horizontal flue pipe

Does the plumbing code say you can not run a flue pipe horizontally?

 

No!

First theres negative slop on the vent, I see no vacuum relief, gas line should of been hard piped, if the discharge pipe is 3/4 on the temp relief valve that might be ok but there might be to many 90's on it. If it goes down to 1/2" its no good.

Read this wrong, you can run pipe horizontally

 

Not sure what's code, but I had a natural draft water heater with a vent that went up 18" , elbowed horizontal 12" and tee'd into a 6" pipe that it shared with my furnace. Sometimes it would back draft and spill combustion gas into basement. Could be from dryer running, bathroom vent fan, Air pressure drop. I was getting really high readings on my digital CO detector. WH was old and I got sick of trying to solve it so I replaced with a powervented model.


I'd get a CO detector in there to see if your getting back drafting. Granted my problem was probably more about the 6" shared class B chimney. That's a pretty big pipe for a 40k btu water heater to be orphaned into during the non-heating season.

 

A CO detector isn't a good backdraft detector- it only works if it's both backdrafting AND the burner is running poorly enough to put out significant CO.

Close all the windows then turn on all of the bath/kitchen/other exhaust fans including the clothes dryer, then turn up the temp on the water heater to make it fire, and use a cobweb/thread/strip of paper at the draft hood to see if it's always pulling properly when firing. If you have central air or ducted heat, test it with the air handler on as well as off.

Over-sizing the flue is pretty bad practice for atmospheric-drafted combustion appliances. (And it's almost always "the non-heating season" in Fort Worth TX.) If it's tall enough and you seal the seams of the flue with automotive muffler sealer or other high temp goop it'll still draw. The horizontal runs need to slope upward at a minimum of 1/4" per running foot to meet code, but for extremely short runs as in the picture it's not going to make a huge difference in the amount of exhaust spillage.