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Danish Gas Technology Centre • Dr. Neergaards Vej 5B • DK-2970 Hørsholm • Tlf. +45 2016 9600 • Fax +45 4516 11 99 • www.dgc.dk
Condensing air heaters Technology evaluation
Project report
Maj 2013
Condensing air heaters
Technology evaluation
Mikael Näslund
Danish Gas Technology Centre
Hørsholm 2013
Title : Condensing air heaters
Report
Category : Project Report
Author : Mikael Näslund
Date of issue : 24.05.2013
Copyright : Danish Gas Technology Centre
File Number : 737-72; h:\737\72 kondenserende luftvarme\rapport\luftvarmerapport_final.docx
Project Name : Kondenserende luftvarme
ISBN : 978-87-7795-362-0
DGC-report 1
Table of Contents Page
Summary ......................................................................................................................................... 2
1 Introduction ........................................................................................................................... 3
1.1 Burner .................................................................................................................................. 4
1.2 Heat exchanger .................................................................................................................... 5
1.3 Flue system .......................................................................................................................... 5
2 Laboratory test of one product: Robur G 30 ......................................................................... 7
2.1 Test procedure ..................................................................................................................... 9
2.2 Appliance performance ..................................................................................................... 10
3 Other condensing air heaters on the market ........................................................................ 14
4 Installation aspects .............................................................................................................. 16
4.1 Air and flue systems .......................................................................................................... 16
4.2 Economy ............................................................................................................................ 16
5 Conclusions ......................................................................................................................... 20
6 References ........................................................................................................................... 21
Appendices
Appendix A: Update of DGC guideline 45 and Danish fuel gas code, “Gasreglementet”
DGC-report 2
Summary
Condensing technology is today the state of the art for single-family house
gas heating in Scandinavia. Heating of larger industrial premises and ware-
houses are still dominated by non-condensing technologies.
Condensing air heaters are available on the market but have not yet found
widespread use in Denmark. This report describes and evaluates the con-
densing air heating technology, and test results are reported. The energy
saving using a condensing air heater is approximately 15% compared to a
modern non-condensing air heater and approximately 20% if an old air
heater is replaced.
A condensing air heater was laboratory tested. The efficiency at nominal
load was measured to 97% which is in accordance with manufacturer data.
At minimum load the efficiency was measured to 102.9% compared to
105.7% claimed by the manufacturer. The annual efficiency is estimated to
100%. The emissions were low with a NOx level of 22 mg/MJ or lower de-
pending on burner load. The CO levels were almost zero, with the exception
of a moderate peak (100 ppm) directly after the burner ignition.
The simple pay-back time if the condensing option is chosen instead of a
modern non-condensing air heater was calculated to be between 0 and 5
years.
This project was funded by the the Danish gas companies' Technical Com-
mittee on Gas Utilisation and Installations (FAU GI). Quality assurance at
DGC was made by Jan de Wit.
Thanks to Danheat for making a Robur air heater available for testing.
Thanks also to Stefano Caverzaschi at Robur in Italy for technical com-
ments regarding the Robur air heater.
DGC-report 3
1 Introduction
Condensing technology is today the state of the art for single-family house
gas heating in Scandinavia. Heating of larger industrial premises and ware-
houses and tool shops are still dominated by non-condensing technologies.
Condensing air heaters are available on the market, but have not yet found
widespread use in Denmark.
Both independently operating unit heaters and central air heating systems
are available in high-efficiency condensing designs. The manufacturers
claim 103 – 106% steady-state efficiency for condensing units and 90%
steady-state efficiency for new non-condensing appliances. This means en-
ergy saving of approximately 15% at nominal load using a modern condens-
ing unit instead of a modern non-condensing unit alone. Retrofitting old
heater technology with condensing technology increases the energy savings
even more. The old Ambirad Centurion air heater with atmospheric burner
and open flue has an efficiency of 85% according to Ambirad product de-
scription. Using a condensing air heater will then reduce the fuel consump-
tion by approximately 20% if an old air heater is replaced.
The main differences between non-condensing and condensing designs are
in the heat exchanger, the burner and the flue system, i.e. all important ap-
pliance parts. General images of a non-condensing and a condensing unit are
shown in Figure 1. The top images show the Lennox LF24 with approxi-
mately 90% efficiency. The burner is of a forced draught type and the heat
exchanger is a simple tubular heat exchanger where the flue gases flow on
the inside. The heat exchanger surface is smooth without any heat transfer
enhancing fins etc.
