Expert: James Ball - 2/1/2009

Question
QUESTION: I have purchased and installed a 118,000 BTU tankless water heater in my dad's house.  The 80% efficient 100,000 BTU furnace and the old tank water heater are vented through the chimney.  The furnace with single wall 6" galvanized pipe and the water heater with 3" galvanized pipe.

The home is in the Chicago area and was constructed in the mid 1950's.

The new water heater can be naturally vented with a 5"dia. vent.

The question is whether I can make my 5' run of new 5" venting and reduce down to 3" before it goes into the chimney or do I need to pull out the old 3" vent and increase the hole through the concrete block and tile liner?

As you can imagine, I am not looking forward to resizing the penetration into the flue.

Your advice is greatly appreciated.

Steve

ANSWER: Hello Steve, what does the new water heater say about venting? if it says that it should go into the chimney then yes you have to open up the hole and before doing so I would have a local Certified Chimney Sweep (www.csia.org, there you can look up by zip code to find one near you) take a first hand look at what you have they should be able to help open that hole or at least tell you if the chimney is safe to use for any thing.
Was the tankless installed correctly? if not that could give you a lot of trouble, check with your plumber.
James

---------- FOLLOW-UP ----------

QUESTION: James, thank you for your quick response.
My father had the chimney inspected a few years ago when the new furnace was installed.  The chimney sweep approved the new furnace without installing another liner.

The manufacturer (Bosch) of the new tankless water heater says that this unit can be naturally vented and calls for a 5" diameter vent pipe.  As stated before, I was hoping that I would not have to increase the existing 3" opening into the tile liner.

Based on your response, I understand that I will need to increase the opening into the tile.  I was planning on using either a series of holes and chip out or rent a diamond core bit.  Any words of wisdom?

Also, not sure what you meant regarding the installation of the water heater itself.  All gas piping and water piping, including pressure relief valve and isolation valves were sized and installed correctly. Did you have something specific in mind?

Answer
Welcome back Steve, when opening a flue to make it bigger you can damage the flue and it should have a trea cotta thimble, and the hot water heater should go in above the furnace pipe. You say My father had the chimney inspected a few years ago when the new furnace was installed.  The chimney sweep approved the new furnace without installing another liner. A lot can happen in a "few years" is the flue sized properly for what you have now? this system need to be installed correctly and vented properly. the 5" pipe from the Bosch may be to small to fit in the flue you have.
get this system inspected, both the gas install and the venting you might need to reline the chimney with this new install.
James