Radiant heating, an all-electric heat pump, and a different understanding of residential comfort.
Something unexpected happened after we installed hydronic radiant heating in our own home. We began noticing that we weren't as cold when we went outside. Even after several hours away from the house, the warmth seemed to stay with us. My partner noticed it too. We found ourselves wearing fewer layers in winter, as though our bodies had acquired an additional reserve of warmth.
It's a curious sensation, and one I haven't found adequately explained in the literature on radiant heating. We all understand the pleasure of standing in sunlight on a cold day, or the difference between entering a warm room and a cold one. But this seemed to be something else: a more sustained sense of thermal well-being, as though our bodies were no longer beginning every encounter with the cold at a disadvantage. We feel, for lack of a better expression, more thermally resourced.
The experience has changed how I think about residential comfort. As architects, we spend considerable time considering the visible qualities of a building—its proportions, materials, daylight, and relationship to the landscape. Yet the thermal environment may be one of the most intimate architectural conditions of all. It influences how we inhabit a room, how we move through the house, and perhaps even how we experience the world after we've left it.
The Difference Between Heating Air and Heating Space
Most Americans are familiar with forced-air heating. A furnace or heat pump warms air, which is distributed through ducts and delivered into rooms through registers. The system responds to a thermostat, cycling on and off as the air temperature rises and falls. It is effective, familiar, and relatively straightforward to install in buildings designed to accommodate it.
Radiant heating works differently. Warm water circulates through tubing beneath the floor, gently raising the temperature of the floor surface. Heat is transferred to the room through a combination of radiation and convection, warming the surrounding surfaces and occupants rather than relying primarily on the circulation of heated air.
One of the important distinctions is mean radiant temperature, a measure of the combined thermal influence of the surfaces surrounding us. Our bodies continuously exchange radiant energy with walls, floors, ceilings, and windows. When those surfaces are cold, we lose heat to them, even when the air temperature seems perfectly adequate. When they are warmer, we can feel comfortable at a lower air temperature because our bodies aren't surrendering as much energy to their surroundings.
This is why a room with radiant heating can feel so different from one heated by forced air, even when both thermostats register the same temperature. There are fewer perceptible drafts, less temperature stratification, and no recurring blast of warm air followed by a gradual cooling period. The warmth is gentle and remarkably even. In a well-designed system, the equipment largely disappears from consciousness.
What remains less clear is why my partner and I seem to carry that warmth with us after leaving the house. There may be explanations involving skin temperature, circulation, thermal adaptation, or the body's response to prolonged periods of comfort. I wouldn't claim that radiant heating literally stores additional heat in the body for hours. But the experience has been sufficiently consistent, and sufficiently noticeable to both of us, that it has become one of the most surprising benefits of our renovation.
It also raises an interesting question: how much of what we call comfort is captured by the temperature displayed on a thermostat?
An Architectural Problem in Search of a Mechanical Solution
Our house in West Marin is approximately 1,500 square feet. When we purchased it, the ground floor consisted of a concrete slab-on-grade, while the upper floor was framed with exposed Douglas-fir beams and tongue-and-groove wood decking. That construction creates a beautiful finished timber ceiling for the rooms below, one of the house's most appealing architectural characteristics.
It also presented a problem. There was simply nowhere to conceal conventional ductwork without compromising the ceiling. Introducing soffits, chases, or dropped ceilings would have diminished the very quality we wanted to preserve.
This is a familiar challenge in California's midcentury modern houses. Many were built with exposed post-and-beam structures, tongue-and-groove ceilings, and minimal cavities for mechanical distribution. Their architectural character depends upon the direct expression of structure and materials. Retrofitting conventional forced-air systems can be surprisingly destructive, particularly when there is no attic or basement available for ductwork.
Our house presented another constraint: there was neither natural gas nor propane. Whatever heating and cooling solution we developed would need to be entirely electric.
These conditions led us toward hydronic radiant heating, but they also forced us to think carefully about how the system would be constructed and how summer cooling would be accomplished.
Radiant Heating Without a Concrete Slab
Radiant floors are often associated with concrete slabs, in which hydronic tubing is embedded directly in the concrete. The thermal mass of the slab stores considerable energy, producing stable temperatures but also making the system relatively slow to respond to changing conditions.
Our approach was different.
Over the existing ground-floor slab, we constructed a sleeper subfloor assembly incorporating a vapor membrane and Rockwool mineral-wool insulation. Above this, we installed a grooved plywood radiant subfloor system manufactured by Uponor, with PEX tubing fitted into the channels. The system is similar in principle to Warmboard, but was considerably less expensive.
This allowed us to introduce hydronic heating without pouring another concrete slab. The insulation helps direct heat upward into the living spaces rather than losing it into the existing slab and ground, while the relatively low thermal mass allows the system to respond more quickly than a conventional embedded-slab installation.
The construction is particularly relevant to renovations. It offers a way to introduce radiant comfort into existing buildings without substantially altering their structure, although the additional floor buildup, thresholds, stairs, and door clearances must be carefully coordinated.
For us, it was a practical solution to a difficult architectural problem. We could preserve the timber ceiling, avoid conventional heating ducts, and create a quiet, unobtrusive source of warmth throughout the house.
But heating was only half the challenge.
One Heat Pump, Two Different Ways of Conditioning a House
We selected a reversible electric air-to-water heat pump manufactured by SpacePak. Unlike a conventional air-to-air heat pump, which delivers conditioned air directly through ducts or indoor refrigerant units, an air-to-water heat pump transfers energy into or out of a circulating water system.
In winter, the heat pump produces warm water that circulates through our radiant floors. In summer, the refrigeration cycle reverses, and the same equipment produces chilled water that circulates through fan-coil units located throughout the house.
