Boiler Efficiency

Apodemus

BUFF said:

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).

Surely there can easily be situations where the radiators are managing to maximise their heat transfer and the limiting factor on the return temperature is the heat lost from the system, not the initial input temperature?

InvertedVee

bob2302 said:

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.

Hi Bob,

I think I understand. I guess that like me you have a smart thermostat connected to the boiler with a digital interface, which automatically varies the flow temperature according to the size of the changes in room set temperature.

My Tado programmer and boiler are linked with Worcester's EMS-bus interface. So if I programme a large change in the room set temperature, the Tado calls for a higher boiler flow temperature. If I programme a small change, I get a lower flow temperature.

Here's a screenshot from my Tado app for yesterday.

The palest shading was when the programmer was calling for no heat.
At 0600, the house temperature had cooled to 18.8C and the first set temperature was 19C. The Tado called for the lowest amount of heat from the boiler. I have arranged this to be around 45C.
At 1030 the sunshine began to warm the house so for the next 3 hours the Tado called for no heat.
At 1430 the set temperature is programmed as 19.5C, so the Tado called for moderate heat.
At 1630, the set temperature is programmed as 19.8C.
Between 1717-1735, the Tado called for maximum heat (in my case this is 55C flow temperature) because the rate of warming in the house was beginning to slow.
At 2000, the set temperature is programmed as 19.8C.
At 2200, the set temperature is programmed as 19C.
At 0000, the set temperature is programmed as 17.5C.

I like the fact that the Tado has an unlimited number of programme times and that I can nudge the set temperature by a fraction of a degree. I rarely notice that the room has become chilly. And all this happens with the lowest flow temperature possible. So althoughI set my maximum flow temperature to 55C, that only occurred for 18 minutes yesterday.

I'm using about 30kWh/day for heating and hot water at the moment. (See signature for more info).

BUFF

Apodemus said:

BUFF said:

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).

Surely there can easily be situations where the radiators are managing to maximise their heat transfer and the limiting factor on the return temperature is the heat lost from the system, not the initial input temperature?

The only circumstance where I can really see that happening is where TRVs are starting to close off radiators limiting the amount of loss from the system & hence a higher return temp than previously if the boiler can't modulate down - but that should occur at the same room temp(s). irrespective of boiler flow temp. (there are always going to be slight deviations but I am trying to keep to a simple model). However, if you have dropped the flow temp by e.g. 5C rather than 1 C I would still expect to see a lower return temp. than previously.
If you have a modern boiler & controls (I did ask earlier for specifics, also see InvertedVee's post) then the boiler should be able to modulate it's output down to as near the actual heat demand as it can so that hopefully should not happen too often but if it does then the boiler would start to cycle.

Apodemus

BUFF said:

Apodemus said:

BUFF said:

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).

Surely there can easily be situations where the radiators are managing to maximise their heat transfer and the limiting factor on the return temperature is the heat lost from the system, not the initial input temperature?

The only circumstance where I can really see that happening is where TRVs are starting to close off radiators limiting the amount of loss from the system & hence a higher return temp than previously if the boiler can't modulate down - but that should occur at the same room temp(s). irrespective of boiler flow temp. (there are always going to be slight deviations but I am trying to keep to a simple model). However, if you have dropped the flow temp by e.g. 5C rather than 1 C I would still expect to see a lower return temp. than previously.
If you have a modern boiler & controls (I did ask earlier for specifics, also see InvertedVee's post) then the boiler should be able to modulate it's output down to as near the actual heat demand as it can so that hopefully should not happen too often but if it does then the boiler would start to cycle.

I think I am suggesting the opposite situation - where the rooms are cold enough that the radiators are delivering the maximum possible heat from the circulating water and the return temperature is low.  Turning the boiler flow temperature up (or down) will simply deliver more (or less) heat to the rooms, without influencing the return flow temperature.  This scenario probably plays out on the warm-up phase of most boilers in poorly insulated houses until such time as the rooms warm up to the point where the rate of heat transfer from the radiators falls and the return flow temperature to the boiler starts to rise.

[Deleted User]

Apodemus said:

BUFF said:

Apodemus said:

BUFF said:

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).

Surely there can easily be situations where the radiators are managing to maximise their heat transfer and the limiting factor on the return temperature is the heat lost from the system, not the initial input temperature?

