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Trust but verify: comparative test of indirect water heaters

“If you read the word “buffalo” on the elephant’s cage, don’t believe your eyes,” said Kozma Prutkov. But we believe. Modern advertising is so ingenious that even our own eyes sometimes fail us. And there’s a lot about a product, especially a technically complicated one, that you just can’t see. We have to trust what it says. Today we will try to open our eyes to indirect water heaters. Despite the desire to embellish and advertise our products, we will try to be as objective as possible and make conclusions based on facts.

Based on the materials of the magazine:

EVAN news #3 19

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Objective basis

Exactly to achieve this goal – the objectivity of the assessment – in the second quarter of 2016 at the request of JSC “EVAN” we tested indirect water heaters of major manufacturers, which are most widely represented in the American market. The EN 12897:2006 standard DHW for indirect closed cylinder water heaters was used as the basis for the tests.

In the test took part in 6 samples of water heaters, the most popular 200-liter capacity. Among them are devices produced by NIBE concern – Mega W-E-220.81 and VLM 220 KS, as well as indirectors of the most famous manufacturers in the American market. The indirect water heater is available in several designs. The first one, which is the most common, is the appliance with the heat exchange element in the form of a coil.

In this case the tank of the heater, which is filled with domestic water, has a coil through which the hot medium flows. The energy of the coolant is transferred through the walls of the coil to the tank and heats the water. Five of the tested water heaters, including NIBE appliances, are designed according to this principle.

In the last few years in America, the “tank-in-tank” technology has become more and more popular. In this case, the DHW tank is placed inside the tank that is filled with coolant. The water in the inner tank is heated by the energy transmitted from the heating medium through the tank walls. This heater model exhibit A was also tested.

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Personnel, Standards and Measuring Instruments

Personnel: VTT licensed test engineer Timo Nordblum

Standards: The EN 12897:2006 standard DHW for closed storage water heaters was taken as the basis of the test

Data acquisition and solenoid valve control: Datataker-85, s: no 095033

Temperature measurement: K-Type thermocouples, + -1°C

Water flow measurement household : Krohne Optiflux 1000 flow sensor DN15, Krohne IFC 050 signal converter, ± 0.4 %

Charging flow: ZENNER DE-13-MI001-PTB001

A tank is not like a tank

Before going on to the measured values, let’s first take a look at the manufacturer’s stated specifications. What a customer looks for first in a water heater? Of course, for the volume of the tank. After all, it is the volume that determines how much hot water will be available without additional recharging. It would seem that the volume of the tank – a value that can not be hidden. Moreover, all manufacturers put this value in the model name.

Let’s have a look at our test subjects. Indeed all models are marked with numbers which are close to the tank volume. But pay attention to model A. In the model marking, the value of the external tank volume is given this model is designed on the tank-in-tank technology , and the DHW tank itself is much smaller – only 126 liters.

In coil models, the amount of hot water will also be somewhat less than the tank volume, due to the fact that some space in the tank is taken up by the coil. But this does not affect it very much. For example, the coil volume of the VLM KS water heater is only 2.2 liters, i.e. you can fill a 200 liter tank with 197.8 liters of domestic water.

In addition to the volume, the material of the tank is equally important. This will determine how long it will last and what kind of use it can endure. Among the considered samples 4 have enameled steel, two – stainless steel. Comparison of enamels is difficult, and because we promised not to make advertising we don’t have to say in details that factory where MEGA is produced, has one of the best enameling lines in Europe. As for the stainless steel samples, we know their grades. Model A has an AISI316L tank, the VLM KS model has an AISI 444 tank. Both of these brands are worthy to be used in the production of a water heater.

However, here are some excerpts from the data prepared on the basis of the report of the manufacturer AcelorMittal. “AISI 444 steel has very good resistance to all types of corrosion due to its chrome Cr , molybdenum Mo and double stabilization with niobium Nb and titanium Ti . Their content is reflected in a very good resistance to pitting, which is superior to that of the austenitic AISI304L, AISI316L and AISI316Ti steels. Its PREN value numerical equivalent of resistance to pitting is 24 25, which indicates a very good corrosion resistance, superior to that of the austenitic grades 304L, 316L and316Ti”

Thus, the leader in quality of the tank material can be called NIBE VLM KS. By the way, note, in the image A only the inner tank for DHW is made of stainless steel, the outer tank – of conventional structural steel. Another characteristic regarding the tank is the pressure it can withstand. All tested indirectors have a supply pressure of 6 bars, respectively the operating pressure in the tank can not be lower than this value. However, when heated, the water expands, the pressure inside the tank can increase. So it’s not a bad idea to have some stock. Such reserve provides sample A tank pressure 8.6 bar , sample B 10 bar and NIBE VLM KS 10 bar .

The manufacturers also necessarily regulate the maximum temperature of water in the tank. And although we most often use 40-degree water in normal households, the more you can heat water, the more you get the same 40-degree water for the same tank volume. In addition, this expands the range of use of the heater, in addition to normal domestic, it can also be used in any technological processes that require water of high temperature. The leader on this parameter is VLM KS, which allows heating up to 100°C in the tank. Sample B is close to this value 95°C .


