I am planning to convert to a tankless water heater but have a few questions to make sure I am on the correct path.

I live in a rowhouse in Philadelphia where natural gas is used for heat and my water heater tank is located in the basement by the front of the house where the water (and gas) enter the house. I want to relocate the new tankless water heater to more central location that is closer to the points of demand. I am planning the placement to on the wall about a foot away from the stack and below the kitchen. I want to avoid cutting through the exterior wall for the venting and was hoping I can run the piping along side the stack (not sure about the spacing requirements) inside the interior wall that separates the kitchen and dining room on the first floor and bathroom and bedroom on the second floor and vent through the roof.

The questions I have are: 1) it is copper piping throughout and was thinking to replace the 3/4" feeding with PEX but maybe it would be helping to keep copper to help the water warm up a bit before reaching the heater. Is it better to keep it copper since PEX is a better insulator. 2) Not sure whether to use concentric or not, would be great to only cut one hole, but maybe I want to add some exhausting venting from the bathroom upstairs which means I might need add a separate exhaust pipe anyway. If do not go with the concentric can I use the exhaust piping to vent the bathroom exhaust too? is there a backstop.

Any information would be appreciated.

  • 1
    What is driving the desire to go tankless here? May 4, 2019 at 18:08
  • Adding space to the basement and family of 4 with one shower hot water supply is becoming an issue. Also, hoping to save energy but the is the third priority.
    – ecco88
    May 4, 2019 at 23:16

1 Answer 1


You cannot combine an exhaust vent for a tankless heater with other vents. For one thing there would be a possibility of the exhaust from the WH going the wrong way in the other vent. The WH must have a dedicated vent.

Also depending on the model the vent may have a requirement for a certain length of vertical run immediately adjacent to the heater before you have a horizontal run. (Tankless WHs with power vents may not have this requirement.)

It will be very costly to retrofit a new vent path for a tankless heater through multiple stories and out the roof. Reconsider your reluctance at venting out a side wall. This would almost certainly be much better, simpler and cheaper. A new roof penetration is an invitation to a roof leak because plumbers in general don't properly flash a retrofit roof penetration. Ours leaks!

Even though the current location is not ideal it may be best to use the same location as the current a gas fired tank. Note that the tankless water heater draws a lot more gas when it is on (up to 4 or 5 times more) than a 40 gal or 50 gal gas tank. The gas supply line and the gas meter must be large enough to supply this amount.

Different models of tankless WH have different sizes and types of vent. It would be a pretty thick wall that would allow one of these vents to be inside a wall.

If the copper water supply piping is in good shape, there would be no reason to replace it with PEX. It may help to put foam insulation around the hot water line wherever you can.

Thirteen years ago we replaced our 40-gal tank with a minimal Bosch tankless WH because twice when our tanks failed they flooded our 1-story house on a slab. If your WH is in the basement, I doubt it is worth it to change to a tankless.

Tankless WHs require periodic descaling with vinegar and they do not supply the flow rate of hot water that most Americans are used to. If you are having a problem with running out of hot water for multiple simultaneous and sequential showering, the best solution is to install low flow shower heads and encourage shorter showers.

  • In regards to the exhaust venting I was hoping there were some back flow stoppers that prevent that from happening and if not , you are right I really need rethink my reluctance about going directly out. If I could link an exhaust into the WH exhaust it maybe worth it. Otherwise it isn’t.
    – ecco88
    May 6, 2019 at 21:30
  • Also, I have retrofitted the shower head and trying to limit in showers. The reason I am asking about whether or not I should use PEX to feed because I understand that copper might help the feed “warm” a bit better than PEX before it goes into the WH. I might just land on relocating the WH tank or tankless to a more central location. We do have a household of 4 if that helps.
    – ecco88
    May 6, 2019 at 21:35
  • What exactly are your points of dissatisfaction with the current hot water supply? Do you have a 120 V receptacle near the current location of the water heater or near where you would move it to? Modern tankless WHs require 120 V power to operate the controls. May 7, 2019 at 4:24
  • Space - I live in the city so it’s kind of a big deal to gain extra space even in the basement. I also want to move WH tankless or tank to a more central location closer to the points of demand. Somehow water is expensive here. I know that modern tanks are at a point where standby is negligible and haven’t ruled it out. I am really wanting to move the heater to a central location and tank or tankless is a secondary decision. I will still have to vent the tank anyway.
    – ecco88
    May 8, 2019 at 9:51
  • A tankless heater will save space and floor space especially. In warm climates one can use a tankless heater which mounts on an outside wall. The exhaust vents out a slit in the front of the unit, but this is probably not recommended where you are. Where is this house located? May 8, 2019 at 10:23

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