Checking alternative energy options

By Bob "Hot Rod" Rohr,

Radiant expert

As heating fuels continue to rise in cost, homeowners across the country are looking for other options. In many areas, wood is still a viable option.  Outdoor wood furnaces (OWFs) are being manufactured by more than a dozen companies in the United States. Typically these are seen in back yards housed in a metal structure looking a lot like a small storage shed. Water is warmed in a drum and piped underground via insulated pex lines to the building or multiple buildings.

Most often a hydronic coil is inserted into the forced-air system to distribute the heat. If the home has an existing hydronic system, so much the better. Dhw or pool heat exchangers are common options.

Independent test show the typical owf operates in the 40-50% efficiency range when burned with dry hardwood.  Manufacturers are working to increase the efficiency and lower the emissions as more and more OWFs are being sold and installed. One manufacturer alluded to 10,000-per- year sold under his brand. Hmmm. Certainly possible, I suppose. Ten different brands were on display at a local FarmFest I attend every year in southwest Missouri. At least several of the brands had updated models with lower emissions on display. Pellet, corn and biomass burner styles were also displayed.

Over the past few years the years, I have been installing and servicing gasification boilers of European origin.  Basically, these units recirculate the flue products through a ceramic or refractory-lined chamber. There, additional heat energy is extracted by initiating pyrolitic gases in a ceramic-lined chamber running in excess of 1,000 F. An additional pass through the water jacket transfers this energy to the boiler water. This gasification cycle extracts additional heat and cleans up the emissions considerably.

During my visit to the ish show in Germany last spring, I visited with at least 10 manufacturers of gasification boilers. Many of the larger German boiler manufacturers we know and love offer gasification boilers in Europe. Nicely built, and well engineered gasification models were on display at the Buderus and Viessmann booths.

OWFs typically have a large capacity of water contained in the unit design. Capacities of 100-300 gallons are not uncommon. Most OWFs are unpressurized vessels. If the unit is over-fired, the water boils and steams without causing a pressure increase in the vessel. Being an unpressurized vessel, no pressure relief valve is required. Of course, the standby losses are higher with a hot tank of water vented to the great outdoors.

The typical gasification boiler is built more like the hydronic boilers with which we are familiar. The water capacity is around 50-60 gallons. Most often this type of boiler would be paired with an accumulator, or as I like to call them, “buffer” tanks. This provides a bit of energy storage space to carry through periods when you are not burning.  It also can help out with small micro loads, just as a “buffer tank” would on a gas- or oil-fired boiler system.  Many of the “buffers” I spotted in Europe have multiple coils for solar, wood or fossil fueled boiler inputs. You could also pull a radiant or hydronic load off one of the internal coils.

Piping a solid fueled boiler or appliance in with an existing fossil fueled system takes some planning.

There seem to be two separate camps as far as the best piping arrangement. Parallel piping seems to be most common in the installation manuals I have seen shipped with wood-fired boilers. I prefer the primary/secondary piping approach. Partially because I am familiar with the design concept and all the advantages and options it provides. The p/s approach is pump intensive but allows you to better match the circ to the load or source better than a single pump approach.

Regardless of the method you chose, solid fueled boilers need return temperature protection just as a conventional non-condensing boiler does. Primary secondary allows this to be addressed with some controls with which most hydronic installers are already familiar. A simple 140 F control is included in some gasification units to prevent the boiler circulator from running until the boiler temperature hits 140 F. A method to watch and react to the return at the boiler return piping sits better with me. I’ve used the Grundfos MixiMiser as a boiler circ and return protection device. The pump speed changes to address the return temperature as opposed to the bang-bang thermostat control. Taco also has a variable speed control that would accomplish this.

Variable speed controls that we use for injection mixing work, also. Choose the pump color to suit your fancy and use the after-market control. The parallel “pipers” use three- way thermostatic valves to provide return protection.

The controls and wiring of multi- boilers, “buffer tank,” multi-temperature systems seem to be a challenge to many installers. It can be perplexing to figure out a rational way of turning various circulators on and off as the system operates.

Recently, I came across www.hearth.com. At the “Boiler Room” section I discovered a group of DIYers and installers with some very interesting control set-ups.

Several have digital displays on their programmable logic control (plc) packages to show temperatures and functions. One poster from Vermont has a sound card that mimics “Come On Baby Light My Fire” when the temperature at his boiler drops. Another “Jersey” fellow has a series of chimes and beeps to indicate conditions.

If wood or biomass burning systems excite you, drop on by for some clever and interesting viewing at that site.

I discovered the ideal control during a recent trip to Germany. It has nine relay outputs, two of which are variable speed and 15 sensor inputs. It’s programmed -- at the factory before shipping -- to accomplish exactly what is required to tie together solid fuel boilers, solar, buffer tanks, back-up heat sources and any assortment of loads. I don’t know if they have plans to “Americanize” it with regard to voltage and ul requirements. I’ll keep an eye open for any news.

Bob “Hot Rod” Rohr has been knee-deep in plumbing, heating and solar work since he was a kid. He has learned a lot of simple ways to install, repair and update hydronic systems. Check out his new, downloadable “Cool Tips from Hot Rod” at www.showmeradiant.com.