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I am trying to figure out if we can add a 199,000 BTU tankless water heater to our house/system.

We currently have a dryer (B in figure) and gas stove (C in figure). The water heater will be at A, where there is currently a 140,000 BTU gas boiler, but the boiler will be removed and heating is being switched to electric heat pump mini splits.

Schematic of gas system

I have been trying to use this guide to come up with the required pipe sizes, but not sure how having bigger than required pipes might affect things.

Using this, I calculated the demand for each point: Table of distance and gas demand

Then, using the table in the linked to guide:

  • The pipe going all the way to C would need to be 1/2"; however, it is 3/4"
  • The pipe going to Y would have a total demand of 51 cfh, so requires 1/2"; however it is also 3/4"
  • The pipe going to X would have a total demand of 232 cfh, so requires 1"; it is 1"
  • The pipe going from X to A has a demand of 181 cfh for 26', so requires 3/4"; it is 3/4"

Key Questions

  • Does this design work? It seems like it does to me, but not sure about splitting the demand for A into two pieces: meter to X and then X to A.
  • Is there any need to change the pressure at the meter? Currently the meter pressure is 6 WC. The hot water heater states that it needs a natural gas pressure of "3.5 in. wc - 10.5 in. wc". If I understand the tables above correctly, there should only be a 0.5 in wc drop, so if it's 6 at the meter, it will be 5.5 at the hot water heater so everything is fine.
  • If there are problems with this design, would removing the dryer help at all?
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    Tankless Gas doesn't have exactly the same issues as tankless electric. But as you can see, it does get complicated. Are you sure it really makes sense to switch? I sort-of understand when it is a new build with limited space, but a new tank could go presumably go in the same place as the old tank (boiler). Did the old boiler not produce enough hot water? Why are you switching? Jan 12 at 17:33
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    Is the current boiler electric or gas? If it's gas what BTU demand? You only need to ensure you can handle the difference between the current boiler and the new tankless.
    – FreeMan
    Jan 12 at 17:57
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    The impression is that you have enough flow capacity in the system to accomodate all the consumption. Having 3/4" pipe rather than 1/2" would mean that the dryer and the kitchen range would be able to draw more flow if they had the consumption rate to do that. They almost certainly don't, but if they did this could lower the gas pressure at X and so reduce the max flow the WH could pull. But this is very unlikely. This hould be fine. We ahve a 120 kBTU/h WH and a 100 kBTU/h furnace pulling on the same 3/4" line 10' to a 1" line. The new furnace is not giving an error code. Jan 12 at 18:59
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    Do you know the rated flow capacity of your gas meter? It is my understanding that if you get new appliances with more demand that exceeds the current meter, the gas suppliers will upgrade the meter at no charge to the customer. Of course, one always wonders if alerting them to upgrading the appliance suite wit higher possible total possible consumption could make them insist that one needs a larger line from the meter, but this is probably worrywartism. But will this tankless WH be used for heating the house as well as for domestic potable hot water? Is it rated for space heating? Jan 12 at 19:13
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    In case you hadn't considered it: you're also allowed to run new pipe from the meter directly to your water heater location. It could serve only the water heater and you leave the rest of the existing system as-is. Also, check the manual for the water heater. Some time ago Rinnai advertised being able to supply some of their heaters with 1/2" pipe. It's possible your heater can tolerate more pressure loss, and thus smaller pipe, than the sizing tables would suggest (but this only applies to the portion of the piping that serves only the water heater).
    – Greg Hill
    Jan 12 at 21:56

1 Answer 1

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Based on the 100 ft longest length from your system, the 1" pipe from meter to X is only good for a flow rate of 195 cubic feet per hour (from Table 1216.2(1), but you also fail under the other tables). This is inadequate for your 232 cubic feet per hour demand (assuming that's the correct demand).

It seems like you haven't figured out the "Longest Length Method." 100 ft is the longest length in your system. The flow rates vary as you've tabulated, but for navigating the tables you use 100 ft as the length for everything.

Bigger pipes are a good thing. They imply a smaller flow velocity (flow velocity is flow rate divided by cross sectional area). That translates to a smaller pressure drop.

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    I have to womder whether those tables and methods lead to unnecessarily large pipes. I am aware that I have limited experience and no formal qualifications, but I would say "put it in and expect that it will work" would be a reasonable, cost effective way to proceed (unless of course an experienced professional would say not). These 199 kBTU/h units would give good service even if they could not achieve max gas flow. Jan 12 at 22:23
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    @Jim, I'm no expert either. I do know that the IFGC has a "branch length method" that allows smaller sizes, but the meter-to-X mentioned in the answer still busts under that method. He could accept a larger pressure drop and find different tables, but I'm not sure if there are minimum outlet pressure constraints beyond the appliance requirements.
    – popham
    Jan 12 at 23:46
  • @popham, how does the meter-to-X break in that method? From the table, 50' of 1" pipe can meet a demand of 284, while my total demand at that point is 232. jim, are there any downsides to putting in and seeing if it works? Thanks to you both.
    – r_alanb
    Jan 14 at 13:46
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    @r_alanb, your table reading assures that the pressure drop at X is no greater than 0.5" WC. Assuming it's exactly 0.5" WC, the pressure drop will be greater than the 0.5" WC downstream from that node. There's an interesting argument to be made that by using the table twice, you can use a 0.5" WC drop table to validate a 1.0" WC drop system. I'm curious if a building official would come along for that adventure.
    – popham
    Jan 14 at 16:54
  • I might just have to find out as not having to change any of the pipes or meter would be great.
    – r_alanb
    Jan 16 at 3:04

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