“Radiant Heating 101” is a guide to one of Wright’s most used heating systems. People who live with radiant heat agree, there’s nothing like a warm floor on a cold day. Here we give an overview of the system. At the bottom of this page are links to articles provided by building stewards that furnish information on specific experiences. Each situation is unique, but hopefully this information will give a better understanding of this system, and how to maintain, repair, or, if necessary, replace it.


The first house in which Wright used “gravity” or radiant heat was the first Jacobs House in Madison, WI, in 1936. The system installed at the Jacobs House was originally steam-filled. Later, Wright would specify that the radiant system be filled with water. The advantage of water over steam is that steam creates an environment within the pipes filled with humid air, which encourages the corrosion of the pipes from within. Water-filled pipes keep air out of the system, limiting corrosion. In Wright’s houses the piping is typically placed in a layer of crushed stone, below the concrete slab.

In the Building Conservancy’s spring 1998 Bulletin, Gerald Lee Morosco, AIA, noted the following:

Initially, black iron pipe was used and later other ferrous metals and copper. However, it has become apparent that in some houses, the pipe specified in the original construction specifications was not that which was actually installed. Also, inconsistencies with the original construction documents have been documented relative to the size, placement and arrangement of the pipes. Though these possible inconsistencies are somewhat inconsequential relative to the short-term solution proposed in this article, it is important for you to document the actual materials used and the placement and arrangement of the pipes in your homes as carefully as possible. This information can frequently be obtained via construction records such as receipts and photographs.

Jacobs House, Madison, WI, Wright testing heating pipes, July 1937. Courtesy of the Art Institute of Chicago, full credit at bottom of page.

Jacobs House, Madison, WI, Construction view of heating pipes for living room, 1937.

The Nuts and Bolts of the System

Going into a Usonian utility space for the first time can be an intimidating experience. Given that Wright was trying to keep costs down, particularly in early Usonians, the space allotted to mechanical support was usually the bare minimum required. A space may accommodate more systems than just heating. For the purposes of this discussion, though, we’ll focus on a boiler-fed radiant heating system and its typical components.

A hot-water radiant heating system is a closed-loop system. In other words, the same water circulates through one or more loops and is replenished only if there is a loss of water in the system for some reason. A system may be laid out in different ways to provide greater control of heat. Here we are calling the components of these layouts loops and zones.

  • The simplest layout is a single-loop, single-zone  system (above, left). The water leaves the boiler, circulates under the floor, heat is gradually dissipated and the pipe returns to the boiler for reheating.
  • A single-loop, multiple-zones layout (above, right), is where a single pipe leaves the boiler and divides so that pipes can go directly to more remote locations before heat is dissipated.
  • A multiple-loop, multiple-zone layout is basically a combination of two or more of the single-loop, multiple-zone layouts. This type of layout will have multiple manifolds gathering multiple pipes.

Contractors may use different terms for loops and zones. For instance, the diagram shown below in “Helpful Tips” was created by J.A. Sweeton for the Sweeton House. It can be seen that the layout is for a two-loop system, gathering multiple pipes into two manifolds. Sweeton uses the term “panel” for each pipe. These panels are the same as the “zones” we’ve been describing; he uses the term “panel” instead of zone.

The following are components of a typical system. The components included in a specific system, and their locations within the system will vary:

A. The piping passes through a boiler’s coil where the water in the pipe is heated.

B. There is a burner attached to the boiler, this is where the fuel is ignited and a flame extends into the boiler to heat the water in the boiler coil.

C. A circulator pump attached to the piping loop, either before or after the boiler, pushes (or pulls) the water though the boiler. There should be an isolation valve on the far side of the pump from the boiler. This allows the system to be shut off, so the pump can be replaced (if necessary) without draining the system.

D. A temperature sensor that is wired to thermostats may be attached to the supply pipe. This temperature sensor is connected to a controller the turns the boiler and/or the pumps on and off based on the room thermostats or when the water temperature falls below the set minimum..

E. Back-flow preventers are on the auto-fill pipe and the supply lines of each radiant loop pipe, to keep water from flowing back into the domestic water supply via the auto-fill pipe or backward in the system.

