The Masonry Heater Comes to Life

A lot of things have happened since my last entry and I hope to post some updates in the next few weeks. The year is drawing to a close and we are definitely in mid winter here in Carlisle, PA. While I do appreciate all four seasons, I must admit that winter is not my favorite season. Having said that, I have been looking forward to this winter more than any other because we would finally get a chance to try out the masonry heater. If you've been reading this blog from the beginning you'll recall that the masonry heater was something we decided to splurge on without ever having experienced what it would be like. Well, the winter weather meant it was finally time to experience what a masonry heater has to offer. We've been thinking about it for more than a year now. Would it live up to our potentially inflated expectations?

All dressed up for Xmas (Click image for more photos.)

Since our masonry heater was brand new, it was necessary to go through a curing process to slowly drive any excess moisture out of the mortar. The curing process consists of a series of small fires that grow progressively bigger over a period of about a week to ten days. By the end of this period, the fires are essentially full size.

As we began the curing process, we noticed some black soot or creosote on the interior of the firebox. This is out of the ordinary as the firebox in a masonry heaters is supposed to burn clean after every burn. After doing a little research and a few calls to Brian Klipfel, we hypothesized that our wood was not dry enough. After locating some dry wood and burning a test fire, it was obvious that our wood source was the problem. Although I was aware that masonry heaters required good dry wood, I was surprised to see how much more ferocious was the fire when proper wood was used. As I had read, the fire burns very hot and fast. When fully established, the fire burns like an inferno with large flames rising deep into the upper chamber. It's quite a beautiful sight.

A fully developed "inferno" (Click image for more photos.)

Because the fires burn so hot and fast, the whole process is complete in about two hours. During this time, the area of the heater directly around the firebox doors is very hot. During the next four to six hours, this energy slowly flows through the bricks and stone until the entire heater is toasty warm. Approximately 10-12 hours after the fire, the heater is warm all over and radiating warmth throughout the house. This continues for another 10-12 hours at which point the heater, while still warm to the touch, is no longer effectively heating the house. After a little experimentation, we have fallen into a routine where we burn a fire first thing in the morning (around 6:00 am) and enjoy the warmth for the rest of the day. By the time we retire to bed (around 9:30 or so), the heater is beyond its peak and is slowly starting to cool.

I find the operation of the masonry heater to be absolutely fascinating. It's simple, elegant, and amazingly effective. Of course, being a wood burning appliance, you do need to deal with wood and ash on a regular basis. I find that my fire-building routine runs approximately 15-30 minutes a day. This includes cleaning the glass and sweeping up afterwards.

Once we had established a good routine, I couldn't resist making some temperature measurements. After all, what's the fun of being a physicist if you don't make some measurements? So I brought home a thermocouple (for high temperatures) and some temperature sensors for an experiment. I placed the thermocouple inside the firebox sticking straight out above the wood and then placed three temperature sensors at difference locations on the face of the heater. One was right next to the firebox doors, one was about a foot above the doors, and one was in our bedroom where the back of the heater makes up one of the walls.

Temperatures inside the firebox (Click image for more photos.)

The results of these experiments were impressive. The temperature inside the firebox reached over 1,200 °F throughout the burning phase. This is the temperature at which secondary combustion takes place. It is this secondary combustion where the unburned gases and particulates are burned to create more heat and less pollution in a masonry heater. The temperatures are supposedly even higher in the upper combustion chamber; unfortunately, I had no way of measuring these temperatures.

If you look at the temperature graphs, you'll see that the firebox temperatures drop fairly quickly after the fire goes out. The energy that is absorbed in the firebox slowly works its way through the walls of the masonry heater and into the stones on the surface. Thus, the temperature of the surface of the heater takes many hours to increase. Looking at the graph, you will see that it takes about eight hours for the majority of the heater's surface to reach its maximum temperature of around 140 °F while the wall in our bedroom reaches a maximum temperature of around 100 °F. Notice that even after 24 hours, the surface of the masonry heater is still over 100 °F.

Surface temperatures (Click image for more photos.)