-guide-greenheck

You are repeating what I said, bends and length are important. But THAT IS IT. I tell you one more time, dont change the logic. Going Up and down and zig zag is bad only because of bends and longer length. If you go from the hood entrance to the exterior wall, it does not matter if you go up or down, as long as the number of bends and the length are the same you have the same dissipation. What matters is the length and # of bends (of course shape of bends matters as well but we are assuming the same type of bends). If going down from the hood to connect to the remnant duct of the downdraft gives a shorter length and fewer bends than over the attic (where sometimes there are constraints that make you zig-zag), then the crawl space is the best choice. I am an Engineer and I run turbulent CFD (Computational Fluid Dynamics) simulations daily.

....I am replacing a 36 in GE Monogram downdraft in granite counter tops. My current one is 20 years old, no problems with it but replacing the glass cook top in new counter tops so replacing the downdraft at the same time. We just realized that the vent hook up in the wall is positioned higher on the old one than the Universal replacement venting system, where it connects into the wall higher up with the old one, the new one goes lower down before connecting to wall venting...can the connecting vents coming out of the wall be modified to use the new system?

It is ok to have bends and a downdraft for your overhead vent.

Also note, that I lived in a house with just a microwave over the oven for years! No vent fan at all and it was ok.

I also owned a house with a regular downdraft stove. A lot of people use them and are happy with them.

There is NO problem. A standard fan will pull all of your smoke through your new hood and down and out of your existing pipe. If a little condensation happens after your thanksgiving dinner. You will be fine as it will quickly evaporate. (Like it always did with your downdraft oven).