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Can I use a split aircon AHU as a fan coil unit for a chilled water (hydronic) aircon system? Answered

I have been developing the idea (in my head) to build a chilled water aircon system using recycled parts.
I have been considering using discarded "split aircon" evaporator units (AHU or fan coil unit or indoor units...diff nomenclature depending on geography) as the AHU's for my system.
My question is simple (but the answer could be complicated) ....... would it work? Furthermore, does this make sense and would it work efficiently?


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Jack A Lopez
Jack A Lopez

Best Answer 5 years ago

Yeah. I guess that will work.

Although I want to make sure I have the right picture in my mind for this.

I mean you throw out this phrase, "split aircon evaporator unit", like everybody knows what one of those is.

Or maybe you only want comments from people who speak the same jargon you do?

Anyway, I am imagining this thing you call "split aircon evaporator unit" is superficially similar to a car's radiator.

Basically it is a big bundle of metal tubes, through which liquid coolant flows, and the tubes are sort of woven together with an array of thin metal fins and air spaces. Moreover there is usually a fan, placed nearby, for the purpose of blowing air through the metal fins and air spaces, so that the air and the metal fins can exchange heat with each other.

A general word for this contraption is "heat exchanger", and more specifically it is the kind that exchanges heat with moving air. I don't know if this kind of air-cooled (or air-heated) heat exchanger has a general name, like "air-coupled heat exchanger", but it should because this style of heat exchanger is very common.

In fact a typical vapor-compression refrigerator has two of these air-coupled heat exchangers. One is called "evaporator", and it is the place where refrigerant evaporates, at low pressure, absorbing heat. The other is called "condenser", and it is the place where refrigerant condenses, at high pressure, releasing heat.

Superficially the evaporator and condenser look very similar. These are just bundles of tubes and fins, with a fan nearby blowing air over them.

Actually the evaporator and condenser could possibly be made to be interchangeable, if it were not for the fact that the condenser must be made much stronger, to withstand the higher pressures, at which the vapor is made to, uh, condense.

Getting back to your question about your, uh, What was it called? Split archon vaporator unit?

If this thing, this air-coupled heat exchanger, is a bundle of tubes of fins, and you can connect plumbing to it for to push cold water through it, through the tubes which formerly carried boiling refrigerant, then...

Yes. It will get cold.

Yes. Blowing air over it will make the air get cold too.

I am guessing the performance of this setup will depend mostly on how cold you can make the coolant-water. If you can make the coolant-water as cold as boiling refrigerant, then the performance, in terms of total heat removed from the air, will be comparable. Maybe better, since water probably has better thermal conductivity than the oil-like refrigerant it is replacing.


Answer 5 years ago

Hi Jack,

Many thanx for taking the time and effort to give me such a detailed and thorough explanation.

I am sorry if I got too "jargony" .......... I guess being a newbie on the site and the fact that I have been researching this concept of mine, to death, got me in the jargon mode.

I understand all that you have said and while the basic principles made sense to me, I was wondering if I was missing some important info/pitfalls of trying to use equipment designed for a completely different cooling principle in my system. You have helped a great deal !

Thanx again for your insights and time!




1 year ago

Hi there Jack,

Thanks for sharing the info.
I too was thinking of trying a similar project to water cool a small room in my apartment, with a used "split air conditioner, 16,000BTU-fan-coil"(heat exchanger $200) into which, chilled water would be piped.

I would be using an upright chest freezer (size: 500 x 500 x 800mm, $380) with dual temperature functions i.e. 1) -5 to -18 and 2) 0 to +5 deg C.
I shall be placing an aluminium tank into the chest freezer that would be filled with water (or some other fluid) and chilled to about 5 deg C, a submersible water pump $50, would be placed into the tank bottom and the outlet piped to the wall mounted remote controlled aircon fan-coil.
The return water from the fan-coil would be piped back into the tank, and would recycle in this system.
I would be cooling the room (size: L3 x W5 x H2.5 meters) to a temperature of about 25 deg C, so, I think the chilled water tank capacity would be reasonable for this purpose.

I'm now looking how to make the connection for the fan-coil and the water-pump to start together when the remote control is switched on.
I hope it works well enough. :-)