Difference between Heat Pumps, Fan Coils & VAV boxes

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Can someone explain to me the difference between a fan coil unit and a heat pump, Im referring to the ones that you find in shopping centres and malls?

I understand that both require power to run the motor that distributes the conditioned air, how im interested to know the mechanism used. Im guessing that in larger buildings like malls there is some kind of centralized air handling unit with a duct network? Or are these more like stand alone systems to condition a specific tenant (not connected to any main ductwork)? If so what is the fuel used to provide the heat? Which is more efficient?

From what I have seen VAV boxes are installed where a duct is terminated, say in a room, and I guess from a control point if offers more flexibility.
 

kwired

Electron manager
Location
NE Nebraska
Can someone explain to me the difference between a fan coil unit and a heat pump, Im referring to the ones that you find in shopping centres and malls?

I understand that both require power to run the motor that distributes the conditioned air, how im interested to know the mechanism used. Im guessing that in larger buildings like malls there is some kind of centralized air handling unit with a duct network? Or are these more like stand alone systems to condition a specific tenant (not connected to any main ductwork)? If so what is the fuel used to provide the heat? Which is more efficient?

From what I have seen VAV boxes are installed where a duct is terminated, say in a room, and I guess from a control point if offers more flexibility.

could be either. At malls each occupant likely has their own heating/cooling system for their space and is likely stand alone. Larger buildings under single management may have central heating/cooling plant and then heated or chilled water or some antifreeze media is pumped to fan coil units where heat is exchanged to condition the space.
 

normbac

Senior Member
There are numerous setups for these a hotel would have all in one shoved into your room halfway There could be a single roof top unit Usually 3 phase. Also units similiar to your residence with a exterior condensor and interior FAU. Usually 208volt fau and 3 phase condensor and usually always duct run
 
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broadgage

Senior Member
Location
London, England
A fan coil unit consists of a fan that blows room air over a coil of finned pipes that carry heated or chilled water.
Some types have a single coil supplied with hot water, these can only heat the air.
Others types have a single coil supplied with chilled water, these can only cool the air.
Yet others have 2 coils and and heat or cool the air.
The electrical loading is small, consisting only of a fan and usually a controller that operates valves to control the flow of heated or chilled water.
Allmost allways single phase.
The chilled and/or heated water would be supplied by remote central plant, with appreciable energy use.

A heat pump cools or heats the space it serves by means of a compressor.
When heating the space, it cools either the outside air, or circulating water.
When cooling the space it heats either the outside air, or circulating fluid.
The electrical load is substantial and in all but the very smallest units is likely 3 phase.
In the UK these are often known as "versatemps" though this is a trade name, other makes exist.
A number of heat pumps are often connected to a common water circulating circuit which is maintained at about room temperature. If the water becomes too warm, it is cooled by being passed through a large outdoor heat dump. If the water becomes too cold, then it is heated either by boiler plant, or more economicly by low grade reclaimed heat, perhaps from extract air.

A VAV box (variable air volume) uses air supplied from a remote AHU, normally at a temperature lower than the room temperature.
A damper or other device regulates the volume of air into the room, thereby controlling the temperature.
The electrical load is very small indeed, and may be supplied at 24 volts.
Some types use air pressure for the control and dont need any electricity supply.
The remote AHU will of course use appreciable energy, as will any chiller or boiler plant to heat or cool the air.
 
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Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
FCU fan coil unit is generally a fan, a coil, (heating and/or cooling) and a vent to exhaust the conditioned air. Different from an air handler unit in that it generally doesn't use duct work. A VAV is a valve that controls the amount of flow in the downstream ducting. Often the incoming air is conditioned (hot or cold) and then the VAV has supplemental heating or coiling coils that "boost" the temperature for that section of duct. They are usually controlled by local thermostats.

A heat pump in not really comparable with the above pieces of equipment. It is comparable to a furnace, or a condensing and evaporating unit. It is a reversible condenser, evaporator. In the Summer it sucks heat from the inside coil and transfers it to the outside coil, and in the Winter it sucks heat from the outide coil and transfers it to the inside coil. Depending on the amount of heat energy available at the source, supplemental heat is often required.
 
T

T.M.Haja Sahib

Guest
A VAV box in a HVAC system is meant for energy conservation by controlling the volume of conditioned air supplied.In contrast, the earlier HVAC systems used conditioned supply air of constant volume.
 
Thanks for the replies!

So for a cold place like say Toronto, wouldnt be much harder for a heat pump to extract heat from outside during winter months? I guess in extreme conditions you need some kind of supplemental heating.
 
T

T.M.Haja Sahib

Guest
No matter how cold outside,a heat pump may be able to extract some heat out of it,provided it is suitably designed.So you may approach a HVAC manufacturer with your design parameters for his suggestions.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Thanks for the replies!

So for a cold place like say Toronto, wouldnt be much harder for a heat pump to extract heat from outside during winter months? I guess in extreme conditions you need some kind of supplemental heating.

Typically you don't see air to air heat pumps in real cold climates but rather ground water based systems as the water temp is relativly warmer year round.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
Typically you don't see air to air heat pumps in real cold climates but rather ground water based systems as the water temp is relativly warmer year round.

Op, in case you don't understand what Texie means, it is a heat pump, but the coils are placed in the water. Much better for tranferring heat than air is. And yes, regarding supplemental heat. I live in Florida and we install supplemental heat here. The temperatur drops below 35 degrees about 6 times a year and it gets used for a couple of hours those days, but code requires. So Toronto would need a huge unit to provide enough heat on a cold day.
 

kwired

Electron manager
Location
NE Nebraska
No matter how cold outside,a heat pump may be able to extract some heat out of it,provided it is suitably designed.So you may approach a HVAC manufacturer with your design parameters for his suggestions.
The heat pump in question would need to be designed to handle the heating load at a specific minimum outdoor temperature. This will usually result in a unit that is oversized enough there are other problems or inefficiencies. There eventually becomes a temperature where efficiency and reliability of the back up heat is better than the heat pump so back up heat is depended on for these times.

Op, in case you don't understand what Texie means, it is a heat pump, but the coils are placed in the water. Much better for tranferring heat than air is. And yes, regarding supplemental heat. I live in Florida and we install supplemental heat here. The temperatur drops below 35 degrees about 6 times a year and it gets used for a couple of hours those days, but code requires. So Toronto would need a huge unit to provide enough heat on a cold day.

Most air to air heat pumps around here have no problems heating at around 20 degrees - efficiency is reduced but HVAC guys are not locking them out until at least temp hits 20 degrees - some even less. If heating demand is not keeping up, the second stage of thermostat is eventually reached and back up heat kicks in. User does not necessarily realize it kicked in either.
 
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