Water heat your house.
Water is the most effective way to transfer heat around your house. Unfortunately the plumbing for it is not so easy and to get your average plumber to see the light can be somewhat frustrating if not impossible. If you are considering hydronic heating, make sure the plumber you hire knows what they are doing. Don’t take their word for it. Ask to see a previous job and ask the owners if they are happy and if the system works. If you are confident using copper and silver solder (a high temperature gas torch is needed) you could do the job yourself, but the welding must be good and preferably pressure tested before filling the system. Any small mistake could mean failure.
Some people put heat pipes in the slab however it is difficult to regulate the heating and a hot slab can become quite uncomfortable and take days to cool off.
Choosing a stove or boiler for the job should be done with care. The average slow combustion style wet back cooker or wood heater is not designed to heat enough water to run a hydronic system. The stove must be capable of producing about 20Kw on good dry firewood. The average slow combustion cooker with a wet back will produce about 3Kw to 7Kw. Most electric HWS operate on about 3Kw.
Slow combustion stoves have a fire box lined with special fire bricks to keep the heat in and so keep the wood burning at a slow rate with almost no air. The result is a very economical use of fuel. However the down side is that, as the fire is starving for oxygen (O2) it burns the carbon dioxide (CO2) in the air as well, producing Carbon Monoxide (CO) which, as we all know, is quite toxic.
Boiler stoves generally are designed primarily to heat water and cook second and so the fire box is made of a double skin steel jacket around all sides of the fire. The water in the jacket keeps the fire cool and so it must burn at a faster rate to keep going. Lower levels of toxic gas production but the down side here is greater fuel consumption and a fair production of carbon dioxide, (a green house gas). Still one can argue that it is better to produce it at home than buy electricity and produce it somewhere else. Or, we could just buy a bigger doona and a few more woolly jumpers and help save the planet!
Having grappled with the big picture it is time to sharpen the chainsaw.
Normal slow combustion hot water services have an insulated tank to store about 40 to 50 gallons (about 200 litres). Water is heated in the “wet back” or water jacket in the stove and rises to the storage tank. The heavier cold water sinks to replace the lighter hot water producing a natural convection known as “thermosyphon. The cold water supply is fed into the tank at the bottom and and hot water is drawn off at the top. These systems are low pressure as most wet backs or water jackets are not made to take mains pressure. Therefore there must be a header tank with a ball valve beside or just above the tank to supply cold water at low at low pressure to the system. All components of the system must be made of corrosion resistant materials such as copper, stainless steel or vitreous enamel.
Hydronic heating uses a “closed circuit” system similar to a car. The fire is the heat source or engine and the radiators dissipate the heat. The stove or boiler heats the water in the storage tank by thermo syphon normally but the water is moved around the radiator system with a small 12 or 240 volt pump. Water is used as the “coolant” and must have corrosion inhibitor added to prevent the stove jacket and the radiators from rusting. If you live in a cold climate or have solar panels included in the system you should have anti-freeze in the coolant as well. Our system has 400 litres of water and 100 litres of ethyl glycol as the coolant. Ethyl glycol is the green liquid used in most car engine cooling systems. The higher the percentage of ethyl glycol used the lower the freezing point of the coolant. So, if your minimum temperature drops to minus five degrees Celsius (big frost) you will need at least 20% ethyl glycol. The supplier can tell you what percentage you need for a given minimum temperature. Don’t cheep skate. A split pipe at the crack of dawn will see most of your coolant down the drain before you can get out of bed. Or worse, if you go away for a week, the house cools down and you return to find your carpets have turned to green slime.
The boiler if it is also a cooker will be in the kitchen. A central position is preferable with the hot water storage tank as close as possible. Keep in mind the fuel consumption and have a ready supply of fire wood. Second hand boilers are rare but we picked one up for about $2500. A new Bosky will set you back about $7000. There are various boiler/cookers on the market. Choose carefully and get professional advice. The wrong stove could be a costly mistake.
Kev Wignell Trading in Melbourne are experts in this field. They have a good reputation and are very helpful.
