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Consider a large auditorium, a church, or some other very large, essentially one roomed building with a high ceiling. Suppose that the building has many entrances which enable cold air in/hot air out and traffic in and out of the building is unavoidably large.

I would imagine that any attempts to control the temperature of such a large building would be very inefficient in terms of energy and cost, particularly because warm air rises.

Assuming that it's very cold outside and we're interested primarily in keeping the building warm at the ground level so that it is comfortable for humans to work and interact, what is the best method to keep such a large, essentially one roomed building with a high ceiling warm when outdoor cold air exposure is frequent and unavoidable?

When I say "best," I'm interested in balancing energy, maintenance, and monetary costs over the life of the building.

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Your question sort of has two parts: How to supply heat, and how to keep it in.

Large open rooms with a high ceilings are most efficiently warmed with radiant ceiling heat. Warm air rises, which renders forced-air systems inefficient because the pumped heat ends up at the ceiling and the coldest part of the room is near the floor where you actually want the heat. Radiant floor systems are limited to about 87F because they are in contact with occupants, and so their peak output may not be enough to keep the space comfortable. They also lose more heat to convection than radiant ceilings. (See this ref.)

As for keeping heat in, besides solid insulation/barriers: air doors (a.k.a. air curtains) are the standard solution in high traffic passages.

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    $\begingroup$ Do you have any thoughts on the use of ceiling fans? I've been in a few cold buildings with fans that provided a noticeable warm downdraft. $\endgroup$ – Dan Jan 24 '15 at 4:48
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    $\begingroup$ Ceiling fans are certainly a good measure to offset convection with tall ceilings, especially with poorly designed convective heat sources. Though often you will notice that forced air systems have ducts along the ceiling and so they're designed to keep warm air from pooling up high during their heat cycle. $\endgroup$ – feetwet Jan 24 '15 at 15:10
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Another option to the radiant solution above would be under floor air distribution systems (UFAD) using displacement ventilation. These systems require a raised floor plenum which works well in theatre style auditoriums, where you often have this anyway. The plenum essentially becomes your supply duct.

The systems are sized to condition the occupied zone only, say the first 2m over the outlet. So no matter how high you ceiling is, you'll end up heating only a 2m high volume.

Unlike in radiant systems, which heat people directly and do not change the air temperature, the outside air component is treated. Whether this is an advantage or not depends on the required outside air volumes, where these volumes are introduced relative to the occupants (i.e. are you blowing cold air on someone?) as well as expected comfort levels.

Opportunities for energy savings in UFAD systems

  • Lower supply temperatures when heating, higher supply temperatures when cooling
  • The conditioned volume is limited to the occupied zone no matter what the ceiling height is
  • Supply velocity is lower, however supply volumes are increased; there might be some value in optimisation here and thinking about perimeters and internal zoning
  • Lower pressure drop in supply
  • Throw in some heat exchanger to capture the heat from the return air

A few other things to note:

  • Lower velocity and higher volumes taken together means that the number of outlets might increase and that these systems cannot handle the same peak loads as conventional systems; this will be important for perimeter zones. The building fabric must be optimised to limit peak loads (losses/gains through glazing)
  • Lower velocities also means less problems with draft (in theory at least)
  • If the acoustic zoning doesn't match the mechanical zoning, things will get a bit tricky; acoustic separation might require walls being taken down through to the plenum
  • If you have a tiled flooring system, the outlet diffusers can be swapped around which has multiple benefits: for one, it provides a very flexible room layout that can be changed at any time; in addition, the systems introduce some flexibility for occupants to make themselves comfortable by relocating diffuser tiles
  • Wikipedia has an article which provides a general overview of UFAD systems and one for displacement ventilation

What is "most efficient" depends on many other things besides space heights. UFADs can be a good solution in certain situations but same is true for e.g. radiant systems. What you'd definitely like to avoid is ending up conditioning a 4m high volume of air.

A few UFAD images from the wiki page to save you a click: UFAD concept section

UFAD stratification

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Here are some ways to keep the warm air from escaping:

  • Revolving doors prevent a constant stream of air exchange between the inside and outside.
  • Air lock areas - a double-set of sliding doors can also prevent continuous air-exchange.
  • Positive pressure by the entrance, usually supplied by a fan heater above the inner entrance door ensures the main bulk of air stays inside and the air from the fan heater escapes.

To keep the lower areas warm, the warm air at the top of the building can be blown down to floor-vents at very slow speed. Either directly or via a heat exchanger so that the heat from the stale air is transferred to a fresh stream of air from outside. This is also a relatively efficient way to heat the air in the first place. A heat-pump can also be used to heat the floor using under-floor heating pipes. This helps to prevent heat-loss from the warm air into the floor and gently keeps the physical mass in the lower area warm.

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