The bottom image shows an Ambirad UESA condensing air heater. The
heater is marketed under at least three brands: Ambirad, Benson and Rez-
nor. It has a single-stage burner firing horisontally towards the heat ex-
changer inlet. This model, as well as other condensing air heaters, has a heat
exchanger in two clearly visible parts. It resembles some early condensing
boilers for single-family houses which also had a second condensing heat
exchanger.
DGC-report 4
Lennox LF24
Ambirad UESA
Figure 1 A traditional non-condensing (Lennox LF24) and a condensing
(Ambirad UESA) unit heater
The manufacturers of new highly efficient non-condensing and condensing
air heaters often mention the following characteristics for the products:
High efficiency.
Low temperature rise for the air. This means better warm air distri-
bution, reduced stratification and increased comfort.
Modulating premix burners.
Room sealed flue solutions.
1.1 Burner
Burners in condensing air heaters are often modulating premix burners.
They offer a constant excess air ratio in the modulation range. The emis-
sions of CO and NOx are also low.
DGC-report 5
1.2 Heat exchanger
The heat exchanger design is also responsible for the temperature and flow
profile at the unit air exit. The images in Figure 2 from Ambirad and Robur
brochures show the warm air temperature distribution in the heated room.
The top left images show the distribution from an old air heater compared to
the distribution from a new design shown in the lower left image. The image
to the right shows a thermography picture from a Robur air heater. The flow
and temperature field is similar to the more general flow field presented in
the Ambirad images. This kind of improved temperature distribution in the
room could also lead to a reduced heating demand.
Figure 2 Illustrations from Ambirad and Robur describing improved warm
air distribution with new air heater designs
1.3 Flue system
The condensing unit air heaters are all modern regarding the air and flue
systems. Normally, room sealed, closed systems are used for stationary in-
stallations. However, open flue solutions are often mentioned as an option.
Modern non-condensing air heaters are also delivered with room sealed flue
systems. Figure 3 shows examples of flue solutions from the manufacturer
Mark.
DGC-report 6
Figure 3 Examples of flue solutions for modern unit air heaters
(Source: Mark)
The acceptable flue system lengths for the Robur G30 are taken as an exam-
ple of appliance specific lengths.
Table 1 Flue system lengths specified for Robur G30 condensing air
heater
Category Description Limit
C13 Room sealed, horizontal flue terminal. The fan is located up-stream of the burner/heat ex-changer.
Air: 8 – 20 m Flue gases: 8 – 20m
C33 Room sealed, vertical terminal. The fan is located upstream of the burner/heat exchanger.
Air: 20 – 30 m Flue gases: 20 – 30m
B23 Open combustion, no draught diverter. Vertical flue system The fan is located upstream of the burner/heat exchanger.
Flue gases: 17 m (=80 mm)
30 m (=110 mm)
C53 Room sealed split system. The fan is located upstream of the burner/heat exchanger.
Air: 1 m
Flue gases: 13 m (=80 mm)
30 m (=110 mm)
DGC-report 7
2 Laboratory test of one product: Robur G 30
An air heater was tested in the DGC laboratory to verify the manufacturer's
claimed efficiency and to investigate the overall characteristics.
Robur G 30 unit heater was chosen and the Danish importer Danheat made a
unit available for testing.
The tests comprised:
Steady-state efficiency
Emissions
Figure 4 shows four external images of the Robur G30 unit and a view of
the heat exchanger. The top images show the heater as installed while the
two bottom images show the heat exchanger when the front louver and the
rear air fan are removed for better visibility.
DGC-report 8
Figure 4 Views of Robur G30 air heater
The premix burner is mounted in a large cylinder. The burner modulates
between 13 and 29 kW with constant excess air ratio. On top of the cylinder
are two almost vertical heat exchangers with fins on both the flue gas and
the air sides (lower left image). The material is an aluminium alloy. The last
part of the heat exchanger, where the condensation occurs, begins at a box
on top of the two vertical tubes. The flue gases flow inside four corrugated
and flexible stainless steel tubes directed towards the air inlet of the heater
DGC-report 9
(lower right image). At the bottom, close to the air inlet, these tubes are
connected to a box where condensate also is collected and drained. The flue
gases leave the air heater through the outlet at the top left corner of the ap-
pliance. The air flow through the heater is managed by an axial fan. The
capacity is 2300 – 2700 m3/h within the burner modulating range. The tem-
perature increase is stated to 16°C at minimum fan speed and 32°C at max-
imum speed. All data according to Robur data sheets.
