Cheap duct refrigerant cooling coil with external air conditioning unit 2

[Todrianth]

Hi,

I would like to build my own cooling system by buying cheaper individual components (China), rather than paying for very expensive out of the box AHU solutions that charge insane up to 20x more just beacause of company name. I know how to do the ducting, install the fan and the filter, and all I need to do now is to build/buy a reduction from the rectangular section of the cooling coil to circular section of the ducts. Then I believe I have to link the cooling coil to an exterior unit, which cools the refrigerant passing through the cooling coil. I basically want to do a clasic split air conditioning system but without the split, instead using a duct cooling coil so I can distribute the cold air evenly throughout my space, without having to endure local high speeds from the split unit and uneven temperature distributions/delay.

So I was wondering if someone else tried it, that is to buy separate components and then weld the cooling coil with the exterior unit. Maybe modified second hand split air conditionend systems ?

About the automation I'm thinking that the outdoor unit should have an out of the box setting for refrigerant upper/lower temperature starting or closing the compressor. Regarding the fan I'm thinking to control it via basic indoor temp sensor. The part I haven't figured out is how to link the exterior unit on/off switch to the temperature sensor working in tandem with the fan.

Thanks!

 

[moltenmetal]

I built my own ducted split system because I have radiant heating- no forced air furnace. The air handler, which consists of a filter grille, coil, condensate collection pan, fan and control transformer all in a handsome painted steel box, was much less expensive than the compressor/condenser unit that goes outdoors- that unit is the major cost driver in such a system if you do your shopping right. The biggest cost adder is easy to avoid if you can buy wholesale, which I was able to do- most Joes walking in off the street are going to get soaked with a hefty mark-up by the distributor. My air handler does have a TXV, which gives an efficiency boost. The outdoor unit for my system doesn't differ at all from the ones used with in-furnace evaporator coils around here, other than requiring that I buy a longer lineset. The square to round transition in my case was easy- it was a rectangular box with two fish-lock collars on it for spiral pressure duct, acting as a "tee" at the discharge of the air handler. I found a licensed refrig tech who brazed, vacuumed, pressure tested and did the refrigerant top-up charge required due to my longer lineset. Works like a charm- and because I did the calcs, it is actually properly sized, does a great job of dehumidification which is the biggest battle around here, and does not short cycle.

 

[Todrianth]

I also have a radiator system therefore I don't need any heating.

I have made a more self explanatory sketch for better understanding:

My doubts are at 1,2,3:

1: I suppose the fan should have some sort of connection for the on/off button to the controler. Do all have or might I just buy the wrong fan ?
2: I don't have a clue on how to connect the exterior unit with the temperature sensor such that the unit (aka compressor/unit fan/laminator) will turn off in tandem with the fresh air fan. Do I need a two conection sensor ?
3: Are there any exterior units to be baught without the interior split unit or must I buy a SH one and modify it myself ? What about the heating mode (reversed refrigeration cycle), can that be manually turned off so that the unit will work only on cooling ?
*4: Any alternatives cheaper/simpler than this are appreciated.

Thanks for your responses!

 

[moltenmetal]

I just took the simplest route, which was cheap enough at wholesale prices. The prepackaged AHU came with the 24VAC transformer to power the thermostat loop, and a contactor to turn the fan on and off. Connecting it was a no brainer. Going off memory: three signal conductors ran from the thermostat to the AHU's controls and a pair of conductors ran from the AHU down to the compressor/condenser unit. I don't know if the AHU fan has a run feedback that determines when to tell the outdoor compressor/condenser that it's OK to turn on or not, but I do know that the t-stat has a "fan only" setting which allows you to run the fan without enabling the compressor/condenser.

I used one of this company's multiposition air handlers:

http://www.adpnow.com/product-literature/?link=air_handlers

The air handler is mounted horizontally in my attic, with spiral pressure ducting as headers and 6" insulated flex running down to each outlet diffuser- the adjustable circular ones are a good idea, because they can be sealed up tight in winter. I insulated the HELL out of the ducting and air handler so I don't lose too much precious cooling in the unconditioned attic space.

Note that the air handler I used is a "normal" unit, meant for normal sized ducting. There are high pressure units which are intended for use with ducting that can be snaked through existing wall and ceiling spaces etc., but of course the heat of compression from that high pressure fan is an efficiency hit. I would imagine these units would be noisy too.

The air handlers can be fitted with hydronic coils instead of evaporator coils if you want to ruin your hydronic system by adding forced air heating to it and all the associated disadvantages that come with it.

