Tankless Coil Water Heaters: Pros and Cons

photo: energy.gov

Install a tankless coil water heater and take advantage of your hydronic boiler, gas, or oil-fired furnace to heat your home and domestic water at the same time.

Check out our buying and installation tips for tankless coil systems and residential heating, including the pros and cons.

What is a tankless coil water heater and how does it work?

A tankless coil water heater is designed to perform as a coil-type heat exchanger to heat domestic water. They are installed in the boiler or furnace, and for some types, it is a slide-in option. 

As the coils are immersed inside the hydronic boiler, they become a part of the home space heating system. The heat is transferred from hot water inside the boiler to water that is running through the tankless coil exchanger, and further to the water fixture, for dishes, showers... There is no water storage, so water is heated on demand providing a continuous hot water supply.

Tankless coil devices are designed to provide optimal heat transfer while saving money on the total installation cost. If the cold water flows too quickly through the coil, it will lower the outgoing hot water temperature. You can control the temperature by installing a flow regulating valve on the incoming cold supply, 

Pros and cons

The advantage of the tankless coil systems is the absence of the storage tank and standby heat loss, which eliminates the need for purchasing a separate heating system - resulting in money savings. Water is heated on demand, flowing through the heat exchanger whenever the hot tap is open.

The tankless coil can also work as a supplemental heater.

A home heating system has to work to produce hot water. This is the main drawback, as the system is less efficient when space heating is not required during the summertime. The solution to this problem switching to a traditional electric or gas water heater during the summer.

Water heating  becomes more efficient when the home heating system is used more frequently during cold weather.

This is why tankless coil water heaters are primarily seen in colder climates.

Moreover, the coil system is combined with the storage tank to help tankless coil water heaters overcome problems with low efficiency during the warmer months. Now, the whole system has to be upgraded with the pump and other controls. The boiler will not operate all the time, only when there is a call from the thermostat for heating.

The pump will run colder water near the bottom of the storage tank through the tankless coil and move it to the top providing a stable temperature. The circulation will stop when the temperature on the thermostat is satisfied.

One of the reviews or studies done by " Consumer Guide to Home Energy Savings" is that in non-winter months, tankless coil water heaters can consume three BTUs of fuel for every one BTU of hot water these units provide.

Problems and how to solve them

As other HVAC systems, tankless coil water heaters also have problems.

If you notice that the water flow is lower than usual, mineral deposits or rust might have built up in the coil exchanger. This is one of the frequent problems as the tankless coils are built from small size pipes, which are subjected to clogging. To avoid further problems, the unit has to be flushed.

Use the white vinegar, citric acid, or any other available solution, and pump it through the tankless coil, therefore flushing the mineral deposits out. It is essential to have the isolation valves installed to isolate it from the potable water supply and make the service easier. This is not a simple DIY project, the project needs a specialized pump, so the recommendation is to call a  plumber.

If the coil leaks, it might result in potable water contamination with the boiler water, or even worse, it might damage the boiler. Leaks are caused due to the failed connections in the coil tubing or where there are plumbing connections. The pressure and the amount of hot water will be reduced.

An improperly adjusted mixing valve is another reason for the insufficient hot water supply, except for the mineral deposits.

As explained before, tankless coil water heaters are recommended for homes located in northern and colder areas, where the furnace and boilers operate most of the year. A supplemental water heating system should be provided in warmer regions, where the heating season is shorter.

Tankless coil and indirect water heaters

photo: energy.gov

While tankless coil water heaters are designed to heat water on demand and without a tank, indirect water heaters require a separate insulated storage tank. They also use a furnace or boiler to heat a fluid which then circulates through a heat exchanger inside the storage tank, heating the potable water. From there, hot water is delivered to your shower or a fixture.

The advantage of such a system is higher efficiency because the furnace doesn't have to turn on and off more frequently, saving you energy. You can even wrap a storage tank with extra insulation for additional benefits.

Indirect water heaters also have drawbacks. The main one is the cost, as you will spend over $1000 for the good quality one, which is more expensive than a tankless coil or other types.