This project is my dad's $10 solution to a $500 solution to a $25,000 problem. As I have previously mentioned around the site, my Dad owns an electric 1979 Ford Courier pickup, and is cool enough to let me drive it around. We absolutely love it, and wouldn't trade it for a Tesla Roadster, but one of the few problems with electric cars is heating and cooling. In a gas car, heat is provided by the 80% of the gas that is wasted as heat, and air conditioning is provided by a crankshaft-driven compressor system. Many EVs use hair dryer elements and fans for heat, and some, ours included, feature a powerful gasoline-burning heater.
(Update from 4-22-08: I'd forgotten that I mentioned the gas heater on here. Last fall the gas tank and heater were removed, and a ceramic heater was built in. It works great, although not quite as fast, and doesn't use gas.)
However, air conditioning is trickier because the shaft of an electric motor doesn't always spin. Some have used a compressor driven by the motor shaft anyway, while others have turned a compressor using a separate motor. Finally, my dad came up with part of the concept for this system. It pumps ice water through an evaporator core, which has fans that blow air through it. It is very simple, but we found what we were looking for at Sporty's Pilot Shop. They sell air conditioners built into ice chests for prices ranging from $475 for a basic model to $625 for a 24V, dual fan model.There is also an ArcticAir unit for $4750 with a full compressor unit. However, we like our $10 version better. I saw the ArcticAir display at EAA AirVenture Oshkosh this summer, and our unit is more compact and puts out cooler air. All you need is materials, basic construction/assembly and wiring skills, and a bag of ice. Let's go!
Update, 5-12-08: 100,003 views! Yay! I'm no Kipkay, but I'm still proud.
(Update from 4-22-08: I'd forgotten that I mentioned the gas heater on here. Last fall the gas tank and heater were removed, and a ceramic heater was built in. It works great, although not quite as fast, and doesn't use gas.)
However, air conditioning is trickier because the shaft of an electric motor doesn't always spin. Some have used a compressor driven by the motor shaft anyway, while others have turned a compressor using a separate motor. Finally, my dad came up with part of the concept for this system. It pumps ice water through an evaporator core, which has fans that blow air through it. It is very simple, but we found what we were looking for at Sporty's Pilot Shop. They sell air conditioners built into ice chests for prices ranging from $475 for a basic model to $625 for a 24V, dual fan model.There is also an ArcticAir unit for $4750 with a full compressor unit. However, we like our $10 version better. I saw the ArcticAir display at EAA AirVenture Oshkosh this summer, and our unit is more compact and puts out cooler air. All you need is materials, basic construction/assembly and wiring skills, and a bag of ice. Let's go!
Update, 5-12-08: 100,003 views! Yay! I'm no Kipkay, but I'm still proud.
Step 1: Background and How it Works
This project is very similar to the ArcticAir Package Unit. In fact, I attached two pictures of it I took at AirVenture this past summer. It looks almost identical to ours, and we built this without ever seeing a picture of the inside! The basic concept is to use a boating bilge pump to circulate iced water through a heater core, then use 12V fans to blow air through that core, which cools the air and pulls out water through condensation.
Advantages: Very compact and portable, lightweight without the ice, no environmentally not-so-friendly chlorofluorocarbons, hydrogenated chlorofluorocarbons, or hydrofluorocarbons, very quiet, and operates off 12VDC, AKA a cigarette lighter. The only disadvantage is that it the ice will melt after 30-60 minutes of operation, depending on the size of your cooler. However, it was built for an EV, so we are only ever out for an hour or two maximum, and the ice lasts longer when it's not running. The third image on this step shows the operation. Have I convinced you to build one yet?
Advantages: Very compact and portable, lightweight without the ice, no environmentally not-so-friendly chlorofluorocarbons, hydrogenated chlorofluorocarbons, or hydrofluorocarbons, very quiet, and operates off 12VDC, AKA a cigarette lighter. The only disadvantage is that it the ice will melt after 30-60 minutes of operation, depending on the size of your cooler. However, it was built for an EV, so we are only ever out for an hour or two maximum, and the ice lasts longer when it's not running. The third image on this step shows the operation. Have I convinced you to build one yet?
Step 2: Materials
You can't very well build this without materials to build it with, can you? I have included the prices we paid for them in italics, as well as prices you might pay and places to get them.
