Well, the short answer is because the manufacturer says that it can't be done.

The longer answer is that it is a powered vent, not atmospheric, and is near condensing. (assuming we are talking about non-condensing units)

They require stainless steel, silicone gasketed vents.

You may absolutely not get creative with these vents - you may not substitute other materials, you may not share a vent.

The condensate is acidic and will eat up standard vent material.

Now, a condensing unit uses PVC to vent, thereby making them less expensive to vent.

The longer answer is that it is a powered vent, not atmospheric, and is near condensing. (assuming we are talking about non-condensing units)
...
The condensate is acidic and will eat up standard vent material.

I B slow... never heard about the acidic angle before... not clear to me why a tankless WH makes different "exhaust fumes" than a gas furnace...?

It's not different. It's the same.

Gas appliances produce water vapor as part of their exhaust. In atmospherically vented appliances, (roughly 65% efficient) the exhaust is so hot that this vapor doesn't condense until it's outside.

On high efficiency appliances, the exhaust is much cooler and therefore the water vapor condenses - sometimes in the flue. On appliances that are above about 85% efficient, the appliance is actually capturing the heat from the condensing vapor - which is why they are called condensing units.

The condensed water vapor isn't just water. It's acidic. Over time it will etch concrete and put holes in metal flue pipe that's not stainless steel. You will notice that 90% efficient gas furnaces, for example, have a plastic drain pipe as do condensing water heaters. They also have stainless steel secondary heat exchangers to survive the acid condensate.

On a tankless water heater that's not condensing, most of the water vapor is vented out before it condenses, but there is some condensation in the flue. That's why the vents must be stainless steel and be designed to drain the condensate.

I think that you are going to see more condensing tankless units that vent with PVC just as a way of solving venting issues.

Lazypup,

The question was why you can't connect a tankless heater to a furnace vent.

Your answer, although factually correct, isn't appropriate to a tankless heater.

Jake,

You are correct about the tankless water heaters, however we must also consider that most gas furnaces have a barometric damper on the flue and code prohibits connecting a power vent burner to a flue that serves a burner with a barometric damper.

Are we confusing barometric dampers with draft hoods and draft induced appliances?

If so, you can common vent a atmospheric vented appliance with a draft hood along with a draft-induced appliance such as an 80% furnace. All the AGA, NFG, CSA code books along with the venting companies with sizing charts show you how.

But of course draft induced/power vented tankless water heaters are category III appliances, thus the required SS, gasketed dedicated vent.

With exception to the Navien and the new Noritz condensing that is Cat IV which is PVC/ABS

ZL700 and LazyPup,

See my first post. I believe I covered all of the issues regarding a tankless and a furnace with enough detail to provide the "why" behind the "It can't be done".

Pup,
This has nothing to do with the OP, but we haven't been installing a furnace with a damper in my area, by code, for almost 30 years. We require at least 85% efficient furnaces and most go 90+.

Jake
You seem offended that we posted on an open forum.

Didn't realize this was a controlled answer forum.
If you have read any of Mr. Fixizin prior posts you would know he doesn't like a simple answer because a case of skeptism and need to understand.

BTW minimum furnace efficiencies are 80 not 85%.

Zl700,

Nope, not offended at all. And on a different topic, I wouldn't be so inclined to follow-up. The problem is that SS vents are expensive and so some people try to get creative with them. At least one person substituted double wall. LazyPup mentioned using double wall as well in a different post. So, when The Pup goes a little off topic and then you follow him, I think it's important to clarify - particularly on this topic.

Although I'm responding to Mr. Fixizin, I am writing with other people in mind. I am less concerned about his theoretical question than I am about people who skim these answers for "permission" to be creative.

RE: 80 vs 85% afue furnace. Okay you got me there... I generally lump all 80% non-condensing furnaces together as 85%. But the point was that we haven't been installing furnaces with a damper for a long time.

Cool
Your a great contributor to the site, and a even-keeled tankless proponent, good to have in this industry.