The only circumstance where I can really see that happening is where TRVs are starting to close off radiators limiting the amount of loss from the system & hence a higher return temp than previously if the boiler can't modulate down - but that should occur at the same room temp(s). irrespective of boiler flow temp. (there are always going to be slight deviations but I am trying to keep to a simple model). However, if you have dropped the flow temp by e.g. 5C rather than 1 C I would still expect to see a lower return temp. than previously.
If you have a modern boiler & controls (I did ask earlier for specifics, also see InvertedVee's post) then the boiler should be able to modulate it's output down to as near the actual heat demand as it can so that hopefully should not happen too often but if it does then the boiler would start to cycle.

I think I am suggesting the opposite situation - where the rooms are cold enough that the radiators are delivering the maximum possible heat from the circulating water and the return temperature is low.  Turning the boiler flow temperature up (or down) will simply deliver more (or less) heat to the rooms, without influencing the return flow temperature.  This scenario probably plays out on the warm-up phase of most boilers in poorly insulated houses until such time as the rooms warm up to the point where the rate of heat transfer from the radiators falls and the return flow temperature to the boiler starts to rise.

There are numerous industry articles that suggest that very few UK domestic boilers are operating in a condensing mode. The main reasons for this are the boiler set temperature and the fact the new boilers are being added to heating systems which were configured for a 80C flow and a 12 degree flow to return temperature differential.

I believe that you are correct in saying the the return temperature will inevitably rise. This is what informs the boiler that it needs to modulate downwards. When the lowest modulation point is reached then the boiler will cycle on and off usually in a 6 times/hr cycle. Some expensive gas boilers now have a 10:1 modulation ratio.

Mstty

[Deleted User] said:

Apodemus said:

BUFF said:

Apodemus said:

BUFF said:

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).

Surely there can easily be situations where the radiators are managing to maximise their heat transfer and the limiting factor on the return temperature is the heat lost from the system, not the initial input temperature?

The only circumstance where I can really see that happening is where TRVs are starting to close off radiators limiting the amount of loss from the system & hence a higher return temp than previously if the boiler can't modulate down - but that should occur at the same room temp(s). irrespective of boiler flow temp. (there are always going to be slight deviations but I am trying to keep to a simple model). However, if you have dropped the flow temp by e.g. 5C rather than 1 C I would still expect to see a lower return temp. than previously.
If you have a modern boiler & controls (I did ask earlier for specifics, also see InvertedVee's post) then the boiler should be able to modulate it's output down to as near the actual heat demand as it can so that hopefully should not happen too often but if it does then the boiler would start to cycle.

I think I am suggesting the opposite situation - where the rooms are cold enough that the radiators are delivering the maximum possible heat from the circulating water and the return temperature is low.  Turning the boiler flow temperature up (or down) will simply deliver more (or less) heat to the rooms, without influencing the return flow temperature.  This scenario probably plays out on the warm-up phase of most boilers in poorly insulated houses until such time as the rooms warm up to the point where the rate of heat transfer from the radiators falls and the return flow temperature to the boiler starts to rise.

There are numerous industry articles that suggest that very few UK domestic boilers are operating in a condensing mode. The main reasons for this are the boiler set temperature and the fact the new boilers are being added to heating systems which were configured for a 80C flow and a 12 degree flow to return temperature differential.

I believe that you are correct in saying the the return temperature will inevitably rise. This is what informs the boiler that it needs to modulate downwards. When the lowest modulation point is reached then the boiler will cycle on and off usually in a 6 times/hr cycle. Some expensive gas boilers now have a 10:1 modulation ratio.

So what efficiency are they running at, 80% higher or lower as they are not setup correctly.

[Deleted User]

Take a look at this graph below. The maximum efficiency % without condensing appears to be c.87.5. If the boiler flow temperature is 55C with a 20C flow to return differential then an efficiency % of 96.5 is theoretically possible. 

It is important to understand the difference between gross and net efficiency. Most boiler efficiencies these days are Net. This link explains what some might regard as mis-selling:

https://boilerservices.net/technical-articles/gross-and-net-efficiency/

BUFF

Apodemus said:

BUFF said:

Apodemus said:

BUFF said:

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).

Surely there can easily be situations where the radiators are managing to maximise their heat transfer and the limiting factor on the return temperature is the heat lost from the system, not the initial input temperature?

The only circumstance where I can really see that happening is where TRVs are starting to close off radiators limiting the amount of loss from the system & hence a higher return temp than previously if the boiler can't modulate down - but that should occur at the same room temp(s). irrespective of boiler flow temp. (there are always going to be slight deviations but I am trying to keep to a simple model). However, if you have dropped the flow temp by e.g. 5C rather than 1 C I would still expect to see a lower return temp. than previously.
If you have a modern boiler & controls (I did ask earlier for specifics, also see InvertedVee's post) then the boiler should be able to modulate it's output down to as near the actual heat demand as it can so that hopefully should not happen too often but if it does then the boiler would start to cycle.