Materials:
Ice Chest-free, had it on hand If at all possible, get one with a hinged lid. Free from a storage shed or a dumpster. Or, the Igloo Ice Cube 14 looks like it would work,well, as do the Cool 16 and the MaxCold 24. We used an old 12 quart cooler, and it fit a 7 pound bag of ice.
Heater Core-free, salvaged from a '77 VW Rabbit we're parting out You can find these on eBay for 99 cents to $20, or from an auto store for around 20 bucks, or at an auto salvage yard. Also, I haven't tried it, but Ufixitautoparts sells heater cores for under 5 bucks apiece.
Box Fans and Blade Guards-free, from stock in basement They're sold out of $5 120mm 12VDC fans at All Electronics, but Jameco carries these for $12.95 each. Newegg has a nice assortment, too. Under $10 on eBay.
Bilge Pump-$10.44 for a 500GPH unit at Wal-Mart The Attwood V500 was at our Wal-Mart for $10.44-you can get a similar pump for under $10 on eBay.
Hose-free, had it in stock Ours came from an auto-parts store, but it can be found at hardware and auto-parts stores for a dollar or so for a few feet.
12V plug-free, chopped off a car accessory Cut one off an old phone charger or other device, or $5 at Radio Shack, or $3.75 at All Electronics.
Caulking-free, from the stock in the shop Can be found near the bilge pump, or from a hardware store. A couple bucks.
Piano hinge (depending on cooler)-free, in stock Only necessary if your cooler isn't hinged. A couple bucks at the hardware store.
Inner tube piece (optional)-free, blown tube You may or may not need this-see step 7. If you do, use a blown one, or another piece of rubber, or come up with a substitute. You did save the last blown tube for future projects, didn't you? A couple bucks, tops.
Assorted wire, wire nuts, and screws-free, in stock Depends on what you have in stock and where you get it. It's all at the hardware store, too.
Ice-free, freezer's ice maker If you need me to tell you where to get ice, you shouldn't be doing this project.
Tools:
Screwdrivers
Drill
Cutting devices
Obligatory safety spiel: Cutting devices cut. Don't cut yourself on them. Drills drill. Don't drill a hole in yourself, my dad says it hurt when he did it once. Screwdrivers don't really do anything, but don't throw them into running jet turbine engines. 12V doesn't do much, but watch out. Oh, and wear safety glasses while you're at it.
Materials:
Ice Chest-free, had it on hand If at all possible, get one with a hinged lid. Free from a storage shed or a dumpster. Or, the Igloo Ice Cube 14 looks like it would work,well, as do the Cool 16 and the MaxCold 24. We used an old 12 quart cooler, and it fit a 7 pound bag of ice.
Heater Core-free, salvaged from a '77 VW Rabbit we're parting out You can find these on eBay for 99 cents to $20, or from an auto store for around 20 bucks, or at an auto salvage yard.
Box Fans and Blade Guards-free, from stock in basement They're sold out of $5 120mm 12VDC fans at All Electronics, but Jameco carries these for $12.95 each. Newegg has a nice assortment, too. Under $10 on eBay.
Bilge Pump-$10.44 for a 500GPH unit at Wal-Mart The Attwood V500 was at our Wal-Mart for $10.44-you can get a similar pump for under $10 on eBay.
Hose-free, had it in stock Ours came from an auto-parts store, but it can be found at hardware and auto-parts stores for a dollar or so for a few feet.
12V plug-free, chopped off a car accessory Cut one off an old phone charger or other device, or $5 at Radio Shack, or $3.75 at All Electronics.
Caulking-free, from the stock in the shop Can be found near the bilge pump, or from a hardware store. A couple bucks.
Piano hinge (depending on cooler)-free, in stock Only necessary if your cooler isn't hinged. A couple bucks at the hardware store.
Inner tube piece (optional)-free, blown tube You may or may not need this-see step 7. If you do, use a blown one, or another piece of rubber, or come up with a substitute. You did save the last blown tube for future projects, didn't you? A couple bucks, tops.
Assorted wire, wire nuts, and screws-free, in stock Depends on what you have in stock and where you get it. It's all at the hardware store, too.
Ice-free, freezer's ice maker If you need me to tell you where to get ice, you shouldn't be doing this project.
