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Old 03-10-12, 12:34 PM   #1
xotet
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Default Using portable A/C for DHW Heater

Hi Folks, first off all I´m apologize for my bad english, I´m writting from Spain. This is the most interesting forum found over the net talking about Heat pumps DIY and A/C hacking.

The goal of my project is to heat more efficiently the 150 liters of the water heater. Currently i´m using AC resistence (2Kw), I think the efficiency is under 1.

I will like to hack a 9000BTU portable A/C, replacing the Air/gas condenser with an water/gas HX, and installing a TXV and a filter before the evoporator.


I spent a lot of hours reading the A/C operation and theorics about TXV operation and selection, but I have a few doubt´s.

What´s the way to determine the Evaporation temperature?
Can I expect the Gas temp on HX Inlet, according refrigerant propierties? (I will like to use R407C or R410A, very difficult to find hardware working with R22 here)

I will like in two stage project first approach with mecanical TXV, and on a second stage control the expansion with an EEV, maybe with arduino acting as a procesor to get the COP as much higher as possible.

Any advice, comments etc... are welcome.


Thanks in Advance

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Old 03-10-12, 02:16 PM   #2
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I have used a 12000btu portable air con unit. The initial problem was the unit was failing to start when the room temp was below 16C. I got around this by putting a 5k resistor in the ambient temperature sensors place. This way, it thought the room temp was 35C, so would always run. Since then I have removed the control board and put in an etc200b controller and put the probe on the evap hx. This is set at -2C and cuts the compressor for 2 minutes. This also has a 2 minute restart, so that the compressor doesnt lock up. The condenser hx is in a large 80 litre container with water in and 100mm insulation. This is then pumped around the solar coil on my 300 litre thermal store. I have set the controller to maintain the temperature at 40-45C. This then runs my domestic hot water and 2 floors of heating. I have low surface temperature radiators, so 40C is enough heat to hold 21C when it is -3C outside.
Ice was the biggest problem,but I have got around this by putting a flap on the blower outlet from the evap hx with a microswitch connected to the etc200b main power. When the evap hx isnt full of ice, plenty of air flows and holds the flap away from the microswitch. Once too much ice builds up on the hx, then the airflow drops until the flap cannot be blown away from the microswitch. The instant the flap hits the microswitch, the compressor stops and goes into its 2 minute defrost cycle. The -2C is ok if where you live doesnt have an ambient temp below about 4C, but the low tech flap/switch gets around hx's not being iced up, but just below -2C - like first thing on a cold morning.
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Old 03-10-12, 04:40 PM   #3
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Nexsupreme thanks for your inputs, your system seems to be very cool, could you please post some photos??? Also I have a few questions:

-What´s the refrigerant are you using???
- How controlling the subcooling and superheating??
- Are you running with capilary tube or with TXV??


I only talked about heat pump but I´m planing to support the heater tank with Solar energy, If the day was sunny the HP dosen´t run the tanks where heated with solar, this will be done with a 3 way valve between the HP HX and the sollar collectors.
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Old 03-10-12, 07:29 PM   #4
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Quote:
Originally Posted by xotet View Post
The goal of my project is to heat more efficiently the 150 liters of the water heater...
Too bad you don't have R22 units in your area. Have you looked in scrap metal yards? They usually get old units that can be made to work. Normally I would suggest to start small, and go with R290, and keep the cap tube that the unit came with for starters, when you replace your condenser with a water-to-gas HX. You will have challenges doing just that... then improve it with the addition of a TXV.

If you go with a R410a or R407C system and a TXV meant for R22, you can probably still get reasonable performance with R290, by adjusting the TXV for the proper flow.

I think it's great that nexsuperne is able to get his DHW & heating by putting the air conditioner condenser coil in a tank of water, but I think that a home built tube-in-tube or better, a properly sized brazed plate HX will yield better efficiency, which is what this is really all about.

But try to keep your initial unit as simple as possible, and improve from there... you have much to learn.

-AC_Hacker
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Old 03-10-12, 10:56 PM   #5
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My HP is on r407c, has a cap tube, and really is just 'quick and dirty' just because thats how it was made.I removed the casing and then basically put the condenser into a container with a lid to be able to pass water through it. It isnt a hack as such, because I havent cut into the refrigerant line. I will post some pictures.
AC is quite right about the hx, and when I can snare my local aircon engineer and bribe him with beer, then I will get a proper refrigerant to water hx brazed on, and loose the container.He has the Maap gas, the nitrogen, gauges and the r407c license.

I have a 4m2 30no 58mm x 1800mm evacuated tube solar array that feeds the store. For 8 months of the year, I need no other backup heat, and in July (northern hemisphere) it regularly shuts down the circulator at 90C. Thats after 4 showers and a couple of baths a day! The backup heater was electrical resistive at 2kw. This would boost the tank to 80C on cheap rate electric overnight, and then switch back on if the temp fell below 45C in the day.

