Fan coils can easily be hidden by installing them within walls or ceilings
By Roy Collver
“Fan coil” as it relates to hydronic heating and cooling is a catch-all term. Multi-zone ducted heating/cooling fan coils are usually defined as “air handling units” and are considered fan coils. This article deals with less involved hydronic terminal units that range from simple suspended heat-only unit heaters, up to heat/cool unit ventilators and simple ducted systems. When I plug the words “fan coil” into my search engine, I get 144 million hits in 0.59 seconds. Many of them are electric or heat pump units, but a large percentage are hydronic. Let’s simplify and get down to just the basics of how they work and where and when they are best deployed.
How things work
Hydronic heat is delivered to a space in four basic ways: 1) radiation 2) conduction 3) convection 4) forced airflow. Many hydronic heat terminal devices use a combination of the first three. Despite the names, radiant floors and radiators deliver heat through conduction and convection as well. Convectors are designed to induce airflow over a finned tube or coil via the chimney effect of hot air rising through an enclosed cabinet containing a hot water coil. Room air travels into the cabinet bottom and is heated by the coil before it exits through the top. More output requires bigger coils and taller cabinets—increasing airflow and delivering more heat. It wasn’t long after electric fans were invented that bright people decided to affix them to convectors, greatly increasing their heat output without having to increase their size. Enter the fan coil.
Getting creative
Fan coils are made in a variety of shapes, sizes, and configurations. Fit them in closets, recess them in ceilings or walls, make them free-standing and decorative, or hide them away in basements and attics. They can be ducted or discharged directly into occupied spaces. Some only have a heating coil, some have a heating coil that can double as a cooling coil, and some have one coil for heating and another one for cooling.
They can be used as spot heating and cooling to fix many comfort issues and solve design challenges like rooms with large amounts of glazing. Look at different manufacturer’s offerings and you will likely have some “AHA!” moments as you discover what’s available.
Fan coil basics are pretty simple. In heating mode: 1) a circulator pumps a heated fluid stream through a coil 2) a fan pulls or pushes air over the heated coil 3) energy is transferred from the heat source to the coil to the air stream. The temperature drop through the coil depends on water temperature and flow through the coil vs. the air temperature and flow over the coil. It is a two-part process that requires tight control for efficiency and comfort reasons. Poorly designed air moving systems can waste energy, create thermal and physical discomfort, and noise. There is a big quality difference between manufacturers and models. High-quality fan coils will be provided with internal acoustic insulation and quiet fans can hardly be heard from a few feet away.
Look at all of the design criteria, architectural desires and constraints, configuration, physical size, available water temperature, heat and/ or cool, condensate removal, two pipes vs. four-pipe, air filtration requirements, integration with the exhaust or fresh air, control requirements, system integration and compatibility with boilers, pumps, etc.
Consider the design restrictions: noise, blowing air discomfort (velocity, temperature), budget restraints, desired level of comfort, and distance of air throw (projection and spread for unit heaters and un-ducted fan coils, or face velocity for ducted systems).
There is a wide span of choices, but the good news is that the variety of equipment available allows designers to pick and choose.
Use your resources
The best starting point is to pick a fan coil that you think will work in your application—then drill down into the technical data supplied by the manufacturer. That would be the literature with all of those charts and graphs and multiplication factors for airflow and water flow and heat output, along with the pictures of all of the options and configurations. Keep in mind that the numbers are intended as “maximums at design conditions.” When you get into the design stage, things can get tricky because we are seeing fan coils in our market from all over the world now. Terminology can be confusing between manufacturers. Some common terms and their abbreviations used in Canada when designing with fan coils include:
- (EAT) Entering air temperature for heating = return air entering fan coil
- (EDB) Entering dry-bulb air temperature for cooling = return air entering fan coil
- (EWB) Entering wet-bulb air temperature for cooling = return air entering fan coil
- (OAT) Outlet air temperature = discharge air leaving fan coil
- (EWT) Entering water temperature = hydronic supply water temperature entering coil
- (LWT) Leaving water temperature = hydronic return water temperature leaving coil
- (AWT) Average water temperature = average hydronic water temperature across coil
- (ESP) External static pressure = air pressure developed by the fan across factory installed components
- (CFM) Cubic ft/min = volume of air pushed or pulled through fan coil
- (USGPM) U.S. gallons/min = water flow through hydronic coils
- (MBH) 1,000 BTU/h (used to detail heat output of hydronic heating coil)
- (HP) Horsepower for fan motors (describes the size of the fan motor, used to define airflow of different models)
This is not even a complete list. Designers need to understand the input data required for equipment selection. Remember this article is not an engineering guide. Look at the technical data, and if you encounter some odd terminology or units of measure, contact the manufacturer’s technical representative for clarity and design assistance. Remember the Gimli Glider!
Manufacturers simplify things by narrowing the choices. By providing some “givens” airflow, EAT, EWT, static pressure, the manufacturer “pins” certain factors and allows you to pick the rest. Their charts or software programs include data for “normal” systems with a few multiplication factors in case your system design is slightly different than their normal. Stay within their parameters and you will be in good shape, but often you will need to interpolate between two choices. One issue that seems to be getting resolved with most manufacturers is the need for their BTUH vs. EWT output charts to include lower water temperatures. Traditionally, the charts used 180F or even 200F EWT—which flattered the output numbers but made the design more difficult for lower temperature heat pumps or condensing boilers. Many manufacturers are now adding columns to their charts that take EWT down to as low as 120F, some even lower—well done.
As much as I like fan coils, they have moving parts and need regular service. Can you maximize the advantages of hydronics and do the same job with panel radiators, radiant floor, or convectors?