Tools:
Screwdrivers
Drill
Cutting devices
Obligatory safety spiel: Cutting devices cut. Don't cut yourself on them. Drills drill. Don't drill a hole in yourself, my dad says it hurt when he did it once. Screwdrivers don't really do anything, but don't throw them into running jet turbine engines. 12V doesn't do much, but watch out. Oh, and wear safety glasses while you're at it.
Step 3: Mark and Cut Holes
An ice chest/cooler has a double lid with a cavity. This is convenient because we can cut separate holes for the fan and heater core.
On the underside of the lid, mark the outline of the heater core, then go in about a quarter inch and cut out a rectangular hole through ONLY the first layer. This will allow airflow through the core, but still make it easy to attach.
Next, you will need to mark the inside of the fans and cut out two circular holes. We originally planned to use a hole saw to cut the holes, but found that we didn't have a bit that big, so we chucked a saw blade into the Dremel Rotary tool and zipped it out, of course wearing safety glasses (hint hint).
Tada! You now have a rectangle on the inside and two circles on the outside. Now that you cut out these lovely holes, lets fill them in.
On the underside of the lid, mark the outline of the heater core, then go in about a quarter inch and cut out a rectangular hole through ONLY the first layer. This will allow airflow through the core, but still make it easy to attach.
Next, you will need to mark the inside of the fans and cut out two circular holes. We originally planned to use a hole saw to cut the holes, but found that we didn't have a bit that big, so we chucked a saw blade into the Dremel Rotary tool and zipped it out, of course wearing safety glasses (hint hint).
Tada! You now have a rectangle on the inside and two circles on the outside. Now that you cut out these lovely holes, lets fill them in.
Step 4: Attach Heater Core and Fans
The next step is to attach the fans. This is fairly straightforward--simply drill a small pilot hole at each corner of each fan, and put a small screw through the bottom hole into the plastic lid. Don't block the top hole if you want to add a fan guard later.
To attach the heater core, we used silicone caulk. A bead all the way around seals the lid and provides plenty of bonding force to hold the heater core in place. Make sure to get the core centered, straight, and with the nozzles pointing in. If your lid is hinged, you will want to do a test fit before attaching the core to ensure that the nozzles clear the edges. If your lid is not hinged, it may be easier to attach it first. We ended up cutting the output off to make it fit better.
To attach the heater core, we used silicone caulk. A bead all the way around seals the lid and provides plenty of bonding force to hold the heater core in place. Make sure to get the core centered, straight, and with the nozzles pointing in. If your lid is hinged, you will want to do a test fit before attaching the core to ensure that the nozzles clear the edges. If your lid is not hinged, it may be easier to attach it first. We ended up cutting the output off to make it fit better.
Step 5: Attach the Bilge Pump
The ArcticAir unit just leaves their bilge pump just dangling from the hose, but we wanted it to be more secure. The pump has a twist-off base, so we twisted it off and Gorilla Glued it to the bottom of the cooler. Make sure that it is angled so that you can easily run a hose from the pump output to the heater core input. We attached the base directly to the bottom of the cooler, which works fine, but we probably should have put some spacers in to increase water flow underneath. Also, the picture just shows where it goes. We haven't attached that hose yet, so ignore it.
Step 6: Attach the Lid (Optional)
If you used a cooler with a hinged lid, skip this step. Otherwise, read on.
By hinging the lid, it makes it easier to open for loading ice and letting air flow in during operation. It also prevents the lid from sliding off and dribbling water out of the heater core while driving. You can use whatever you want for a hinge-a rubber strip glued on, a couple of cabinet hinges, whatever. We used a piano-type hinge that we found in our stockpile of random stuff. It goes all the way across the back, and allows the lid to flip all the way back, but still close completely.
By hinging the lid, it makes it easier to open for loading ice and letting air flow in during operation. It also prevents the lid from sliding off and dribbling water out of the heater core while driving. You can use whatever you want for a hinge-a rubber strip glued on, a couple of cabinet hinges, whatever. We used a piano-type hinge that we found in our stockpile of random stuff. It goes all the way across the back, and allows the lid to flip all the way back, but still close completely.
Step 7: Plumbing and Fan Guards
This is fairly straightforward. Connect a hose from the output of the bilge pump to the input of the heater core. In most cases, it doesn't matter which nozzle is used as the input. We just let the output dribble back in, but if the noise bothers you, you could attach a hose to the output. This would also be used as a drain hose.
