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I'm adding a second bathroom back-to-back with the existing bathroom at my house and I'm trying to understand how I plumbed the additional bathroom incorrectly because I'm losing pressure when I turn both showers on at once. Note that I'm losing more pressure when I turn on the hot water than the cold. Here's my setup:

Cold water supply
I have a 1" copper cold water supply line coming into the crawl space of my house from the meter (I don't know the exact pressure, but the city I'm in says it's an average of 53 psi city-wide). Cold water then branches from the 1" pipe to a) the hot water heater with a 3/4" cold supply line b) the kitchen with a 1/2-inch line c) the existing bathroom with a 1/2" line. The 1/2" cold-water supply to the bathroom is roughly 30' in total distance from that 1" main supply line if you add up all the 1/2" piping to the shower, bathroom faucet and toilet (there are multiple 90-degree turns en route to these fixtures, but nothing out of the ordinary I don't think, though I think more turns means more pressure loss).

Hot water supply
As I said, there's a 3/4" cold water supply line that goes to the tankless hot water heater. Then the hot water exits the tankless in a 3/4" pipe as well. About 6 feet from the heater the hot water tees to a) the shower and sink in 1/2" supply lines running roughly 20 feet in total to those fixtures (again with some 90-degrees en route, but nothing seemingly out of the ordinary) b) the kitchen with 1/2" supply lines running a fairly long distance of perhaps 75 feet or so.

The Problem
I thought it would be ok to tee off of the 1/2" line that goes to the existing shower/bath to run 1/2" supply lines to the new shower right "behind" the existing shower. However, now that I've done that, when I turn on the existing shower/bath, and then turn on the new shower, the existing shower/bath pressure drops down significantly ("significantly" is based on sight because the shower itself just loses a ton of pressure when you look at it). Same happens with the cold water, but not nearly as much as the hot.

Last comment that I think might be relevant: when I say I turn on the new shower, what I'm actually turning on is a non-flow-restricted "compression" fixture. Is that perhaps what's causing this pressure drop in the existing shower? That compression fixture cranks water out at roughly 6 gallons per minute (I just filled a 5-gallon bucket nearly the entire way in 48 seconds to test it out). Maybe the hot water heater can't keep up with that? I have a Navien 240a that I think does 8.7gpm, which would be close, but still short, of the total number of gallons I'm using in my test here (1.8gpm for existing shower and ~6gpm in the compression fixture).

If that last comment seems really dumb and something I should've shared at the beginning, my apologies. If it's the compression fixture that's causing all of this because it cranks out so much water and so the issue will likely be resolved when the 1.8gpm shower (I'm in California) is installed thanks for taking the time to read this far and correct this ignorant amateur plumber.

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  • as they say, bigger is better :P. How long are the lines to the shower, are they on second floor ? Commented Jun 8, 2022 at 19:27
  • @isherwood thank you for taking the time to reply. Do you agree with Greg's reply below that since the hot water heater doesn't have enough throughput it doesn't matter what I do here with the piping as long as I go with low flow shower heads (note I "have to," at least legally, use 1.8gpm shower heads here). Appreciate your guidance/input. Commented Jun 9, 2022 at 1:44
  • @Ruskes it's about 12 feet, with 4 elbows (it's not efficiently laid out), from where the 3/4" hot water heater "out" pipe splits to the 1/2" inch to the shower/bathroom and 1/2" to the kitchen. I could reduce that run to probably about 5 feet and make it all 3/4" inch to the "base" of the shower if I thought it would help. At this point I don't think, unless someone said I was going to fail inspection (I might yet!), that I'd go back and run separate 1/2" lines to each shower from a 3/4" pipe. Commented Jun 9, 2022 at 1:50
  • I completely missed the mention of the water heater being tankless. That changes things a bit. Also, just "water heater". Hot water doesn't need heating. :) Commented Jun 9, 2022 at 12:57
  • Ha, a lifetime of shame for always qualifying with "hot!" A mistake I will not make again! Commented Jun 9, 2022 at 14:31

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While the piping could be a problem, you noted a big difference between the cold and the hot water. Before we consider the piping let's have a look at the water heater.

There is a limit to the volume of hot water a tankless heater can source. The exact amount depends mostly on the water input temperature, the temperature setting, and the heating capacity of the heater (BTU or kW). Most manufacturers provide a table or a graph of "temperature rise" showing the expected performance for each of their models. A plumber could (should!) use this resource to ensure that the right heater model is chosen for each job.

The published flow of Navien NPE-240A of 8.7 GPM is based on a temperature rise of 45° F. See the graph below from Navien Installation & Operation Manual.

output flow data

If your ground water happens to be about 60° F and if the heater is set for 120° F then you're really operating at 60° F rise; the data show you can expect about 6.5 GPM with those parameters. Your flow measurement with the bucket shows that the single shower is consuming almost the entire capacity of the heater.

When too much demand is placed on a tankless water heater, meaning more flow (gallons or liters per minute) and/or higher temperature, you'll get one of two outcomes. Which one it is depends on the design of the heater.

  • you get the flow you're asking for, but the temperature is lower than desired, or
  • water continues to be delivered at the set temperature, but nowhere near the flow or pressure you expected.

Given that your complaint is about loss of pressure, rather than cold water, I'll guess your heater is designed for the latter. (My Rinnai water heater is, too.)

Now, a brief word about the pipe sizing. I don't have ready data as to flow rates and pressure drops for 1/2" pipe with X length and Y elbows, but common practice would consider a 1/2" pipe undersized for feeding two showers simultaneously.

However, you're probably not in the mood to replace the water heater with one that can belt out 12 GPM at 60° F rise to feed both of those high-flow showers. Then the fact of the pipe being undersized isn't relevant. If you switch both showers to low-flow heads of no more than 3 GPM each then both the heater and the piping will be able to accommodate them. It sounds like you ultimately intend to install a 1.8 GPM model, so all should work out fine in the end.

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  • Greg, I'm not sure if I'm supposed to add comments like this, but thank you so much for the detailed explanation. I felt like it wasn't so much the 1/2" piping because the cold, as I said, caused less of a pressure drop and the run of 1/2" piping is much longer for the cold than hot (with everything else being equal). And that Navien chart says it all because to test this out I turned the hot all the way to the hottest it would go, which would mean the hot was attempting to get well above 100 degrees. Commented Jun 9, 2022 at 1:36

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