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I'm a homeowner with a gas water heater that was installed in 2010 (before I moved in) in Central MD. It is a 71 gallon tank. State Select.

Sometimes the water is normal hot temperature, while other times it is cooler. There's no leak or anything else that looks odd on the tank.

https://i.imgur.com/hMuNvSX.png

Is it perhaps something with a thermostat that is causing inconsistent temperatures? Is it worth trying to have a plumber do a diagnostic / repair vs just replacing it, given the age?

I have a family of 5, and I read that 60+ gallons is recommended for that size. But when browing on Home Depot, I only see commercial grade heaters above 50 gallons, so I'm not even sure what to look for. Also, do I need to get a tall / non-tall one based on the size of my current tank?

Any advice is welcome. Thanks

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    You write "Sometimes the water is normal hot temperature, while other times it is cooler." then "I have a family of 5...". Doesn't the second explain the first? i.e. The hot water reserve will "sometimes" be exhausted by the time the 5th person goes to shower (or whatever)...right?
    – AA040371
    Aug 20, 2023 at 12:46
  • The water being cooler doesn't seem to be due to the use of the water though. e.g. it's not as if it's cooler because we just took 3 showers. But I can't get an exact pattern of it. Aug 20, 2023 at 13:05
  • I see...although lacking a pattern may make it hard to troubleshoot (hence the frustration, I am sure!). Speaking anecdotally for myself, my two 40 gal water heaters have last almost 20 years because (I assume) I perform the suggested maintenance suggest over at FamilyHandyman every year like clockwork (search on "family handyman water heater maintenance"). One other thing to check is your anode rod...if it needs replacing, this could easily explain the sporadic performance you are experiencing. Best of luck...
    – AA040371
    Aug 20, 2023 at 13:12
  • Tell your family members to take shorter showers. Run the tank until failure. In a rental I own there is a 80 gallon electric water heater that is about 40 years old works fine!. No point in replacing until it starts leaking. Aug 20, 2023 at 18:14
  • Could you install a flow and/or temperature sensor to do some testing?
    – Huesmann
    Aug 21, 2023 at 16:30

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There are a bunch of different issues that can affect how well a hot water heater performs. Some are design issues (is it big enough for your typical load?), some are installation (are the pipes connected properly? (Yes, they can be connected improperly - I had that...)), and many are due to age (which gets to maintenance issues, which I suspect most people ignore).

That being said, I can explain the replacement issue. 71 gallons was a typical size in 2010. At the time, you could basically use any size water heater you wanted, they were almost all the same efficiency (80% for gas, 100% for resistance electric) and they were extremely simple devices that had only changed incrementally (e.g., better insulation) for the past 50+ years. If you wanted more capacity, you got a bigger water heater. Done.

Now things are different. In ye olden times, if you wanted to buy a more efficient appliance vs. a less efficient appliance, you could, as a consumer, look at the total cost of the appliance (fixed cost, easy), installation (one-time cost but with tremendous variability depending on individual situation - type of house, related upgrades needed (electric supply, new exhaust vent, etc.), local labor costs, etc.) and fuel costs (impossible to predict 100%, but can be estimated for the life of the unit based on expected usage and current fuel costs (electricity or gas)). You then factored in additional advantages or disadvantages of each appliance (e.g., natural gas - works even when electricity is out; electric - if you convert everything else to electric then you can drop the natural gas service and stopped worrying about carbon monoxide) and made your own decision.

Now, the US government, as governments often do, has made some decisions for us. In order to meet broader goals (at least nominally...), energy efficiency has been forced upon us. In the case of water heaters, this takes the form of specific energy efficiency requirements. Effectively this has made a split into two groups of residential tanked water heaters - 20 - 55 gallon and 55 - 100 gallon. Within each fuel type, the specifications vary.

