Boiler Efficiency

bob2302

There's been a lot of publicity recently about turning boiler flow temperatures down to 60C. I find this a bit confusing because I thought that the important thing was to keep the return temperature low.

I use my controller to bring the room temperature up in half degree steps, which keeps the return temperature well below the condensing limit. Would I really benefit from lowering the output temperature?

My concern is that more water would have to be pumped through the radiators - potentially flushing the warm water through and raising the return temperature. The boiler would also use more electricity in the pumps and receive more wear and tear.

[Deleted User]

Yes. The lower the boiler set temperature the lower return flow temperature. Boiler efficiency increases as return flow reduces.

bob2302

That diagram plots efficiency against return temperature. My understanding is that flow temperature is the outgoing temperature. Radiators fill with hot water top-down, so at modest levels of heating, the outgoing temperature has little to no effect on the return temperature.

[Deleted User]

bob2302 said:

That diagram plots efficiency against return temperature. My understanding is that flow temperature is the outgoing temperature. Radiators fill with hot water top-down, so at modest levels of heating, the outgoing temperature has little to no effect on the return temperature.

You are correct. Most people refer to flow as the water flowing out of the boiler and return as water returning to the boiler. Can we agree that it is the temperature of the water when it returns to the boiler after it has been around the CH system that matters?

The problem with most systems - where a condensing boiler has just been added to an old heating system- is that the original temperature differential was 12C, and modern heating systems with bigger radiators are designed with a 20C differential in mind. 

I am not sure what you are hoping to achieve by playing around with the thermostat in the way that you describe?

My system boiler is set at 55C and our base temperature is 18C. The boiler just does its thing. There is very little boiler cycling. I have just checked and my gas usage for the past 12 months and is 4667kWh  (based on smart meter readings) for a 4 bed detached property. It is significantly less than it was 2 years ago when I just set 70C and forgot about it.

bob2302

Dolor said:

I am not sure what you are hoping to achieve by playing around with the thermostat in the way that you describe?

My controller supports  6 changes a day, so I use them all to avoid any big steps in temperature. Bigger step changes cause the radiators to fill completely with hot water leading to a much higher return temperature.

lohr500

bob2302 said:

Dolor said:

I am not sure what you are hoping to achieve by playing around with the thermostat in the way that you describe?

My controller supports  6 changes a day, so I use them all to avoid any big steps in temperature. Bigger step changes cause the radiators to fill completely with hot water leading to a much higher return temperature.

I'm not sure I follow your logic. 

Nebulous2

bob2302 said:

Dolor said:

I am not sure what you are hoping to achieve by playing around with the thermostat in the way that you describe?

My controller supports  6 changes a day, so I use them all to avoid any big steps in temperature. Bigger step changes cause the radiators to fill completely with hot water leading to a much higher return temperature.

Have you measured the return temperature? 

Some boilers give a figure for that, mine doesn't.

Some people on here have bought thermometers that fasten onto a pipe and measure the temperature as it returns to the boiler. That depends on your system, the flow through your system, and other factors such as the outside temperature. 

BUFF

bob2302 said:

There's been a lot of publicity recently about turning boiler flow temperatures down to 60C. I find this a bit confusing because I thought that the important thing was to keep the return temperature low.

I use my controller to bring the room temperature up in half degree steps, which keeps the return temperature well below the condensing limit. Would I really benefit from lowering the output temperature?

My concern is that more water would have to be pumped through the radiators - potentially flushing the warm water through and raising the return temperature. The boiler would also use more electricity in the pumps and receive more wear and tear.

Lower flow temp from the boiler inevitably leads to a lower return temperature to the boiler & therefore higher efficiency (assuming a condensing boiler operating in the condensing zone).

The downside is that running lower temperatures means that it can/will take longer for the room(s) to heat due to lower output from the radiator(s).
Yes, your pump will also run longer & use slightly more electricity but running longer, lower actually reduces wear & tear on the system & is more efficient (especially if your boiler & controls support advanced modulation so reduce cycling).
Which exact boiler & controls do you have?

Cardew

BUFF said

Lower flow temp from the boiler inevitably leads to a lower return temperature to the boiler & therefore higher efficiency (assuming a condensing boiler operating in the condensing zone).

Exactly! That is what weather compensation control achieves by reducing the flow temp from the boiler as the outside temperature increases. 

bob2302

BUFF said:

Lower flow temp from the boiler inevitably leads to a lower return temperature to the boiler & therefore higher efficiency (assuming a condensing boiler operating in the condensing zone).

That's not inevitable, it depends on how much heat you need the radiators to put out. At modest power the top of the radiator and the inlet side get hot, but the outlet pipe remains cool. Most people can't stay in this regime all the time and so benefit from turning down the temperature.

BUFF

It is. We are talking about the flow & return temps from/to the boiler & not individual radiators (although the same should follow unless you have e.g. a TRV that has closed).
If I drop the flow temp from the boiler by e.g. 5C it is inevitable that the return temp to the boiler will also drop (although not necessarily by the same 5C).