Should I replace two 75-gallon (gas) water heaters in series with a tankless heater?

Question

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.

Answer 1

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:

https://www.thisoldhouse.com/plumbing/21124306/how-to-repair-a-shower-anti-scald-pressure-balancer

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.

Answer 2

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.

https://www.youtube.com/watch?v=Bm7L-2J52GU

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.

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* 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" :)

Answer 3

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.

Answer 4

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.