jbrooks
Mechanical
- May 4, 2006
- 3
Hi
I'm new here and quite surprised by the lack of threads on high-efficiency boilers (furnaces) (someone correct my terminology - here in UK, the thing in your house that makes hot water is a 'boiler').
In my work installing / maintaining mainly 'large residential' heating systems, I'm subject to certain government regulations on thermal efficiency. There's a classification of boilers called 'SEDBUK' which ranges from A (best) to G (worst). Today, unless it's virtually impossible due to structural issues, Band A or B boilers are mandatory in UK. To achieve this rating they MUST be condensing type.
There are several inherent problems with the policy:
-it says too little about efficient control systems (weather compensation, for example;
- I believe it does not take sufficient account of the impact of 'full-life costing';
- especially in retrofit situations, there is a big (and largely-unrecognised problem with high Return temperatures to the boiler, resulting in no condensation!
It should be self-evident that if you have a boiler that can modulate its output to match load, then you should be able to reduce the Flow temperature under automatic control. In reality, most smaller boilers have only a single, manually-adjusted Flow setpoint, so weather compensation is hard to do for a start! (It makes no sense to drive a condensing boiler at 75 or 80 degrees Celsius then immediately downstream have a blender to reduce Flow to 45 degrees for baseboard heating - but believe me - it happens!)
It should also be self-evident that if you have a complex machine with expensive electronic components, fans, pumps, etc., it will cost more to maintain in parts, labour, truck rolls and (especially) energy than an all-or-nothing cast-iron furnace with one moving part in its gas control valve. The extraordinary thing is that no-one seems to have done a comprehensive investigation of full-life costs for heating equipment in Europe. It's just become received wisdom that 'energy efficiency is good' whatever the cost, even in additional energy!
Return temperature management is absolutely critical in condensing boilers. Unless the temperature in the heat exchanger is below Dew Point, there will be NO condensation! Unfortunately, dew point inside a boiler is arounds 56 degrees Celsius, which is a long way below the 'conventional' Flow temperature of 75 - 80 Celsius. (This is partly why Weather Comp should be such a big deal - but isn't!). Further aspects of this issue are radiator sizing / performance and pipework design. If the radiators (or other heat-sinks in an installation) are designed for a delta-T of 50 with respect to average room temperature, then they've got to be at 70 degrees to heat the rooms up in a reasonable time. Most systems are 'two pipe', so that all the heat sinks are connected across flow and return pipes. All well and good, except that if radiators connected near to the boiler release too much hot water into the main Return to the boiler, this will quickly raise its temperature to only a few degrees below Flow temp - so no condensing! Generally, and especially in retrofit situations, it appears that far too little attention is paid to Return temperature management, with the result that boilers that are intended to deliver high efficiencies dependent on the contribution from condensation (maybe 8 - 10 percentage points of the total) never actually achieve this due to elevated Return temperatures.
I'm interested to learn whether these musings strike any chords in other places, especially the US, where rising gasoline (and presumably also gas) prices have hit the headlines.
I'm new here and quite surprised by the lack of threads on high-efficiency boilers (furnaces) (someone correct my terminology - here in UK, the thing in your house that makes hot water is a 'boiler').
In my work installing / maintaining mainly 'large residential' heating systems, I'm subject to certain government regulations on thermal efficiency. There's a classification of boilers called 'SEDBUK' which ranges from A (best) to G (worst). Today, unless it's virtually impossible due to structural issues, Band A or B boilers are mandatory in UK. To achieve this rating they MUST be condensing type.
There are several inherent problems with the policy:
-it says too little about efficient control systems (weather compensation, for example;
- I believe it does not take sufficient account of the impact of 'full-life costing';
- especially in retrofit situations, there is a big (and largely-unrecognised problem with high Return temperatures to the boiler, resulting in no condensation!
It should be self-evident that if you have a boiler that can modulate its output to match load, then you should be able to reduce the Flow temperature under automatic control. In reality, most smaller boilers have only a single, manually-adjusted Flow setpoint, so weather compensation is hard to do for a start! (It makes no sense to drive a condensing boiler at 75 or 80 degrees Celsius then immediately downstream have a blender to reduce Flow to 45 degrees for baseboard heating - but believe me - it happens!)
It should also be self-evident that if you have a complex machine with expensive electronic components, fans, pumps, etc., it will cost more to maintain in parts, labour, truck rolls and (especially) energy than an all-or-nothing cast-iron furnace with one moving part in its gas control valve. The extraordinary thing is that no-one seems to have done a comprehensive investigation of full-life costs for heating equipment in Europe. It's just become received wisdom that 'energy efficiency is good' whatever the cost, even in additional energy!
Return temperature management is absolutely critical in condensing boilers. Unless the temperature in the heat exchanger is below Dew Point, there will be NO condensation! Unfortunately, dew point inside a boiler is arounds 56 degrees Celsius, which is a long way below the 'conventional' Flow temperature of 75 - 80 Celsius. (This is partly why Weather Comp should be such a big deal - but isn't!). Further aspects of this issue are radiator sizing / performance and pipework design. If the radiators (or other heat-sinks in an installation) are designed for a delta-T of 50 with respect to average room temperature, then they've got to be at 70 degrees to heat the rooms up in a reasonable time. Most systems are 'two pipe', so that all the heat sinks are connected across flow and return pipes. All well and good, except that if radiators connected near to the boiler release too much hot water into the main Return to the boiler, this will quickly raise its temperature to only a few degrees below Flow temp - so no condensing! Generally, and especially in retrofit situations, it appears that far too little attention is paid to Return temperature management, with the result that boilers that are intended to deliver high efficiencies dependent on the contribution from condensation (maybe 8 - 10 percentage points of the total) never actually achieve this due to elevated Return temperatures.
I'm interested to learn whether these musings strike any chords in other places, especially the US, where rising gasoline (and presumably also gas) prices have hit the headlines.
