The manual will say something like
Flushing the heat exchanger with a descaling solution if mineral build up is evident. Scale build up will shorten the life of the water heater, descale heat exchanger thoroughly and repeat annually depending on mineral content of ground water.
(From Bosch 2400E NG user manual)
or maybe something like
Periodic descaling may be necessary in areas with high
mineral content in the water. Scale buildup in the heat
exchanger may result in lower flow rates, error codes of
A7 and E9 and boiling sounds (knocking and banging)
in the heat exchanger.
(From Bosch GWH 2400 ES NG User Manual)
so
Under what situations should a tankless hot water heater be descaled?
When mineral build up is evident on annual inspection.
Does it depend upon how hard your water is?
Yes
If descaling should be done periodically, how can I determine the right frequency for my situation?
By inspection.
If your 80 gallon tank turns cold at an unacceptable rate, either you have a broken hot water heater (like, a broken siphon tube), or you really, really like high water flow. For instance a lo-flo 1.5GPS shower head, given 2 parts hot to 1 part cold, should last 80 minutes.
I would start by looking at the water flow rate out each faucet (i.e. by sticking a gallon bucket under there and timing it with a stopwatch), and comparing that to best practices for efficient homes. Your problem might be right there.
Obviously you should be able to get the lion's share of 80 gallons of water out of the heater before it turns cold. It may be broken. For instance there's supposed to be a pipe or baffle to assure that incoming cold water is deposited in the bottom of the tank. If this has corroded and is dumping cold water at the top of the tank, your outlet pipe will tend to gulp up this cold.
You will not be able to buy another 80 gallon water tank. New tanked water heaters of that size must be "heat pump" types - they use half the energy but make a lot of noise. Also since they are pumping ambient heat into the water, they chill the area around the heater. Your HVAC system will have to work harder in winter, summer will be a wash.
Another complication with tanked heaters is they must now be kept at 140F or hotter to prevent formation of legionella, the bacteria responsible for Legionnaire's Disease. It was always a problem, but now we know it, so now it is a requirement. This higher temperature means more insulation losses and less efficiency. It also will cause scalding, so it absolutely requires new blending spigots with anti-scald features - traditional 2-knob setups cannot be used. This most likely means a low-flow shower valve.
Tankless heaters are excellent if you can provision the large electrical service. They are actually more efficient, since they only heat the water you use, and only when you're using it. Since they don't have an hour to preheat the water, they must do all their heating on-the-spot, so they need more powerful heaters. But you also don't need to pay them to preheat water for an hour or keep it at temperature. Tankless heaters also don't pay for insulation losses. Local heaters can be moved quite close to the point-of-use, so you are not heating a long hot-water-pipe run - nor waiting for it! However to keep power requirements sane, you must keep flow requirements fairly modest. Since they don't store hot water, legionella is not a factor, so you can heat to the more modest and safer (for scalding) 100-110 degrees F.
Using a tankless and a tanked together makes no sense. Putting the tankless after the tanked is useless since the tank must be kept up at 140 to stop legionella, so the tankless would never run until the tank is exhausted. Putting the tankless before the tanked might slow exhaustion of the big water tank, but probably not by enough to matter, and at a huge energy cost.
Best Answer
This kind of connection does not need pipe dope because a plastic washer or seal is compressed between two hard metal rims. The threads are not exposed to water pressure and are not part of the sealing so pipe dope would not be useful.
Twelve years ago I bought and supplied to the plumber who installed my tankless heater a set (hot and cold) of Webstone 50443 service and isolation valves. I had bought two of these Bosch heaters as discontinued models on sale at Home Depot, intending to put one in my house and the other in a rental house I owned, but I chickened out on doing it myself and hired a plumber. He acted like he had never seen valves of this type. These are specifically for tankless heaters and allow cutting off the cold water supply and the hot water piping of the house from the water heater and then opening up the heater for draining and for descaling with vinegar.
I had it descaled once in 12 years of service. The plumber who did the descaling was not familiar with these valves and wrenched one in the wrong direction over the stop. Luckily this didn't break it and I was able to remove the handle and restore normal operation. He used a pump in a bucket to circulate a couple of gallons of white vinegar but decided to turn the heater on with the vinegar circulating. I don't think that is a good idea.
I wonder whether descaling is necessary for most installations. If yours is new just use it for 10 years and then think about descaling.
What model is your tankless water heater? Mine is a Bosch Aquastar 125 (?), a discontinued model with a standing pilot light that does not require electric power.
Noritz seems to think that there is no one universal descaling time interval. http://www.noritz.com/blog/biggest-myth-tankless-water-heater/