The resulting radiant heating density was a workable 19 Btuh/ft2 (60 W/m2) supplied to the room and an estimated 4 Btuh/ft2 (13 W/m2) of edge and back losses. The rest of the capacity required for the project (8.5 mbh/2.5 kW) was provided by in-floor radiant heat. The room wound up with two 8-ft/2.4-m lengths of baseboard (one on each side) that deliver a total of 6.2 mbh (1.8 kW) at the design water temperature (145 degrees F/63 degrees C average) and flow. The in-floor heat solved the cold floor problem and preserved wall space for bookcases. The design philosophy was to maximize the amount of baseboard that would fit under the windows and a few feet to either side where bookcases would not be located and meet the balance of the load with in-floor radiant heat. Pictures are normally mounted higher on the wall.Īfter considering the relative costs for baseboard and in-floor radiant heat along with the need to avoid a cold floor surface, the addition was designed with a hybrid baseboard and in-floor heating system. That risk is actually fairly small because supplementary tubing in a wall usually runs only a few feet up the wall. Tubing in the walls might also be susceptible to physical damage from activities like hanging pictures. Since the owner rearranged the furniture after construction was complete, supplementary tubing in the walls might have been a poor choice. Tubing in the walls would have to coordinate with the furniture layout. Tubing in the ceiling or the walls could also have been used. Conventional hot water baseboard was selected. Therefore, an in-floor heating design for the project would require some form of supplementary heat. The addition has almost as much building skin area as a larger (wider) structure that would have 50% to 80% more floor area.Ī heating load density of 34.7 Btuh/ft2 (109.5 W/m2) is difficult to meet with in-floor radiant heat and a carpeted floor. That load density is higher than a typical house because the addition is relatively small. That heating load translates to 34.7 Btuh/ft2 (109.5 W/m2). Those values include a 10% safety factor and 15% for pickup after night setback. The calculated heating load for the project came to 14.7 mbh (4.3 kW) composed of 9.9 mbh (2.9 kW) transmission loss and 4.8 mbh (1.4 kW) infiltration loss. The opening required for airflow would make the bottom shelf unusable. If bookcases were butted against baseboard, they would have to be modified to let air in at the bottom. The wall space required for the bookcases limited the wall space available for baseboard heat. The owner wanted free-standing bookcases. The proposed furniture layout also influenced the hvac system selection. Avoiding that problem required either some type of in-floor heat, or insulating and heating the crawl space. Nevertheless, the owner was concerned about discomfort (cold feet) from a cold floor surface. The crawl space is enclosed by foundation walls, and the band joist is caulked and insulated (R-30), so the crawl space stays warmer than the outside air. The room is used as a home office, so the owner would spend many hours sitting at a desk. The addition is built over a well-insulated (9 in/23 cm fiberglass batt/R-30) but unheated crawl space. The availability of boiler capacity and the relative ease of piping hot water over to the new addition quickly narrowed the heating system choices to hot water baseboard or in-floor radiant. If boiler capacity proved inadequate, money would be spent upgrading the envelope of the existing house rather than adding boiler capacity. In light of those changes, the boiler was assumed to have enough capacity to support the new zone. In addition, the new construction replaced two large, leaky windows with a well-insulated wall, reducing heat loss from the existing house. Both those changes reduced the load on the boiler. Since that time, the walls and ceiling were insulated, and a separate gas-fired hot water heater was installed. The boiler was installed when the house was built (around 1958) and provided both space heat and domestic hot water. The addition was to be heated by adding a zone to the existing boiler. It was (and is) heated by a gas-fired hot water boiler. A second companion to this article, also available on PME's Web site, compares functional aspects of perimeter baseboard and in-floor radiant heat.Īpplication to the Project The project that precipitated this article is an addition to an existing house. Others result from comparing well-designed radiant systems to poorly designed other systems. Some of those claims are truly attributable to radiant heat. Proponents of radiant heat claim many benefits over other types of heating systems.
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