The distinction is important. We aren't circulating cold water through the floors, which would introduce condensation concerns and provide limited cooling capacity. Instead, the chilled water serves fan coils designed to transfer heat from indoor air into the water, with provisions for collecting condensate when dehumidification occurs.
This arrangement allows us to use two different distribution strategies, each suited to its purpose. Radiant floors deliver exceptionally gentle, quiet heating in winter. Fan coils provide responsive cooling during summer, without requiring the extensive ductwork associated with a conventional central forced-air system.
The concept is not new, and SpacePak recognizes these applications in its product literature. But there's a considerable difference between a system being theoretically capable of something and having a complete, well-documented method for making it perform reliably in an actual building.
We discovered that difference firsthand.
The Engineering Behind the Comfort
The SpacePak equipment offered the essential heating and cooling capabilities, but its documentation left many practical questions unanswered. How should the system transition between heating and cooling? How should water circulate through the different distribution networks? What arrangement of pumps, valves, and buffer tanks would provide stable operation? And how should the controls coordinate the heat pump with the demands of individual rooms?
These weren't incidental details. They were fundamental to making the system work as an integrated whole.
We ultimately engineered much of the hydronic infrastructure ourselves, developing the arrangement of circulation pumps, valves, holding tanks, and associated controls. We experimented with several thermostat and control platforms before settling on a system manufactured by HBX, which provided the functionality we needed.
This process required considerably more investigation and experimentation than specifying a conventional residential heating and cooling system. The components were available, and the underlying principles were well established, but the complete solution had to be worked out.
The experience revealed an interesting gap between the capabilities of contemporary building technology and the ways those capabilities are commonly implemented. Manufacturers often develop remarkably versatile equipment, yet the responsibility for integrating it into a coherent architectural and mechanical system can fall to the designer and installer.
For a custom home, that integration is particularly important. The objective isn't simply to make the equipment function. It's to make it operate quietly, efficiently, reliably, and with as little intrusion into daily life as possible.
Comfort Without the Usual Energy Penalty
The completed system has performed remarkably well.
Our approximately 1,500-square-foot house has been extraordinarily comfortable through both winter and summer. During the colder months, we virtually never turn off the radiant heating. In summer, the cooling system operates throughout the day and sometimes into the night, depending on outdoor temperatures.
Despite that sustained operation, our household utility bills have generally been around $400 per month in winter and $200 per month in summer.
These are whole-house operating costs, rather than separately metered heating and cooling expenses, and they aren't a controlled comparison with another mechanical system. Nevertheless, given California electricity prices and the consistency of the interior environment, we've been very pleased with the results.
Part of the efficiency comes from the characteristics of heat-pump technology. Rather than generating heat through electrical resistance, a heat pump moves thermal energy from one place to another. Hydronic radiant floors are particularly well suited to this approach because they can deliver comfort using relatively low water temperatures, which can improve heat-pump efficiency.
Equally important is the relationship between the equipment and the building. The radiant floors provide stable heating without the temperature swings associated with intermittent forced-air operation. In summer, the fan coils deliver cooling where it's needed. The entire system operates with remarkably little noise or perceptible air movement.
The result is a house that feels consistently comfortable without constantly reminding us how that comfort is being produced.
A Broader Opportunity for California Homes
I suspect this approach deserves much wider consideration, particularly in the renovation of California's midcentury modern housing stock.
These houses often possess extraordinary architectural qualities: exposed timber ceilings, generous glazing, intimate relationships with gardens and landscapes, and structural systems that give their interiors a directness rarely found in more conventional construction. Yet many were built with relatively primitive mechanical systems, limited insulation, and little accommodation for contemporary heating and cooling equipment.
Improving their environmental performance can be difficult without compromising their architectural character.
An all-electric hydronic system offers one possible answer. Radiant heating can be incorporated into new floor assemblies, while appropriately located chilled-water fan coils can provide cooling without extensive ductwork. The approach isn't suitable for every building, and the design requires careful attention to insulation, moisture, equipment sizing, controls, and distribution. But it can be particularly compelling where exposed structure and finished wood ceilings make conventional ducted systems undesirable.
There is also a broader opportunity for new construction. As California continues moving toward all-electric buildings, architects and engineers have good reason to reconsider the assumptions that have shaped residential mechanical design for decades. A single heat pump serving complementary hydronic systems can offer flexibility that isn't always apparent when heating and cooling are considered as separate problems.
The important work lies in designing the complete system, rather than merely selecting the equipment.
The Architecture of Feeling Well
What stays with me most about this experience is the unexpected quality of the comfort itself.
We began with a set of architectural constraints: a beautiful exposed timber ceiling we wanted to preserve, an existing concrete floor, and a house without gas service. Solving those problems led us toward a mechanical system that was unfamiliar, technically demanding, and ultimately more rewarding than we anticipated.
And somewhere along the way, we discovered that living with radiant warmth seems to have changed our relationship to cold itself.
I can't yet explain precisely why we feel warmer for hours after leaving the house. Perhaps future research into thermal perception and physiology will offer a clearer account. For now, it remains a shared and rather wonderful observation.
Architecture is often discussed in terms of what we see: form, composition, material, and light. But the buildings we inhabit also act upon our bodies in ways that are quieter and sometimes more profound. They shape our experience of temperature, sound, air movement, and physical well-being, often beneath the threshold of conscious attention.
Our experience with radiant heating has made me more attentive to those invisible dimensions of architecture. And on a cold winter morning, when I leave the house feeling unusually warm and find that the sensation stays with me, I'm reminded that some of the most meaningful qualities of a building are the ones we carry away with us.
