Combi boiler cycling

Reed_Richards

meat_n2_reg said:

Changed flow temperature to 60c took alot longer to reach target temperature , at 65c flow takes approx 45 mins and at 60c took over an hour 
Going to try flow of 70c for comparison, not sure if that's too high for condensing mode ?
I am recording gas usage for each setting
For reference heating is on for 1.5 hrs morning and evening so only 3hrs a day , so maybe a 70c flow might work better?

It's the return water temperature that matters for condensing so whether 70 C flow is too much depends on how your heating system is set up and how much of a temperature drop there is between flow and return.

The amount of heat you need is very dependent on how cold it is outside so if you are trying to make comparisons make sure that the outside temperature is much the same on each day you compare.

FreeBear

meat_n2_reg said: Going to try flow of 70c for comparison, not sure if that's too high for condensing mode ?

They say that condensing mode kicks in with a return temperature of about 55°C and efficiency should be around 87%. Running at a higher return temperature, say 70°C will see efficiency drop to ~85%. Going the other way, if you can get down to 40°C, you should see the efficiency go up to ~94%.

See https://www.heatgeek.com/condensing-boilers-efficiency/ for more information..

The return temperature will depend on how well balanced your radiators are, and also the temperature differential between inlet & outlet. Normally, with a flow temperature of 70°C, radiators are set for a 20°C drop. Reduce the flow temperature, and the drop reduces accordingly - With my boiler set to a 50°C flow, the radiators drop ~10°C to give a return temperature of ~40°C. Not tried cranking the flow temperature up to 70°C yet. Might try it if/when it starts snowing.

Reed_Richards

FreeBear said:

Not tried cranking the flow temperature up to 70°C yet. Might try it if/when it starts snowing.

That's called Weather Compensation.  Some boilers will do this automatically, with the right controller.

Lorian

Get a cheap probe thermometer and attach to the return pipe near the boiler. At 70c almost certainly won't be condensing much. The alternative is to stick with 60c and just make it start a bit earlier.

The 4000 will modulate down as the return temperature rises.

FreeBear

Reed_Richards said:

FreeBear said:

Not tried cranking the flow temperature up to 70°C yet. Might try it if/when it starts snowing.

That's called Weather Compensation.  Some boilers will do this automatically, with the right controller.

Yup. £30 for the Viessmann sensor, plus a lot of hassle running a cable from it to the boiler, and then having to pay a Gas Safe engineer to open up the boiler to do the final connection - The Vitodens 050 will do the weather compensation automatically if it is enabled.

Or I could just use the home automation system to increase the max flow temperature as and when it gets icy outside (already have an outdoor sensor hooked up). Might even add WC to the OpenTherm code that I use to control the boiler.

Reed_Richards

Any boiler that supports OpenTherm connected to a controller that supports OpenTherm should be able to do Weather Compensation, I believe.

meat_n2_reg

Just measured return temperature

At 65c flow return is 45c
At 60c flow return is 40c

So evething appears balance

Is 40c return temperature low enough to get high efficiency?

Regards Steve 

QrizB

meat_n2_reg said:

Is 40c return temperature low enough to get high efficiency?

Yes, I would say that a 40C return temp is fine.

FreeBear

meat_n2_reg said: At 65c flow return is 45c
At 60c flow return is 40c

Is 40c return temperature low enough to get high efficiency?

50°C will return ~90% efficiency, 40°C should get you about 94%. Not a lot in it, and the difference between 45°C and 40°C will be even less.

With my system, I found dropping the flow temperature too low, whilst boosting theoretical efficiency, was actually taking a lot longer to heat the property up. Quickly decided that a flow temperature of 50-55°C reduced gas consumption by a wide margin, so more than made up for the loss of ~2% efficiency.

If 65°C gets you up to temperature quickly enough, I'd say stop chasing every crumb of efficiency.

meat_n2_reg

Looks like 65 flow is the best combination for my house 

Regards Steve