Running Radiant Infloor off a Water Heater #FAIL

I am doing a bit of a science experiment in my house and it involves radiant infloor heating, my water heater and REHAU RAUPanel. The entire system is also being monitored by a smart control  that I built specifically for hydronics. Please note I work in the HVAC industry and this is a deliberate experiment to see how different systems work under different situations.

I under no circumstance recommend you install your system the way that I have.

In my house I had a very cold room and wanted to heat it easily and the fastest way for me to add some serious heat to the room was to do it with three things; RAUPanel floor panels, a water heater and a cycle timer.

Lets quickly explain those three items.

  • RAUPanel: Made by REHAU it is a very low profile radiant panel that goes on my floor and can deliver up to 32BTU’s per square foot using 110 degree water.
  • Water Heater: The area I am heating is actually not that big and the total heat loss is around 2800 BTU’s. I chose to use my water heater because I am not yet ready to install a boiler plus the Water Heater provides me with 108 degree water off the mixing valve which is perfect for my application. ( Please note there are all kinds of reasons NOT TO run radiant off a water heater )
  • Cycle Timer: Because I connected my radiant infloor system to my Water Heater I need to make sure that the water does not sit and ‘stagnate’ in the pipes in my floor. The cycle timer cycles my radiant infloor system every 24hours and prevent me from getting things like legionnaires disease

How has my system worked so far? Well from a heating stand point it has done exactly as I intended. The RAUPanel as promised is delivering on average 22 BTU’s per square foot with on average 104 degree water and the overall comfort of the room has improved immensely. It is now the most comfortable room by far.

There is however one tiny snag in my system and that is when my wife does Laundry or I run the dishwasher. Checkout the graph below to see what I mean.

Can you tell when my washing machine kicked in? Water Supply Temp went from 106 to 73.61
Can you tell when my washing machine kicked in? Water Supply Temp went from 106 to 73.61

When we run our washing machine we are not only cleaning our clothes we are robbing all the heat out of our radiant infloor system. #FAIL.

Why is this happening? Lots of reasons.

  • Water Heater Can Not Keep Up: My Water Heater was never designed to wash my laundry and heat my radiant infloor and clearly it does not have enough hot water to do the job. A water heater is not a boiler and when you try to use it as one bad things will and can happen.
  • Bad Piping: The way my system is piped with no heat exchanger the water heater actually pulls the heat out of my floor. Why no heat exchanger? Because as noted this is a science experiment 🙂
  • Control Issues: I am using a custom built smart control which monitors my system and it could use a few tweaks. Namely I need to write some code to email me alerts when the system temperature drops dramatically.

Moral of the story? A water heater works pretty good as a ‘heat source’ but remember its not a boiler and although my installation meets all local codes with a cycle timer that does not change the fact that when I have high water loads I am sucking all the heat out of my floor reminding us all that local codes are a base line and do not represent the best or most efficient option.


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