We live in the Midwest in a large home that was built 20 years ago. At the time, we installed two 75-gallon gas water heaters, and the plumber did this in series, not parallel. We also have a system of pipes for hot water recirculating to each of our fixtures.

We seem to recall that we never got the kind of hot water capacity one would expect with that kind of system; certainly now we notice it's a problem. Sometimes, we have plenty of hot water (regardless of outside temps). Sometimes, when there are 6-8 of us in the house, we know that the hot water will be out after the first or second shower. And often we don't have enough hot water when just 2 of us shower in sequence.

We want to finally address this. The current hot water tanks are less than 10 years old. Our options include: (1) just replacing the hot water heaters; (2) replacing them and repiping them to be in parallel instead of in series; or (3) replacing them with one tankless gas. Obviously the last option is the most expensive, but we're willing to do it if it works. We anticipate we will likely move out of the house in 15-20 years. We're less worried about efficiency and much more worried about wasting our money on another system that won't work.

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    I would want to understand the problem before throwing money at it. You're asking us about a solution to something we don't understand. Do both units heat? What are their set temps? If you disable one, what happens? One 75 gallon heater should handle 4-6 people without issue unless you're complete shower gluttons. :) My family does well on one 40 gallon unit.
    – isherwood
    Commented Apr 19, 2023 at 19:15
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    I suspect your dip tubes are broken or your series-installing plumber took them out thinking they'd be a problem. Dip tubes let colder water enter at the bottom and hot water leave from the top. I can't imagine going through 2 HWH's of water quickly if the tubes are in place. Parallel is if you want two people to be able to shower at the same time.
    – Tiger Guy
    Commented Apr 19, 2023 at 19:17
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    I agree with the other two comments. There is another problem here that should be found. Two big tanks and two people, they should almost never run out of hot water, even showering more than 12 hours a day, each.
    – crip659
    Commented Apr 19, 2023 at 19:21
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    Why wouldn't the dip tubes explain past behavior? Recirc shouldn't be a problem unless there's some peculiarity about your system that we don't know about.
    – isherwood
    Commented Apr 19, 2023 at 20:11
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    In series would mean the cold water goes in first tank, then the hot water from tank 1 goes in cold water inlet on 2nd tank then it goes to faucets. Is that what you have ?
    – Traveler
    Commented Apr 19, 2023 at 20:57

4 Answers 4


A completely different cause (unrelated to the water heaters) might be failed anti-scald valves in one or more of various fixtures around the house.

Due to design of such valves, the fixture does not need to be running for it to feed cold water back into its hot water line (if it has failed), which might cause your showers to turn cold when you haven't used anywhere near the amount of stored hot water you have. If you run water at a fixture until the hot pipe is hot, and then have someone check the temperature in that pipe when you turn on the shower, you might find the culprit. Another common case (and perhap the only way where you can't touch the pipe because it's in the wall) is that "if shower A is running, Shower B works fine, but if shower A is not running, Shower B turns cold" which makes Shower A suspect of having this fault. If you can find somewhere to touch the pipes that's more certain...

Normally you can replace the cartridge in the fixture with the problem.

This link to This Old House (no affiliation) is the opposite problem, but the principle is similar - an improper connection being made between hot and cold:


This can also be the fault mode of recirculation setups that send water back on the cold water lines, if the check valve that should keep that flow strictly hot -> cold fails, so that cold -> hot. With dedicated recirculating lines that you can valve off, that should not be an issue with recirculation shut off.

  • Interesting. If the anti-scald valve is the problem, would replacing the cartridges solve the problem? We do have anti-scald valves for 5 fixtures. (BTW, took one shower post-shutting off the recirculator valve and pump on that one line and it was terrific. Waiting to see if that will be replicated several more times because our problem has generally been intermittent.)
    – Shelly Lee
    Commented Apr 20, 2023 at 17:30
  • Replacing the cartridge in the faulty one (or ones) should fix it, if that's the cause. I would not replace all 5 on general suspicion, just look for symptoms at each one and replace only those with problems. Which might be none, if it turns out to be the recirculation system.
    – Ecnerwal
    Commented Apr 20, 2023 at 17:39
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    I would just remove all the anti-scald valves so you can get the water temperture you asked for. Commented Apr 20, 2023 at 18:59
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    Ah yes, ignore/remove/disable modern safety measures and then claim no responsibility when someone too young, old, or temporarily less mobile than usual who can't adjust the water or get away from it gets second degree burns that were completely avoidable. Clap....clap.....clap. What a brilliant plan. And of course far more expensive than just replacing any failed cartridges, too.
    – Ecnerwal
    Commented Apr 20, 2023 at 19:15
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    Update: First hot water heater in the series was not lighting! (Swear that wasn't the case!) We replaced thermocouple which solved that problem. I refurbished the master shower stall Grohetherm valve (20 yrs old) by replacing the check valves and the temp cartridges. We had an air hammer sound in the shower when it was off when running the bath or flushing the toilet, and now we don't. I took the recirculating system off line by turning off the pump and closing an in-line valve. We now have lots of stable hot water! I will now check the valve on recirculating system to see if that's an issue.
    – Shelly Lee
    Commented Apr 27, 2023 at 14:29

We live in the Midwest in a large home that was built 20 years ago. At the time, we installed two 75-gallon gas water heaters, and the plumber did this in series, not parallel

That is the correct way to do it. This will become more clear once you understand how water heaters work, in particular, how hot water stays separated.


