50

60

70

40

55

12 Hours6 HoursLighting of Fire

75

65

De

gre

e (

Fº)

Room Temp

Temp at Hearth

Exp. Burn Control Burn

F E E L I N ’ T H E B U R N :

DATA ANALYSIS: Temperatures recorded over a series of both control and experimental burns

Additional thermal mass allows the system to store more heat

6 Hours 12 HoursLighting of Fire

10

5

Diff

eren

ce in

∆T

Deg

ree

Exp. Burn

Control Burn

Wood Stoves, Thermal Mass and Temperature Lag

Exp. Burn Control Burn

Side Surface

Top Surface

Exp. Burn Control Burn

Outside Temp

Inside Temp

Lighting of Fire

100 °F

200 °F

300 °F

400 °FSurface Temperatures

3 Hours

Lighting of Fire

Deg

rees

(°F)

Deg

rees

(°F)

s

306 Hours 12 Hours

6 Hours

A 40 pound stack of bricks placed directly on top of the wood stove will keep the room 5 ° F warmer than the control condition two hours after the peak of the heating cycle.

Hypothesis:

Alex Brooks | Jessica Benner |Luke LarsenECS 1 A. Kwok | T. Collins

University of Oregon | Winter 2013

We sought to understand the effects of adding thermal mass to a wood-burning stove. Our goal was to determine if it was possible to increase the efficiency of such a system through an external intervention.

A direct comparison of average room temperatures from the control and experimental burns related to the respective outdoor temperatures

A direct comparison of the surface temperature of the stove to the surface temperature of the experimental thermal mass

A relationship that compares the change in temperature (∆T) from inside to outside over the course of each burn for both the control and experimental conditions

A direct comparison of the temperature at the hearth versus the average room temperature over the course of each burn

Data Logger Locations

Conclusion: The research disproved our original hypothesis. Our results show that a 40 pound stack of bricks placed directly on top of the wood stove did not keep the room 5 ° F warmer than the control condition two hours after the peak of the heating cycle.

This experiment has revealed the challenges of introducing thermal mass into a heating system in an effort to regulate it. We found that at the scale studied the bricks actively detracted from the heating potential of our finite parameter of 10 pounds of mixed soft wood. We found that the thermal mass was absorbing heat as intended but failed to re-radiate in a beneficial manner.

If the thermal mass had been more saturated with heat, meaning that we had fueled the fire with more wood, we assume that the system would have had a higher heating potential. Additionally, had we placed a small fan to circulate the air over the bricks, the thermal mass would have re-radiated more effectively.

Punch “the warm spot finder”

FeelingtheBurn:WoodStoves,ThermalMassandTemperatureLag

LukeLarsen

DepartmentofArchitectureUniversityofOregonEugene,OR97403

lflarsen@uoregon.edu

JessicaBennerDepartmentofArchitecture

UniversityofOregonEugene,OR97405

waltersd@uoregon.edu

AlexBrooksDepartmentofArchitecture

UniversityofOregonEugene,OR97405

brooks4@uoregon.edu

ABSTRACT We sought to understand the effects of adding thermal mass to a wood-burning stove. Our goal was to determine if it was possible to increase the efficiency of such a system through an external intervention. Our sample sets were drawn from evening burning sessions in winter months, when heating is most needed to establish thermal comfort. We established a baseline for efficiency, and thereafter tested the stove’s room- heating capacity with 40 pounds of additional thermal mass, in the form of brick. The results from our study will impact the way that people heat their homes using radiant combustible sources. We found that at the scale studied the thermal mass absorbed heat and made the temperature more consistent, but did not then efficiently re-radiate to improve heating performance.

Fig. 1 - Heat flow through wood stove system

1. INTRODUCTION This project found its genesis in the backwoods of rural Colorado where a member of the research team lived in a

primitive cabin with only south facing glazing and a wood stove to heat his living space. In an interest to increase the efficiency of the nighttime heating, he piled river stone on top of the woodstove hoping that the additional thermal mass would increase the heating lag thus helping him to control the thermal comfort of the space. The research team decided that it would be a worthwhile exercise to rigorously test this idea.

Fig 2. Rustic cabin as project inspiration

We sought to understand and potentially improve the wood burning system. Both owners and designers will be able to use the results of this study to inform their use of wood stove heating, and to empower control over the thermal environment. The building that we used for our wood burning experiment is a small 40-year-old lofted studio structure, located in the Marine West Coast climate of the Willamette Valley. Temperatures can vary from cool to warm, with dry summers and cool, wet winters. Under-insulated and drafty, the space would benefit from the increased performance of a system that heats more efficiently and for longer time periods. The designer intended to heat the space primarily with baseboard radiators but also included the wood stove as a supplemental and atmospheric heat source. Our intent is to

increase the prominence of the wood stove’s role in this system.

Fig. 3 - Study Site Exterior

2. HYPOTHESIS A 40 pound stack of bricks placed directly on top of the wood stove will keep the room 5° F warmer than the control condition two hours after the peak of the heating cycle.

Fig. 4 - Experimental brick arrangement and control condition

3. METHODOLOGY & EQUIPMENT To test the effect of thermal mass added to a wood stove, we compared the averaged temperatures recorded in the space during two experimental burns with thermal mass added to temperatures recorded during a control burn without. The data was collected over a 3-week period. We placed two data-loggers next to the wood stove, at a distance of 4', directly to the right and left of the stove. We placed three more data-loggers at distances of 15', 20', and 25' from the stove--one on a bookshelf, one on the kitchen counter, and one in the lofted bedroom, respectively. As a control we placed one final data-logger outside under a dry, shaded eve. We fueled each burn with 10 pounds of mixed soft wood.

