Hot water heater and furnace venting

Can someone remind me of the proper way to vent these two appliances. One should be above the other and I can never remember which one. In this case they are almost horizontal. Whould anyone call this out?

vent code.jpg

Smaller BTU unit should be on top. I’m leaning more towards not calling it out. I think I would turn both units on for a while and check the area w/co2 monitor. If negative, then I’d let it go.

Is the bottom of that vertical pipe open or closed?

Bottom is closed, thanks for the info. No Co2 present.

As always check with local code officials. Very rare but some localities do not allow coventing of water heaters and furnaces.

That installation doesn’t appear to be professional. But as long as they are exhausting the same fuel, it should be OK.

Here’s a link that’ll answer all your double venting questions.

Thanks for the link, that will come in handy.

It is important to understand why the appliance with the smaller BTU output connnects on top of the appliance with the higher BTU ouput.

The reason has to do with what happens when both units are firing and venting at the same time.

If the arangement was reversed with the higher BTU on top, as the applaince vented the flow of gasses would from the higher unit would take precedance over that of the lower connected and lower BTU output appliance. This would basically chock off the lower output appliance.

Here is a rather crude illustration that I just made to illustrate the point.

By putting the higher output appliance on the bottom the hotter gasses can help lift the relatively cooler gasses from smaller unit up the stack.

For the installation in the pictured in the first post, It probably works only because the water heater is wyed where as the furnace T’s in. The addtional turbulance from the furnace T is likely enough to “get under” the water heater gasses.

Personally, I would like to see a bit greater distance between where the two appliances connect. Not only does it look like there is room but it would also give a greater slope to the water heater horizontal section of vent which should be at least 1/4" per foot or greater. Greater is better.

Its hard to tell from the picture but the furnace flue looks like it has a negative pitch. -X