What's the typical COP of a central A/C?

[dude]

The Central A/C system in the US is typically rated in SEER which is defined as BTU in season / kWh in season.

Anyone know the COP (coefficient of performance or watts cooling per one watt of input) of the typical two ton(24,000BTU or 7kW) central system?

For comparision, a cheap, small window A/C(1.48kW) has a COP of 2.846 (9.7 EER)

Is it more efficient to use a window A/C in one room where I need it or cool the whole house with the central?

[chemi-cool]

Hi Dude.

Windows unit are usually of low cop/eer.

Every manufacturer must write on the nameplate the cop/ eer.

To convert: COP=EER/3.412.

It will be cheaper to use a mini split for each room then a central. You always cool/heat empty rooms.

With the mini splits you only cool/heat occupant rooms.

I use 4 unints in my house for many years and its comes out very cheap on poewr consumption.

Chemi

[baker]

There was recently a discussion on alt.hvac on this subject and the general concensus there was that there is no substitute for central air. I disagree. Central air is a neat and easy solution and has the advantage of being able to introduce outside air, but individual ductless splits are more efficient and do not have an oversizing problem, especially if using inverter ductless splits.

Correct central air design requires compromises that can be avoided with individual splits. If I was building a house to sell, I would install central air. If I was building a house for myself, I would use individual Daikin inverter splits.

[Peter_1]

Central air is a neat and easy solution and has the advantage of being able to introduce outside air, but individual ductless splits are more efficient and do not have an oversizing problem, especially if using inverter ductless splits.

Correct central air design requires compromises that can be avoided with individual splits.

If you have a correct central air design, what compromises do you see then eny further?
If you use a AHU with a watercooled coil, VAV boxes and speed regulation on the main fan in the AHU, then in my opinion this is a very good solution.

[chemi-cool]

Hi Peter and Baker.

None of you said anything about initial and running costs.

I agree on one thing only - the looks of the system.

Splits are ugly, take space in the room and if they have a water leak, it will be on a telly or a computer.

A central unit cost more to install, to run and the house have to be designed for it. Most houses are not.

any problem- no air condition in the house. Repairing it will cost more.

In a private home, lets say 5 people, they spend most of their time in the living room and kitchen. Why to waist energy on cooling empty bedrooms?

And if you work at home and spend most of the day in your study, why to cool the empty living room.

If something goes wrong with one split, the others still work and the cost to put it back to work is less.

The only advantage of central unit is fresh air but some of the splits manufacturers thought about it and left an opening at the back for fresh air. The problem is controlling that air ang cleaning it.

I have checked and provedthe subject for years and I can not be covinced that I am wrong.

Right now, I'm alone in the house and only a smallunit of 7500 btu is working and its cool and nice here.

Chemi

[Peter_1]

Hi Peter and Baker.

None of you said anything about initial and running costs.

To remove the heat from a space, you need energy for that. Whatever system you use, you always need to remove the same amount of energy and you need therefore electrical energy which will be heat load divide by COP. If this is with a split or an AHU doesn't matter.

A central unit cost more to install, to run and the house have to be designed for it. Most houses are not.

It's true, it can designed better in a new house but it 's perfect possible in existing houses. We did it already many times. It all depends the way they build houses. For example: the way they construct houses in Belgium and the US is completely different.
I was in the US some years ago for training and due to the bad weather, we had some free days. We rented a car and looked around in the LA area how they build houses. We talked there also with 2 HVAC contractors.

any problem- no air condition in the house. Repairing it will cost more.

VRV or VRF --> same problem.

In a private home, lets say 5 people, they spend most of their time in the living room and kitchen. Why to waist energy on cooling empty bedrooms?
And if you work at home and spend most of the day in your study, why to cool the empty living room.

Not with a VAV system. Rooms are only heated/cooled when occupied.

The only advantage of central unit is fresh air but some of the splits manufacturers thought about it and left an opening at the back for fresh air.

Bringing to much fresh air is also spoiling energy. Other advantages: humidifying, drying, controlled fresh air intake, better air filtering, less noise (almost perfect silent), only grilles (aesthetic better)

[baker]

I think that we are being too general in our terms. Central air covers a huge range of technologies but I was thinking of the standard domestic product, not the more sophisticated types that Peter is referring to.

The typical domestic unit with a fixed speed compressor and fan for the ducts is sized for optimum performance. Where I live this means a design for 36C DB, 24C WB. Every year we will have several days when it over 40C, and humid. A correctly designed system will not handle this. Sizing for the rare hot days gives humidity problems. Admittedly, a more sophisticated central system can overcome this, but in our market, it is a lot cheaper to go with separate splits.

[superheat]

Window units are cheap. Most do not last for the 5 years on the warrenty.

It does not take too much sophistication to wire an extra fan relay to a dehumidistat for humidity control.

Zoning is not that expensive either.

In the southern part of the US, cooling those empty rooms is not an option. I have seen houses with several mini splits. They do not do a very good job on one room only. The extra heat from the house still leaks into the room you are cooling.

In southern CA, AC is optional. A friend of mine from Florida was transferred to LA a few years ago. She does not have AC.

[frank]

With central Air you also have a control problem.

Where do you put the thermostsat? in the main room?, mixed air return air duct? With this type of system you will not get correct control in each individual room so the placing of the stat is important. Some rooms will be overcooled and some undercooled. COP ratings can only be calculated from energy input and energy output. A system that gives 5kw of refrigerating effect from and input of 2kw has a COP of 2.5.

If I was building a house for myself, I would use individual Daikin inverter splits.

Got to agree on this one

[Peter_1]

With central Air you also have a control problem.

Not with a VAV where every room has his own thermostat. You can do this also very simple with a 3 way air valve (Y -joint) and a bypass over the AHU.

[frank]

I've never been much of a fan of VAV. We have a customer who has a large office (2 floors) and the whole area is served by central plant on VAV boxes and perimeter heating. Lots of complaints of low humidity and temperature variances. Seems that over the past 10 years the system is getting worse and nearing the end of it's life cycle (25 years old). Maintenance costs are increasing and complaints are also increasing. When some employees move around the site and get to spend time in other office areas with VRV they seem to realise the diffence and are not happy moving back to the main office block.

[chemi-cool]

VAV was a bad thing from the beginning.

Air volume should be constant.

Over here, from some stupid reason, its getting popular to install before the air diffuser, temp controlled damper which regulate the air volume.

I saw it many times in private homes, central ducted system.

When I was asked to fix any of these units, I always cut the dumper motor wires, set the AHU on automatic fan speed and no more complaints.

They tell me that finally it works fine

VAV for you.

Chemi