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Venting a tankless water heater

Conrad | Posted in Energy, Heating & Insulation on May 20, 2006 02:53am

I’m about to install an Aquastar 240FX tankless heater in my basement. It will be mounted to the side of the mansonry chimney that currently vents the tank-style heater only (furnace is direct vent). When a new furnace was installed five years ago, the contractor put a flexible flue liner in the chimney saying the 8×8 flue was too large to create an adequate draft for just a water heater. I know I have to use class III stainless steel to vent a tankless heater, but can I just use a 90 degree elbow into the clay flue liner (sealed with furnace cement) or do I need to run stainless steel all the way up the 15 foot chimney and out the top? I’d really rather not pony up for all that stainless steel if I can help it. Does anyone make a flexible stainless liner, and is that legit for a tankless heater? Also, how much condensate do these units make? Can I just let it drip into the ash pit of the chimney, or will there be so much liquid that I need to pipe it to a drain? The installation will be in an area where looks are not important and a little water running across the floor to the floor drain is acceptable to me. I do have the option to horizontally vent if that would be superior, but it would be more work and probably require a longer venting run than up the chimney. Any and all advice you can give me would be most appreciated.

Conrad

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  1. User avater
    rjw | May 20, 2006 03:37am | #1

    Looking at the installation manual for the 250 (couldn't find one online for the 240 you mention) the stainless vent must "terminate outside" which, to me, doesn't include "in the chimney."

    See, http://www.boschhotwater.com/Portals/7/TechManuals/250SX_English.pdf

    Note that you can side vent it, however, so there might be a better route for the vent than up the chimney.


    Fighting Ignorance since 1967

    It's taking way longer than we thought

    1. Conrad | May 20, 2006 06:00am | #3

      The 240SX is a re-badged Takagi TK-1 and the installation instructions that came with it were not nearly as detailed as those for the Aquastar 250. Thanks for pointing me in that direction. A diagram there clearly shows that I must line the entire chimney with stainless steel.  Looks like I'll be trying to figure a way to do a horizontal vent. Anybody have any experience with this? Which brand of vent pipe do people perfer; z-flex, protech, or heat fab? Thanks again.

      Conrad

      1. Karl | May 20, 2006 09:29am | #4

        I have mounted two takagi's outside the house and one inside. So far so good on these outside installations. Venting is a breeze, just add the exterior vent cap and you are done.Is this an option in your case?Karl

        1. Conrad | May 20, 2006 08:20pm | #5

          From where I can best mount the unit, it is either up the chimney or I have to vent horizontally about 12 feet between first floor joists to the nearest outside wall. Ceiling height is 6.5 feet. The flue gasses from these units can reach 400 deg. F, and I'd rather not singe what little hair I still have on the top of my head walking under the unit while it's in action. Add to that the clearance to combustibles issue and horizontal venting becomes a challenge.  Any other thoughts anyone might have?

          1. Karl | May 20, 2006 11:16pm | #6

            When I put in a takagi TK2 in my basement I had to cut a 12" by 12" vent in to the exterior wall. The vent is a 12 x 12 stainless box that couples onto a round 4" vent. Will your run down the joist bay allow cutting a square that big in the exterior wall.The vents are pricey stuff being stainless steel so I try to locate the heater on the outside wall and reroute gas and water pipes as needed.In my experience the most important part of a tankless installation is to put it as close to your kitchen sink as possible and use the smallest pipe you can tolerate. (I used 3/8 copper tube to my kitchen) as there is a 3 second delay between turning on the h2o and the burner reaching operating temp. Since you are starting with a 3 second fixed delay it pays off to reduce the volume of cold water you have to purge from the pipes as a means of keeping the total delay for hot water manageable.In one house I ended up putting a one or two gallon electric heater under the sink to make the delay in getting hot water from the tankless heater non existant. It was a pretty good solution and saves a lot of water by having hot water the instant you open the tap. I am not sure how much it adds to actual energy consumption. It is a 110 volt heater that plugs into a 15 amp outlet and pipes in with two flex lines.Karl

