Thread: Two appliances connected into single vent

The furnace is induced draft, the water heater natural. They are connected at the Wye. I know that IRC G2427.3.3 (4) allows these two appliances to join into a common vent connector. The water heater vent is located below the furnace vent (at the Wye). I checked the IRC and it says “where two or more vent connectors enter a common gas vent… the smaller connector shall enter at the highest level”… (IRC G2427.10.4). These seem to be the same diameter. I can't find anything in the IRC code that mandates the water heater vent must be installed above the furnace in this configuration

So my question is: Which one should be at the top? Why? Where can I find a reference

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IFGC 503.10.4
Where two or more vent connectors enter a common gas vent,chimney flue, or single wall metal pipe, the smaller connector shall enter at the highest level consistent with the available headroom or clearance to combustable material.
Vent connectors serving Category 1 appliances shall not be connected to any portion of a mechinical draft system operating under positive static pressure, such as those serving Category 3 or 4 appliances.

As I understand it the smaller BTU flue should go on the top.

The water heater should be seperately vented into the chimney above the power vented furnace.

I think the furnace is a category I, according to the IRC definitions of a Cat I appliance -- (aside from a natural vent) other Cat I's can be a mechanical draft system; specifically an "induced draft system". This furnace is induced draft and provides removal of gasses under non positive static vent pressure (i.e. the fan is in the located between the appliance and the vent). This I believe is what makes it a Category I. ??
Anyway, If I have this right, then I still struggle with believing that the installation is OK. Their must be some other guidance when you have this combination. I think the prudent thing to do is install the lower BTU (water heater) on top, regardless of vent size.... But I still have nothing that says this would be correct in this case??
Am I making too much of this?

The most dangerous installation allowed by code.

Originally Posted by James Risley

Note in the photos of the original post that the natural vented water heater is vented in a maner which allows the hot rising gasses to have a more directly upward pathway to the vertical stack. The induced flue is not dependent on the nature of hot gasses to rise and can therefore have the more horizontal pathway.

James - Good point. You know I did not even consider that. --Space is pretty limited in there, not sure if they would have enough room to move the wye down further...

To clarify: an induced draft CAT I appliance DOES vent under natural draft. The 'induced' part is simply to induce a draft at startup. Once the appliance fires and heats a properly sized/ maintained flue, the vent pressure switches from positive to negative so these appliances do NOT blow into the chimney. Now, because they are so efficient, there is relatively little heat to generate a draft, esp. with high mass masonry chimneys. They should only vent into type B-vent or listed liners, which are low mass.

Now, the whole thing about smaller appliances entering the chimney above the larger is stated in the gas code commentary as a means to increase the vent rise off the appliances collar. All these theories of the smaller needing to be above the larger due to flow issues are just that--unproven theories. In the photos provided, the water heater's vent connector should rise vertically as high as it can then slope over to the manifold at 1/4" per foot. This long slope is worse than an initial vertical rise. You need to generate a 'head of steam' so to speak before encountering any offsets.

The code does allow the use of a manifold such as shown here. This is the method I use. The fact they used a 'wye' in lieu of the more common 'tee' shows some level of concern on the part of the installer. You can rotate that wye flat, too.

Now, what David is referring to is the potential for this common vent to do two things:
first, during startup, the furnace vent, being under some positive pressure, can spill out the water heater's draft hood until the chimney primes and establishes a draft (negative vent pressure). During this period, you are getting CO into the home.

Second, should something block or restrict the chimney, that furnace will happily vent out the WH draft hood without tripping any of the three safeties in the furance. There is a way to correct this, which is taught in the NCI course of Carbon Monoxide and Combustion Analysis. This is the course that will teach you when, where and how to test for CO and what to do with the results. David is an instructor along with Jim Davis.

Bob

Originally Posted by James Risley

I once built an all electric home for a gas company employee.

Jim - now that's FUNNY!