The reversed refrig cycle is an air-sourced heat pump arrangement that the mini-split compressor/condenser units that are typically paired with ductless fan-coils typically offer as standard. However, I didn't use a mini-split compressor/condenser as they're expensive and have a poor SEER relative to the larger outdoor units. I used a regular compressor/condenser unit which is typically coupled to a coil in a natural gas furnace, made by Keeprite if I recall correctly- it has worked very well for five or six years so far, so we'll see how long it lasts. The outdoor units typically used with gas furnaces can be configured for heating as an air source heat pump, but that adds cost and complexity. It would be handy for heating during the "shoulder" seasons if your off-peak electricity is cheap enough, but for me the payback versus using my modulating condensing boiler for heating would have been well over ten years- just wasn't worth the bother.

 

[Todrianth]

I just realized that I also need somehow to control the indoor inlet air temperature. Since the refrigerant will have rather constant temperature and with the outdoor air temperature varying, I could get very cold temperatures/drafts indoors when outside temperature drops say from 36 to 28 C or at night. The outdoor unit should calibrate the compressor power depending on the outdoor temperature variation...

 

[moltenmetal]

No. The A/C system is what controls the inlet air temperature- by cooling the air in the room...you can't independently control it.

The outdoor unit will cycle on and off as the thermostat tells it to, unless you go for a variable speed unit, and frankly I'm not sure how those are controlled. The TXV will regulate refrigerant flow to the coil to maintain a certain amount of superheat in the vapour leaving the coil, which reduces the load on the compressor and saves energy relative to a plain orifice. Temperature control inside the house in a normal A/C system is on-off, using a simple switch-type thermostat.

The outdoor unit is always over-sized except for the peak demand (highest design differential temperature between indoor and outdoor) when it will run continuously. The key is to pick an indoor unit matched to the needs of our outdoor unit. You also need to pick an outdoor unit which doesn't have vastly more compressor/condenser capacity than you really need, otherwise it will "short cycle"- it will run only for brief periods and hence allow too few air exchanges over the coil before it shuts off again to do a decent job of dehumidification- and in my climate, humidity is the thing you really need to control to be comfortable. Most HVAC people over-size units because they think that will make owners happy- it will cool the house off from a peak temperature much faster of course, as well as handling the hottest day or two you encounter every year without allowing the temperature to rise much above setpoint- but then it will do a crappy job of maintaining temperature and humidity level for the entire rest of the year.

 

[Todrianth]

It all sounds rather complicated. The system I'm trying to build myself is very simple:

(0)outdoor
(1)-> main circular duct
(2)-> filter (round fittings)
(3)-> in-line axial fan (round fittings)
(4)-> cooling coil (round fittings) with two pipes going out of it (in-out for refrigerant)
(5)-> pipes go to exterior unit (basic heat pump cycle unit)
(6)-> adjustable air inlets for achieving confortable air speeds
(7)-> temperature controller

Each of these components is an individual object, no box, no premade stuff, just bought of alibaba or something.

Fitting them together with circular piping is easy.

More problemaic are:
(4) -> welding cooling coil pipes to exterior unit pipes (hire a specialist). Afraid of refrigerant spilling with time.
(5) -> couldn't find where to buy individual exterior unit from. thinking of buying a second hand one that comes with that interior split unit (which I don't need, throw that away)
(5) -> what kind of controller outputs/signals will such outdoor unit have ? can't seem to grasp how it works (automation-wise) and how can I connect it/modify it to a simple controller to fit my needs.
(7) -> simple analog thermostat which I turn and then the system goes full-blown with cold air. (haven't got this figured out without a sketch, again figuring out controllers)

I've calculated the cooling load so choosing the cooling coil and the exterior unit shouldn't be a problem (7.5 kW/24000 btu for both the capacity of the coil and exterior unit since pipe heat losses are neglijable)

Thanks for your answers moltenmetal but since I'm not native english speaker and maybe just plain stupid, I would need simpler pinpoint explanations.

Cheers!

 

[Todrianth]

In a nutshell, this system is exactly as a traditional - romote control, split air conditioning system (air-to-air heat pump) but without a split, that is, giving an even distribution of air through circular ducts in the living space (one big 150 sqm/1616 sqft room).

Ideally I would have that remote control still in use, so I can use the system for heating aswell, so I can undersize my radiator heating system, but I don't know how to do that also, or if it's possible without the split itself...

 

[boo1]

just buy a mini split and install it your self