Fire it up!: Commissioning the hydronic heating system.
Before getting into most of the particulars about the hydronic system and the Walltherm commissioning I figured that I should discuss a little about my choices of radiators. There were none available locally. Hydronic radiators are a specialty item so had to be ordered in regardless of brand name. Jaga makes low temperature radiators. They have a large surface area and provide high BTU at low temperatures. Some of them have ECM fans to boost output. The price was beyond the scope of my build so I abandoned that idea fairly early after I decided that I wanted to use hydronic heating. My HVAC contractor (Adam Rickert, Hot Water Systems), recommended Softline radiators by Stelrad (http://www.expressradiant.ca/compact-series). They are a low volume radiator and the manufacturer provides BTU output for low water temperatures. The BTU output from the radiators was determined from load calculations provided by Passive Design Solutions. Moving water around at lower temperatures leads to less energy loss in the distribution lines even when they are insulated so I sized the radiators based on 120 F water temperature.
Controlling the temperature was the next question. For an hydronic system there were several options. Thermostats in each room with zone valves on a manifold would take up a lot of space in the mechanical room and required more controls and electronics than a simple TRV (Thermostatic Radiator Valves) controlled system. I decided that TRVs were a great option for a low energy house and really simplify the distribution at the manifolds (Photo 1): They require no electricity and open and close based on temperature response. Photo 2 shows one of the installed radiators with the TRV installed at the bottom right corner of the rad. Coupled with a constant pressure pump (Photo 3), they provide all the same benefits as using variable speed pumps and zone valves without all the electronics.
Commissioning the hydronic system was a lengthly process. First the system had to be filled with water. Once the tank was filled, ball valves to the radiator distribution manifolds were opened and lines were filled with water. Because the radiators are controlled by TRVs they had to be fully opened to allow water to enter the radiators. Once the radiators were filled, they had to be purged of air. Each radiator has a air bleeding valve installed for this purpose. With the TRVs opened, the pump was set to provide 14 ft of head. Each supply valve was adjusted at the manifold to give 1 gal/min since the calculated load in BTU/hr was based on that flow rate.
When the wood stove is burning, the Logix24 tank could easily hit 80 C. Distributing high temperature water is wasteful from an energy perspective and would lead to large temperature swings in each zone due to cycling of the radiators. The Taco iSeries outdoor reset mixing valve (Photo used in the system does two things to help solve this problem: First, since the hot water in the return manifold still contains useful energy it can be reused. The valve mixes some of that water back into the supply manifold as hot water is drawn from the Logix 24 tank. Secondly, the valve also uses an outdoor reset sensor to determine how much water from the hydronic return manifold it needs to mix with hot water from the Logix24 tank to modulate distribution temperatures as outdoor temperature changes. As the temperature outside rises, the water temperature to the supply manifold decreases. As it gets colder, the water temperature rises. The system dynamically changes the radiator temperature in response to the energy loss in the building. This setup should be more efficient than a non-mixed system and temperature response should be more even.
In addition to the Walltherm connection to the tank, we installed several 4500 W electric elements to provide a grid connected heating source. This provides much more flexibility then using wood alone. Electric elements can be used during the heating season when wood is not being burned and to keep the domestic hot water coil inside the tank ready for hot water use. Electricity will be used when going on vacation and during the shoulder seasons when heating with wood will lead to overheating in the living space. The water inside the tank is highly stratified. Water at the top of the tank may be at 60C while the bottom may be at 25C. This provides a way to heat ("charge") the tank partially with hot water and leave "storage" room at the bottom of the thermal store for wood heat. I pre-designed the control system and my electricians (Trevor leonard, Mike Molloy, 709 Electrical) made some modifications so it would conform to the electrical code. In brief, a temperature controller which uses a sensor (sensor placed about 1/3 of the height from the top of the tank) turns on/off the electric elements as it sees fit much like a normal tank water heater. It is a fairly robust system that doesn't require any user input once configured. When wood is being burned, the water temperature in the tank rises well beyond the set point temperature of the controller. As long as the tank is charged above the set point, the elements will not come on. Otherwise, the temperature controller and elements control the tank temperature.
The manufacturer of the stove provides all the safety equipment needed to run stove safely. There is a 30 PSI pressure relief valve (Photo 4). This valve is plumbed from the stove to the outside. Should the water pressure ever increase beyond 30 PSI, the valve will evacuate the hot water to the exterior of the house. Likewise, if water hits 95 C in the water jacket a capillary temperature sensor opens a valve attached to a copper lance that is plumbed directly to well water at about 7 C which will cool the water jacket (Photo 5). The damper in the stove is controlled by a thermostatic control. As temperature rises, the stove damper (attached by a chain to the control arm of the thermostat) starts to close and air supply to the stove decreases (Photo 6).
To extract heat from the stove, the boiler charging set (PAW, Grundfos alpha 1 pump.) is thermostatically controlled and starts around 68 C. The pump was tested before the initial burn by turning the thermostat dial down until the pump turned on. Initially, the pump was noisy ( a good sign there was still air in the lines.) We had to use higher pressure water from our well pump set up to force air through the supply/return and into the Logix tank where it could escape through an automatic air vent. With all the testing and verification out of the way it was time to light the stove.
Building a fire is tricky at first but once you get the hang of it, its not that hard (Photo 7). Everything depends on the bed of hot embers filling the area around the injector plate at the base of the firebox. Starting with kindling and then adding larger wood works. After about 20-25 minutes, the water in the boiler is about 50 C and the flue temperature is over 400C. Actuating the gasification process is just a matter of pulling a lever on the side of the stove and voila!!!! Gasification! The temperature in the water jacket starts to raise significantly and the pump comes on. The flue temperature drops to around 150 C as heat from the flue gas is absorbed by the stove heat exchanger. Kindling and 6 pieces of wood burn about 2.5 hours. I estimated that there was a temperature rise of 72 F based on the temperature gauges on the tank. This is equivalent to about 23 kWh of heated water. After this time, the whole tank has a temperature of about 149 F (Photos 8 and 9). Since the mixing valve tempers this water to around 95 F (this time of the year) that is more than enough for heat and hot water for the next day. During a 2.5 hour burn, the house received about 5.6 kWh worth of space heat. The stove body and water in the stove act as a radiator until the next day. The radiators on the main level won't come on at all while the stove is burning and for some time after. They typically come on the next day if its not sunny.
Overall I am pleased with the system. The house is amazingly comfortable with the low temperature radiators. The only thing I am sorry about is that it is almost the end of heating season for our house according to the WUFI model....so it may be next year before I really get to use it extensively.