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I installed one years ago. I love it. It takes up very little room, is outside and provides limitless hot water.
As the original poster installed her water heater two years ago, she should have many years left on the warranty so she probably doesn’t want to install a new one. Also, tankless water heaters are appreciably more expensive than the tank style.
Great advice thank you so much very much appreciated.
As you may be aware a couple of years back, Australia has severe drought and serious water storage concerns. Various state governments virtually mandated max 4 minute showers, gave free water proof timers and it is now very socially unacceptable to have a long shower. Things like watering your garden, hosing your concrete and washing your car were actually banned and strictly enforced.
I have in my house manual that Australia’s water resources are limited and precious and to respect this with shower times. At least half my guests come from countries you can’t even drink water from the tap but many just don’t care and have ridiculously long showers, don’t put the exhaust fan on and leave water EVERYWHERE. What can you do? I tell them, do the internal eye roll, wait for my money and move on.
I would suggest getting a shower timer, recommend guests ‘respect the environment’ have limit showers to 5 mins and install a water saver head and ignore any bad review. My showers are usually less than 3 minutes, even if I shave or wash hair, but weekends I have baths. These use less water and are better for the environment.
http://www.couriermail.com.au/news/queensland/land-of-the-four-minute-shower/story-e6freoof-1111113123662?sv=a1ed751869715080703fd6819c93972f
I would find out how long the hot water lasts in the shower and let them know. Let them know its a new standard hot water heater, not a cheap one. If they both want hot showers they should take responsibility for each other.
Cliff
The hot water lasts longer for those who prefer a cooler shower, so there’s not way to tell people exactly how long the hot water will last.
Thanks Cliff I will do that. Good advice.
Food for thought…after you’ve explained that the water temp has been adjusted (basically nothing else you can do). Suggest the time limit (mentioned above) or a water conservation shower.
When I lived on a boat the shower was 1. Turn on water 2 turn off 3 lather 4 turn on water 5 rinse. 6 water off
depending on their background they may buy into the water conservation idea. (Or totally ignore you and make snarky comments in their review)
This is just one more thing to add to the idea box for the discussion
Our friend Mearns who owns the famous private island in Belize and used to post here, had a great solution to the long shower dilemma. He was on a rain tanks system like I am. Simply install a heating timer to turn off the hot water at 5 minutes. The sudden cold blast would cause people to leap out of the shower so fast it would make their heads spin, 
No, no, no! If they are the ones causing the issue, then do not refund nor apologize like this is your fault. Have they always been spoiled with an endless hot water tank? Since they mentioned that they shave etc. in the shower - were they insinuating that they acknowledge they take long showers?
I’m really surprised the topic of how long they are showering never came up. I understand you didn’t feel comfortable asking. But could you still send a text asking for a little more info since the plumber said nothing was wrong. Ask in a way that you’re trying to figure out if something is still wrong or maybe they take longer showers than the average person.
My guests have to wait to shower one after another if someone is taking really long showers. I have lived in so many different places and have never lived in a place where I had endless hot water.
Thank you. My guests have left now and hardly mentioned the hot water, have left a lovely note said they loved staying and will give us a good review. So maybe I was worrying unnecessarily. Do appreciate your concern and response. Linda
Can you go there yourself and take a shower and see how long it takes before it runs cold? I have one of those little 3 minute hourglass timers on my shower and ask them to keep them to 3 minutes and turn off the tap while brushing their teeth. Mainly because I am on a rainwater tank and have to pay to get it filled up when it runs dry.
I’m on limited hot water too but also try to conserve water and power for the sake of the planet. I have this as one of my house rules:
“Our family home and this studio share one electrically heated water cylinder - please limit showers to 10 minutes so there is enough hot water for everyone.” and have only had one cold shower in 200+ stays. The guest’s shower is super powerful too which I think helps 
I suggest the local beach park which has a solar heated shower with niagra falls pressure.
i was thinking all of this as I was reading these comments. I’m sure CA has it’s fair share of drought regulations too. Put up a waterwise sign. Guests, being on holiday, will probably ignore it but you can at least point it out if they complain.
Even in Qld, when the govt introduced that 4min “rule” I think every woman who washes her long hair had a groan at the bureucrats who came up with that. I do recall those years of keeping a bucket in the shower so you could use the run off to water plants.
Easiest solution? Replace the shower head with a MUCH lower flow one. Normal CURRENT showerheads only deliver 2 gallons/min but you can get lower flow ones too. Also, replace the faucet aerators with flow restrictors. These two things could double or TRIPLE your time before you run out of hot water.
It seems no one mentioned the water temperature setting. I am not familiar with your boiler, but the capacity will also depend on the usage of hot water. So if the water temp is set at max (for those guests only), you’ll be able to have tem mix the hot and cold water so they can use for longer showers / more people. I also like Narwhal’s idea, more of an initial cost though.