Water is heated for two purposes in residential applications. The first is to heat water for domestic use. Such a cleaning, washing, cooking, showers and bathing. The second is to heat water to be a medium for carrying that heat to a room then releasing the heat into that room to keep us warm.
Water heater| hot water tanks
Heating water for domestic use is done with hot water tanks, tankless water heaters, or indirect water heaters.
Tankless water heaters
Tankless water heaters are becoming more common in North America, although these have been used in other parts of the world for years. This kind of water heater does not have any water in a tank. If you do not have a tank then you will not have to continually keep that water heated as the water will always cool even when not in use. The cooling process is caused by heat from the water warming the air in the flue, then the hot air rises and is displaced by cooler air which is in turn heated by the hot water (convection heat just like baseboard heating). This air will flow up the flue and spill out into the room. Also the water in the tank will cool as heat conducts through the tank and into the room. This cooling process is called standby loss and because of this process your water heater is at best 55% efficient.
Tankless water heaters are more efficient than hot water tanks because of the heating process. A hot water heater will heat the bottom of the tank and the center of the tank, with most of the energy leaving the tank up the flue. Where as a tankless water heater heats a coil of pipes inside of a heat exchanger similar to that of a boiler. This heat exchanger is made up of small tubes that heat up very fast.
A tankless water heater will also modulate the burner. So if you need only a small demand the tankless heater will only give you a small flame. And if you need a larger demand a tankless water heater will give you a larger flame.
Hot Water Tanks
Hot water tanks are the most common form in North America to heat your domestic water. These tanks have been around for a very long time and have not changed much until recently. Most homes use a 40 to 50 US gallon water heater. Between high energy costs and pollution many people are switching to better ways to heat there water.
A hot water heater requires a chimney for the flue gases to flow to the outside (except power vent and direct vent water heaters, these can be piped out the wall). Water heaters also have an anode rod to protect the tank, a dip tube to allow the cold water to flow to the bottom of the tank, a gas valve, burner and a relief valve. NEVER plug a relief valve. This is the largest cause of water heater explosions. Water when heated to boiling changes to steam. When this happens the water expands 1700 times and with no where to go the tank has to blow up. Usually the bottom is what bursts and the water heater becomes a rocket shooting through the roof in a two story building.
For more info got Hot water tanks:
Indirect water heaters--An indirect water heater uses a boiler to heat the water that flows through the inside of a coil of pipe which in turns heats the domestic water through conduction. These are very efficient, have a fast response time and out last a standard water heater more than three to one. The response time is very quick giving you more than enough hot water. In my opinion these are the best water heaters on the market. The biggest problem is that you need a boiler to heat the water therefore the cost can be high compared to a normal water heater. If you are renovating and going to install hot water heating of any sort, such as a fan coil instead of a high efficient furnace, then this is the way to go
Water heating | Heating your home
Heating water to heat your home is best done with a boiler. The reason I say this is that a boiler is meant to heat water in an efficient manner, while your water heat is not. A standard water heater has an efficiency of 65% to 70% including standby losses. A standard boiler runs at about 80%. The boilers I install have an efficiency rating of between 85% and 98% depending on the system and time of year. So you could use a water heater to heat your home (check with local codes), but the lifespan is much less and your heating bill will be much greater. Not worth the 00 to 00 difference between the two. (my opinion). You could use a tankless water heater, but you will need two or three if you are heating your domestic water as well. Still compared with a condensing boiler you are only saving less than 00
