Hot Water Heating
Hydronic heating. What exactly does this mean? Hydronics is the use of water, be it in a liquid or gaseous (steam) form, as a medium to transfer heat. The water is heated in a boiler, and distributed via a system of pipes, like duct work for forced air. Water has some interesting qualities that make it a far superior heat transfer medium compared to air. It’s denser (more mass) and consequently has a higher “heat capacity”. In fact, water has one of the “highest specific heat capacities among common substances”.
It’s also plentiful and non-toxic. We’re lucky to have such a beneficial resource!
Hot water systems are highly adaptable and can be categorized into three basic design categories – 1) high-temperature, low-mass; 2) high-temperature, high-mass; and 3) low-temperature, high-mass
A high-temperature, low-mass design would be your basic hot water baseboard system. Hot water, usually around 180 Degrees F is circulated through the pipes to the baseboard heat emitters. The baseboard itself is composed of a length of copper pipe, wrapped with aluminum fins, all of which is enclosed in a metal housing. As the hot water moves through the pipe, heat is transferred to the aluminum fins by conduction (both copper and aluminum are highly conductive metals, hence their use in this application). The baseboard enclosure is designed to foster a slight convective current with the surrounding air – as the fins heat up, the warm air rises out of the top, drawing the cooler room air through the bottom. There is a small amount of heat energy dissipated into the room by means of radiation, but the vast majority is due to the convective nature of the baseboard design.
The benefit of high-temperature, low-mass system, is a rapid response rate to heating demands; the high temperature of the water quickly heats the lightweight baseboard assembly; expediting the heat transfer into the room. In hybrid hot water systems, such as those composed of a mixture of radiant heat and baseboard, the baseboard is typically installed in areas that are seldom used, like spare bedrooms and offices, where a rapid increase in temperature would be desired.
As opposed to the “active” convection inherent in a forced hot air system, a baseboard system’s convection is very subtle, and not prone to distributing particulate matter. Hot water systems are an excellent choice for those suffering from allergies.
From a cost perspective, hot water baseboard systems are less expensive to install compared to forced air systems; duct work is more labor intensive than pipes. But if central air conditioning is being considered, forced air offers the additional benefit of having a preinstalled infrastructure.
High-temperature, high-mass systems are somewhat unique. They are composed of radiators (a high mass type of emitter) supplied with water temperatures between 150 – 180 Degrees F. Commonly found in Europe, hot water radiators are rarely used in the United States, and most of these types of designs are usually converted steam systems in older homes. While still an effective form of home heating, benefitting from many of the aforementioned attributes, radiators are bulky and considerable design concessions have to be made when room layout is considered.
This now brings us to the low-temperature, high-mass hydronic system. Most commonly found in the form of radiant heating systems, this genre of design offers unparalleled levels of comfort and efficiency.
Radiant heat has been used for centuries. The earliest known form was used by the Romans who called it hypocaust.
The hypocaust was composed of a “furnace” (basically a stove burning wood) and a series of hollow chambers beneath the floor of the living space. Heated air from the stove found its way through these spaces, and eventually up and out of the home through a chimney system. While not an efficient means (refer to the drawbacks of utilizing air as a heat transport medium) of heating, the basic principles behind this design would eventually find their way into modern radiant heat systems.
Radiant heat, in its modern form, utilizes relatively low temperature water (approximately 100 Degrees F) circulated through an array of tubes adhered directly underneath, or imbedded, in the floors of the living space. Unlike baseboard systems, which primarily rely on convection, the heat from the hot water is CONDUCTED through the pipe and directly into the floor. The warm floor then warms all the objects in the room, people included. You end up with a completely silent, even, means of heating a space through the two most efficient means possible – conduction and direct radiation. Infrared photographs of rooms heated with radiant systems show the highest level of heat energy at the floor level. The floor itself acts as a giant radiator, relying on the high amount of mass to radiate heat in the primary living space of a room; very little energy is wasted heating the ceiling area (a detriment of forced hot air systems).