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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?

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  • $\begingroup$ The gas is on and off periodically while the water and air are circulated continuously, correct? $\endgroup$ – Chet Miller Oct 7 '19 at 15:28
  • $\begingroup$ @ChetMiller water is not circulated while gas is not burning, so the radiators are simply cooling off. Air is circulated as normal indoors air without any special ventilation. This is a brick living house with mediocre heat insulation and current outside temperatures between 0-10C. $\endgroup$ – Gnudiff Oct 7 '19 at 16:10
  • $\begingroup$ This is the way such systems were designed in the UK maybe 20 years ago. Modern ones use a digital thermometer to measure air temperature not an on/off thermostat, and the boiler automatically adjusts the water temperature to provide the correct amount of heating, probably using a PID controller or something similar. On my current system, the water temperature might be anything from about 30C to the maximum of 85C. Note, the water temperature for the taps, shower, etc, is set independently, so you never get scalded by water at 85C from a tap! $\endgroup$ – alephzero Oct 7 '19 at 16:18
  • $\begingroup$ My advice to the OP would be to get a smart meter(s) and experiment with the water setting, checking consumption and house temperature. $\endgroup$ – Gert Oct 7 '19 at 17:31
  • $\begingroup$ @Gert thank you. While I appreciate the idea, installing a smart gas meter might be somewhat involved, as it is only something the gas company may do. $\endgroup$ – Gnudiff Oct 7 '19 at 18:00
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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.

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    $\begingroup$ Hmmm... you posit your conclusion with quite certain aplomb but your model is quite simplistic. For example, it doesn't take into account any ventilation losses, caused by opening and closing of doors and windows and imperfections in sealing off doors and windows. My advice to the OP would be to get a smart meter(s) and experiment with the water setting, checking consumption and house temperature. $\endgroup$ – Gert Oct 7 '19 at 17:30

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