Can You Get Radiant Heat in an ADU Without a Boiler?

By Buz Artiano, BuildX | October 2026

Watch: Designing Concepts & Tools for Net Zero Homes w/ Gwendolen St. Sauveur | BuildX Podcast #18

You want warm floors for your ADU in the winter. Somebody told you that heating the floor requires a boiler, and then somebody else told you your ADU is going all electric. Those two things contradict each other so now you're even more unsure. Then the site work came back and the foundation is probably going to have to be a slab, and now you're thinking that you definitely need floor heating because a concrete floor is the coldest surface anyone will ever stand on.

My name is Buz Artiano, founder of BuildX. We've built dozens of ADUs all over Massachusetts, mostly on the South Shore and in Plymouth County. We handle the entire process from design and permitting to construction. We've worked on lots where septic capacity decided the bedroom count, where zoning setbacks determined where we can build, and where ground conditions forced a certain type of foundation. Homeowners usually think about decisions on mechanicals last, but that's the one construction locks in first, and most of the time nobody tells you that.

Radiant heat isn't about the boiler. Radiant heat is about hot water moving through tubing in the floor under your feet. A boiler is one way to make that hot water, but it's not the only way. On the BuildX Podcast we put the question to Gwendolen St. Sauveur, a mechanical engineer who designs net-zero homes, and she told us about a piece of equipment we hadn't heard of at that time. We are going to explain what that machine is and when it's worth it for an ADU. We're also going to explain when you have to make the decision because you can't go back and add it later.

What Replaces the Boiler in an All-Electric ADU?

An air-to-water heat pump. It works similar to how a mini split on your neighbor's wall works. The only thing that changes is where the energy goes.

A conventional heat pump is air to air. An outdoor unit and an indoor unit trade refrigerant back and forth, and that refrigerant expands and contracts. The phase change is where the energy gets gathered, and the indoor head then dumps that energy straight into the room air. An air-to-water heat pump keeps the outdoor unit, keeps the refrigerant, keeps the phase change, and only changes the last step. Instead of using that energy to heat or cool air, it uses it to heat or cool water.

Once there is a tank of hot water available, a radiant floor has everything it needs to function. It makes no difference where the hot water comes from. A propane boiler and a heat pump both provide the floor with heated water. St. Sauveur told us that heat pumps for radiant floor heating is a trend she is seeing more of in her market.

Does Going All-Electric Rule Out Radiant Heat?

No. It just means you don't use a boiler.

Homeowners hear all electric and picture electric baseboards, or a big mini split head on the wall, and nothing else. An all-electric requirement is a fuel thing. It governs what burns on your property. It doesn't say anything about how heat travels once it is inside the walls. Radiant floors, ducted air, and wall-mounted heads are distribution methods, and one heat pump can feed any of those three things.

You need to know where your requirement for your ADU came from before you design around it. Homeowners get told they have to go all electric by a builder, a designer, a utility company, and a building department, and those four companies are not always talking about the same thing. Massachusetts has three levels of building energy code. The Base Code applies statewide, and an individual town can vote to adopt the Stretch Code or the Specialized Code on top of it. The rules for your ADU depend on which town you're in, and the Department of Energy Resources publishes a map of what towns go by which code at mass.gov.

How Does an Air-to-Water Heat Pump Actually Work?

It moves energy from outdoor air into water in a tank, and the storage in the tank is the whole thing.

The outdoor unit gathers energy the same way every heat pump does. Inside, rather than blowing warm air into a room, a heat exchanger transfers that energy to water held in a large insulated tank. That tank is called a buffer tank, and the buffer part is what's important. A buffer tank is a reservoir of heated or chilled water that goes between the equipment making the temperature and the system delivering it.

The reservoir is why the system works differently than heads on the wall. Air carries heat and stops doing that when the fan stops. Water holds on to it. The heat pump can run when it's the most efficient, and the floor can just use the water from the tank when the rooms need it, and they don't have to happen at the same time. St. Sauveur, who came to net-zero design out of mechanical engineering, called the difference between what air holds and what water holds the reason this whole method works efficiently, and pointed out that European builders have been doing this for years.

The Buffer Tank Is the Part That Changes Your Options

One tank makes radiant floor heating possible. But you're not limited to one tank.

The same air-to-water heat pump chills water just as well as it heats it. That means you can have a two-tank arrangement, a hot tank for heat and a cold tank for cooling, both of them operating off the same outdoor unit. St. Sauveur went a step further and described a third tank you can use that preheats domestic hot water off the same equipment as well. Preheating means the water gets to the water heater already partially heated, so the water heater doesn't have to work as hard.

