Types of stove boiler
Here we will discuss various types of stove boiler: backboilers and top boiler, clip-in boilers and wrapround boilers and whether they should be made from steel or stainless steel.
Steel or stainless
Boilers are generally made of either steel or stainless steel.
If the boiler is steel then the water must contain rust inhibiting chemicals (known as inhibitor) to stop the steel boiler from quickly rusting.
If your boiler stove directly heats your hot water as well then the boiler should be stainless. Directly heating hot water means that the water flowing through the stove boiler would be directly linked to the water in your hot water tank that it is heating, which is the same water that you use when you run a hot tap. In this case the water cannot contain rust inhibiting chemicals and so the boiler must be inherantly rust proof in the first place.
Clip in or wraparound
In terms of construction there are generally two types of boiler - clip in or wraparound
Clip in stove boilers
A clip in boiler is usually a simple thin rectangular metal box that replaces the firebricks at the back of the stove. A clip in boiler can be retrofitted to the stove - you just take out the rear firebrick, knock out precut holes to let the 4 tappings points on the boiler through the back of the stove, and fit the boiler in place of the firebrick. The heat from the fire passes into the water filled clip in boiler and then on to your heating and hot water system.
Because clip in boilers are quite small they cannot produce as high an output as a wraparound boiler, but they are very versatile as they can allow you to fit a boiler to a stove that is already installed.
Wrapround stove boilers
A wrapround boiler, like the name suggests, runs along the back of the stove firebox (again in place of the rear firebrick) and then 'wraps' down each side of the firebox. These boilers have to factory fitted as it would not be possible to get them through the doors of the stove after it is made. Some wraparound boilers are simply added into an existing firebox in the place where the firebricks would normally go (Hunter herald central heating stoves are a good example), but some wraparound boilers form the inside of the firebox itself (Stratford boiler stoves are a good example).
Some wrapround boilers also extend over the top of the firebox to form a baffle. This increases the surface area of the boiler and so can increase the amount of heat transferred to water. Again Stratford boiler stoves are a good example of this type of boiler.
Aquatherm Eco stoves have an igenious take on this idea as the boiler has a large tube running across the top of the fire box, the tube connects to each side of the boiler (and so is water filled) and is covered in metal fins to improve heat transfer.
Stoves with wraparound boilers can produce high heat outputs - up to around 90,000 BTU. The only real disadvantage is that if the boiler breaks then it is not possible to replace it (like it is if you have a clip in boiler), but it is sometimes possible for a metal worker repair or replace the boiler in situ.
Backboiler or top boiler
Most boilers are backboilers. This refers to the fact that the boiler is in the firebox itself, but does not necessarily mean that the boiler is just at the back of the stove (although historically this is probably the case). Backboilers seem to work well at transferring heat from the fire to water.
A theoretical problem with backboilers is that wood burns most efficiently in a hot firebox (300-400+ degrees). Because the boiler is always going to be quite cold and is taking heat from the firebox (that is what it does after all) this will result in a colder firebox and therefire might reduce burning efficiency.
A way around this is the top boiler. Here the heat is transferred to water not in the firebox from the flames but from the very hot flue gases beyond the firebox. This should raise the firebox temperature thus improving efficiency. Examples of this type of boiler stove are the Scanline 500 Aqua, the Oranier Hydra and the Dunsley Yorkshire boiler stove all of which work at a very high efficiency for boiler stoves.