Actual performance of Condensing Boilers (furnaces)
Actual performance of Condensing Boilers (furnaces)
(OP)
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.





RE: Actual performance of Condensing Boilers (furnaces)
Doug
RE: Actual performance of Condensing Boilers (furnaces)
Why would anyone want a hot water heater?
Here, we heat cold water to get hot water. So, we call them just water heaters, and, more popularly, "geysers."
If anything, one would heat hot water only to produce steam, and that would be a boiler, for all purposes, right?
(tongue firmly in cheek!)
RE: Actual performance of Condensing Boilers (furnaces)
If you live further South where the need for air conditioning exists then most of the time you have a central forced air heating unit that either runs on natural gas or electric. If you are in a rural area your furnace might run on LPG instead.
I think most of the "boiler" aplications are in the North East of the US.
I have lived in Nebraska, Colorado, and Illinois and they are not common in any of those areas.
I have noticed that with rising gas costs, Heat pump usage is on the rise.
And here we heat our water in "hot water heaters". I never thought about it before but that does sound dumb. We keep our geysers in Yellowstone Park, not in the house.
Later
StoneCold
RE: Actual performance of Condensing Boilers (furnaces)
My main reason for posting here was to discover other's views about condensing burner technology, whether used for heating hot water, radiators or for baseboard heating (in Europe, usually referred to as 'underfloor heating' - oh dear, 2 nations divided by a common language once again :=) ).
RE: Actual performance of Condensing Boilers (furnaces)
Regards
StoneCold
RE: Actual performance of Condensing Boilers (furnaces)
It was realised a while ago that the condensation that could occur in boilers was water produced in the combuston process and that significant amounts of energy were being wasted by allowing it to exit the flue as steam. Thus, condensing boilers (and recuperators on conventional boilers) were introduced to lower the fluegas temperature below dewpoint and recover the latent heat from the steam as it condensed. Two problems: condensate and wet fluegas are quite corrosive, especially if there is any sulphur in the fuel. So HXs and flue components need to be resistant (stainless steel prefered. Second problem is how to reduce the Return temperature below dewpoint. This is especially tricky in retrofit situations where part of the original design was to achieve the opposite, to prevent 'back-end rot' in the boiler. Hence my original question, to gather other's views and experience.
RE: Actual performance of Condensing Boilers (furnaces)
RE: Actual performance of Condensing Boilers (furnaces)