I'm planning on installing a tankless water heater that maxes at 160K Btu. I only have a 1/2" line from the meter to the furnace(75K) in the same room. The city told me that there is 2lb of gas from the meter to the house. The sizing charts that I found say that 1/2" 2psi black pipe will handle the btu draw for both the furnace and the water heater. I know I will need to use a regulator at the water heater to step down to 7" w.c. or so. I will also need to adapt the 1/2" line to 3/4" for the water heater. My questions are:

  1. Will the 1/2" 2lb line be enough?
  2. Should I convert to 3/4" before the regulator to the water heater?
  • So the gas company says you have 2 psi from the meter to the house, but are all the lines in the house at 2 psi? That is, does the furnace have a regulator to drop the pressure to 7 inches W.C. (0.25 psi)? Commented Mar 7, 2019 at 19:37
  • Yes, the furnace has a regulator. I can tap off the line before the regulator.
    – Chuck W
    Commented Mar 7, 2019 at 19:51
  • Then I would say that the existing 1/2" line should supply the additional 160 kBTU/h that the tankless heater would require, but what do the tables say? What kind of water heater was installed before and what size burner or was it electric? Commented Mar 7, 2019 at 20:05
  • I currently have an electric water heater that is on its last leg I believe. Since I have to replace it anyway I thought this was a good time to change to a gas heater. The heater I'm going to install is a Takagi condensing unit, so just pvc venting out the wall 3 ft away. The table says 753 cu ft for 1/2" over 40 ft, which is the longest measurement including elbows, seems like a lot!
    – Chuck W
    Commented Mar 7, 2019 at 20:46
  • Are you sure the tables you are referencing give the gas flow rating in "cu ft" (of course this would have to be cu ft / h or some other unit of time). The tables I have seen give the capacity in BTU/h. Commented Mar 7, 2019 at 20:51

2 Answers 2


2 psi!

Where do you live? Yes 2psi on 1/2" iron can deliver your stateted load a whopping 250 feet!

You can use a 1/2 in regulator down to 7" up to about 15 feet before the tankless. And reduce at the unit.

Also check the tankless for acceptable input pressure, you may not need a reg but most likely you will.

Something like this is totally acceptable not in reference to code enter image description here

  • I live in a small town in central Missouri. So, it doesn't matter if I put the regulator on the 1/2" line or on the 3/4" line after upsizing? The manual for the heater states min gas pressure of 5 w.c. and max of 10.5
    – Chuck W
    Commented Mar 7, 2019 at 21:01
  • What line are you thinking about upsizing to 3/4"? The water heater may be delivered with 3/4" fittings assuming that the gas will be at 7" WC. If so you would only need to run 3/4" from the regulator to the water heater . . . and the water heater may come with the regulator attached already. If so, then it would seem to me that all you would need would be an adapter to transition from 1/2" pipe to 3/4" at the input of the regulator. Just be sure any regulator supplied with the heater takes 2 psi input. Commented Mar 7, 2019 at 21:15
  • I don't know about your local codes regarding the piping practice but from a strictly load handling point of view 10' of 1/2" is at 7" wc will give 227,000btus.
    – Joe Fala
    Commented Mar 7, 2019 at 21:20
  • Sorry, maybe I worded that wrong. I didn't know if I should just run 1/2" all the way to the heater (including a 1/2" regulator) then change to 3/4 to connect to the heater or if I should change to 3/4 after teeing from the existing 1/2" line and use a 3/4 regulator and then run to the heater.
    – Chuck W
    Commented Mar 7, 2019 at 21:21
  • Joe - the 1/2" line to the furnace is about 30ft, I would have to tee off of that to keep from running a separate line to the meter.
    – Chuck W
    Commented Mar 7, 2019 at 21:22

If the tables say the existing 1/2" piping will supply what you need (assuming 2 psi all the way to the appliance), then I would just go ahead and install it with the existing gas piping. But be sure there is not a regulator just where the gas line enters the structure or up in the attic that drops 2 psi to 7" W.C. (0.25 psi). I would check with the builder or city permitting office.

What kind of water heater was installed before--what size burner or was it electric?

We have 7" WC gas from the meter. Our 13 year old very basic Bosch Aquastar tankless NG fired WH (~117 kBTU/h) came with a regulator which I had the plumber leave in place, but maybe the one our Aquastar came with accepts only standard low ~7" WC input. Alternatively, maybe this regulator is not needed for 7" WC input. IIRC the installing plumber thought that he should remove the regulator, but I nixed the idea because I could not find any statement in the installation instructions to the effect that the regulator was not needed if the house supply was 7" WC. I had bought this WH at a big box and carefully read the instructions intending to install it myself, but I thought better of it and hired a plumber.

A new tankless WH like yours might come supplied with a regulator which accepts 2 psi gas input. Check the instructions.

  • 1
    In my neck of the woods we only ever get 7" maybe 14" from the utility. 2 psi would be a dream come true. We're allowed up to 2 psi but never get it. These days people are building homes with loads well over a million btus and frankly I'm jealous that I have to run 2" gas pipe in a resi application.
    – Joe Fala
    Commented Mar 7, 2019 at 20:39
  • Yeah, I was planning on having a 3/4" line ran direct from the meter but was happily surprised when the city told me the meter supplied 2psi, but I wanted confirmation that the 1/2" line was enough.
    – Chuck W
    Commented Mar 7, 2019 at 21:03

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