Most stoves can heat a modest quantity of water – sufficient for baths or washing up. Higher outputs are also available for central heating. This section explains the things you need to know if you decide to opt for this facility.
1. Water Heating - a direct system for domestic hot water.
Many stoves offer a water-heating facility. The simplest systems usually deliver an output equivalent to an immersion heater (2 – 3kW, or 7,000 – 10,000 BThUs) and will meet most household needs for bath water and washing-up.
Before opting for such a system you should consult your installer to make sure that the location of your domestic hot water tank is suited to such an arrangement.
Low output boilers operate on a ‘gravity’ system whereby water is made to circulate round and round continuously between the boiler and the storage tank, driven by the density differences between the hotter and cooler streams. The force is weak and only works well when the tank is reasonably close to the boiler, is at a higher level than it, and the two are inter-connected with pipes at least 1″ (25mm) in diameter. The pipes also need to be well insulated so that the temperature (and density) differences between them are maintained.
If you are simply replacing an earlier system with a new one, most of these considerations should already have been addressed.
The illustration at right shows a direct system – in other words the water coming out of your hot tap has been heated directly by the fire while passing through the boiler. For this reason the boiler must be made from a rustless material otherwise it would corrode rapidly and flakes of rust would come out of your hot tap.
To prevent this from happening, boilers for direct systems are manufactured from copper or stainless steel. If they are made from mild steel or cast iron lined they must be lined with glass or vitreous enamel.
2. Water Heating - indirect system.
In the illustration at right, two new components have been added to convert what was a ‘direct’ system to an ‘indirect’ system. The calorifier is a coil of copper tube inside the hot water storage tank, and the new feed/expansion tank keeps the calorifier (and also the boiler in the stove) filled with water.
We now have two completely separate circuits, known technically as ‘primary’ and ‘secondary.’ The ‘primary’ is the closed heating circuit containing a trapped volume of water topped up as necessary by the feed/expansion tank. The ‘secondary’ supplies the hot-water taps.
It is essential to adopt an indirect system when central heating is planned since panel radiators are made of mild steel and are subject to corrosion. In a direct system, rust particles would appear in the hot tap water. An indirect system eliminates this problem and also allows the water circulating through the primary circuit to be dosed with anti-rust and anti-freeze compounds. With this in place we can also now fabricate the boiler itself in mild steel. The cost-saving over copper or stainless is substantial particularly on the much larger size of boiler needed to drive radiators.
3. Adding radiators to an indirect system
Once an indirect domestic hot water system is in place, it is a simple matter to add radiators and a pump.
The panel sizes are selected according to the heatloss of the area being served.
The pipe sizes and pump capacity needed to serve any given situation are beyond the scope of this site but can be found in any textbook on the subject.
4. Fit a thermostat and protect your boiler.
Nothing shortens the life of a boiler faster than running it cold when wood or peat are being burned. Under these conditions moisture in the smoke condenses onto the exposed cold surfaces of the boiler and causes rapid failure of the metal. (Modern high-output boilers are made from ordinary mild steel for cost reasons.) The answer is to keep the temperature UP as high as possible and for practical purposes this generally means above 50ºC.
The way to do this is to control the circulating pump via a “low level thermostat.” The thermostat connects to an electrical power supply and the pump and is attached to the return pipe from the domestic hot water storage tank. This set-up has benefits all round. As well as protecting the boiler it saves you from having to remember to turn off the pump when you want a bath, since it always gives priority to domestic hot water over the radiators.
Be warned that most multifuel stove manufacturers consider their warranty void UNLESS you have this type of protection fitted.
5. Bleeding the radiators
Usually when a central heating system is filled for the first time, pockets of air get trapped. It is necessary to bleed the air out, otherwise circulation gets disrupted and radiators may fail to heat up. Every radiator has a bleed valve at the top, and if you open the valve with a key (available from any builders’ merchant) you can release the trapped air.
You may need to bleed again a few days later because when water is heated up, any dissolved oxygen it contains is driven out in the form of tiny bubbles and of course is again trapped at the top of the radiator.
Fresh water is rich in dissolved oxygen and is highly corrosive to steel so you should avoid draining and refilling your system once it has been commissioned. It is also good practice to use a corrosion inhibitor such as “Fernox.” Simply introduce it into the primary circuit through the Feed/Expansion tank in accordance with the manufacturer’s instructions.
6. Balancing the system
If your radiators heat to different temperatures when they are full open, the system is running out of balance. To rebalance, you need to adjust the lockshield valves which are located at the bottom of the panel and generally fitted with plastic caps which must be removed.
In the diagram at right, the lockshield valves on the two larger radiators need to be closed fractionally and/or the lockshield valve on the small radiator needs to be opened. By adjusting the resistances applied by the lockshield valves you should be able to balance the system so that all radiators run at the same temperature.
You can check the temperatures with a bulb thermometer. The design temperature for the flow is 180ºF (82ºC.) and for the return is 160ºF.(71ºC.) In practice it is difficult to obtain these temperatures accurately with a multifuel stove because the output is less steady than with an oil or gas burner, but a fair approximation should be achievable.