Central A/c or individual units

Bryanell

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Aug 9, 2005
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We've searched the forum for an answer to this but couldn't find one, if anybody can answer this question or point us at a thread which already answered it - we'd be grateful

We have a two story building in the Capital, with 7 rooms - all of which are used part of the time but none is used all of the time.

Airconditioning options are either fitting each space with an individual airconditioner which can be switched on and off as needed, or installing a central system (or perhaps one central system for each storey) and having individual fan-coil units in each space which can be switched on and off as needed.

Assuming that the difference in equipment and installation costs and electricity supply are not the issue, but that the cost of electricity is - does anyone have any reliable (preferably practical rather than theoretical) experience as to which of the options would be intrinsically less expensive to run??
 

Robert

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Jan 2, 1999
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I would try and size the rooms so you know what size A/C units you need.

Take into account the tropical environment when sizing.
Plus any windows in direct sunlight, kitchen, people etc.

Lots's of websites offer sizing calculators. Do a search in Google.
http://www.eere.energy.gov/consumerinfo/energy_savers/calculator.html

I have found that the A/C people in the DR tend to oversize.
They have the bigger the better mentality.

One of the main functions of any A/C unit is to remove excess humidity from the air. An A/C that cools too effectively must cycle its motor on and off more frequently, giving the unit's condenser element inadequate opportunity to dehumidify the air. The starting and stopping increases mechanical wear and reliability. Also paying higher electricity bills .

Just last week I had a "A/C engineer" telling me I needed a 3 ton (36,000 btu's) unit for a 400 sq ft room. Ridiculous, he was just looking for more money and obviously couldn't careless if the install was efficient or not.

One thing I have learned in the DR, try and become an expert in everything that involves you handing over money to a contractor :)
 

Snuffy

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May 3, 2002
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That is an unfortunate reality to life here. You can be the smartest damn business person on the block and they will come in and try to up sell you ten times beyond what you need...and then look at you like you are stupid for rejecting their ideas. Do they really believe you are that stupid and why would they risk losing your business which he obviously did?
 

Bryanell

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Aug 9, 2005
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The house has seven rooms and the only cost I'm interested in is the consumption of electricity - central system vis-a-vis individual units.
Let's take it as read that the airconditioners are suitable and used prudently -does the central system BTU per BTU cost more to run than the individual units?
 

rellosk

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Mar 18, 2002
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Bryanell said:
The house has seven rooms and the only cost I'm interested in is the consumption of electricity - central system vis-a-vis individual units.
Let's take it as read that the airconditioners are suitable and used prudently -does the central system BTU per BTU cost more to run than the individual units?
I would think it's a given that central air (unless you set up a system with seven zones), would cost you more to run. Especially since you said that some of the rooms are not used. With central air, you cool all the rooms in a specif zone. You can buy vent covers and cover the vents when you don't need to cool a room, but you are still wasting energy pushing the cold air to the vents.


PS: This is my opinion, I am have no education in thermodynamics and my only experience with central air is in the US, not the DR.
 

Robert

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Jan 2, 1999
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Bryanell said:
The house has seven rooms and the only cost I'm interested in is the consumption of electricity - central system vis-a-vis individual units.
Let's take it as read that the airconditioners are suitable and used prudently -does the central system BTU per BTU cost more to run than the individual units?

Impossible question to answer on the info you have given.
What piece of string is longer, the red or the black one? :)

What are the size of the rooms?
What are the rooms used for and how often?

Of course 7 smaller units can consume more than one large central air unit.
Then again, if you only use 3 of the small units on and off, then of course they will be cheaper in consumption terms.

Without the "real" practical info, the best you will get is "theoretical" guesstimates.
 

easygoin

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Jan 2, 2005
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Zone systems are the way to go for your application

First off,

Square-foot of structure.
r-factor of insulation, walls, ceiling, Windows.
Now you have a basis to start with. What temperature to maintain... overall.
Now you're into tonage and CFM for what the air handler can feed.
determined tonage and CFM you start running your ductwork as far as airflow per rooms in each zone coming off the air handler.

NOW it all depends on how many zones you would like to have in the structure? Each ZONE will have a dedicated sensor and dampener per zone.

Then you have a programmable t/stat in the most desirable area... to control each zone. Usually set the closest of the most used, living room.

Now you may ask? Well what happens if I only want to cool three rooms, what happens to all of the extra CFM the air handler produced for all seven rooms? You run a recapture dock back to the return side of the air handler to accommodate the extra airflow that is not being used at the moment.

If you do not do that you will eat up your compressor.

The reason why is refrigerant is a liquid to start with entering the evaporate or and needs heat to boil off and turn into a cool gas coming back to the compressor...... liquid is noncompressible and pound the valves number one and in the meantime take oil out of the Crank case of the compressor. so that is why you have to do this..... to boil off refrigerant.


Just remember, most people in this industry do not even have a clue so very careful. ;)
 

Bryanell

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Aug 9, 2005
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Robert said:
Impossible question to answer on the info you have given.
What piece of string is longer, the red or the black one? :)

What are the size of the rooms?
What are the rooms used for and how often?

Of course 7 smaller units can consume more than one large central air unit.
Then again, if you only use 3 of the small units on and off, then of course they will be cheaper in consumption terms.

Without the "real" practical info, the best you will get is "theoretical" guesstimates.

Thanks for the info. You seem to confirm my initial feeling that having an individual unit in each room that can be activated as and when necessary, would overall use less electricity than having a central compressor running constantly when cooling is required in say only one or two of the rooms at any given time.

As I'm not involved in the cost of purchase and installation of the equipment anyway, my concern is the possible difference in the consumption bills.
 

Bryanell

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Aug 9, 2005
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Thanks easyg....

easygoin said:
First off,

Square-foot of structure.
r-factor of insulation, walls, ceiling, Windows.
Now you have a basis to start with. What temperature to maintain... overall.
Now you're into tonage and CFM for what the air handler can feed.
determined tonage and CFM you start running your ductwork as far as airflow per rooms in each zone coming off the air handler.

NOW it all depends on how many zones you would like to have in the structure? Each ZONE will have a dedicated sensor and dampener per zone.

Then you have a programmable t/stat in the most desirable area... to control each zone. Usually set the closest of the most used, living room.

Now you may ask? Well what happens if I only want to cool three rooms, what happens to all of the extra CFM the air handler produced for all seven rooms? You run a recapture dock back to the return side of the air handler to accommodate the extra airflow that is not being used at the moment.

If you do not do that you will eat up your compressor.

The reason why is refrigerant is a liquid to start with entering the evaporate or and needs heat to boil off and turn into a cool gas coming back to the compressor...... liquid is noncompressible and pound the valves number one and in the meantime take oil out of the Crank case of the compressor. so that is why you have to do this..... to boil off refrigerant.


Just remember, most people in this industry do not even have a clue so very careful. ;)

I get the idea, thanks. Fortunately my only concern is the cost of the electricity which the airconditioning will consume and it seems that the individual units will be more cost efficient in that respect than a central system particularly since some of the zones are closed rooms and others are open plan.
Most grateful for the time and effort of all the responders.
B