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News > Features > 01/01/2009  

Forced air hydronics  - by Roy Collver

Although radiant floor heating is the sexy darling of the New Hydronics set, there are many other great ways to use wet heat. A common system uses tubing in the basement floor to warm the concrete slab (creating a comfortable and useful living space downstairs) and a fan coil to heat the upper floors, allowing the easy addition of air conditioning and ventilation. Fan coils come in a variety of shapes and sizes, with many looking identical to your typical forced-air furnace, minus the gas piping and venting. Instead of a gas or oil burner and heat exchanger, a fan coil has a finned heat exchanger coil we run heating fluid through. As the air blows across the coil, it picks up heat from the fluid and is sent through ductwork, distributing the heat through the house much like a traditional furnace. Although most fan coils use standard sized and shaped metal ductwork, for some years now some manufacturers have been making high velocity fan coils that operate at higher than normal static pressures and use quite small, insulated flexible distribution ducts, allowing them to easily to hide their bones in standard wall and ceiling construction with few traces  a good choice in many applications. Any forced-air finned coil that uses hydronic heating fluid as a heat source can be classified as a fan coil. These devices include furnace-type fan coils, compact high velocity units, hydronic unit heaters, air make-up units and wall, floor, or ceiling mounted forced flow units. For variety, it doesnt get much better than this! The beauty of fan coils is that you can design systems to use them in a number of configurations and you can size them to take advantage of each type of system according to its best operating characteristics. Designing a system There are six basic factors you should consider when designing a system that will affect the heat output and operating characteristics of a fan coil. These are: 1) The minimum entering air temperature available 2) The maximum desired discharge air temperature required (A combination of factors 1 and 2 will tell you the maximum heat input required to the fan coil.). 3) The maximum desired air volume/velocity that must be heated on its way through the coil 4) The desired entering fluid temperature 5) The available fluid volume/velocity you have to flow through the coil 6) The desired temperature drop of the fluid as it passes through the coil Any fan coil you are considering to use in a hydronic system should have supporting technical literature that will allow you to calculate all of the six factors listed above. What degree of importance do we place on these factors? The minimum entering air temperature is always fixed by the application itself. For a residential heating system fan coil, the entering air temperature is the heating system return air temperature (usually 15 to 18?C). If the heating system brings in fresh air from outdoors or from a heat recovery ventilator, there will be a further drop in entering air temperature. If the fan coil is used to temper 100 percent incoming outdoor air; the entering air temperature will depend entirely on the physical location of the building; perhaps -30?C in Ottawa and 0?C in Victoria  check your ASHRAE weather data for the correct temperature in your area. The maximum required discharge air temperature will also depend on the application. For air make-up applications, the discharge air temperature will be the same or slightly less than the desired ambient air temperature. For heating applications, the discharge air temperature will be above the desired ambient air temperature. How much higher it should be will depend on how often we want to cycle our heating coil, based on how much air temperature differential we can live with. This is an important point, and a big advantage to using fan coils. With the exception of variable speed and modulating input units; fuel fired forced-air furnaces have one input and by necessity, must short cycle at warmer temperatures in order to not overheat the space. They also have a much higher heat rise than most fan coil units so the discharge air temperature is higher. Both of these factors can lead to occupant discomfort. Moving target The maximum desired air volume/velocity that you need be able to heat on its way through the coil is a bit of a moving target. At times, ventilation requirements will dictate this number, but normally it will be a function of how much heat (or cool) needs to be delivered to a particular space. The desired entering fluid temperature used to be a real easy thing to figure out. All you had to ask was: How hot can the boiler go? This was before we had to actually pay real money for energy. Now the best question is, can I run this sucker with a condensing boiler? Surprisingly, with fan coils, the answer is yes. We can make that condensing boiler condense ALL the time  just let us size it properly for minimum supply temperatures and maximum fluid temperature drops. Another advantage to designing for lower water temperatures is that other low temperature heat sources such as solar panels and geothermal heat pumps can be included in the mix  something we will see more of in the near future. The desired temperature drop of the fluid as it passes through the coil also used to be a real easy thing to figure out. All you had to ask was: How can we maintain a 20-degree temperature drop across the coil (to satisfy those high temperature boiler guys)? Nowadays, we are faced with designing systems to a higher standard and with a little more finesse. Again, condensing boilers change everything here. We want to try and design with as much temperature drop as possible so the return fluid to the boiler is as cool as possible, promoting better condensation and higher efficiencies. The real deal here is: fan coils are multi-tasking devices that can give the astute hydronics designer a serious amount of options. If you arent using them now, you should really investigate all of the different types and sizes of units available from the various manufacturers. You might be surprised at how often you reach into that particular tool box for an application solution.

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