Coffee Maker Heater for Seed Starting





Introduction: Coffee Maker Heater for Seed Starting

I needed a seed germinating heater for my cool night time greenhouse. In trying to find a DIY one on line I came across several coffee maker heater's used to make hot water. This kinda was what I was looking for so I decided to build one of my own. I want to tell you I am very satisfied with this project. For the passed two months I have pushed 130 degree water through 58' of tubing for 24 hours a day. This does add a little heat to the greenhouse but not much. I have not tested mine but it might keep a small greenhouse from freezing. My greenhouse is 6' by 9' and I try to keep my night time temp between 50 and 60 degrees. It takes about two hours to heat the water from a cold start. I have spent less than $60.00 on the entire project. I do not know the cost of running this setup. This setup could also be used for supplying hot water for personal use. Just shorten the return hose. With a thermostat you can set the water to several different settings.

Step 1: Bucket

I used a regular 5 gallon bucket and lid, from Home Depot, to store my pump and return water line. These buckets are inexpensive and can be found in various stores. You might even have one around the house. You will need to cut two holes in the lid. I ran the pump power cord and outflow hose out the same hole. The other hole was for the return water line. I am also using a Deep Fryer thermometer to keep an eye on the temperature of the water.

Step 2: Coffee Maker Heater

The heater is a simple on/off type of coffee maker. Mine was one I had just sitting in a closet. You can buy a new coffee maker from Wal-Mart for under $10.00 or look at a resale shop. I did not remove any wires from the heater. I did leave the switch in so I could turn on/off just the heater. If you leave the switch in line remember to tape around the base to cover the exposed wires. I found out the hard way that this was a missed step. There is a back flow valve, in the hose, that prevents the boiling water from backing up. Your pump will need to be hooked to this side. At one time before I got my pump I was testing the heater out. I found out the heater will heat the water to boiling and shoot it down the exit side. At this time it will draw in water from the cool side. This action make the heater a pump also. I would advise you to not use it in this way. The water coming out the heater is boiling and may lead to burns if you have an accident.

Step 3: Low Flow Pump

I am using a small fountain pump, low flow, to push the water through the heater and hose. I found this pump at Home Depot for under $20.00. I did have a problem, at first, when I was testing without the thermostat. There is a cover, were you hook up the hose, that came off with the very hot water. I just wired it back on and have had no trouble with it coming off. At first I did not have a thermostat to regulate how hot the water would get. In two hours time I was able to raise 68 degree water to over 200 degrees. A little to hot for what I needed.

Step 4: Thermostat

I found an electric hot water thermostat, lower heating coil, at Home Depot for under $9.00. I applied high temp grease, white lithium, to the contact area and taped it to the bucket. I figured this would help make a better contact area. I did not need to cut into the bucket. I took one leg of the heater cord and cut it then I wired it into the thermostat. Then duck taped it to the bucket. There was no need to mount anything inside the bucket. I am not able to set the thermostat to a set temperature. I am able to use it to get to my desired operating temperature and to maintain that temperature. After I took this picture I found out it is upside down. In this setup I am sure you could get 5 gallons of hot water at 180 degrees maybe higher in 2 hours time.

Step 5: Vinyl Hose

As you can see I ran the hose along the top of my shelf. I then placed spacers to hold the seed trays off the hose. This is one continuous hose about 58' in length. Not sure how long you can make it before you lose to much pressure. The hose I used is 3/8" ID and 1/2" OD vinyl. To connect two hose ends together I used 3/8" OD ridged tubing and two small screw clamps. I got most of it from Home Depot or from my shop. The hose is rated for 140 degree hot water. If you are going to be running a hotter water you might get a higher rated hose. The return side just empty's back into the bucket. Because of the warm water and sunlight, on the water tubes, I added some bleach to the water to reduce the growth of algae. I will probably run the temp up around 140 to 150 degrees. I will have to wait until I have some seed trays planted to test the temperature of the soil.

Step 6: Thermometer

The thermometer I used was for a deep fryer. I like for it's long prob. Most any one will do as long as it can be inserted into the water.

Step 7: Materials and Tools


  • Bucket with lid
  • fountain pump (low flow)
  • coffee maker heater and wiring
  • thermostat
  • thermometer
  • hose to fit pump
  • small clamps
  • small tube for splicing hose
  • duct tape
  • electrical tape
  • lithium grease
  • string or wire to fasten hose to shelf


  • knife for cutting holes in lid and hose
  • wire cutters
  • screw driver
  • Not sure I have presented everything. This is my first post. If you have any questions please ask. If you have any suggestions please make them. I am a retired shop teacher and like to tinker around the house.



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12 Discussions

Hi Guys! Some of the things I learned over the late winter seed starting. The most important was I did not need all the hose running the full length of my shelf. (12') I trimmed it to only cover about half that length. I did not need it under my plants after they were started. This helped with not having to run the coffee heater as much. I have added and heater, to the greenhouse, using the hot water from the seed starter. Check out my new addition.

Update; Found out my first thermometer was reading to warm. Replaced with two new ones. Temperature of my hose water was running around 70 degrees. Stepped my thermostat up to 140.Now hose temp is running from 85 to 90 degrees. Planted two small trays with cauliflower and broccoli seeds. A good temp for germinating these seeds is around 85 to 90 degrees. Takes around 5 to 10 days. My broccoli was breaking ground in four days and the cauliflower was 5 days. I am very satisfied with this setup so far. Tomato and peppers will be started mid Feb. Update later.

great instructable, I built something similar last year living in a cooler winter climate makes it difficult to maintain a constant temp for seed starting especially for exotics. By adding a $19 dollar temp. controller, two coffee heating units, and using recycled aluminum radiators, I was able to keep not only the air temp but also the soil temps constant.

3 replies

Thanks for the comment and the idea of using the radiators. Now to find a radiator to help with heating the air. Again thanks for the suggestion.

I ended up using the aluminum evaporators out of two broken central air units I had behind my shop, after removing the radiators I took a Sawzall and split them lengthwise between the cores, ending up with 4, 24" x 24" pieces per unit, I then ran all 8 pieces In a series on my shelves, then wired PC fans under each radiator. One other modification was I built a plate out of 1/8" aluminum and drilled holes to fit both heating elements making sure to insulate against a short circuit, fit the assembly into the bucket so that they were not only heating the water flowing through them , but the water in the bucket as well.

Very interesting way to re-use/re-purpose a coffee maker! Makes me want to see what's inside of mine. Does it lose much heat on it's trip through the tubes? Is 140f kinda hot for seed starting?

1 reply

Good Evening. I have not tested the temperature setting on a seed tray yet. I am not sure what to set it at. As I start planting my seed trays I am planning on keeping an eye on there temperature. It will also depend the type of tray I use. Some of my trays I direct seed in and some I start the seeds in small pots and place them in a tray to hold them. I figure if I can maintain a soil temperature of 65 to 70 degrees it will be close to what the outside garden soil would be at planting. I should be OK. I will use a lower shelf for seeds that need a cooler temperature to germinate. I think I will do a test tray and see what the outcome is. I will post the results. Thank you for commenting on my post.

Clever idea. This could be used to heat an aquaponics tank so your tank water doesn't come in contact with the heating element. It wouldn't be as efficient as direct heating of the tank, but it would prevent metals from the heating element from being transferred to the tank.

1 reply

Thank you for the comment. I would suggest you get the longest hose and make your connections outside the tank. This way, if you have a leak at the connection, your water will not be contaniated.