The Robur G series condensing air heaters are available in 4 sizes with nom-
inal capacities between 30 kW and 90 kW.
2.1 Test procedure
Test conditions for condensing air heaters are specified in the European test
standards [1] and [2]. DGC has not previously tested air heaters and the test
procedure did not fully comply with the test standards. The test set up is
shown in Figure 5. The air heater is placed between two wooden shields
which direct the flow to the ventilation system. The warm air enters into the
400 mm hose and is finally evacuated through the laboratory ventilation
system. This is necessary to keep the laboratory temperature within reason-
able limits. The image to the right shows the air intake and the flue system.
Laboratory air is taken directly into the air heater. The flue system is a split
option where combustion air is taken from the laboratory. The combustion
air temperature is then equal to the laboratory temperature.
Figure 5 Robur G30 air heater test set up
DGC-report 10
The air heater was connected to gas and electricity. The burner was checked
and not adjusted to Danish natural gas. The current installation procedures
for premixed burners in Denmark suggest no burner adjustment to current
gas quality. The appliance should keep the factory adjustment to pure me-
thane, G20.
The steady-state tests were done during 20 minutes after a stabilization peri-
od. Gas input and air temperatures were measured. A standard flue gas anal-
ysis including O2, CO2, NOx and CO measurements were also made.
2.2 Appliance performance
The efficiency was measured in the four operating points possible to set in
the control box. These operating points are shown in Table 2.
Table 2 Operating conditions in testing of Robur G30 air heater
Operating point Burner input Fan speed
1 30 kW (Max) Max
2 22 kW High
3 22 kW Low
4 16 kW (Min) Min
The laboratory air temperature was 22 – 25°C, which is within the test
standard limits. The O2 content in the flue gases varied from 4.5% at maxi-
mum gas input to 5% at minimum gas input.
The measured flue gas temperature and efficiency are shown in Figure 6. It
is clearly seen that the flue gas temperature in general is never below the
dew point. The condensate is formed due to cold heat exchanger wall tem-
peratures. There is almost no condensate found at maximum gas input while
the condensate formation at minimum load was measured to 0.5 l/h. At
maximum burner input the flue gas temperature is in the 80 – 85°C range
while it is 60 – 65°C at minimum fuel input. The flue gas temperatures are
above the dew point in every operating condition. Condensation takes place
due to wall temperatures below the dew point.
DGC-report 11
Figure 6 Robur G30 air heater laboratory performance data
Robur claims an efficiency of 105.7% at minimum burner input. This was
not measured in the DGC laboratory tests. The maximum efficiency in the
DGC tests was 102.9%. The tests were repeated, but without any change in
the maximum efficiency. The efficiency was calculated according to Euro-
pean standards [1], [2]. The efficiency for condensing units is the sum of
two parts. The first efficiency part is based on temperature and excess air
ratio only. The second efficiency part is the energy from the collected con-
densate volume during the test. The spread sheet for the efficiency calcula-
tion used by Robur was also used for comparison. No differences were
found.
One explanation may be the gas quality. The Robur data is obtained with
pure methane. The DGC laboratory measurements were done with Danish
natural gas with a higher Wobbe number. This means that the burner input
is higher and the inner wall temperature in the condensing part of the heat
exchanger may be slightly higher. A higher wall temperature will reduce the
condensate formation and the efficiency gain from the condensation. The
flue gas temperature is the same as in the data from Robur which indicates
that it is the condensate formation that affects the results.
Robur has suggested other possible sources to the difference in efficiency.
The most likely seems to be the air temperature. Air temperatures below
20C would increase the condensate formation. However, the air tempera-
0
20
40
60
80
100
0 5 10 15 20 25 30 35
Tem
pe
ratu
re (
C),
Eff
icie
ncy
(%
)
Gas input (kW)
Robur G30 air heater - flue gas temperature and efficiency
Flue gas temperature
Efficiency
DGC-report 12
ture during the tests should be 20±5C as stated in the test standard /2/. The
standard seems to allow a fairly wide temperature range for a condensing
appliance. Another suggested explanation is that some formed condensate
did not drain properly and remained in the heat exchanger but the repeated
tests showed almost identical condensate formation rates.
