Flue slope

Question - I’ve seen this type of venting 3-4 times a month in older houses and never had anyone complain. The bottom metal flue is the 80% furnace with an uphill rise from it to the newly lined masonry chimney (stainless steel liner). The upper metal flue is the water heater with an upward slope from it till right before the chimney - then it turns down to drop into the furnace flue and they become 1 flue entering the chimney.

My question is - I’ve not worried about this before but recently started wondering something … We always say we need a 1/4" p/ft uphill rise on flues, so do you think the downturn of the water heater flue will cause backdrafting of the water heater.

Sorry forgot the picture in my previous post


Possibly. Anything that defeats the means of exhaust getting out of the building properly can lead to backdrafting. By any means, that set up is not right and needs correction.

UPC 511.2.9 says that the wye shall be considered as part of the common vent and 511.2.12 says that the rise is measured from the draft hood to the centerline where the two vent gas streams come together.

In that regard, you do not have a continuous rise and it should be considered defective, IMO.



[FONT=Comic Sans MS]This venting is incorrect, any horizontal run in the vent should slope upward (never downward) at least 1/4" per foot. Correction is needed.[/FONT]

It might work fine But it is wrong . Write it up if it gets fixed great if it does not get fixed and some one dies You should be off the hook .
I call this out every time I see one .
CRA Cover Roy’s A $$…

Absolutely incorrect…repairs are needed.

It may let condensation “roll” down furnace flue pipe.

That’s what HVAC tech said caused this.


All fixed.


Come on, Dan. Youse should have seen this before. I have seen it dozens of times. HWH should be at a good slope, under the furnace flue. Those old Brookside homes are tough to inspect.

Regardless this type of Post Teaches the newre Homie what to look for .
Thanks Dan …

Another makeshift shortcut way of doing things.
A little close to the PVC pipe isn’t it?

Call it out.

Good one Dan.

Gas and Oil Appliance Venting

Gas fireplaces are factory-built systems. The manufacturer’s listing and instructions will preclude attaching any other appliances to it.
Multiple gas or oil furnaces or boilers, as well as hot water heaters, can be vented into one flue. There are a few rules to mention:

  • The rules apply to listed appliances. While I have never seen an unlisted gas or oil furnace in my life, if you have one, you are referred back to the rules for solid fuel burning appliances- one per flue.
  • If venting two or more appliances on the same flue, you have to know the flue can handle it, as determined but the BTU input and other factors.
  • Both or all appliances have to be on the same floor. So, no furnaces in the basement or room heaters on the second level of your home.
  • The connectors for the appliances have to be offset. They can’t come into the flue at the same height, and especially never directly across from each other.
  • The smaller of the two connectors go into the flue above the larger one (usually meaning the hot water heater).
  • As a general rule, don’t mix “natural draft” appliances and “fan assisted” appliances on the same flue. This rule is more complicated than this, but if this is your case, be sure you refer to the manufacturer’s instructions. Call and HVAC company and make them show you to your satisfaction it’s right. Don’t take anyone’s word for it, see it in writing.