For electricity, the result is extremely clear. It is impossible to get > 100% efficiency out of a resistance electric heater. So the result is to require a heat pump electric heater (which is on the order of 200% efficiency, when compared to resistance heating) for larger than 55 gallons. Note however that, for ease of installation (which == cost of installation), most heat pump electric water heaters are installed entirely indoors. Which negates some of their energy savings in winter months...but that is ignored in the broad ratings for reasons.

For gas, it gets a bit more complicated. A typical water heater (or furnace) was 80% efficient for quite some time. In order to meet the new requirements, a 55-100 gallon water heater needs to be significantly more efficient. How much more efficient? I'm not sure! My hunch is that they're aiming towards the equivalent of a 90% - 95% efficient furnace. Otherwise known as a condensing furnace, which makes use of much more of the heat produced (good) but produces much lower temperature exhaust gases (not always so good). Why is low temperature exhaust not such a good thing? Because heat rises and (basically) goes up and out of your house with no extra work. With higher efficiency, there may be extra work needed (power venting) and/or shorter/different exhaust venting and/or handling of condensate (water is a natural product of burning natural gas).

In fact, in a quick search I have not yet found any water heaters that comply (as far as I can tell) with the new standards. They are either much more expensive and don't comply anyway, or they are officially listed as commercial instead of residential. (Yet another game governments and manufacturers play - see also "light trucks vs. cars").

The bottom line is that if you:

  • Want to replace a gas water heater with a gas water heater (to stick with the same fuel type, avoid having to increase electrical service, etc.
  • Don't want to spend on the latest & greatest highest efficiency water heaters (which, if you can find them, will likely cost quite a bit more than the old stuff and likely require new exhaust venting, which can cost quite a bit depending on your particular house)

then you can either make do with one 50 gallon tank or install two 40 gallon tanks. Yes, that's a thing. Call it a loophole. Or just call it normal - people have done that (or 2 x 71 gallons even) in the past if they needed more capacity than a single tank can provide.

How to connect them? I'm not sure. A good plumber should know what to do. You will need to split the gas supply (relatively easy), split the cold water supply (easy), combine the hot water output (easy), and join the two exhaust vents together (easy if you have a high enough ceiling above the tanks, not so easy otherwise).

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    Thanks for the information. I'll stick with a single tank for space purposes / less complexity. Hopefully 50 would be enough. On the label I posted here, I see a "thermal efficiency" of 80. I assume that means 80% efficiency. On heaters listed on home depot, their efficiency is just listed as "high", and I'm not sure waht that even is. Aug 20, 2023 at 23:03
  • A straight combustion appliance (at least with natural gas) is ~ 80% efficiency. In other words, 80% of the heat produced by burning the fuel is used to heat the water and rest is wasted. To get to 90% (or even 95%) requires a condensing appliance. That requires a somewhat more complex appliance and different types of exhaust ducts (cooler output air so it doesn't necessarily go out of your house unassisted but it must go out to avoid CO2 or (worse) CO problems) and has to handle condensate (with hot exhaust the water in the exhaust doesn't condense inside your exhaust ducts because the Sep 19, 2023 at 17:09
  • air is very hot). The result is that for a typical replacement gas water heater you stick with 80% (if you are that fanatical about energy saving despite the up-front cost then you would go all the way to an electric heat pump water heater). Sep 19, 2023 at 17:10
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A bit late to the discussion, but another possibility is that your local electric utility has installed a time-of-day control on your water heater so they can help reduce peak demand by turning off the electrical supply to high-load electrical devices when needed. Typically they don't switch it off for a long continuous duration. But depending on when you use significant amounts of hot water, the timing might be that you're using a lot of water when there isn't electricity available to immediately heat the incoming water back up to normal hot water temperatures.

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  • It's not clear to me why the electric utility would install a time-of-day meter on the gas water heater. Yes, there may be an electric ignition and/or control system, but neither of those would strike me as high-load. Now someone else might experience this problem with an electric water heater, but this particular asker wants to know about gas.
    – mdfst13
    Mar 18 at 0:23

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