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Efficiency of central heating

Posted: Tue Oct 19, 2021 8:46 pm
by Grumble
I’ve got a simple question, but I suspect the answer might not be simple. What’s the most efficient temperature to set my central heating to, given a combi boiler?

Re: Efficiency of central heating

Posted: Tue Oct 19, 2021 8:52 pm
by bob sterman
You mean the temperature of the boiler? Not the room thermostat temperature.

I think it's common to set the boiler output for the radiators at 75°C

https://www.viessmann.co.uk/heating-adv ... ing-boiler

Re: Efficiency of central heating

Posted: Tue Oct 19, 2021 9:31 pm
by Grumble
bob sterman wrote:
Tue Oct 19, 2021 8:52 pm
You mean the temperature of the boiler? Not the room thermostat temperature.

I think it's common to set the boiler output for the radiators at 75°C

https://www.viessmann.co.uk/heating-adv ... ing-boiler
Yes, that’s what I meant. Is it more efficient to heat the house quickly and shut off the boiler but have very hot radiators, or be slower but have the boiler run for longer.

Re: Efficiency of central heating

Posted: Tue Oct 19, 2021 9:31 pm
by dyqik
Grumble wrote:
Tue Oct 19, 2021 8:46 pm
I’ve got a simple question, but I suspect the answer might not be simple. What’s the most efficient temperature to set my central heating to, given a combi boiler?
On general principles, as low as you can while getting enough hot water to heat your house and supply your hot water.

Assuming that the pump for the radiators isn't a massive power draw, then to first order it'll be most efficient to run the pump all the time with the lowest heat output from the boiler. That may require some rebalancing of the radiators to get even heat.

Having the radiators and pipes hotter means higher losses to outside through the walls behind the radiators, or from the pipes to unheated parts of the house. Heat reflective stuff behind the radiators can help some as well.

But I can imagine that there's complications with gas boiler efficiency.

Re: Efficiency of central heating

Posted: Tue Oct 19, 2021 10:08 pm
by philip
All modern boilers are condensing boilers, they achieve efficiency by using the incoming water to condense the flue gases. This is simple when heating your hot water as the incoming water is cold, but when heating your radiator circuit the incoming water is the warm water returning from the radiators. For your boiler to operate efficiently the radiator return water has to be cool enough to cause condensation, about 50-55°C is often quoted.

Re: Efficiency of central heating

Posted: Tue Oct 19, 2021 10:13 pm
by Grumble
philip wrote:
Tue Oct 19, 2021 10:08 pm
All modern boilers are condensing boilers, they achieve efficiency by using the incoming water to condense the flue gases. This is simple when heating your hot water as the incoming water is cold, but when heating your radiator circuit the incoming water is the warm water returning from the radiators. For your boiler to operate efficiently the radiator return water has to be cool enough to cause condensation, about 50-55°C is often quoted.
Hang on, the return water should be 50-55°C or the outgoing?

Re: Efficiency of central heating

Posted: Tue Oct 19, 2021 10:29 pm
by philip
The return water temperature. The heating system should be "tuned" so that the return water is cool enough, this involves setting pump size/speed, radiator size, outgoing temperature, etc.

Re: Efficiency of central heating

Posted: Tue Oct 19, 2021 10:59 pm
by dyqik
Lowering the heating output temp would make sure of that, as well as the other efficiency things I mentioned above.

Re: Efficiency of central heating

Posted: Wed Oct 20, 2021 6:09 am
by bjn
Egads, manually tuning this stuff is a nightmare. Surely you want a digital control system that will automagically balance your radiators and water temperature to achieve maximum efficiency while keeping you toasty? It’s a classic thing that should be solvable by a few cheap microprocessors.

Re: Efficiency of central heating

Posted: Wed Oct 20, 2021 8:20 am
by Grumble
Here’s an article stating that the lower the temperature you run at, the more efficient you will be, and increasing supply temperature only when outside temps drop below freezing.

Agrees with the general principles laid out by dyqik (and agreeing with my preconceived ideas, so it must be right)

https://www.building.co.uk/how-to-opera ... 40.article

Re: Efficiency of central heating

Posted: Wed Oct 20, 2021 11:44 am
by dyqik
bjn wrote:
Wed Oct 20, 2021 6:09 am
Egads, manually tuning this stuff is a nightmare. Surely you want a digital control system that will automagically balance your radiators and water temperature to achieve maximum efficiency while keeping you toasty? It’s a classic thing that should be solvable by a few cheap microprocessors.
Thermostatic radiator valves aren't even that complicated, for the balancing side of things.

I think the main reason microprocessors aren't so heavily used in home HVAC is the lifetime expected, the skills of the average maintenance person, and the variety of installations. This is important because it's pretty easy to build a system that runs away in some not so common state or that doesn't respond quickly enough - there's long delays and hysteresis in the system, and uncontrolled inputs (solar gain, windows that open*), which makes control not so simple.

Oh, and the annoying feature of US systems where the power and control signals for the controllers are 24V AC.

* See Life, the Universe, and Everything for details...