Last week I wrote here how I plan my off-grid house to be built. Today I will explain in more details my ideas on how to supply heat to the house, how to ventillate it and how to make domestic hot water.
In my climate the most of the energy our houses use goes to heating, so a reliable off-grid supply of heat is a must.
As I already mentioned, I want the house to be built as close to passivhaus (passive house) standards as possible. Insulating the walls will not be difficult, but as the walls will be built with wooden frame, they won’t store much heat. This will make using all the passive heat sources (like waste heat generators — TV, computers, kitchen stove, etc.) a bit more difficult.
The main source for heat of the house will be solar gains, i.e. — passive solar heating. The southern wall will have 3 large windows (two of those will be fixed). I will later add solar air collectors, so the amount of utilized solar energy will be greater. On the roof I will have place for solar thermal collectors. In summer this heat will be used to produce domestic hot water, and in spring and fall will also provide supplementary heat for heating.
I will also make a very cheap solar thermal collector — a long piece of garden water hose will be placed just below roof tile. It will not be very efficient (large surface with no insulation from above will be the cause of large heat losses), but since the area will be very large, the mount of power generated should be significant.
The main source of external heat will be a fireplace with water mantle. I’m not sure if this is the proper name of such device, but I’m sure you’ll understand what I mean. This fireplace will heat water that will later be stored in large heat storage tank. Because of that I will be able to use it only from time to time, while the heat will be used all the time.
I want to use wood as the fuel for my heating as it’s very easy to produce your own. There’s nothing really difficult in having your own coppice willow field, to have sustainable supply of fuel. On the other hand it’s also easy to have stored enough fuel for three or four years. You can buy the wood cheap when it’s wet, and season it so that it will be very dry — burning of wet wood is inefficient.
The heat will be transferred to the house interior using underfloor heating. If the power of such heating in bathrooms will be inefficient, I will add a radiator or two.
All those four heat sources and receivers:
- fireplace with water mantle,
- underfloor heating,
- solar collectors,
- domestic hot water,
will connect in the large storage tank.
In the beginning I will install a small storage tank (200 liter / 55 gal. capacity) because it will be cheaper and easier. Eventually this tank will have at least 3 m³ (almost 800 gal.) capacity. Near the bottom of the tank a heat exchanger will be connected to solar thermal collectors, near the top there will be a copper coil that will warm the domestic hot water.
Such a tank will allow me to store almost 200 kWh (kilowatt-hours) of heat, equivallent to over 655,000 BTU! Once the tank is loaded with heat (i.e. the water inside is heated up to 95C (203 F)), it will store enough heat for a week! If I will reach the passive house standards, the house will require less than 1 kW of heat (3.4 BTU/h).
The tank will also be equipped with electric heater, as energency heat source. And it will allow me to be compliant with Polish building codes, which say that the fireplace should not be the only heat source in a building.
I will have forced ventillation with heat recovery. I want to have ground heat exchanger, but this probably will be added later. I won’t have to build many chimneys for natural ventillation, which will save me some money. The chimney will have only two ducts: for fireplace and for the kitchen stove. Yes, I will have wood-fueled kitchen stove!