My partner and I have a small home in the SF Bay Area. To try to reclaim some indoor space, we're planning on replacing our tank water heater (currently in the house) with a tankless unit that we're going to put outside.

The company we're planning on going with has given us quotes with and without permits. The price with permits costs $1,300 more partly because they said the inspector/code will require them to run a dedicated gas line to handle the tankless unit even though our existing gas line has enough capacity. I don't mind the charge as much as I mind having a potentially unnecessary, unsightly gas line that requires drywall to be ripped/replaced in a finished room in order to route the gas line. (Our house has no crawl space.)

While I generally prefer to do things with permits to make sure the job is done well/safely, I realize building code doesn't always make sense, and we've had our share of bad experiences with overzealous inspectors.

I'd love people's advice on the following:

  1. Do we have enough natural gas capacity in our house to support a new tankless hot water heater and our other gas appliances (HVAC furnace, dryer)? I believe we have a 1" supply line, although I measured a 4.5" circumference or 1.43" OD for the gas supply line, which doesn't match the expected OD for a 1" or a 1.25" gas supply line.
  2. Is it true that building code would require us to get a new gas line regardless of the sizing of the current gas lines we have?
  3. If you were getting this work done, would you get a permit pulled for this work?

Thanks in advance for any help anyone can provide!

EDIT: I looked up the BTU/hr. ratings of my dryer and furnace. They are 18,500 and 60,000, respectively. The two tankless models I'm considering (Navien NPE-210A2 and NPE-240A2) have 180k and 199k BTU/hr. ratings. Given what I see here and the runs of gas line that we have/will have (20-40 feet), it's going to be pretty close. If I want to run everything at once (unlikely), we would need a new line.

EDIT 2: Clarified the reason behind the new gas line (required by inspector/code) and that the bigger issue for me is having an additional gas line and having to replace drywall vs. price.

EDIT 3: I located the meter under the sidewalk and took a picture. It appears to be a "Sprague 175" gas meter. I also bought a pair of calipers and measured the line coming out of the meter (vs. gas lines in my home). It appears to have a OD of 1.0685", which corresponds to a 3/4" gas line.

Sprague 1753/4" gas line

EDIT 4: More updates. According to PG&E (gas supplier):

Upon review of your address, we see that the gas service is for 425k btu's or less. Are you adding need appliances that will increase your load? If you need a larger gas service at the location please submit the application below

I also asked my tankless WH person about my findings re: the 3/4" supply line and whether I would still need a dedicated line. His response:

You would still need a dedicated line from that tee in your garage to the tankless. It is unnecessary which is why I have issues with pulling permits on something like this, but they will require it.

If pge got involved they’d want you to have the meter moved in your garage as they are not allowing sidewalk mounted meters anymore.

  • What else uses gas? How much? can all things run together?
    – Solar Mike
    Commented Aug 2, 2021 at 7:38
  • 1
    You may want to consult an attorney about the liability you might face by making unpermitted changes to your home before attempting this.
    – jwh20
    Commented Aug 2, 2021 at 9:17
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    I see you've added some pictures. The Sprague 175 seems to be rated for 175K BTU. You're instant hot water will take all of these BTUs and you should avoid running your furnace or dryer while someone is using hot water. A new gas line is the right answer.
    – MonkeyZeus
    Commented Aug 4, 2021 at 15:09
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    @JimStewart My bad. I must have missed "underground gas line from the meter to the structure" initially. Yes, your comment is correct.
    – MonkeyZeus
    Commented Aug 4, 2021 at 16:31
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    Have you given any thought to what may happen when you want to sell your house and the buyer wants to bring in an inspector? Commented Aug 7, 2021 at 17:55

1 Answer 1


Locating a gas fired tankless WH outside gives two huge benefits. First, there is no flue that penetrates the ceiling and roof. The exhaust comes directly out of a slit near the top of the unit. The combustion air also comes directly from the outside air as opposed to an inside location where provision must be made to supply combustion air.