I think I am suggesting the opposite situation - where the rooms are cold enough that the radiators are delivering the maximum possible heat from the circulating water and the return temperature is low.  Turning the boiler flow temperature up (or down) will simply deliver more (or less) heat to the rooms, without influencing the return flow temperature.  This scenario probably plays out on the warm-up phase of most boilers in poorly insulated houses until such time as the rooms warm up to the point where the rate of heat transfer from the radiators falls and the return flow temperature to the boiler starts to rise.

I am still not convinced that this happens in anything like the majority but there are so many potential variables (how cold is the room at start/end, radiator size & system design criteria, boiler flow/return temps, boiler age/condensing or non etc. & critically, controls*)  that it is possible that it may happen in some, especially with older boilers.
 
*The whole point of modulation & especially with load/weather compensating controls is to automatically match boiler output to heat demand (assuming that the boiler can modulate down that low).

Dolor said:

I believe that you are correct in saying the the return temperature will inevitably rise. This is what informs the boiler that it needs to modulate downwards. When the lowest modulation point is reached then the boiler will cycle on and off usually in a 6 times/hr cycle. Some expensive gas boilers now have a 10:1 modulation ratio.

Viessmann 200 can do 17:1

[Deleted User]

BUFF said:

Apodemus said:

BUFF said:

Apodemus said:

BUFF said:

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).

Surely there can easily be situations where the radiators are managing to maximise their heat transfer and the limiting factor on the return temperature is the heat lost from the system, not the initial input temperature?

The only circumstance where I can really see that happening is where TRVs are starting to close off radiators limiting the amount of loss from the system & hence a higher return temp than previously if the boiler can't modulate down - but that should occur at the same room temp(s). irrespective of boiler flow temp. (there are always going to be slight deviations but I am trying to keep to a simple model). However, if you have dropped the flow temp by e.g. 5C rather than 1 C I would still expect to see a lower return temp. than previously.
If you have a modern boiler & controls (I did ask earlier for specifics, also see InvertedVee's post) then the boiler should be able to modulate it's output down to as near the actual heat demand as it can so that hopefully should not happen too often but if it does then the boiler would start to cycle.

I think I am suggesting the opposite situation - where the rooms are cold enough that the radiators are delivering the maximum possible heat from the circulating water and the return temperature is low.  Turning the boiler flow temperature up (or down) will simply deliver more (or less) heat to the rooms, without influencing the return flow temperature.  This scenario probably plays out on the warm-up phase of most boilers in poorly insulated houses until such time as the rooms warm up to the point where the rate of heat transfer from the radiators falls and the return flow temperature to the boiler starts to rise.

I am still not convinced that this happens in anything like the majority but there are so many potential variables (how cold is the room at start/end, radiator size & system design criteria, boiler flow/return temps, boiler age/condensing or non etc. & critically, controls*)  that it is possible that it may happen in some, especially with older boilers.
 
*The whole point of modulation & especially with load/weather compensating controls is to automatically match boiler output to heat demand (assuming that the boiler can modulate down that low).

Dolor said:

I believe that you are correct in saying the the return temperature will inevitably rise. This is what informs the boiler that it needs to modulate downwards. When the lowest modulation point is reached then the boiler will cycle on and off usually in a 6 times/hr cycle. Some expensive gas boilers now have a 10:1 modulation ratio.

Viessmann 200 can do 17:1

Indeed it can. I confess that I had mentally discounted the 200 as it is really not a practicable replacement boiler if separate DWH and CH is required unless the homeowner is prepared to do some re-piping (according to the guy who installed an Atag for me).

bob2302

You are all over-complicating this. It's a 12 year old basic Worcester-Bosch, I don't have anything sophisticated, I don't even have thermostatic valves.   I'm simply keeping the overall heat output low-enough that the water temperature falls to luke warm before it exits the radiators.

If I turn the flow temperature knob low, the living room radiator (which gets most of the heat) runs luke warm  from top to bottom, if I turn it to high I get a narrow band of very hot water at the top, but it's still about the same at the bottom. It's simple physics, hotter water has a lower density and a greater energy density which gives better stratification, less mixing and a longer transit time.

I don't have an exact temperature value, but I have a length of unlagged copper pipe next to the boiler, the peak temperature feels well below 55C - closer to skin temperature I'd say. The position of the flow temperature knob makes no perceptible difference.