Isolaton Valve Kits (AKA Service Valves) are a great accessory for any tankless water heater purchase. They will save you money on the installation of your tankless water heater and they make future maintenenace of your tankless water heater both easy and convenient.
When you install your new tankless water heater you will certainly need both a hot water shutoff valve and a cold water shutoff valve. You will also need to plumb in a pressure relief valve (PRV).
With traditional tank heaters you can easily drain the tank for regular maintenance. But with tankless water heaters there is no drain to open up at the bottom. So, you will need some other plumbing strategy that will allow you easy access to the internal pipe of the tankless water heater.
Gas tankless water heaters have a single pipe that starts as the incoming water line, then wraps a bunch of times around the gas-fired heat exchanger and then exits as the hot water line. So, the only ways to maintain this water line is to either use a set of isolation valves or build in some extra valves and fittings to do the job. (Or course, you can always just dismantle your plumbing system, but that is not very convenient!)
We have been told time and again that the cost of the Isolation Valve Kit is less expensive than the many valves and fittings you would need to purchase to get the comparable functionality.
And, the labor to install the Isolation Valve Kit is much less than if you go without it!
Well, once you have your Isolation Valve Kit installed you are now in a positon to easily maintain your tankless water heater. With the Isolation Valve Kit in place you can easily run a Descaling Solution through the heater. Or, you can easily blow all of the water out of the line with compressed air if you need to stow your heater away for the winter.
In any case, planning ahead with an Isolation Valve Kit will have a big payoff.