Introduction: Hot Water Heater for My Greenhouse

Hi guys! My name is Kenneth Sissney and I am a retired Shop teacher living my dream working at Walmart. In my spare time I like to garden, take care of my chickens (14) and work in and on my greenhouse. The first part of this year I made a hot water seed starter using a coffee pot heater element. If you would like to take a look at it here is a link. This instructable is about a heater I made using the hot water from my seed starter. I got this idea for firewizerp in a comment he made about my seed starter. Being is is the middle of September I do not have any stats ready to share right now. As It gets cooler and I am able to do some temp measurements and I will share them in the comment section. I hope you enjoy and please leave a comment.

Step 1: Heating Coil

This is my AC coil that I am using. I had an old window AC that was over 40 years old. Make sure there is no freon left in it. After cutting it out of the old case I had to straighten the fins. Being stored and moved many years there was some fin damage. Finding a screw driver and getting me a good chair I set out to fix the fins so more air would pass through. It took me several weeks to get it done. I was doing this during the hottest part of the summer and the wife would not like me doing it at the dinning room table. Being cheap I didn't go and get a fin comb. LOL

Step 2: Attaching Base and Fan Mounting

In the first picture I have attached the base to the coil. If you look close I added a board to the frame of the coil. This is what I attached the base to. In the second pic you can see the beginnings of my fan mount. This is an old chicken nesting box. After cutting it into ( third pic) and removing the sides (forth pic) for better air flow I was ready to mount the fan in front.

Step 3: Mounting the Fan

In the first picture I needed to remove a small section from the bottom. After getting it placed I needed to make a covering for the mount. I was able to use some 1/4 inch hard sided foam core board we got from work (trash). In the second picture I have removed the front fan guard. I placed two wooden cleats (behind the foam board) and screwed them to the foam board. Next I screwed the back fan guard into the cleats and into the bottom board of the fan frame. This made a very strong mount for the fan. The fan was a extra we had laying in the shop.

Step 4:

In the first pic you can see the placement of the fan mounting. In the rest of the pics you can see I am enclosing the coil and fan. What I am using is 1/4 inch corrugated plastic sheets. You can buy these from Lowe's or Home Depot. I got mine from the trash so look around for things you can use. After cutting the side and top panel I screwed them on with sheet rock screws. I turned the fan on high and ran the cord out the front before putting on the top panel.

Step 5: Completed Unit

In the last picture you can can see that I have used white tape. Any color would do that is all I had. I prefer Gorilla tape over Duck tape. To me Gorilla tape has better glue and holds longer. All I was trying to do was close all the places unheated air could get in.

Step 6: Tools and Supplies

In the following list is the tools and supplies that I used to build this project. There are many materials out there that you can substitute for the ones I used. You could substitute 1/4 inch plywood for the foam board and the corrugated plastic. As I said I found a lot of the things I needed just by looking around and asking the neighbors.


Phillips screw driver

Band saw or saber saw

skill saw

hack saw (to cut out the coil) make sure it is empty of freon before you cut into it.

power drill for drilling screw holes in the coil frame

utility knife for cutting the corrugated board


Phillips screws

various sizes of plywood

1/4 inch hard sided foam board

1/4 inch corrugated plastic

gorilla tape

8 inch table fan with base removed

AC coil

Step 7: Final Thoughts

As you can see from the picture I have placed it in my greenhouse. This is right under my seed starting area of the greenhouse. I have hooked it into my coffee pot heater and even ran water through the hose and coil. Found I did not have my hose clamps tight enough. After fixing the leaks I ran it with the heat turned on. The water heated up and ran at 120 degrees. The only bad part was the temp in my greenhouse was reading 102 so I was not able to get a true reading of the temp of the air blowing out. When the weather turns a lot cooler I will update with a better reading. I am in the process of locating a thermostat to run the fan only. Please ask any questions and I will try to answer them. If you have any suggestions please feel free to leave them. Thanks again for looking.

Step 8:

This is really just an update. Had some problems with the coffee water heater. First one the switch burned out, replaced it with another and it developed a hole. Replaced the second one with a heater that was a little smaller. Heated well but needed to be the larger one so I replaced the switch on the first one. Works again. As you can see,in the first picture, I was losing to much heat from the water. I covered most of the back. Works even better now. For Christmas I received a Lux thermostat. This thermostat allows me to run the heater fan only when It needs to run. This way the water can recover some of the lost heat. The thermostat was bought from Ace hardware. I do not know how much it costs.I keep the thermostat set at 50 degrees. This morning the outside temp was 27 with the temp in the greenhouse at 51. Hope this helps.

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