Image Credit: David Goodyear The fixed location of the through-the-wall thimble dictated placement of the boiler. The variety of stove pipe lengths available from ICC simplified installation. Connections at the boiler are hidden by a removable side panel. Once the system was plumbed, and could hold air pressure over a weekend, it was filled with water — 243 gallons, not including the water to fill the distribution lines to each radiator. Softline radiators by Stelrad require a low volume of water, resulting in less energy loss in distribution lines. The TRV valve that controls heat is on the lower right. The manifold for the heating system. A single Taco constant pressure pump provides all the power needed to distribute hot water to 12 separate zones. Below the pump, the Taco iSeries three-way valve mixes return water with supply water from the tank to modulate target temperatures based on an outdoor reset. Boiler controls include a thermostat (orange, at center) and a pressure relief valve on the left.
Editor’s Note: This is one of a series of blogs by David Goodyear describing the construction of his new home in Flatrock, Newfoundland, the first in the province built to the Passive House standard. The first installment of the GBA blog series was titled An Introduction to the Flatrock Passive House. For a list of Goodyear’s earlier blogs on this site, see the “Related Articles” sidebar below; you’ll find his complete blog here.
The Walltherm is the showcase of the Flatrock Passive House heating system. As mentioned in previous posts, it is a wood-fired gasification boiler. With a efficiency of 93%, it dumps about 12.5 kilowatts into hot water while only 2.5 kW is emitted to the room.
It took four of us to move it with a hand truck into the living room. With the wall thimble already in place (see Image #2 below), there was only one place for the boiler to go. However, limitations in the lengths of double-wall stove pipe made the placement a little challenging. This being said, Excel ULTRABlack manufactured by ICC comes in a variety of different lengths, and they also have slip lengths to accommodate many set ups. So we placed the stove so the back was 10 inches away from the wall (minimum offset to combustibles, according to the manufacturer).
With the stove in place it was time to plumb in the various connections (see Images #3 and #4 below). Adam Rickert was on site to do the work. Connections include a supply and return from the tank, a thermostat that turns the pump on and off, a temperature gauge, and a manometer.
One issue we ran into with the stove is that the threads are made for European fittings. Although NPT threads fit fine, they bottom out completely once tightened since they are mating with a non-tapered fitting. The fittings required large amounts of teflon and leak lock to seal adequately and the results weren’t foolproof.
Multiple attempts to pressurize the system with air presented leaks between mated fittings. A local company, Island Hose and Fittings, carries Dowty Bonded Seal meant for this application. A bonded seal is really just a washer with a rubber seal bonded to the center hole of the washer. We switched directions and used Dowty seals instead since they provide a more robust seal.
The system was pressurized and remained pressurized over a weekend, so we felt that it was good to go for filling. The system was filled with water from the well using the automatic refill connected to the tank. It took almost 1.5 hours to fill the tank! At 243 gallons, plus the water needed to fill the distribution lines to each radiator, there is a lot of water! The well didn’t run dry so it’s a good sign that the well recovery flow rate recorded by the driller was probably pretty accurate.
With the installation complete, it was time to test the system.
Before getting into most of the particulars about the hydronic system and the Walltherm commissioning, I should discuss my choices of radiators. Nothing was available locally. Hydronic radiators are a specialty item so they had to be ordered regardless of brand name.
Jaga makes low-temperature radiators. They have a large surface area and provide high BTU output 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 of Hot Water Systems) recommended Softline radiators by Stelrad, a low-volume radiator (see Image #5 below). 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, and I had 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 simplified the distribution at the manifolds (see Image #6 below). They require no electricity and open and close based on temperature response. Coupled with a constant pressure pump (see Image #7 below), they provide all the same benefits as using variable-speed pumps and zone valves without all the electronics.
Commissioning the system
Commissioning the system was a lengthy process. First, it 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 feet of head. Each supply valve was adjusted at the manifold to give 1 gallon/minute since the calculated load in BTU/hour was based on that flow rate.
When the wood stove is burning, the Logix24 tank could easily hit 80°C (176°F). 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 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. Second, 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.
Setting up the electrics
In addition to the Walltherm connection to the tank, we installed several 4500-watt electric elements to provide a grid-connected heating source. This provides much more flexibility than 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. We’ll use electricity 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 60°C (140°F) while the bottom may be at 25°C (77°F). This provides a way to heat (“charge”) the tank partially with hot water and leave “storage” room at the bottom for wood heat.
I pre-designed the control system and my electricians (Trevor Leonard and Mike Molloy of 709 Electrical) made some modifications so it would conform to the electrical code. The control circuit is a little beyond the scope of the blog, so details are omitted for now.
The manufacturer of the stove provides all the safety equipment needed for safe operation, including a 30 PSI pressure relief valve (see Image #8 below). 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 (203°F) in the water jacket, a capillary temperature sensor opens a valve attached to a copper lance that is plumbed directly to 7°C (44.6°F) well water. 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.
To extract heat from the stove, the boiler charging set (PAW, Grundfos Alpha 1 pump) is thermostatically controlled and starts around 68°C (154°F). 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 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.
Time for a fire
Building a fire is tricky at first but once you get the hang of it, it’s not that hard. Everything depends on the bed of hot embers filling the area around the injector plate at the base of the firebox. The trick is to start with kindling and then add larger pieces of wood. After 20 to 25 minutes, the water in the boiler is about 50°C (122°F) and the flue temperature is over 400°C (752°F). Actuating the gasification process is just a matter of pulling a lever on the side of the stove and, Voila!
The temperature in the water jacket starts to rise significantly and the pump comes on. The flue temperature drops to around 150°C (302°F) as heat from the flue gas is absorbed by the stove heat exchanger. Kindling and six pieces of wood burn about 2.5 hours. I estimated there was a temperature rise of 72°F based on the temperature gauges on the tank. This is equivalent of about 23 kWh of heated water.
After this time, the whole tank has a temperature of about 149°F. Since the mixing valve tempers this water to around 95°F (at least during the spring) 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, so radiators on the main level won’t come on at all while the stove is burning. They typically come on the next day if it’s 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.