Miele downdraft airflow issue

8 years ago

We just installed a Miele 30" downdraft to go with our 30 Miele induction cooktop. The installation is in an island, and for a variety of reasons that was our only option. I understand that downdrafts cannot be as effective as hood vents, but the airflow from our Miele downdraft is very low.

The set up is that the 10" duct comes out the front of the downdraft into a 90 degree elbow into the crawl space. There it makes an another 90 degree elbow for a straight run of about 9 feet to the exterior vent. Our contractor installed a 700 CFM in line blower in the crawlspace near the exterior vent. He tested the airflow at the exterior vent and got about 300 cfm on low, 500 cfm on medium and about 700 cfm on high. He was using an airflow meter that attached to his smart phone. I'm not sure how accurate those are. While I could feel the airflow at the exterior vent, it did not seem like as much as 700 cfm, but I don't have a reference.

At the downdraft itself, the airflow is minimal even on high and the "boost" setting. On high there is barely enough air to pull in a strip of paper towel held close to the inlet. I boiled water in two pots, one on a front burner and one on the back burner. Little to no steam from the front burner went into the vent, a little of the steam from the back pot went into the vent.

I am having a hard time reconciling the cfm readings at the exterior vent, which seem very reasonable, with the lack of airflow inside. The contractor says all the connections are taped with no airleaks, but I have not been able to check that myself.

Any suggestions? We can install a higher cfm blower, but I want to make sure we are not missing something else that is causing the low airflow.

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Comments (8)

zneret
8 years ago
Are there two motors, one at the downdraft, and another inline, or just the inline?

If there's a motor for the downdraft, I'd suggest removing the inline and see how that goes. The setup in my house is a Faber Scirocco with 26''+ of 6" ducting, and 2 degree bends and it vents a lot, just not effectively.

If there's only one fan, I would disconnect upstream and see what suction is like.

Before all that, perhaps check the fan specs and see what its intended application is for.
Joseph Catlett
Original Author
8 years ago
There is only one motor, the inline blower. The downdraft controls the speed, but does not include a motor.
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zneret
8 years ago
last modified: 8 years ago
With the fan on full, does the air flowing out feel strong as you put your had against the vent outlet? It should feel "3-4x" what a bath fan is pushing out.

If so, then air is coming in somewhere but not at the head. If you are sure of no leaks (you will have to go into the crawlspace and check it for yourself), I'd disconnect the duct at or near the downdraft (easiest at an elbow - just need a screwdriver and something to cut the tape) and see how strong suction is at that point. If it's strong, keep working your way upstream to check for blockages.
kaseki
8 years ago
last modified: 8 years ago
A blower rated for 700 CFM at zero static pressure (where they are usually rated) cannot flow 700 CFM at the end of a duct run with a downdraft device on the other end. Further, the air velocity at the duct exit should vary across the exit aperture, so any estimate of flow rate there would require multiple measurements and an integration over the aperture area.

What is the total area of the downdraft assembly entry aperture(s) (all of the openings that the downdraft unit can pull air through) that are exposed above the cooktop?

This looks like a lot less area than a 10-inch circular duct has. The mesh filters that appear to be in the openings will have more restriction than implied by their blockage fraction.
cabinathome
5 years ago
Jkjjcc - I am planning to get the Miele 30 induction and 30 inch downdraft DA 6881. Is that the one you have? It sounds like the blower for your Miele downdraft was installed in the crawlspace, so it runs more quietly and you have more cabinet space below the cooktop. That's what I'd like to do. However, the Miele manual shows their DAG600 can only be installed at the front or the back of the downdraft, which means it's in the cabinet below the cooktop if on the backside it then will infringe on the legroom for seating at the island. It looks like you got a different blower instead of the Miele one. Has everything worked out for you? Otherwise, I am thinking to get the Best Cattura downdraft. Thanks much.
Greet Heremans
4 years ago
cabinathome - I am in a similar situation thinking about getting the Miele 30" induction and downdraft. Would also like to install the blower in the crawlspace. What did you end up doing?
cabinathome
4 years ago
last modified: 4 years ago
The plan is to install the DA600 blower at the back of the DA6881 downdraft. The blower can't go into the crawl space. The duct attached to the bottom of the blower will then go down through the floor and into the crawl space and make one turn, and then a long run out. CFM should be fine. To get as much counter space at the back of the island, I am placing the downdraft flush to the induction cooktop, so no gap between. I also increased my counter to 48 from 42 inches. I should have 10-12 inches unobstructed overhang for seating at the counter. I did investigate the Cattura downdraft system where there is an attachment that lets the blower to sit in the crawlspace. So there is that option. I just didn't like how it looked with the Miele induction cooktop.
Greet Heremans
4 years ago
Fantastic. Thank you so much!