Last year I built a wood burning water heater from an old propane bottle, put that into an oil barrel and then filled the space between with vermiculite. The bottom 2/3 is firebox, the top 1/3 has a 5/8" soft copper coil wound into it. This is my heat exchanger and runs back to the store via a 24 plate 100kw heat exchanger. I have various sensors on it, so that the controller works out where the best heat source is and uses that. The woodburner runs on old pallets that the couriers wont take away when they deliver here, so there is no shortage of wood.

I've only been running the HP for a week, but already it is easy to see the huge financial savings compared to resistive or oil heating.

I got my 1 ton air con from freecycle, the guy bought it 10 years ago, but stopped using it 7 years ago when it got noisy. I got it back to mine, blew it through with the airline, cleared the dust and it worked perfectly! Funny thing is a friend of mine was asking if I could build an ashp from an aircon, and until 2 months ago, I couldnt have. The guys on this site have been extremely helpful, and they do know their stuff.

I am on a mission to make my house as cheap to run as possible. I already have 500 watts of solar pv, and a 1kw wind turbine, connected to an old compaq r3000h pure sinewave 48 volt to 230 volt uninteruptable power supply. This puts out 2.5kw and runs the lighting, cctv, all the circulator pumps for the heating systems. When the power goes out here, we still have light, heat and can make hot food. The HP hasnt been tested on the UPS yet, but I always have the solar or woodburner to keep us warm and in hot showers.
All of my lighting is LED, with lumen outputs higher than halogen bulbs, but on only 9% of the power. These are on PIR detectors, so no lights get left on when not needed.
I put in a grey water harvesting system a few years ago, made from a 1000 litre IBC and a pressure boost pump for running water on a boat (12 volt). Total cost was under £100 ($160us) It runs on rain water, and when that runs out, the water from the bath/showers gets diverted into it. We flush the toilets and wash the cars with it. We now use as much water as 2.2 people, yet there are 4 of us.
My truck runs on waste vegetable oil, not diesel, so again, people throw that stuff out, and I go driving for free! But thats a whole other story.....

Last edited by nexsuperne; 03-11-12 at 10:03 AM..
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Old 03-11-12, 03:45 AM   #6
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nexsuperne

Good work with your renewable energy & heat pump. Once you hack a shell and tube HX onto the unit that will really make it nice. The veggie oil truck is very cool as well. We as well have good results with the solar hot water and heat pump. Just tonight we were visiting neighbours watching movies and he had spoke of paying $1.33/Ltr of furnace oil for a small house. Ouch. We'll get him switched over to Geothermal this summer.

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Old 03-11-12, 07:47 AM   #7
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I have just got deal of the century through ebay. I wanted a hydronic fan heater for my lounge, so found this on ebay, starting at £10. No one else bidded on it. Smiths Hydroflame Brass Trim Classic Inset Dual 580mm x 460mm Radiator; fires retails for £506! Absolutely brand new, still in the box.

Some pictures of my install. I am waiting for a couple of temperature sensors to arrive, so I will make it all look pretty later....

Change that. I cant get it to select the jpgs I want to add.....

Last edited by nexsuperne; 03-11-12 at 09:54 AM..
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Old 03-11-12, 09:45 AM   #8
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Hi Folks, thanks for your inputs.

Quote:
Originally Posted by AC_Hacker View Post
Too bad you don't have R22 units in your area.
But try to keep your initial unit as simple as possible, and improve from there... you have much to learn.
-AC_Hacker
This is a great idea. I´m focused now only on the HX, multiplate HX have Higher performance but have higher pressure drops. I Will like to start testing coaxial HX (condenser).

Should start with the same lenght of the original condenser???


On another hand I´m worried with the subcooling, when the water of the tanks is cold there isnn´t any issue, but when the difference btween the water temp and refrigerant flowing throug the HX for Ex Water temp 42ºC and the refrigerant temp is 50, is too close, not all of the refrigerant will change from gas to liquid, reducing system performance and overloading compressor.


Here is the full system idea HP only works as Backup.
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Old 03-13-12, 06:48 PM   #9
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Today I buyed a Olimpia portable A/C, for only 40€. Before make any mods on it I´ve tested the operation, here are the numbers:


Ambient temp = 15ºC
Compressor Discharg = 57ºC Max
Evaporator outlet = 7ºC

The first step will remove all te elecronics from the unit, because of the compressor stops according electronic board logic, so I will remove all the electronics and let see the max condensing temp.

Here are some pics.
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Old 03-13-12, 10:36 PM   #10
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Quote:
Originally Posted by xotet View Post
Today I buyed a Olimpia portable A/C, for only 40€. Before make any mods on it I´ve tested the operation, here are the numbers:


Ambient temp = 15ºC
Compressor Discharg = 57ºC Max
Evaporator outlet = 7ºC

The first step will remove all te elecronics from the unit, because of the compressor stops according electronic board logic, so I will remove all the electronics and let see the max condensing temp.

Here are some pics.
Looks like your unit is working!

Is it possible for you to get R410a in your area?

That might be a simple path to success, allowing you to keep the cap tube, etc.

-AC_Hacker

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