The combustion is clean with low NOx emissions and very low CO emis-
sions. The emissions of NOx and CO are shown in Table 3. The CO emis-
sion peaks shortly after the burner starts with a maximum of approximately
100 ppm. The level rapidly decreases to <1 ppm within 2 – 3 minutes. This
is close to the instrument detection limit.
Table 3 Robur G3 air heater emissions
Load (kW)
NOx (ppm 0% O2)
NOx (mg/MJ)
CO (ppm)
CO (mg/MJ)
30 43 22 0.1 0.1 22 35 18 0 0 16 28 14 0 0
The difference between the room air temperature and the flue gas tempera-
ture is shown in Figure 7. The inlet temperature equalled the laboratory air
temperature and varied between 22 and 25°C during and between the tests.
Figure 7 Robur G30 air heater temperature laboratory data
Finally, the electric power demand for operation was measured. For the op-
erating settings the power demand was measured as shown in Figure 8. The
0
10
20
30
40
50
60
70
0 5 10 15 20 25 30 35
Tem
pe
ratu
re d
iffe
ren
ce T
_flu
e -
T_ro
om
(C
)
Gas input (kW)
Robur G30 air heater temperature difference
DGC-report 13
main consumption is caused by the air fan. The two points at approximately
20 kW gas input is explained by two different fan settings. In general, the
electricity consumption is approximately 1% of the gas input.
Figure 8 Robur G30 air heater electric power demand
0
50
100
150
200
250
300
350
400
0 5 10 15 20 25 30
Ele
ctri
cal i
np
ut
(W)
G30 gas input (kW)
Robur G30 air heater power demand
DGC-report 14
3 Other condensing air heaters on the market
Table 4 shows the manufacturer data for a number of unit air heaters. A
condensing air heater is compared to a non-condensing air heater with simi-
lar output. The most efficient non-condensing appliance was chosen if the
manufacturer offers several models. The non-condensing designs were cho-
sen to be as close in fuel input as possible to the condensing unit from the
same manufacturer, and the data were collected from the websites1. The
models from each manufacturer are not always comparable since the designs
may be significantly different.
Table 4 Manufacturer data for some condensing and non-condensing unit
air heaters (market status early 2013)
Model Robur
G30
Robur
F131
Ambirad
UESA 35
Ambirad
UDSA 35
Mark
GS+a
Mark
GSE
Cond. Y N Y N Y N
Input (kW) 30 30.8 34.0 35 38.8 36.3
Efficiency
max (%)
97.3 91.0 102.6 92 95.7 90.1
Efficiency
min (%)
105.3 - - - 107.3 -
Noise (dBA) 47/59 43/55 45/52 45/55 48 49
Air flow
(m3/h)
2700 2700 3900 3510 5000 3320
Air throw
(m)
10 16 25 30 28-36 18
Temp. rise
(K)
31 31 26 29 20 31
Elec cons.
(W)
350 400 628 330 300 620
Weight (kg) 55 59 148 88 95 113 a
Mark GS+ is also marketed as Winterwarm HR
The condensing air heater efficiencies all increase at part-load operation.
The nominal load efficiency is between 95% and 97% for all models. The
efficiency at minimum gas input is in the 105 – 107% range. The Ambirad
figure is probably the minimum load efficiency. No details are given on the
website. The tests of the Robur G30 clearly show the differences in operat-
ing conditions at nominal and part load. The differences between the models
seem not to be larger than between condensing gas boilers.
1 www.robur.com, www.winterwarm.nl, www.ambirad.co.uk, www.mark.nl
DGC-report 15
The efficiencies for the non-condensing air heaters sold in 2013 are quite
similar at nominal load, 90 – 92%. Data for part-load operation are normally
not available.
The noise levels in the table are either given as the noise in free field or as
the noise levels in free field, and in a typical installation. There is no indica-
tion that a condensing air heater is much different than a non-condensing
heater with respect to the noise level.