If the system is not performing to spec, you should maybe contemplate that it might be broken. For instance, if sacrificial anodes have not been maintained, it's possible a dip tube sacrificed instead. Or, one of the heaters may simply be inoperative.

Sometimes, when there are 6-8 of us in the house, we know that the hot water will be out after the first or second shower. And often we don't have enough hot water when just 2 of us shower in sequence.

That shouldn't be possible, unless you have a very unusual sense of what a shower is.

You should be able to get honest 125 gallons of stable hot water out of this setup. Let the whole system reach 'quiescent' (heaters achieve temperature) and then run a test. Go to a spigot, get a bucket and stopwatch and figure how many seconds to flow 1 gallon. Multiply by 125 and see if you can get that many seconds of reliable hot out of it.

Try slapping on California low-flow showerheads and rigid 10-minute timers on showers, and see if that helps. A 10-minute California shower should only use 15 gallons of hot water, tops.

We also have a system of pipes for hot water recirculating to each of our fixtures.

That's not helping. Aside from being wasteful, some of those circulate water onto the cold line! So now, instead of being consistently cold, the cold water line has variable temperatures. So you adjust the hot/cold to the temp you want, and it keeps changing... you think "the hot temperature is not stable" actually the COLD temperature is not stable! That "recirculate back onto the cold line" only works if you have self-adjusting thermostatic mixing valves i.e. the fancy joystick faucets.

Also most of those recirculators are cheap, and may well have failed, e.g. be circulating when they should not, or be leaking cold water back into the hot pipe at inopportune moments.

(1) just replacing the hot water heaters;

Sure, that will accomplish "replace anodes and check dip tubes", kind of an expensive way to do it though.

(2) replacing them and repiping them to be in parallel instead of in series;

"try everything"? No, that'd be worse for sure. Unless you split the house's piping so 1 tank serves half the house and the other tank serves the other half.

or (3) replacing them with one tankless gas

You guys seem really into simultaneous showers, so you might want to get California showerheads if you do that. I've seen tankless gas go not so well. My rule of thumb is 34,000 BTU* per GPM.

* This equipment is rated in "BTUs" per trade industry habits... but actually this means BTUs per hour. If you really want to get into brass tacks (and it might be worth doing so), a "BTU" is the energy needed to heat 1 pound of water 1 degree F. 8.3 pounds of water to a gallon, 60 minutes in an hour. So you can convert GPM into pounds per hour. Multiply by degrees rise in temp desired and you arrive at BTU/hr, commonly called "BTU" :)

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    I've used a recirculating system where it was done right (feeds back into bottom of tank). It actually does help. What it's doing is preheating the pipes and temporarily extending the hot water supply (assuming the hot water heater works correctly and clicks on as soon as it notices it has cold water in the bottom).
    – Joshua
    Commented Apr 20, 2023 at 20:22
  • The calculation above is based on actual heat into the water. A tankless heater (or any heater for that matter) is rated on the gas input to the heater, not the transferred heat. You have to take into account the efficiency when determining how much hot water you will get. Also, if you live at high elevations (my house is at 7500'), you have to derate the output of a gas-fired heater to compensate for altitude. In my case, the output is decreased by about 30%.
    – Llaves
    Commented Apr 21, 2023 at 12:42

Option (0) would appear to be to "address this" by fixing the defective installation, without replacing anything major, since it's very likely an installation problem or a simple part replacement, not a "junk this pair of water heaters, they must be the problem" problem. Water heaters are quite simple. If the tank isn't leaking, replacement is generally the wrong choice.

If ceiling access is low, then cross the bridge of draining and disconnecting one heater so you can lay it down and work on it. If adequate valving to permit that is not present, add it. You have an entire spare water heater, so you won't be without hot water while you fix one, and I have considerable doubts that you need two of this size.

Once laid down, inspect the anode rod and replace if needed, and inspect the dip tube.

As you seem to lack information about the current state of things, consider adding some thermometers when you put the inspected and renewed one in place; You could also increase the set temperature and then add a tempering valve (to lower the output below scalding temperatures) to the output, which aids in effective capacity and also kills off legionella bacteria.

For the sake of troubleshooting the problems, shut off and valve off the recirculation system, until you verify normal operation with it de-fanged.


I never run out of hot water with my tank less water heaters I have a triplex and all 3 units have them 1 could have done it for all. I have a huge jacuzzi in 1 unit and our daughter lives in that one, when my wife and I had it we filled it up never worried about running out of water. Some hard waters can clog the electric ones though, but they have great warranties, but I think electric ones cause somewhat of a hydrogen/oxygen fracturing effect and create that mineral build up, knowing that there are ways to prevent the issue.

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    If you are experiencing electrolysis (the H2/O2 "fracturing" you described) in your water heater something is very wrong. That probably also constitutes an emergency because 1) there is an exposed electrical connection somewhere in the device and 2) When H2 and O2 mix they tend to combine rather energetically if given an ignition source. Shut off power to the device immediately until you have fixed the problem.
    – cat40
    Commented Apr 21, 2023 at 2:05

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