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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.