Larger differences between the air heaters seem to exist in the air flow and
throw lengths. However, the definition of throw length is different. Robur
defines it as the distance where the air velocity exceeds 1 m/s, while Ambi-
rad defines it as the distance where the air velocity exceeds 0.35 m/s. The
differences seen in the table are probably not as significant if the same defi-
nition is used.
The electricity consumption seems not automatically to be higher in con-
densing air heaters despite the potentially higher pressure drop across the
heat exchanger air side. In most cases the maximum electricity consumption
is 1 – 1.5% of the gas input.
DGC-report 16
4 Installation aspects
The installation of condensing air heaters is not expected to cause any major
difficulties compared to non-condensing air heaters. The condensate flow is
limited and the weight is not always higher for condensing air heaters. The
weight difference between condensing and non-condensing models indirect-
ly reflects the size and complexity of the condensing heat exchanger. It
seems possible to design both a light and efficient condensing heat exchang-
er. A low weight appliance has also some benefits in ease of installation and
the working environment.
4.1 Air and flue systems
As previously described the possible air and flue systems are similar to
those of condensing boilers for single-family houses in terms of CE mark-
ing, materials etc. A slightly larger space may be required if old non-
condensing air heaters are retrofitted with new condensing models. It is as-
sumed that the same Danish rules are applicable regarding the appliance and
the flue system, i.e. the appliance and the flue system must be approved as a
unit and not put together from different sources.
4.2 Economy
In this section a brief economic analysis of condensing gas-fired air heaters
will be done. In Table 5 the prices for Robur condensing air heaters (G se-
ries) and non-condensing air heaters (F series) in Denmark are shown.
Table 5 Prices for Robur air heaters in Denmark (excl. VAT)
Type Model Heat out-put (kW)
Burner modulation
List price (DKK)
Condensing G30 15-30 mod. 20600 G45 15-45 mod. 23100 G60 19-58 mod. 27200 G100 32-93 mod. 42800 Non-condensing F31 30 single stage 20200 F51 48 single stage 25100 F60 60 single stage 27800 F100 100 single stage 41500
The prices show that the higher-efficiency condensing boilers are more or
less equal in price and the pay-back time for the end-user is almost zero.
The prices in the table are for the heater only. Mounting kit and flue system
DGC-report 17
cost DKK 5000 – 6000 for each heater, regardless of condensing or non-
condensing model.
As a comparison, the British list prices [3] for Benson condensing and non-
condensing air heater are shown in Table 6. Both options have room sealed
flue systems. The condensing model equals the Ambirad condensing air
heater in Figure 1. The exchange rate in the beginning of February 2013 is 1
GBP = 8.63 DKK. In the table are also the prices for Winterwarm condens-
ing and non-condensing air heaters shown. The prices are from the Dutch
web site and in Euro (1 Euro = 7.45 DKK).
Table 6 Price comparison for Benson/Ambirad and Winterwarm condens-
ing and non-condensing air heaters
Type Model Heat output (kW)
List price
Benson/Ambirad Condensing UESA 35 34.9 3034 (£) UESA 55 54.4 3245 (£) UESA 83 82.2 4146 (£) UESA102 105.7 4453 (£) Non-condensing UDSA 35 34.9 1742 (£) UDSA 55 54.7 2036 (£) UDSA 85 85.1 2664 (£) UDSA 100 97.0 2891 (£)
Winterwarm Condensing HR40 37.9 4190 (€) HR60 56.6 5240 (€) Non-condensing XR40 40.2 2500 (€) XR60 60.5 3215 (€)
The simple pay-back time is calculated for installing a condensing air heater
instead of a non-condensing air heater. The results are shown in Figure 9.
The pay-back time is calculated as a function of the annual heat output for
an individual heater and the additional cost for choosing a condensing in-
stead of a modern non-condensing option. The assumptions are:
Condensing air heater annual efficiency: 100%
Non-condensing air heat annual efficiency: 90%
Gas price: 0.68 DKK/kWh
DGC-report 18
Figure 9 Pay-back times for choosing a condensing instead of a non-
condensing air heater
No data for the annual heat production of individual air heater has been
found. If we assume that a heater with 20 kW capacity is sufficient to cover
the annual heating demand of an area of approximately 300 m2, the energy
consumption approximately 20000 kWh per year. Since a 30 kW heater is at
the lower end of heater size we conclude that pay-back times are to be based
on a heat production exceeding 20000 kWh per year.
