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Hydronic Water Heater

As you may have read in the Radiant Floor Heat section I have installed pipes for floor heating in my bathrooms and kitchen. This is called a hydronic heating system. In this section I will discuss a bit about the the system that I used to heat the floor. Up till this year (2007) I was using my domestic water heater. This worked really well as far as BTU, as long as my wife didn't just pour a tub. I had both bathrooms and the kitchen on one zone. I had misterhouse running a timer to keep the floor at a nice temperature. It turned off over night and while we were at work. This was adequate but took some messing around to get things balanced properly and I would like to have the bathrooms warm all night but did want to heat the kitchen. Here is a look at the schedule that I was using...

Click to enlarge

This is very similar looking to my sprinkler screens. One started from the other, can't remember which came first. Anyhow you can see that it runs three times an hour at 5, 25 and 45 minutes after the hour. For the first hour after not having been running it will run longer (15 minutes) than it would if it had run the previous hour (5 minutes). The system is also configured to run all day on Friday, Saturday and Sunday, someone is always home. On weekdays it runs in the morning and at night.

Late last year I decided to disconnect the system because my little boy started to drink the bath water and I new it was less than clean. I would strongly recommend not connecting your heating system to your domestic water heater, it is just not worth it.

Summer comes and goes and it's starting to cold outside, I am forced to get the floor heat running again. Our kitchen extension is almost unusable without the floor heat. I was originally going to build a heat exchanger for my water heater. It was going to be a fairly simple setup. I was going to run a 3/4" pipe inside of a 1" pipe. Then I just needed to setup one pump to circulate the water through the 3/4" pipe and in / out of the water heater. And another pump circulating the fluid through the 1" jacket and then the floor heat pex. There are a pile of different ways you can do this, you just need to be creative. The point of this type of setup is that you can a "closed" system. So your domestic water doesn't mix with your floor heat water. Again this is important for health reason but also if you are running your fluid in an area that may freeze if the power goes out you can now run an antifreeze of some type (glycol). I know nothing about this.


I did decide to go with a closed system, but not a heat exchanger, my system has it's own heating source. I thought about going with a natural gas water heater, but that becomes a larger issue. You will need to deal with venting and gas supply, etc... Plus it is expensive to get an efficient gas water heater. If you go with an electric water heater there are no  venting issues, anyone can install it, you just need to deal with the electrical part. Plus there is no heat loss with an electrical water heater. A portion of the heat goes up the chimney with a gas water heater. With electric is all stays in the house, and because I only run this system in the winter time I don't care about insulating pipes or the water heater, it all goes to heating the house.

OK so we have decided on what we are going to heat and how we are going to produce that heat. Now we need to look at some technical issues. Please understand I am not an expert, I don't work in the plumbing industry I don't even pretend to understand all of the issues, I am just a guy that likes to play. In my research and experience I know that we need a couple of key pieces to make this system work as a closed system. Here is a list of stuff you may need to purchase:

  1. Heating coil (pex pipe or radiators)
  2. Manifold (to connect all the ins and outs to heat source)
  3. Zone valves (turn zones off and on)
  4. Temperature / Pressure gauges (not required, but nice to have)
  5. Expansion tank (to absorb pressure changes)
  6. Auto fill valve (if connect to water supply)
  7. Air release valve (removes air from system)
  8. Heat source (boiler or water heater)
  9. Various other valves for balancing, draining or filling the system

Here are some pictures of what I have built. I will probably need to do some testing and make minor changes. I am currently run both zones with one control wire, this will be that last part of this project. When I put in the second control wire and temperature sensors I will also clean up the other wiring.

Click to enlarge

On the last picture you can see I also put an air compressor fitting on the system to blow out the lines in the summer time. On the bottom left of the same picture you can see the auto fill valve. With the combination of auto fill and air bleeder valves the system will just run, I didn't do anything special to fill it, just turn the water on and away it went. The auto fill also has a check valve built in so that the circulating liquid doesn't back into the drinking water. I especially like the pressure / temperature gauge, I like watching what the system is doing.


The completion of this project is on hold until I get my 1-wire network working. I will be using ibutton technology to measure temperature in the floor and control each zone. Read more about 1-wire stuff here.


  • Fill the system before turning on the water heater
  • After the water heater is hot bleed off some water to lower the pressure, there is no need to run the system at 50 psi, should sit between 20 and 40 psi
  • I am going to do some research on what to run in the system (glycol or whatever), I know nothing about this
  • I went with an eight gallon water heater, I wouldn't go any smaller than this unless you used an on-demand system that can keep up
  • Do your homework (read about design), don't rely I what I have written, I don't know anything
  • I will add the ability to the software to run on the holidays (by date), my Asterisk box already does this at work for the long weekend outgoing message, so I will just steal the code from that
  • I am also going to do some testing on running the system all the time vs. running only when we are home to get an idea of energy savings if any, to do this I will need to monitor when the water heater turns off and on (again a new project altogether)
  • Have fun and let me know how your project is going

Please feel free to leave a comment about this page

#2 Brad wrote at 05.03.2009 22:28

My system never loses pressure, if anything the pressure goes up. The auto fill valve keeps the water press at about 14psi, then when the water goes through a cold line it contracts which lowers the pressure even more. The auto valve kicks in and fills it up again. Then when the loop warms up it expands, so I am usually running around 30psi.

1. Expansion tank will only let so much pressure go (assuming it is losing air, but it should run out)
2. This is possible, but again it should slow down
3. You might have a leak, if you have a pressure gauge on your system then just turn the pumps off, pressurize the system and let it sit for a couple days. If you lose pressure (more than 5 pounds), I would say that you have a leak.

If you have more than one loop, you could pressurize each one and see if you lose pressure individually. You can do this with air or liquid.

I am no expert, just some ideas.


#1 Ian Egloff wrote at 02.03.2009 12:09

I've put together a small closed loop hydronic system like what you have described.
The in slab Pex was tested to 150psi and then left at 50psi for over a year.
Now that I have filled and tested the pumps and 8 gal electric hot water tank it is losing pressure.
I can't find any water leaving the system & I don't think the floor slab has moved at all so I can't imagine the pex is broken where I can't see it.

I started with 20 PSI & it fell to 7 PSI over 36 hours.
My three main culprits are
1) Expansion tank is slowly letting the pressure drop.
2) Air is coming out of solution & leaving the system through the automatic vents.
3) I have a leak that I can't find.

I'm hoping it is #2 and the system will start to hold pressure once the air is out & I should just keep adding water.

Have you found this in your system?


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