The hot water tank must have a large capacity, at least 80 to 100 gallons (about 400 litres). It must have a mains pressure copper coil inside for the hot water supply to the house. The coil pipe needs to be long enough to allow cold water to absorb heat from the storage as it passes through the coil. If the coil is too short or the water travels through the coil too quickly it will not have time to heat up sufficiently. If the storage tank is too small the cold water running through the coil will cool the stored water too quickly.
Remember, 500 litres weighs 500Kgs or half a tonne. Support it carefully!
Our hot water tank is built in over the pantry, immediately behind the stove We obtained an old 80gallon (360litre) heavy duty copper tank that was taken out of a youth hostel. I unsoldered the top and extended the copper coil inside to get more heat transfer to the hot water supply. I also added extra outlets for the solar panels to flow to the tank separate from the stove out lets. This is not strictly necessary but reduces the likelihood of back syphoning through the panels during the night. All joints were done with silver solder using an oxy acetylene torch. Normal solder is not adequate for strong joints of this type.
Second hand radiators are fairly easy to get. If you have the space the old cast iron ones are fine and make great towel warmers and clothes dryers. The aim is to get as much thermal mass into the radiator and expose as much surface area as possible to the air in the room. This will create strong convection currents of warm air in the room. The new sheet steel radiators are more economical with space but will not retain as much heat when the fire goes out. We found some old milk chillers that proved to be quite effective in the bedrooms.
For the bathroom, we bought an old refrigerated stainless steel milk vat from a closed dairy for $500. It was degassed and we connected the built in waffle style radiator under the bath to the heating circuit to create a permanently hot Japanese bath. It holds a 1000 litres of water from the nearby creek and the stove keeps the bath between 40 and 45 degrees celcius. 200mm x 25mm cedar planks are used for a cover to keep the heat in when the bath is not in use. We change the water once a week and all must shower before their soak in the time honoured Japanese tradition. The stove, when going flat out, will have the bath filled with hot water from empty in two hours.
The plumbing is very important. As for normal thermo syphon systems the pipe running from the top of the boiler to the middle of the tank must preferably climb at a steady rate all the way. The syphon will not operate if there is any drop in the pipe. The return pipe from the base of the tank to the base of the boiler should also have a steady fall however it is not as important as the top pipe. The circuit plumbing is pump fed and so can travel anywhere however any places where there is air likely to be trapped should have bleeders to remove all air from the system. If air is allowed to get back to the pump it will “cavitate” and, if left, will overheat and be damaged. All the radiators should have bleeders on them and all low points should have drain plugs to allow the system to be drained if necessary.
Pipe work to the rooms can be ¾” or 1” and small leaders to the radiators may be ½” It is best to keep the pipes as large as practicable to reduce friction head and increase flow rate from the pump. Insulation inside the house is not important as we are trying to heat the house anyway. If you are starting from scratch it is convenient to run pipes in the slab. I used ¾ Copper for this, as 1” seemed too thick for the slab. It must be fully lagged to allow the pipe to move with heat variance. Avoid running pipes under a timber floor as heat loss and vermin are problems. If you have a roof cavity it is simple to run them over the ceiling and run “droppers” to each radiator. All concealed joints in walls and slabs should be pressure tested before concealment.
The pump is installed between the return line from the radiators and the stove return pipe via a swept tee. The pump is wired to a thermostat in the stove and will only operate when the stove water temperature is above 650C. The pump injects cooled water from the radiators, directly into the base of the stove via the swept tee. This ensures that only hot water is returned to the storage tank and the radiators don’t rob the storage tank of domestic hot water. An override thermostat is fitted to the rise pipe from the stove to the storage tank to activate the pump if the water temperature gets too high. It is important that at least one radiator is left on at all times in case the over ride thermostat activates the pump.
Although the initial set up is expensive and takes time, the rewards are great. Our system has been operating for several years now and the only ongoing cost is fuel for the chain saw and tractor and the occasional sore back from handling too much firewood.
Do I hear mumbling of disembodied energy?
But then there is always the hot tub!!!!!!!