Guest post by Doug Hansmann

In our new house, we intend to maximize our use of solar energy with both a comprehensive passive solar design and an array of four solar hot water panels installed in the back yard.  A couple of months ago we migrated away from a deep sand bed design under the house, even though that would be one possible way to store every BTU of solar energy collected, (see Denise’s post Why We Are Not Using a Sand Bed to Store Thermal Heat )   because such a  system is not easy to control. We are now learning that this lack of fine-tuned heating control in radiant floor heat will still be problematic with the 2- and 4-inch thick concrete slabs we will have in our new house.

Comfortably and efficiently capturing the heat of the sun within four walls takes careful planning. (photo credit: http://www.flickr.com/photos/soloflight/3474350801/)

In our Wisconsin winters it’s not cost effective to use solar energy to do the entire job of heating a house.  I supposed you could design for the depths of winter using a huge array of collectors and a massive storage tank, but what would you do with all that heat the rest of the year?  Like most installed systems, we are going to design for the shoulder seasons and supplement (in our case with propane) as needed.

With the goal of making the best use of our solar collectors, we now have a revised plan which has brought us to a new choice between two fundamentally different approaches to solar-based hydronic heating systems.

It can be tricky to make the most of your solar panels.

1. a slow response system that  provides relatively even heat 24/7.
2. a rapid response system that allows you to dial back the temperature when you are not using certain rooms.

Heat that is provided only through the slab will be a slow response system.  That means that you will be heating rooms when you don’t need to.  If you want your bathroom to be warm while you use it in the morning – no can do.  You will have to chose between a cool room while you take your morning  shower or an unnecessarily-warm room for many hours a day.

These are NOT the radiators you struggled with in your first apartment.

We are opting for rapid response, wall-mounted, low-mass  radiant wall panels in rooms where heating needs are intermittent.  (See the Select line of Myson radiators.)   These will be powered by a propane boiler rather than the sun and will primarily be used on cloudy days in areas of the house the wood stove won’t heat effectively.

We’ll put all of the solar heat into the concrete slabs, but won’t supplement the slabs with any propane at all.

Yes, the low mass radiators will be fueled by non-renewable propane, but even with the slab-only heating option, propane would typically supplement solar to keep the floors warm when the sun isn’t shining. We expect our rapid-response radiators will sip propane moment by moment and room by room, rather than gulping propane to continually heat the entire slab.

Each radiator will have a simple dial control and will provide heat within ten minutes of turning them on.  Our plan is to walk into the room, turn on the light, turn on the radiator.  When we leave –  turn off the radiator and turn off the lights.  It will be a visceral reminder of our use of non-renewable energy, and will give us the opportunity to minimize the minutes of propane usage.

Yes, this tank is ugly -- as it ought to be.

We got the idea for high efficiency radiators from project engineer Andy DeRocher at Full Spectrum Solar,  who is designing our solar hot water system. We like this approach better than the relatively constant warm temperatures that would inevitably be chosen if the slab were our only heat source – temperatures that would exceed what is really necessary in the parts of the house we are not using.

It seems intuitively right to be able to manage our non-renewable energy use closely.  To have the tangible reminder through our fingertips when we are burning propane, and the satisfaction of stopping that use whenever and wherever we can.  We will still be soaking up every last BTU from the sun into our concrete floors and other thermal storage (more on those later), but we won’t be wasting propane to heat empty rooms.