(e-mail to (Bob Carruthers) 






Climate 100 miles north of Seattle (mild)

House 3,800 sq ft 12’ ceilings 11 skylights (1970)


Heat demand 400 therms plus in winter Months (cascade natural gas bill $400)

 Installation paired with existing Well-McLAIN GV6 175,000 Btu condensing gas boiler

Existing Hydronic baseboard (165*f plus operating temp)

Plumbing 1 ¼’ cooper primary loop the three zones being secondary loops (1 ¼”)

Gas boiler has a pair of Taco 007F circulator pumps built in. Wood boiler has one external Taco 007F pump. Three zones each zone has a 009F pump.


Hot water storage 500 gal (converted propane tank ¼” thick wall) R19 6” fiberglass with tin cover

Large Extrol SX series expansion tank  (see pic)


Figure 1 Heating System




Figure 2 Self- activating valve  for cooling loop


Over heat protection for wood boiler

Self-actuating ½” valve connected to cooling loop (Johnson V47AB4C) Normally closed. ($230)

Tripp-Lite automatic power system 12-volt inverter charger 500 watts (car battery) ($165)


Figure 3 Backup power system


The Atmos boiler is designed to operate at 80-90*c by way of a power regulator that controls the draft door and an induced draft fan. At temperature the 007 circulating pump works continuously.


System control

The system shown has an auquastat (dp st) at the output of the wood boiler. When operating temp is reached it locks out calls for heat to the gas boiler by opening the circuit between house thermostats and gas boiler. It also makes a circuit to power a relay that controls a three-way diverter valve that routes water around the storage tank when tank temp is low (start up) giving the house priority. When house does not call for heat the wood boiler heats the storage tank water by way of the valve. In addition there is an aquastat (sp st) on the outlet of the tank that locks out the diverter valve relay when the tank water is hot. The water is then routed through the tank. These aquastats are adjustable and allow fine-tuning. This system uses the Termovar (mixing valve) set @162*f for return water boiler protection ($220) also used back flow check valves to prevent gas boiler from heating storage tank water.

The system works well and switches between boilers automatic


Figure 4 Expansion tank


Figure 5 Plumbing detail



Figure 6 Three-way diverter valve


Figure 7 Relay and valve



When the pipe was insulated it lowered the temp in the garage 15*f it also raised the temp of water in the tank 10*. (195*f)

Uses average 18 cu ft of wood per day



Atmos boiler is the best value I have seen it works well with dry wood and the gasification process is easily achieved.

It will burn 3-4 hours under heavy demand more than 6 hours average and more would be the exception.

When heat demand is not high the gas boiler is more practical.


Heat storage

Radiant heat works better than baseboard system as radiant demands a much lower temp than the 165*f required for baseboard. For example I can hold 500gal at 195*f so that gives me 500gal at 30*useable before it reaches 165*f.  Radiant heat would yield 500gal at 100* of useable heat, before it reached 90*-95*f  over three times as much as baseboard.

 I think that the 500 gal tank is beneficial for efficiency but the value of a larger tank for added heat storage would not be practical.

The tanks can be purchased use from scrap yards ($.13 lb) ($150) Tank is easy to insulate it is supported by a 1” rubber matt salvaged from horse trailer. Down side is cost of expansion tank ($350)

This has been a project I have enjoyed and am pleased with results.





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