Fig. 5 - Fuel sample per burn

We conducted two “control burns,” which set a standard for heating the space without any added thermal mass. After lighting the fire we measured the surface temperature of the stove top and side walls with the Ray-Tech gun every 30

minutes for 6 hours. We tended the fire as was necessary and appropriate until the wood was consumed. We kept the door closed and the vent completely open. Next we conducted a series of two experimental burns with 40 pounds of brick oriented flat to the top surface of the stove. We repeated all monitoring and data collecting procedures from the control condition, except that the Ray-Tech surface temperatures were taken from the top of the brick.

Fig. 6 Data Logger Locations

4. DATA ANALYSIS Our analysis is broken into four comparisons. 1. A direct comparison of average room temperatures from the control and experimental burns related to the respective outdoor temperatures over the course of each burn. (see fig.7)

Fig. 7 - Interior and exterior temperatures

The results show that the experimental burns achieved an overall higher and more stable room temperature in colder outdoor conditions than the control burn, which had a steeper heat gain leading to a sharp peak. This indicates that the control did not maintain the heat at the peak for as long. Over the length of the recording period, the space lost less heat during the control burn. It should be noted however, that the outside temperature for the control was on average 5 degrees warmer come morning.

2. A relationship that compares the change in temperature (∆T) from inside to outside over the course of each burn for both the control and experimental conditions. (see Fig. 8)

Fig 8. zeroed temperature difference between inside and outside

To control for the variability of the outside temperature we found the change in temperature from inside to outside for the experiment (∆Texperiment) and for the control (∆Tcontrol). To control for the difference in initial room temperatures, we zeroed both ∆T data sets for comparison. Control (Tinside – Toutside )= ∆Tcontrol set Experiment ( Tinside – Toutside )= ∆Texperimental set ∆Texperimental set – ∆T experimental initial = ∆T experimental set zeroed ∆Tcontrol set – ∆T control initial = ∆T control set zeroed

The graph shows the ∆T sets zeroed over time. The control burn released more heat into the room whereas the thermal mass entrapped the experimental burn's heat ultimately providing less output into the room. The experimental burn was slower to heat the space but provided a more consistent temperature than the control. Significant dips in both curves can be ascribed to people entering and exiting the space. Note that in the experimental condition the fluctuations were less severe presumably due to the normalizing effect of thermal mass. 3. A direct comparison of the temperature at the hearth versus the average room temperature over the course of each burn. (see Fig. 9)

Fig. 9 - Hearth and room temperatures

The results show that the temperature fluctuated less when thermal mass was added to the wood stove. However, the thermal mass's heat didn’t re-radiate effectively enough to have an overall positive warming effect on the temperature of the space as compared to the control. Note that at 10 hours the two curves cross. This indicates that the control condition is a more effective heating scenario. 4. A direct comparison of the surface temperature of the stove to the surface temperature of the experimental thermal mass. (see Fig. 10)

Fig. 10 - Surface temperatures

The results show that the top surface temperature of the experimental burn was significantly less warm than the control’s top surface temperature. This indicates that the bricks take longer to heat. The fact that the brick surface stayed warm for a longer period than the stove surface once saturated indicates that the bricks more efficiently stored heat. The side temperatures from each burn had similar peak temperatures and cooling slopes which indicates that the conditions were alike. Our results show that a 40 pound stack of bricks placed directly on top of the wood stove did not keep the room 5 degrees warmer than the control condition two hours after the peak of the heating cycle. In fact the room, on average was colder in the experimental condition. 5. CONCLUSION This experiment has revealed the challenges of introducing thermal mass into a heating system in an effort to regulate it. Our research disproved our hypothesis. We found that at the scale studied the bricks actively detracted from the heating potential of our finite parameter of 10 pounds of mixed soft wood. We found that the thermal mass absorbed heat as intended but failed to re-radiate it in a beneficial manner. If the thermal mass had been more saturated with heat, meaning that we had fueled the fire with more wood, we assume that the system would have had a higher heating potential and the thermal mass would have demonstrated a

more positive impact. Additionally, had we placed a small fan to circulate the air over the bricks, the thermal mass would have re-radiated more effectively. It would be great to extend this study by trying to specifically calculate the effect per pound of brick so that users could accurately calibrate their heating system to lag an appropriate amount into the night. i.e. 10 bricks will warm till 2 am. 20 bricks will warm till 4 am. etc. Another area of interest for future study regards the formal arrangement of the bricks. It would be great to model the convective and radiant consequence of different forms. We could create hand warming pockets, glove drying currents, and heat projecting flows. Considering the shortcomings of our findings, we maintain that users can better control their environment through discrete manipulations of thermal mass. This has implications for design. Architects can enable building users to interact with thermal mass systems and improve their comfort. These systems can come to partner well with passive solar strategies. The same mass, which holds immense opportunity for architectural gesture, can be productive both when heated by the sun and also when heated by fire. Both of which are naturally occurring sources of fuel. The first is cyclically available and the second is storable and so it is available on demand, making them a good complementary pair.

Fig. 11- natural fuel, wood and sun

6. REFERENCES Buchner, Klaus, and Hermann Hofbauer. “Optimization of

Ceramic Heat Storage of Stoves.” Energy and Building 2003: version 35, issue 11, 1121-1128. Journal.

Paul, Greg. “Heating with wood.” Natural Home Heating.

Chelsea Green Publishing. White River Junction, VT. 2003.