Solar Hydronic Space Heating

A Case Study

Inside this Article

Carl Bickford's Shop
Carl Bickford's Shop
A 95-watt PV module controls and powers the thermal fluid cycle
A 95-watt PV module controls and powers the thermal fluid cycle.
Two refurbished 4- by 10- foot thermal collectors
Two refurbished 4- by 10- foot thermal collectors designed for horizontal mounting.
Carl Bickford's Shop
A 95-watt PV module controls and powers the thermal fluid cycle
Two refurbished 4- by 10- foot thermal collectors

I’m fortunate to live in a sunny climate that averages 5.8 peak sun-hours per day. But I also live at 5,600 feet above sea level, where the heating season is much more significant than the cooling season (5,700 heating degree-days versus 700 cooling degree-days). Despite this, and that my 620-square-foot shop’s south face receives morning shade from a neighbor’s house and tree, my two solar systems—passive solar heating and a solar hydronic floor system—provide enough heating to make the building comfortable in all but January, the coldest month of the year. Even then, the relatively warm surface temperatures and warm floor allow me to work without gloves in air temperatures as low as 45°F (the lowest recorded interior temperature). During the coldest times of the year, solar heating typically keeps temperatures between 50°F and 65°F. At first light, due to the previous day’s solar heat collection and storage, temperatures are generally 40°F above ambient, a testament to the power of the sun and a little RE technology. Paired with passive solar, my drainback hydronic heating system offers two main benefits:

  • Simplicity. The system is easy to assemble and operate, and homeowner friendly. Distilled water is used as the heat-transfer fluid, and the unpressurized fluid loop is easy to drain and fill. There are no overheating or freezing issues with the collector fluid because the collectors are empty (drained back) when there is no solar heat. This system requires no differential controller, saving about $200 up-front on equipment. When you don’t want heat, just turn off the pump. There’s also no heat exchanger or storage tank to increase complexity and cost.
  • High efficiency. Extremely low collector inlet temperatures of the heat-transfer fluid maintain high efficiency by lowering losses to the outdoor environment. The floor temperature in the winter varies from 50°F to 70°F.

System at a Glance

From the two 4- by 10-foot, flat-plate collectors mounted on the shop’s south-facing wall, water flows through a small drainback tank and then directly into the radiant tubing embedded about 3 inches deep in the 4-inch-thick concrete floor. As it circulates through the tubing, the solar-warmed water gives up its heat to the slab and then returns to the collector for reheating. In turn, the slab transfers heat to the shop’s interior by convection and radiation.

This straightforward system has only two control mechanisms: the sun shining on the PV module that drives the circulator pump, and a manual switch used to turn off the pump (and therefore, the system) during the summer months when additional heating is not needed.

Solar Sizing Strategies & Best Performance

Designing any energy system is all about the load. With a solar-electric system, it’s your electrical loads you’re concerned with. With solar heating, either passive or hydronic, that load is what it takes to heat an interior space. It’s a little less straightforward than sizing a PV system. Knowing the size of the heating load requires intimate knowledge of your climate and your building envelope, including the thermal resistance of all the surfaces.

All heat transfer is driven by temperature difference (delta T or DT). The rate of that heat transfer depends on the DT and the amount of thermal resistance (R-value) at the locations of those temperatures. Lower R-values will result in higher heat transfer rates.

Comments (2)

CrazySteve2418's picture

You may want to try ceramic bead paint. I painted my house interior with it and can feel a difference if I place my hand near the wall. The beads are supposed to reflect the heat. Presently I am contemplating installing vacuum tube solar heating with 4 hydronic radiators. Presently I heat a 2nd home to 45 degrees when not planning to be there. I would like to increase the temperature and not pay for electric or propane heating. I plan to install it myself. I was going to use a heat exchanger as well to preheat the water to my point of use propane water heater. Steve

Michael Welch's picture

Hi Steve. I did some research and found some interesting info on it. I also found an article that pretty much summed up the other pieces I read. Seems there is controversy surrounding claims. Anyway, check out this web piece on the technology's viability:

Michael, Home Power.

Show or Hide All Comments