Thermadoor pop up down draft vent won't run - Thermador UCV36A 36" Universal Cook 'n' Vent Downdraft Kitchen Hood

Generally 40 feet for ordinary atmospheric-vented household water heater...
Straight run to roof.
Atmospheric means no power vent... just hot air drafting upwards
Google search: gas vent height code

Atmospheric venting code depends on 4 basic factors:
Diameter of vent--- bigger vent, more draft
How many water heaters are connected to same vent
BTU per hour rating of water heater-- the more gas that is burned per hour, the bigger the vent needs to be
How straight the pipe is -- are there any horizontal or lateral runs
http://www.ameri-vent.com/files/assets/files/1366372183_Sizing26ApplicationGuide1.pdf

http://waterheatertimer.org/pdf/Bosch_Checking_the_draft.pdf
http://waterheatertimer.org/pdf/checking-the-draft2.pdf

Other factors can include
-wind conditions... higher wind across top of vent pipe on roof can reduce draft, and cause downdraft that blows deadly CO into same room as water heater
-vent termination near high walls, trees, high roof peak etc will reduce draft or cause downdraft
-amount of incoming air supply... if water heater is inside a closed room, then there is no air to draft up the pipe, burner will shut down or burn lazy yellow instead of clear blue
-stuff piled up around water heater that obstructs air intake
-dirty air intake screen that reduces air intake
-vent fans inside same room as water heater that pull air down the vent and cause reduction in water heater draft
-generally elbows reduce draft by 10%
-vent cannot terminate near any window door or air intake for another gas appliance
-bird build nest on top of vent pipe after vent termination falls off
-highly acidic gas by-product causes large amount of condensation inside pipe that burns hole in pipe or drips down and damages combustion parts...blower and inline condensate trap and drain might be required
-multiple water heaters installed on same vent line.

Gene
Water Heater Timers Save Money

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

First make sure everything is clean,from the chimney to the stove.insure all metal flues are tight from stove to chimney. Open damper on top of stove(some are in metal pipes). Open your draft vents, usually on clean out door at bottom. Depending on chimney draft you may need to open clean out door to get it started. Load your kindling and get it started. Add bigger wood as it catches. Turn down venting and drafting as the fire roars. Just don't shut draft before vents. You'll get used to balancing draft and vent as you go. I hope this helps

Are your air filters relatively new? Is a bird nesting in your units combustion air exhaust pipe? Does your draft inducer motor run before and after the gas has ignited? Check and confirm these 3 things first. Good Luck!

Get more air to the unit.
http://waterheatertimer.org/pdf/Bosch_Ventilating_tight_houses.pdf
http://waterheatertimer.org/pdf/Attic-Ventilation-and-Pilot-Outage-in-Gas%20Water-Heaters.pdf
Don't use household vent fans.
Trim back trees, and move stuff away from front of water heater and from around vent pipe.
Pilot outage can be caused by tightly insulated house, poor upward draft on flue, other vents or vent fans drafting air upward and causing downdraft on water heater, wind coming down flue pipe, water dripping down flueway, dirty combustion parts, clogged pilot orifice, failed thermocouple, failed gas control valve, low gas pressure, water or air in gas line, super hot location ... or combination of more than 1 thing.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

sounds like bad gas coils,you have to send the whole model number,it will start with 110. or similar,if the bottom panel comes off,remove it,start the dryer, or in the front panel down near the bottom pop off the cover and look in,if the flame lights and then after 10 to 20 minutes the flame doesn't light you have bad gas coils.when the coils are starting to fail they will work correctly until they heat up and then they won't work until they cool down,the coils are what opens the gas valve.the coils are on top of the gas valve,remove the two phillips screws,pull off the cover,remove the wires from the coils and install the new coils,put it back together and you'll be fine,also while you're in there,get a shop vac and vac out the cabinet,stick it on blow and blow all the lint off of the motor and out of the gas tube,then clean out the duct that the lint filter slides into.if your vent line is a long run from the back of the dryer to the outside,i use a leaf blower remove the vent line from the back of the dryer and blow all the lint outside out of the line,do this once a year,you won't have any fires,your clothes will dry on time and your motor and parts will last longer

When a furnace is replaced with a newer style unit like yours with an inducer motor, they sometimes use the old vent pipe. These old style vent pipes are transite (light weight cement), the moisture created from your furnace exhaust is caught on the cool transite surface and runs back down into the blower blades or wheel bringing a white mineral deposit with it which causes your problem. The vent should nave been replaced with the heater using a metal vent material. You will need to replace your inducer motor assembly and vent pipe to prevent a recurrence.

its would be best to install a damper
whether auto matic
or manuely operated
this is a common problem for the down draft
ive seen some people install things similar
to dryer vent louvers,
but check you exhaust area and still if you can block direct draft
but dont block it so that it cannot exhaust properly

Yes , its much more labor intense but it can be done by venting it up through the wall then through the attic over to the side vents. Make sure to get flexable duct to pull up through the wall.
Phil

I would think you can. Most of the cooktops are 240v and the vents are 120v, so the vent may be feeding off of one of the cooktop's hot wires. My vent is fed from it's own receptacle but is harnessed along with the cooktop's 240v.