A Backwoods Home Anthology
water * circulation. I’m going to see what it would cost to have one made up by a local machine shop.
An alternative to cutting holes in your stove is to remove some of the heat from the flue with a coil of cop- per pipe, either inside it or wrapped around the outside. There are advan- tages and disadvantages to this approach. Pluses include recycling of “waste” heat and no chance of damag- ing an expensive stove. Installation is the same for all kinds of stoves, and you don’t have to work with iron pipe, which some people find daunting.
On the other hand, it’s difficult to make a “flue-robber” work well, with- out making it work too well. Heat is what powers a stovepipe’s draft; remove too much and it won’t draw right. You’ll get excessive soot and creosote.
Heat also dries up the water vapor and pyrolytic acid that are normal products of combustion. When the creosote does dry out, it can start a chimney fire and turn your house into a blast furnace in seconds. That’s why I don’t use a “flue-robber”. Today’s air-tight, highly efficient stoves are designed to emit just enough excess heat to draw properly. There just ain’t much to spare.
If you are running a drafty, old stove that shoots plenty of excess heat up the chimney, it may be safe to use a stovepipe coil. They’ve been around a long time. One of my neighbors has one that’s about 20 years old. It con- sists of four loops of 3/8-inch, copper pipe, coiled around the outside of the stovepipe. Coiling copper tubing per- fectly-so it touches the stovepipe at all points-is an art I haven’t mastered.
The thermosyphon effect
One thing all these indirect systems have in common is they all work on the thermosyphon principle. Rising hot water pushes previously heated water into the tank and draws cold water from the bottom of the tank into the lower end of the heat exchanger.
Figure 4. A properly
Figure 4 is a schematic drawing of a properly plumbed system. Note how high the tank is mounted. If the tank were mounted any lower, cold water from the bottom of the tank would have to rise to get into the stove.
A column of water exerts a down- ward pressure of about 1 psi (pound per square inch) for every 2.2 feet of rise, regardless of the pipe diameter. The thermosyphon effect is not strong. If it has to pull cold water uphill, it won’t work. It’s OK to have the tank outlet level with, or higher than the stove inlet, though. Lots of folks put their tank in the attic.
Let’s look at figure 4 and see how a properly plumbed system works. Beginning near the upper left, we see cold water entering from your source. First it passes a valve, so you can shut the system down for repairs, then a union, which connects pipes the same way garden hoses connect. This
allows removal of parts for repair or modification without tearing the whole system apart. Note-that the valve is on the supply side of the union. Next comes an elbow, then the cold water inlet, a pipe inside the tank, extending nearly to the bottom. Some water heaters have an external cold water inlet near the bottom of the tank. I have seen tanks with as many as five openings on top. If you can’t tell which is the cold water inlet, remove all the plugs, caps, and other hardware and shine a flashlight down each hole in turn, while peering into another one. Light from the cold water inlet will make a small circle on the bottom of the tank. The others will illuminate the whole bottom.
At the lower right of the tank in fig- ure 4 is the stove supply. In its origi- nal incarnation as a gas or electric water heater’, this was the drain cock (or valve). Since we still need a drain