Why Hydronic Radiant Heating Makes Sense
An architect builds hydronic radiant heating into her own home.
An interview with Elizabeth A. Gibb, Architect
Interviewer: Bruce Thaler, Communications Coordinator, Hydronic Industry Alliance
Q. How would you describe your house and why did you decide on hydronic radiant heating?
Gibb: My family had been living and working in the Boston area for over a decade. When we decided to remain in the area for the long term, we began the search for a house. I was very fortunate to have the rare opportunity to build a new home in Cambridge, Massachusetts. This new 4,000 sq. ft. contemporary-style house was built on a tight sloping urban site just over 5,000 sq. ft. In addition to our main living spaces, this 3-story house has a separate studio apartment with its own private entrance. Many decisions occurred over the course of the design and construction of our house. Very early on in the process we decided to install a radiant heating system, a form of hydronic heating. My husband, who is German, grew up in Italy and Germany where hydronic heating is commonplace. I also educated myself on the various aspects of hydronic heating. With radiant heating, a form of hydronic heating, hot water circulates through extremely durable tubing installed in or under a home’s floors, walls or ceilings. This form of heating produces very even, stable temperatures, produces no sound (therefore is completely silent), and from my point of view as an architect is invisible in the interior spaces.
Q. What did the hydronic radiant installation require?
Pouring the 1-1/2" light-weight concrete slab embeds the hydronic radiant tubing.
Gibb: There are various installation options for radiant heat. For our home we decided to embed the radiant tubing in a light-weight concrete slab poured on top of the plywood subfloor. After the construction of the wood floor framing, we installed the radiant tubing in close loops across the entire floor surface. Then we poured a 1-1/2" light-weight concrete slab on top of the regular floor framing. This concrete slab helps provide the outstanding comfort and energy efficiency of hydronic radiant heating. The mass of the concrete retains the heat of the system. That means the homeowner continues to feel the warmth of the heating system at substantial operating cost savings. The concrete slab requires additional structural modifications because now the deadload of the structure must include the weight of the concrete. Radiant heating also requires insulation installed underneath every interior floor. This prevents heat loss and ensures that all the heat from the hydronic radiant system goes to the living spaces above the tubing, and not downwards as well. Because of the insulation in each floor, we also used recessed lighting fixtures that were rated to work with the insulation.
Q. What about air conditioning for this hydronically-heated home?
Gibb: For air conditioning, we installed a high velocity AC system. This system transfers cooled air from an AC unit to the rooms of the house through small, flexible ducts that are able to weave inside the 2" x 6" stud walls. The house does not have an attic or basement so additional planning was required to locate the larger return ducts. The supply for cooled air is a series of 2"-diameter outlets, with the number of outlets determined by the size of the room. It is virtually unnoticeable.
Q. How about the installation costs of hydronic radiant heating?
Gibb: A hydronic radiant heating installation does come with additional costs. For our house, these costs were for the 1-1/2" light-weight concrete slab over the regular floor framing, additional structural framing to carry this added dead load of the framing, the insulation under every interior floor, the separate AC system, and the recessed lighting fixtures rated for use with insulation.
Q. How do the installation costs and benefits balance out?
Gibb: Aside from the unsurpassed heating comfort, there are cost benefits as well to a hydronic radiant installation. These derive from the operating cost savings of hydronics. And, as I’ve mentioned, the mass of concrete slab under each floor retains heat for a long time. Here is how our heating bills for our new home compare to those of our previous condo, both of which were fueled by natural gas. The 1,500 sq. ft. forced-air-heated townhouse condo was in the middle of the condo complex, so the exposure to the outside was limited to the roof, front wall, and back wall. During the very coldest winter months, our heating costs at the condo were about $250-$300 per month (17-20 cents per sq. ft. per month). In our new 4,000 sq. ft. house, which is fully exposed at the roof and the entire perimeter of the house, the heating cost is about $350-$400 per month (9-10 cents per sq. ft. per month). The operating cost savings are significant, and, for my money, the advantages of hydronic radiant heating are more than worth the additional installation costs.
Q. What, specifically, are the advantages of hydronic radiant heating?
Architect Elizabeth A. Gibb specified hydronic radiant heating for her new house in Cambridge, MA.
Gibb: First of all, other than routine maintenance, once you set the boiler’s programmed controls, you just don’t have to think about it any more. The boiler has a thermostat to the outside, so as soon as the outside temperature gets below 55 degrees F., the boiler water rises so that the reaction time of the heating system is quicker and the indoor temperature is steady and does not fluctuate. In addition, hydronic heating allows us to have temperature control over each room of the house. For example, our guest room is rarely used in the winter months, so we keep the door closed and set the temperature in that room at 60 degrees. This considerably cuts unnecessary fuel costs at no loss of heating comfort. And the heating comfort is superb. There are no hot or cold spots in any of the rooms, never a feeling of “It’s too hot” or “It’s too cold.” It is a gentle, even heat that is calibrated to be at its optimum at about 5'6" from the floor. Walking on our wood floors is a pleasure because the floors are always warm and never even cool, even on the coldest days. The bathrooms, of course, have tile and stone floors, so it is always comfortably warm to step out after a shower. I especially appreciate that you don’t see the components of the hydronic radiant system in the home. This heating system is totally unobtrusive. Furthermore, the hydronic radiant heating is absolutely silent; you don’t hear a thing.
Q. How is domestic hot water provided for the house?
Gibb: Domestic hot water is never a problem because our boiler comes with an indirect water heater attached to it with dedicated piping. The hot water for the radiant heating system is in a separate closed loop system, and operates independently from the domestic hot water supply. No matter how many people are staying over at our house, there is always plenty of hot water. And there is no need for a separately fired water heater.
Front view of Architect Elizabeth A. Gibb’s home with hydronic radiant heating.
Q. Now that you’ve lived with hydronic radiant heating, are you happy with your decision to install it?
Gibb: Definitely. It is hard to communicate the high quality of living in spaces heated with radiant heat. Now as the temperatures start to drop, without any effort on my part the boiler turns on circulating hot water through our floors quietly warming our spaces.
Elizabeth A. Gibb is a practicing architect, principal in her own firm (Elizabeth A. Gibb, Architect), and an assistant professor of architecture at Wentworth Institute of Technology. She graduated from the University of Michigan in 1981 with a BA in Biology and in 1982 with an MS in Microbiology/Immunology. She received her MArch degree from SCI-Arc in 1989. Her firm’s work includes a design-build residence. She has taught courses covering the design of the construction detail, integrated building systems, and methods of structural and envelope design. Before establishing her own firm in 2000, Elizabeth practiced with Machado and Silvetti Associates. She oversaw the construction of residential, academic, and institutional buildings, many of which received national recognition, including an AIA Honor Award. She has taught at the Southern California Institute of Architecture, the Rhode Island School of Design, Northeastern University, and the Boston Architectural Center.