We also attached a piece of bicycle inner tube rubber to catch water that drips from the output and that condensates on the core. It was cut to fit around the back edge and a couple inches up he sides, and secured with a mega-rubber band we found. This may be unnecessary if you attach a hose to the output, or if your heater core is configured differently.
This would also be a good time to attach wire fan guards to the fans. Just put some screws through the guards into the top holes.
We also attached a piece of bicycle inner tube rubber to catch water that drips from the output and that condensates on the core. It was cut to fit around the back edge and a couple inches up he sides, and secured with a mega-rubber band we found. This may be unnecessary if you attach a hose to the output, or if your heater core is configured differently.
This would also be a good time to attach wire fan guards to the fans. Just put some screws through the guards into the top holes.
Step 8: Wiring
It would be difficult to make the make the wiring for a project easier than this. There should be a red and a black wire coming from each fan, the bilge pump, and the 12V plug. Use wire nuts to attach them all together, and make sure the pump and fans are going the right directions. The fans should be blowing out, and the pump should be pumping through the hose. After everything is moving correctly, you can solder the wires together, or put a switch in the power cord. We also used small zip ties to hold the wires together and to the fan.
Step 9: Operation
Pretty easy, really. Dump in enough ice to fill the cooler about 3/4 of the way, pour in about a half-gallon of water (thats two liters for the smart people) so that the bilge pump can work, and plug it in! And flip the switch if you installed one. Make sure that you open the lid slightly for return airflow. We found that flipping the handle over to prop up the lid provides plenty of airflow without letting the ice get too warm.
If everything is hooked up right, the pump should be humming away, and the fans should be blowing. The water is chilled enough to cool the air within a few seconds. You can put this in your car with the dead A/C (note: this will cool pickups and small cars. Don't bother on your Ford Excursion), you can hook it up to a wall outlet through a 12V battery charger, or you can clip it onto a small 12V gel cell.
If everything is hooked up right, the pump should be humming away, and the fans should be blowing. The water is chilled enough to cool the air within a few seconds. You can put this in your car with the dead A/C (note: this will cool pickups and small cars. Don't bother on your Ford Excursion), you can hook it up to a wall outlet through a 12V battery charger, or you can clip it onto a small 12V gel cell.
Step 10: Test Results
We took the unit out to the truck for a test run when it was well over 90 degrees Fahrenheit (mid to high 30s for Celsius people). My dad set it in the truck and plugged it in, then went back to the house to get a camera. By the time he returned to the truck, the air inside had already dropped below 80, and the air was much drier, making it also feel much cooler.
I attached a PDF of JPEG of a scan of a notecard that my dad took notes on. What it basically says is the following: When the unit was started, the air in the cab was 95 degrees, parked in the shade after a morning of sun,and the outside heat index was 108 degrees. Within five minutes the cab had cooled to 75 degrees, and the air output was 65 degrees. With two quarts of water that had been refrigerated and 8 pounds of ice cubes, the ice had melted after 40 minutes, leaving 50 degree water, with an output of 65 degree air.
In other words, it works! And it works great! On Sunday, August 12 we brought it to Kansas City for a monthly meeting of our electric auto group. WE met in a small meeting room of a library, and ran the air conditioner off a small 12V battery on a table. Many of the members were impressed that they could feel the room getting cooler, and many tried to buy it from us on the spot. We are also in correspondence with an EV owner from Alabama who is eagerly awaiting the publication of this Instructable (I hope). If you're reading this, you know who you are, and I hope you like it.
I attached a PDF of JPEG of a scan of a notecard that my dad took notes on. What it basically says is the following: When the unit was started, the air in the cab was 95 degrees, parked in the shade after a morning of sun,and the outside heat index was 108 degrees. Within five minutes the cab had cooled to 75 degrees, and the air output was 65 degrees. With two quarts of water that had been refrigerated and 8 pounds of ice cubes, the ice had melted after 40 minutes, leaving 50 degree water, with an output of 65 degree air.
In other words, it works! And it works great! On Sunday, August 12 we brought it to Kansas City for a monthly meeting of our electric auto group. WE met in a small meeting room of a library, and ran the air conditioner off a small 12V battery on a table. Many of the members were impressed that they could feel the room getting cooler, and many tried to buy it from us on the spot. We are also in correspondence with an EV owner from Alabama who is eagerly awaiting the publication of this Instructable (I hope). If you're reading this, you know who you are, and I hope you like it.