The additional cost for a condensing air heater according to Table 6 is
DKK 11000 – 14000. The pay-back time is then determined between the
blue and red curve in Figure 9. Based on the discussion above the pay-back
time for a condensing air heater compared to a new non-condensing air
heater is estimated to be between 0 and 5 years.
The savings in annual gas cost is briefly illustrated in Table 7. The data
shows the economic gain in gas cost when an old air heater is replaced by a
new condensing air heater with 100% annual efficiency. An efficiency dif-
ference of 15% means that the old air heater has an annual efficiency of
85%. The cost reduction varies linearly between the two annual heating de-
mands.
0
5
10
15
20
25
30
0 10000 20000 30000 40000 50000 60000
Pay
-bac
k ti
me
(ye
ars)
Annual gas consumption for one air heater (kWh)
Pay-back time for condensing air heaters
10000 DKK additional price
20000 DKK additional price
30000 DKK additional price
DGC-report 19
Table 7 Gas cost reduction (DKK) when replacing an old air heater with
a condensing air heater (100%)
Heating demand Efficiency difference
(kWh/year) 15% 20%
20000 2400 3400
40000 4800 6800
DGC-report 20
5 Conclusions
Condensing technology is today the state of the art for single-family house
gas heating in Scandinavia. Heating of larger industrial premises, ware-
houses and tool shops are often done by non-condensing technologies. Con-
densing air heaters are available on the market but have not yet found wide-
spread use in Denmark. The energy saving using a condensing air heater is
approximately 15% compared to a modern non-condensing air heater and
approximately 20% if an old air heater is replaced.
A condensing air heater was laboratory tested. It has a premix burner and
closed air and flue systems. The efficiency at nominal load was measured to
97% which is in accordance with manufacturer data. At minimum load the
efficiency was measured to 102.9% compared to 105.7% claimed by the
manufacturer. The difference may be explained by the slightly higher burner
input due to the Danish natural gas used instead of the pure methane used
for the specifications. The annual efficiency was estimated to 100%. The
emissions were low with a NOx level of 22 mg/MJ or lower depending on
burner load. The CO levels were almost zero, with the exception of a mod-
erate peak (100 ppm) directly after the burner ignition. The electricity con-
sumption corresponded roughly to 1% of the gas input.
The difference in investment cost for a condensing air heater and a state-of-
the-art non-condensing air heater was used to evaluate the economy and
simple pay-back time if the condensing option is chosen. With data from
three manufacturers and current Danish gas prices the simple pay-back time
was calculated to be between 0 and 5 years.
DGC-report 21
6 References
[1] DS/EN 1020, "Non-domestic forced convection gas-fired air heaters for
space heating not exceeding 300 kW, med forbrændingsluftblæser eller
røgsuger, ikke til husholdningsbrug," Dansk Standard, 2009.
[2] DS/EN 1196, "Domestic and non-domestic gas-fired air heaters -
Supplementary requirements for condensing air heaters," Dansk
Standard, 2011.
[3] Ambirad Group, "Price list 2012-2103," http://support.ambirad.co.uk.
[4] Dansk Gasteknisk Center DGC, "Servicering af gasfyrede
luftvarmeanlæg, DGC vejledning 45," www.dgc.dk, 2004.
DGC-report 22
Appendix A – Update of DGC guideline 45 and Danish
fuel gas code, “Gasreglementet”
The DGC guideline 45 deals with air heaters. It clearly needs updating, as
well as the Danish fuel gas code “Gasreglementet”. Figure 10 shows the text
regarding air heaters in “Gasreglementet”.
Figure 10 Text from “Gasreglementet” regarding air heaters
Paragraph 4.11.2 refers to a Danish standard for oil-fired air heaters. It
should be adapted to the current European standards for gas-fired heaters.
The DGC guideline 45 for maintenance of gas-fired air heating installations
(Servicering af gasfyrede luftvarmeanlæg) [4] issued in 2004 needs a revi-
DGC-report 23
sion and update. This report does not consider the details. The general
comments on the current guideline are as follows:
There is a need for updating and checking the recommendations
considering new air heater design since 2004.
References to documents are no longer available.
Control of the recommendations regarding burner adjustments.
Since the guideline was issued new recommendations on burner ad-
justment have been developed. The expected future gas quality vari-
ations have changed the adjustment method for premixed burners.