F. An expansion tank contains a bladder that gives water an outlet within the system as it expands when it is heated.

G. If a system has a multiple-zone layout (see below), pipes returning to the boiler are gathered in a manifold into one pipe. Just before going into the manifold, there is a balancing valve for each pipe, which was set at the time of construction. Over the decades it is likely that the valves have become corroded and it is not advisable to try to adjust them (if the system is operating  well there is no practical need to adjust these valves). If adjustments are required, the valve will likely need to be replaced.

H. The water then returns to the boiler to be re-heated and go through the system again

Above is a diagrammatic view of a boiler and related components of a radiant heating system.

Below are photo examples of several of these components.

Manifold and balancing valves for a 5-zone system

Loop back-flow preventer

Gas exhaust, fresh-air intake

Back-flow preventer for auto-fill pipe

Burner on the front of the boiler

Temperature sensor (with red dot)

Circulator pump

Circulator pump and isolation valve with expansion tank behind

Electrical power switch for boiler, with J.A. Sweeton's diagram for system hung below

Helpful Tips

  • Consult a qualified contractor with experience working with radiant systems, and a qualified professional when required, for any adjustments or alterations to the system. Routine survey and maintenance is a key to prolonging the life of a system such as this.
  • Don’t drain your system. While it might seem like clean water would benefit the system, each time a system is refilled, it is an opportunity for air and contaminants to enter it, increasing the potential for corrosion and abrasion of the pipe interior.
  • Don’t adjust balancing valves (like those shown adjacent to the manifold above). They were probably set at the time of installation – over sixty to almost ninety years ago! They may be so corroded that trying to move them will damage or break the valve.
  • Most systems have a pressure gauge mounted at the boiler. If the gauge is reading zero, then this means the system has a leak and you should contact  your heating contractor.
  • You may consider install a water flow meter to gauge water added to the system. Small amounts are expected, many gallons likely indicates a problem.
  • Do you want to know what your radiant pipes are made of?  Look for the manifold where the pipes emerge from the slab, or if you have a simple loop, look for the pipe where it emerges from the slab. If the pipe is magnetic, you have steel or iron piping; if the pipe looks like a penny (shiny, or brown, or even verdigris), you have copper; if the pipe looks like a reinforced plastic hose (usually whitish or semi-translucent ) you have a more modern PEX type system.
  • Create a diagram of your system and post it in an obvious place. J.A. Sweeton did this and it was a great help to current owner Dan Nichols when he was first leaning how his system worked.

When things go wrong

A sure sign of a leak is noticeable increase in your water usage. There may also be physical evidence in the form of damp spots, warm spots or a hissing sound. Again to quote Gerald Morosco:

Radiant heating systems fail for almost as many different reasons as there are different houses, although many of the original systems… continue to function fully as designed. The most common, and most problematic, source of failure is loss of water and system pressure through leaks in the under floor pipes themselves. Most commonly, these failures have been attributed to corrosion of the pipes from the outside-in due to corrosive soils, erosion of the pipes from the inside-out caused by the friction of impurities and the velocity of the water being pumped through the system, and cracks created by settlement of soils and the gravel bed beneath the floor slab. In one Usonian-type house, designed by Wright apprentice Peter Berndston in western Pennsylvania, the system developed leaks when blasting for the construction of a nearby highway in the 1970s caused the gravel bed beneath the floor slab to settle.

The following are links to experiences shared to provide a variety of techniques for working with radiant floors. Please Note: The provision of this information or mention of a specific product or products does not constitute endorsement, recommendation, preference or approval by the Conservancy.

Walker House - Using a Stop Leak Product

Reisley House - Finding leaks by Ultrasonic Detector - and by Ear

Sweeton House - a Radiant Heating Repair Diary

For those with unlocated leaks, get a cat. It will be attracted to the warmth of the leaking water and will curl up on the concrete over it.

— Edith Payne, former owner of the Richardson House


  • Herbert and Katherine Jacobs Residence, Madison, WI, 1937. Frank Lloyd Wright, Herb Jacobs, photographer. Herbert and Katherine Jacobs Residence and Frank Lloyd Wright Records, Ryerson and Burnham Art and Architecture Archives, Art Institute of Chicago. Digital file #197701_220408-022.
  • Herbert and Katherine Jacobs Residence, Madison, WI, 1937. Frank Lloyd Wright, Katherine Jacobs, photographer. Herbert and Katherine Jacobs Residence and Frank Lloyd Wright Records, Ryerson and Burnham Art and Architecture Archives, Art Institute of Chicago. Digital file #197701_220408-023.

Posted April 3, 2023