Tankless Water Heater for Radiant Heat

Thanks to everyone who came out to the ADU on the Build It Green tour. It was fun to be able to show off the place to so many people, and I hope that it left some of you all inspired to see more ADUs.

This post will deal with the tankless water heater and radiant heating, which I've mentioned in other posts, but never fully described.

An ADU water line splits off the water main in the main house basement, and comes to the ADU underground. Some of that (cold) water is directed to the water heater, which heats the water to ~120 degrees whenever we turn the hot water (for a shower, sink, dishwasher). The water heater is "tankless", which means that it heats up water "on demand", only when needed. A tankless, on-demand water heater is inherently is more energy efficient than a conventional 60-gallon holding tank water heater, which keeps 60 gallons of water heated constantly, even when there is no need for hot water.

Meanwhile, a heat exchange heats up more water in the heater in a separate "closed loop" to a lower heated temperature (~70-80 degrees) that runs through the Pex tubing throughout the first and second floors, providing in-floor radiant heat. Radiant in-floor, hydronic heat is the heat source, along with some passive solar heating.

There are three forms of heating: radiant, convective, and conductive heat. Most of us group these three forms together in our head as "heating", but they function totally differently from one another. 

Forced air is convective, and although it is the dominant mechanism used for residential heating in this country, it is not a good way to heat most residences. Radiant heat is generally the most comfortable and effective way to experience and manage heat. 

Radiant heats works totally differently than forced air heat.  Radiant floor heating will heat the surfaces in the house to a particular temperature (~70 degrees), and those surfaces will then radiate that heat outwards towards the occupants, reducing our body's heat loss towards those surfaces.

Radiant in-floor heating is a comfortable way to experience heat evenly throughout a home. Interestingly, with radiant surfaces everywhere, air temperature in a house can actually be lower than what you think is comfortable, and you will still feel warm. This is akin to when you're standing outside on a cold night, with your front side is facing a campfire. That's radiant heat gain working its magic on your front side, and radiant heat loss working on your back side.

Conversely, forced air heat does not heat up surfaces; it only heats the air that is then blown throughout the space, and air a poor heat conductor. That heat is quickly dissipated because air does not have the thermal mass to retain that heat. That energy that was used to heat that air is quickly dissipated and lost. The surfaces of the house will remain cool to the touch. Even if the air is warmed in a space, cold surfaces can still make you feel cold. Cooler surfaces draw heat from your body.

Houses heated by forced air tend to have spots that feel cold- areas far from the vents, or areas near cold surfaces like windows. While in the US, homes ares primarily heated using forced air (a form of convective heat), we are alone in this approach globally. Europe, for example, uses a far greater percentage of other heat types--primarily forms of radiant heat.

My water heater is a Navien condensing combi tankless water heater. It's gas powered, which is a more efficient way to heat water than heating with electricity, but both power sources are available as options for the tankless water heaters. Here's a video of the gas line from the street to the ADU, which is the gas supply that is used for both the water heater and for the gas powered cooking range.



Here's a video of the water heater. As you can see, the water heater plumbing and mechanics are a bit complicated. We'll use the radiant heating in the concrete slab on the first floor to heat the house. There are  radiant loops running through the second floor as well, but we will rarely use them. The concrete, once warmed, will easily heat the whole ADU in the winter. Once warm, it would take days for the concrete (and ADU) to cool back down. The heat demand load on this ADU will be very low, due the ADU's small size, the high R-values in the walls, ceilings, and floors, and the lack of thermal bridging. I'm looking forward to experiencing how the heat system works, and to comparing the gas bills from the ADU and the main house.