But all of these things take up floor space, and floor space is very limited in an ADU. Our ADUs generally run between 600 and 900 square feet. A tank is not a component you hang on a wall between the studs. It has to sit on the floor, it needs space around it for service, and three tanks means they take up more space. That space has to be worked into the plan, not crammed in somewhere later. A mechanical room that isn't planned for ends up taking space from a bedroom, closet, or a pantry, and the person living there will notice that every day.

When Does Radiant Actually Pay Off in an ADU?

Radiant heat pays off when you have to use a slab as your foundation.

That's when you want to seriously look into it. If your lot conditions force you to a slab, the slab turns into an opportunity for floor heating instead of a disappointment. When we brought that up with St. Sauveur, she told us that her firm designs it that way.

The reasoning is this. A slab is the coldest surface in the building by default. It's a large mass sitting right on top of the ground, and it doesn't warm up just because the air above it warmed up. The complaint we hear the most about slab-on-grade buildings is cold floors, and radiant is the one system that solves that problem without just compensating with more warm air. The install works in your favor too. Radiant tubing is laid over the insulation and tied down before the concrete arrives, on a floor that is already being formed, already being insulated, and already being inspected. It goes in during work that had to be done anyway. That's much better than opening a finished floor two years later, that's a much bigger project. That is why we always ask the question during the foundation design, not during the mechanical walkthrough.

Mini Splits or Air-to-Water: Which Belongs in Your ADU?

Neither one is a default we use for every project. The best way to make this decision is to ask the same three questions about both systems and see which answers fit your lot, your floor plan, and how complicated you want to get.

Mini Splits

The heat gets to the room by an outdoor unit that's feeding one or more indoor heads mounted on the wall, and each head pushes air into the room.

It needs a wall space inside for each head and a line path back to the outdoor unit. No mechanical room is required.

It's the right decision most of the time. St. Sauveur's designs come with mini split layouts by default, and she gave us reasons that we also see in the field. Installers are comfortable with them, they are available nationwide across many makes and models, and cold-climate models keep running when it's five degrees outside.

Air-to-Water With Radiant

The heat reaches the room by an outdoor unit that heats water in a buffer tank, and tubing inside the floor distributes the hot water, which warms everything standing on it.

But this requires a mechanical room with floor space for one to three tanks, space around them for servicing, and tubing put down before the concrete floor is poured.

It's the right decision when a slab is already in the plan, when you want heating, cooling, and hot water preheat all from one machine, and when the floor plan has enough space to accommodate the mechanicals without eating storage the resident will miss.

When you compare the two, the trades are easy to see. The mini split gives you simplicity and a lot of options for installers. The air-to-water system gives you comfort at the floor and consolidates three jobs into one piece of equipment, but it takes up square footage and requires specialty labor.

Do You Have to Decide Before the Slab Is Poured?

Yes, and this is the one absolutely non-negotiable thing on the list. Radiant tubing goes under concrete, and concrete only gets poured once. On an ADU that happens in a single day, and once it cures your floor is a finished part of the structure of the home. Adding radiant heating afterward means removing the entire floor you just paid for. There is no retrofit path that keeps the slab, it has to be completely torn up.

Three things have to be settled before that day arrives, and all three belong to design rather than construction. The tubing layout has to be drawn and the detail set during foundation design. The mechanical room has to be located on the floor plan with real dimensions, because the tanks are going to stand in it. And the electrical service has to be sized for the equipment you actually chose, since the panel gets specified long before a mechanical contractor walks the job.

That is the whole reason we raise this early. A homeowner who first hears the words air to water three weeks before the pour is a homeowner making a permanent mechanical decision under time pressure, and that is a poor way to decide anything.

Should Every ADU Get Radiant Heat?

No, not every ADU should have radiant heat. We'd rather tell you that here than let you discover it after paying for a design that points you in that direction.

Mini splits are the default in ADU design for reasons that have nothing to do with cutting corners. St. Sauveur's own firm draws mini split designs as standard and only switches to geothermal, ducted, or air-to-water systems when a client requests it. When a designer who builds net-zero homes for a living starts there, that tells you a lot.

If your ADU is being built with a crawl space or a foundation with a wood floor system, if your mechanical space is already tight, or if the fastest way to a heated ADU is what matters to you, mini splits are the right answer and radiant is an upgrade, not a requirement. Talk to a mechanical contractor who doesn't work for us and has no bias before you commit either way. This decision is very permanent and you should be able to defend your reasoning when somebody asks.

Where the Radiant Decision Fits in Your ADU Timeline

Radiant heat in an ADU has nothing to do with a boiler and it's not an expensive luxury upgrade. It's a foundation question with a mechanical answer, and it's got a deadline. If your ADU build is being built on a slab, you have the option to put warmth into the coldest surface in the building, the floor, but your window to do that closes as soon as concrete is poured. If your site is not headed for a slab, mini splits will heat your ADU properly and you can stop thinking about tanks.