Heater efficiency on maintaining temperature? I have a central heating system with natural gas (atmospheric) heater warming the water going through the closed loop to radiators.
I can set the temperature of the hot water in a fairly broad range, 30-57C.
As I have added a thermostat which turns the heater on/off depeding on surrounding air temperature (set to 21C), I was wondering, if there was difference in efficiency (in terms of gas usage), depending on whether I set the heater temperature higher or lower.
Obviously, the lower I set the temperature, the slower the rooms will heat up, and the longer the heater will run. On the other hand, it will need to maintain lower temperature in the pipes, and probably burn less fuel that way.
Apart from keeping an eye on the gas meter for some time, (and interpreting the readings with adjustment for outside temperature), is there some way to figure out, what might be the best settings for using least gas, or even if it matters?
 A: First order effects: The main driver for energy consumption is simply the temperature difference between inside and outside and the thermal resistance of the house to the outside. This determines the amount of energy that you consume and the exact way how the energy gets into the house doesn't really matter. 
Second order effects: lower water temperature makes for a slower system. Most thermostats have a little bit of hysteresis, so they let temp drop a couple of degrees below the target before turning on the heater pump.  Same happens if you crank up the thermostat or switch from night temperature to day temperature. If the steady state temperatures are the same, low temp water setting will result in a slightly lower average temperature and so overall less energy consumption. It's also less likely to result in short term "overshoots". 
Any type losses are also a function of temperature difference, so the high water setting will generate more losses. This may not matter: as long as the energy stays in the house, it still contributes to heating. However if pipes are in outside walls and not well insulated some of the losses may go directly to the outside without heating the house first. You may also end up with heat in places where you don't want/need it.
So overall the low-water system will be slightly more efficient but it's also less comfy since it's slower. 