Second, if it ever leaks it will not dump water inside. And the regular descaling operation with vinegar is easier outside.

All, or nearly all, modern gas fired tankless WHs require 120 V a/c power to operate the electronic controls and to power a small heater to prevent freezing of the core when ambient temp drops below freezing.

Part of the cost of installing a tankless WH outside is the cost of installing an outside 120 V receptacle if there is not one where you want to place the WH. The outside location is best suited to a climate with little or no freezing weather, but a proper installation can work in a climate with some freezing weather.


I recommend getting the permit and city inspection in spite of the extra charge.


Most houses require a whole house tankless WH with a burner rated at 160 kBTU/h to 180 kBTU/h to replace a 50 gal tank. Such a tank will have a burner rated at about 40 kBTU/h and the original gas line will be too small for the tankless WH.

In some houses the gas plumbing with black iron pipe has a capped off "T" to provide a connection point for future gas appliances. A new 3/4" line from that "T" to the tankless WH would be necessary and sufficient to allow full function of the WH.


I suggest you use a 160 kBTU/h tankless WH if that would satisfy the city inspectors, and let you use the existing gas line. That should be able to heat water for two people. Fourteen years ago we put in a Bosch 125 NG tankless WH with output 120 kBTU/h and it works for two people. I elected to just use the same gas line as that fed the 40 gal NG fired tank. This is an inside model and I had the installer place the tankless WH on the wall where the tank had been. The WH shares a 3/4" gas line with a 100 kBTU/h natural gas furnace.

  • Hi Jim - thanks for the extensive reply! Agreed with the benefits re: moving the WH outside, although we will still have to run a outdoor vent to the roof because of how close the WH will be to the property. I'm not as worried about the extra charge as much as I am about having a potentially unsightly, unnecessary gas line + having to rip and replace dry wall in a finished laundry room to route it. We fortunately have a 3/4" gas line where the current 40-gal tank WH and furnace are.
    – bacchuswng
    Commented Aug 3, 2021 at 7:12
  • I have never seen a tankless WH designed for outside placement that had a round flue going up. All the ones I have seen have a slit on the outside face. I used to visit a relative in Oakland CA that had one like that, i.e., slit without a flue. Is your house very close to a neighbor's house or is it that the only good location is close to a window on your house? If you are going to have a roof penetration, get a roofer to do that part. A plumber is likely to do the flashing wrong Commented Aug 3, 2021 at 8:51
  • Looks like the Navien models you are looking at are rated for inside or outside placement so a vertical flue outside is an option. My choice of a tankless model was made 15 yo while standing in a checkout line at HD. Next to me were two boxes with a sign over them "Discontinued model Bosch tankless WH, price reduced". I took both and had one installed in my house and one in a relative's. In both cases we used the original gas lines which had been feeding a 40 kBTU/h tank. This is a basic unit with mechanical controls and standing pilot light! No mains electricity reqd. But you cannot do that. Commented Aug 4, 2021 at 10:45
  • I have been told that modern tankless water heaters have a gas pressure sensor and will not come on if the gas pressure is below a certain value. If you would decide to use your existing gas lines, it could be that when your gas furnace lights up the gas pressure at the water heater would drop below the lower limit. Last summer we replaced our 30 yo HVAC/gas furnace with one that has extensive electronic controls. Our 120 kBTU/h tankless WH and this 100 kBTU/h furnace share a 3/4" gas line from a central point in the attic. The furnace worked perfectly in last winter's 0 F cold. Commented Aug 4, 2021 at 11:27
  • The conclusion of the above is that your contractor may be correct that your existing gas lines will acceptably feed a 160 kBTU/h tankless WH. The city inspectors may be insisting on unnecessarily large gas lines. Our house has a 1" line* from the meter up into a central point in the attic. From that point there is maybe 12 ft of 3/4" black gas pipe. *OD measured with Vernier calipers 33.4 mm = 1.315 inch. Commented Aug 4, 2021 at 15:12

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