Tankless coils(or just a "coil") use the heat from a boiler(normally oil fired) to transfer heat to the domestic water.
The coil is actually bolted into the boiler, but it's like an entirely separate system.
The coil itself is basically a long run of copper tubing, with fins on the outside of it, wrapped around and around.
The inlet and outlet of the coil are only a few inches apart from one another, but there's quite a bit of pipe in between.
By the time the cold water gets through all of the pipe, it comes out very hot on the other side.
The downside to a coil is that it is directly tied to your heat source. So if you lose heat, you have also lost hot water.
This also means that your boiler will need to maintain at least 160° year round.
More often, the boiler is set to maintain 180°. This isn't really as bad as you may think. The heat loss in the summer is no more than
a hot water heater, and you don't have to pay for the electric of a water heater. Any way you choose to get your hot water,
you're going to have to pay. There are many new hot water products out there as well.
When everything is working properly, a tankless coil should give almost unlimited hot water. There are many problems and
exeptions to this, however. The one device that helps this happen, and prevents near-boiling water from coming out of your
faucets, is the tempering valve.
The tempering valve(or mixing valve or hot water extender) uses a thermostatic element to mix some cold water with the
hot water, slowing the flow of water through the coil and reducing it's temperature. This is another reason for the high
boiler temperature. This reduces the demand on the coil, as well as giving more flexibility to the heating source.
A common problem with the tempering valve is that the thermostatic element in it gets corroded. This causes the temperatures
to be off and needs to be replaced. The normal symptom is that the hot water runs out, or is just luke warm the entire time. If
the element doesn't take care of the problem, the entire tempering valve may need to be replaced.
Warning!: Hard water minerals can ruin the heat transfer capabilities of a coil. This can be fixed for a while,
but it will eventually destroy the coil. The hard water problem needs to be remedied with a water softener. A hot water
heater is another solution, although this doesn't actually fix the problem, it just greatly extends the duration between
service. In very hard water, an untreated coil may need service every week or 2. The water heater might go 2-4 years or more.