          2. User avater
            rjw | May 20, 2006 11:26pm | #8

            >>between first floor joists to the nearest outside wall. Ceiling height is 6.5 feet. The flue gasses from these units can reach 400 deg. F,Don't forget to minatain minimum distances from combustibles (typically 6" with a single wall flue at those flue temps)

            Fighting Ignorance since 1967

            It's taking way longer than we thought

        2. User avater
          rjw | May 20, 2006 11:24pm | #7

          >>I have mounted two takagi's outside the house and one inside.1. Where are you located? Any freeze hazards?>>Venting is a breeze, just add the exterior vent cap and you are done.2. The model 350[xx] instructions call for a minimum 3' flue in all cases. You might want to doble check your instructions.

          Fighting Ignorance since 1967

          It's taking way longer than we thought

          1. User avater
            rjw | May 20, 2006 11:28pm | #9

            Venting combustion appliances involves several safety issues.In my experience, you shouldn't be working with flues if you don't have (i) specialized knowledge or (ii) a determination to read, understand AND follow the manufacturer's installation instructions

            Fighting Ignorance since 1967

            It's taking way longer than we thought

          2. Conrad | May 21, 2006 02:48am | #11

            I couldn't agree with you more about knowing what you are doing before starting. The problem with my unit is that it came with installation instructions that were rather generalized. I got much more detailed information on venting reading the Bosch manual online. BTW, according to the Bosch manual, you can reduce clearance to combustibles to 1" if you run the stainless inside b-vent.

            Conrad

          3. Karl | May 21, 2006 12:57am | #10

            >>I have mounted two takagi's outside the house and one inside.
            RJW,1. Where are you located? Any freeze hazards?>>Venting is a breeze, just add the exterior vent cap and you are done.2. The model 350[xx] instructions call for a minimum 3' flue in all cases. You might want to doble check your instructions.Coastal Northern California location so no significant freeze hazard.I don't know about the 350 model, is that the same as a TK 2. I have 24" of flue prior to the vent termination. The sales rep said it was fine and I didn't think to question further. I haven't had any problems but if I get a chance I suppose I should take a look at the manual.Karl

  2. frenchy | May 20, 2006 04:41am | #2

    Conrad,

       from what I've read it's not proper to vent into anything less than stainless steel. the resulting condensate is extremely corrosive which means most type 2 exhaust is not proper..

  3. hurnik | May 21, 2006 08:29pm | #12

    Just be careful when you vent out a sidewall. I'd looked into one and it became virtually impossible to vent.

    Why?

    Code required a MINIMUM of 4' from the exhaust vent to ANY window or other air intake (so if you have a high efficiency furnace that has an air intake like I do, it became impossible to install).

    Obviously it depends on how your house is constructed, but for me it was impossible to do without violating code.

    Not to mention the $3,000 they wanted to install such a beast.

    Put a high efficiency 50-gallon unit instead for $950 and it uses LESS gas than my 4-year old 40-gallon did and I get quicker recovery time.

    1. User avater
      rjw | May 22, 2006 12:04am | #13

      >>Code required a MINIMUM of 4' from the exhaust vent to ANY window or other air intake (so if you have a high efficiency furnace that has an air intake like I do, it became impossible to install).In my area the code defers to the manufacturer's specifications. For the 90+ plus condensing furnaces and gas log sets for which I've studied the installation instructions, the clearance requirements are usually less than that 4'The 90+ direct vent (exterior air intake and exhaust units) furnaces usually require 12" or 18" height separation between the air intake (lower) and the exhaust, and 6" horizontally - but each model/manufacturer can have different specs.Some models/manufacturers specify that they both be in the same "pressure zone" (i.e. don't draw air from close to grade and vent above the roof.)But, of course, codes vary from place top place.

      Fighting Ignorance since 1967

      It's taking way longer than we thought

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