Notes.pdf151 KBStep 11: Possible Modification and Other Notes
We have some ideas for further modification that we may or may not implement. For starters, we will probably connect a hose to the heater core output. This will eliminate the burbling, trickling sound that is hard on one's bladder, as well as make it easier to drain. The ArcticAir units have a valve inside that you turn to redirect the output to an external hose so that you can drain the cooler out the window onto the flight line without wrestling it through the door. Another option would be a drain plug on the bottom, which is already on many models of coolers. We also are considering a vent system, so that the lid could remain closed while still allowing air to circulate. The ArcticAir units have a louver vent on one side for this purpose. A third modification would be some way to redirect the airflow. ArcticAir units have adjustable flaps over the fans, which could work, or some have a duct hose assembly to redirect the airflow where it is needed. A fourth, and painfully obvious, step would be to enclose all the wiring inside the lid or in a project box to make it look neater. However, for the time being, the zip ties are plenty for us. Also, we may try using Blue Ice-type ice packs, so that it would be reusable and not waste water.
This being a Go Green contest, I should put in a spiel about why you would bother with this. The main purpose is to make it more comfortable to drive a zero-emissions electric truck in Kansas in August. This runs off any 12V power source that can shove out 3 amps, so it can be used in other areas that need cooled, such as a hot workshop. Also, the ice lasts for hours when it's not running, and you can leave it on while you run into the store. The only disadvantage is that you could say it wastes water in the form of ice, but you can empty it out on your garden or lawn. It uses only ice water for cooling, which is about as non-toxic and environmentally friendly as you can get, and it kept a hose, a cooler, and a heater core out of a landfill. In other words, it uses environmentally friendly power to run environmentally friendly coolant through recycled parts in a zero-emissions vehicle. Can you get greener than that? I mean, this is #008000 at its best! Just make sure to recycle batteries.
Thank you for reading!
This being a Go Green contest, I should put in a spiel about why you would bother with this. The main purpose is to make it more comfortable to drive a zero-emissions electric truck in Kansas in August. This runs off any 12V power source that can shove out 3 amps, so it can be used in other areas that need cooled, such as a hot workshop. Also, the ice lasts for hours when it's not running, and you can leave it on while you run into the store. The only disadvantage is that you could say it wastes water in the form of ice, but you can empty it out on your garden or lawn. It uses only ice water for cooling, which is about as non-toxic and environmentally friendly as you can get, and it kept a hose, a cooler, and a heater core out of a landfill. In other words, it uses environmentally friendly power to run environmentally friendly coolant through recycled parts in a zero-emissions vehicle. Can you get greener than that? I mean, this is #008000 at its best! Just make sure to recycle batteries.
Thank you for reading!
Step 12: Tales from the Comments
9 April 2010: I'm adding this step to address some of the comment subjects of the comments...with 384 comments and counting, it's not nearly as easy to just read through them.
1) Dry ice has a lower specific heat capacity than water ice, so while it is colder, it won't last as long.
2) Dry ice sublimates to CO2, which, in a confined space, will at best impair judgment, and at worst cause loss of consciousness. Driving a car requires being alert. I highly discourage using dry ice.
Seems like a good idea, I know--add 12V and that little plate gets frosty cold. But you have to consider the net movement of heat. All that a peltier does is create a heat differential...a difference in temperature between the two sides. It does this by moving the heat from one side to the other. If you put a peltier inside the car, it won't cool anything down, because the same amount of heat that is removed to make one side cool is released on the other side right back into the car. The only way this could possibly work is if it was mounted so that the heat was released outside the car.
Using salt water or alcohol to lower the freezing point, so it will be colder...not really. The temperature is determined by how cold your freezer is. Oh, and salt water is corrosive and will OM NOM NOM your heater core.
Let's take this
Remember the energy flow. If you freeze the ice in your own freezer, all the heat that is pulled out of the ice, and then some, is released into your house from the coils in the back of the freezer. You could use this to cool that hot bedroom, but the house as a whole will be warmed.
