Radiant Heat and On-Demand Hot Water

As I mentioned in earlier articles, our home is generally heated with a wood burning stove, with some supplemental heat gathered through a passive solar process.  Of course we use a more standard heating system as well, which is thermostatically controlled and fired by liquid propane gas.  What is unusual about our system is that it is an open network of water filled pipes, warmed by an on-demand water heater.

By “open network” I mean that the water that runs through our floors is also our potable water.  Potable is water that is used for drinking, bathing, washing, and cooking.  So through a complex array of mixing valves and pumps, our hot water is diverted as necessary to the washing machine or the shower or the floor.

When considering this “green” technology, I spoke with several companies who supply radiant heat parts and services.  At the time (three years ago) this was completely foreign to me and to most of my colleagues here in the building industry.  This was before the price of fuel skyrocketed, before the current economic slump, and before the green building movement had gained any momentum.

The supplier I chose had many years of proven service and experience.  I sent them my building plans, and they generated a detailed diagram of my system along with a complete list of materials.  Fortunately for me, my friend was building his house at the same time, and he was also interested in using a radiant heating system throughout his floors.  We helped each other with every aspect of the learning and installation processes.

For the lower level we tied pex tubing to the re-bar grid before the concrete was poured.  The thermostat for this zone communicates with a temperature sensor that is also submerged in the poured slab.  On the upper level, the pex was strapped to the bottom of the sub floor.  This zone has its own thermostat, which is mounted on the wall about five feet above the finished floor.  Both zones had to have insulative barriers beneath them, since the heat is so effective it actually radiates downward. 

After running the system for a couple years, the two zones have clearly demonstrated their differences.  The lower level zone, being surrounded by a mass (poured concrete), gives us almost immediate results.  In fact, if I set the thermostat for 72 degrees and come back later, I find the slab temperature has climbed to as high as 75 or 76 degrees.  In contrast, the upper level zone takes more time to raise the temperature to the set point of the wall thermostat.

My two zones are connected to a propane-powered on-demand water heater.  During the construction process I heard many differing opinions regarding my proposed system.  The building inspector was intensely curious, and eagerly soaked up every bit of information I offered.  The plumber shook his head and told me the water heater was going to run continuously, and purchasing propane would bankrupt me.  The radiant heat company assured me it would be effective and efficient.  The propane supplier licked his lips and told me we needed to have a 500 gallon tank and a line of credit. 

In the end, the radiant heat company was right.  What the others did not realize is that the water heater only needs to run for less than an hour, until it sufficiently heats the water that circulates through the floor and back into itself.  After that, only one 85-watt pump is necessary to circulate the hot water throughout the floor until the set temperature of the thermostat is achieved.  Even if the pump runs for several more hours, I am basically heating my house with less than one amp.

The propane supplier called me every month during the first year…now they know I only fill my tank once every 12 months.  When they top off the tank at 80% capacity that would be 400 gallons.  A year later the tank generally reads 45-50% or 225-250 gallons.  So in one year I consume only 150-175 gallons. 

I cannot promise the reader the same benefits unless you are willing to do some of your heating with a wood stove.  But still this is a fantastic result considering that a two-person household is using the propane also to cook and to bathe.

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