Dry ice instead of water ice
First, a clarification: water ice isn't 32°F, it's whatever the temperature of the freezer is, anything below 32degF...just like how the steel on your car can be 10°F on a cold day, or 100°F on a hot day. Dry ice can be any temperature below -109°F. Since it is much colder, id would theoretically put out colder air. The two main problems with this idea are:1) Dry ice has a lower specific heat capacity than water ice, so while it is colder, it won't last as long.
2) Dry ice sublimates to CO2, which, in a confined space, will at best impair judgment, and at worst cause loss of consciousness. Driving a car requires being alert. I highly discourage using dry ice.
Visit Our Store »
Go Pro Today »
I'm glad to see that another person built one, and even more so because it's in the application that KoolerAire was trying to gyp $500 for.
My air conditioner is in a helicopter! Yay!
I was wondering if I could pick your brain, get your opinion and what not. I'd like to use this concept, but marry it with the solar ice maker idea. And try to make it into a self-contained air conditioning system that could replace the AC unit for a 28' - 30' class C motorhome. Basically, it seems to me that I would need to scale down the solar ice maker and scale up your design to make it work. Would a 100watt solar panel be enough to run the mechanical parts in your design, do you think?
Could this design work with a larger heat element? Do you think it could cool a motorhome of that size (which is a relatively small space, usually about 95 - 150 sq. ft. of space)? The way I'm thinking is whatever melt ice is coming out of the heat element can be routed back to the ice maker, so it's essentially on this continuous loop with minimal amounts of water lost.
Anyway, sorry for the questions. This is a really exciting tutorial, though. Thanks again for posting it!
The basic design could be scaled to any size. I won't presume to make an estimate on how well it would work to cool a motorhome, but it could at least blow cold air on your comfiest chair.
I'm not familiar with solar ice makers. With the amount of solar panels you're talking total, I wonder how hard it would be to find a high-efficiency traditional air conditioner that you could run off those panels instead.
Super Cooler! Peltier Heat Sink & Fan
http://www.goldmine-elec-products.com/prodinfo.asp?number=G19056
It's kind of like knives: what's safer to work with, a dull blade, or a sharp one?
1) Installed cooling fans and connections inside the lid between the heater core and the top of the lid to keep it neat
2) Two large fans blow over the heater core into the ice chest
3) Added a small fan to blow the cold air out
4) Added a 4" corrugated plastic hose to direct air
Used it for the first time today in our 4 seat airplane. It worked pretty well, even in the 90 degree heat. Dropping the humidity in the cockpit made a huge difference!
I only filled the ice chest half way with ice, water, and a couple frozen water bottles. Next time will will fill it up higher...
Don't use your hand as a clamp.
I put this cooler together for a total of less than $150 .
I had the 12V 230 cfm 4â bilge blower fan (~$30 now, Attwood 1741), a junk car 5âX7âX ~ 2â cabin heater core (~$15???), 4â dryer hose/clamps, wire, connectors, misc parts and pieces.
I purchased a 12V 500 gph bilge pump (Wal-Mart Attwood 4606-7, $18.66), some ¾â hose, a few clamps, a 4â dryer adaptor, some fuses, the intake HVAC diffuser with control lever (Home Depot aircraft supply), a 12 V on off switch, & the 48 qt cooler box with drain (Walmart, $25). Total about~$86 incl tax
It took about 5-6 hours to put together. Worse thing was cutting the insulated cooler to fit the parts in. I used construction adhesive and SS wire/screws for most of it. Total cost<$150+ my time as an educational endeavor.
On a 104F TX day (today here), it cooled the air to 65F until the pittance of water/ice from the refrigerator test ran out. (I didnât purchase a couple of ice bags-will test that next-you put about a qt or so of water in to get the pump/exchanger to work).
4â Fan draw at 230 cfm=3.5 A at 13.6 V; 500 gph bilge pump=1.5 A. I used a 7.5 A fuse.
This is a really great Instructable! It really got my creative juices flowing, thank you so much for that!
In the teardrop, if you are camping next to a stream, you can use two long hoses and a funnel to catch the water flow, divert it to the Teardrop where the pump can take over and pump it up into the teardrop. The teardrop is cooled and then the water is returned to the stream! Call it a "Gone Green A/C Cooler!" Hey, come to think of it, that would supply the Teardrop with a source of water that could quickly be purified using a ceramic filter for consumption!
Is there any substitute for heater core?
Doesn't matter.
Coil of copper tube? Anything like that.