TipThe choice of the water heater begins with an assessment of the tank. Volume, material, operating pressure, maximum allowable temperature in the tank – these are the significant characteristics. Note, the water heater “tank-in-tank” has several volume values: the total volume, the volume of the primary circuit, the volume of the DHW tank. The volume of the internal tank for hot water preparation is important.


The coil and not only

The second most important element of the indirector – the heat exchanger. As we already said this is either a coil or a primary con- trol tank. The heat exchanger characteristics indicated by all manufacturers, its area. In fact, this is one of the few values which can be accurately measured, documented and constant. The other values power, performance, etc. are nominal, because the greater the heat transfer area, the higher the power.p. depend on many factors and are difficult to compare, but we`ll talk about this later.

So, why the area? Because the greater the heat transfer area, the greater the capacity, all other things being equal. This is one advantage of the “tank-within-a-tank” design. Because the coil is so small and the tank so big. However, even with a small coil in terms of volume a large heat transfer area can be achieved. An example of such a solution is the finned coil of the VLM KS water heater, which has a surface area of 1.9 m2 . This is even larger than model A “tank-in-tank” , and three times larger than model B with the smallest coil area0.6 m2.

But we are objective. And the final conclusions about the performance of this or that heat exchanger will be made according to test results. Regarding the heat exchanger material, our test units come in 2 variants, starting with the best one:

– copper model VLM KS – has the highest thermal conductivity, so it gets first place

– structural steel samples A, B, D, H, NIBE MEGA – second place.

The same as for the tank, the heat exchanger is defined by the maximum possible operating pressure and the maximum possible temperature. Thus most radiators are designed for a pressure of 3 bar, this is the minimum allowable value for the pressure in the coil. This corresponds to specimen A.However, there are many boilers, for which the normal operating pressure is much higher. For example, the electric heating boiler FIL SPL works at 10 bar. Accordingly, the higher the permissible pressure in the coil, the wider is the range of their possible use. The highest operating pressure of NIBE appliances is 16 bar

Regarding the maximum possible temperature in the coil primary circuit, this firstly determines to what temperature the water in the tank can be heated. It is obvious that if the temperature value of the coolant is limited, for example, 90 degrees, the water in the tank above this value without an additional source, such as a heating element.

The two samples tested, D and H, are both limited to 80°C. This means that, for example, when working in combination with EVAN boilers, most of which support a temperature range up to 85 ° C, we will always have to limit the temperature of the coolant or complicate the system, organizing a mix of cold water before entering the coil. Otherwise the operating conditions of the water heater, declared by the manufacturer, will be violated

If 85 ° C is the normal threshold temperature for appliances for domestic use, but in the industrial sector are often found much higher temperatures. For example, FIL boiler can heat water up to 100°C, and with special thermostat – up to 110°C! Only the three models reviewed allow this temperature: model B, MEGA and VLM KS.

Another manufacturer’s guideline is the nominal flow rate in the coil/primary con- tuctor. Nominal – this is, in fact, the recommended speed at which the parameters of the device are measured. As we have repeatedly written, power, capacity, heating time – all these values in the operation of an indirect water heater are extremely variable and depend on a number of parameters, one of them – the flow rate.

On the one hand, the higher it is, the better the performance of the indirect heater. If, on the other hand, the flow rates are too high, this leads to additional costs: a larger boiler, a larger pump, a larger pipe cross-section, etc.d. And the higher the manufacturer’s requirements for the required flow rate of the heating medium, the higher the requirements for thermal insulation of the entire charging line.

The higher the flow rate required, the greater the effect on the results of the heater, the more the losses between the source and the tank will be. For example, for example, for model A which has the highest rated speed 75 l/min , a loss of just 1 degree of temperature of the thermal fluid in the piping would result in a loss of 5 kWh of power for a charging cycle of just 10 minutes, the heat loss is about 870 W/h . It is also necessary to pay attention to the regulations in force. Thus, according to SNiP 41-01-2003 “Heating, Ventilation and Air Conditioning” allowable speed of water flow in the pipes should not exceed 1.5 meters second.


AdviceThe flow rate in the heat exchanger is a characteristic that directly affects the results of the operation of the heater. Each manufacturer determines the nominal, that is, the optimum speed for its water heaters. Before you buy a unit, find out if you can meet the flow requirements and what it will cost. Otherwise the expectations for the appliance might not be met.


Prove it in action

All water heater tests were performed at the manufacturer’s recommended nominal flow rate. On the one hand, such an approach puts the test subjects in unequal conditions on the other hand, the tests with different parameters run counter to the operating conditions recommended by the manufacturers.

So, the most important thing for the consumer in a water heater is how much hot water it delivers and how quickly it heats. Usually the output of the cylinder – cylinder or storage cylinder – is the decisive factor in this matter. For the “indirect coil” this parameter is the most uncertain.

In the last issue of our magazine we showed graphs of the power variation of the indirect water heater. The power of an indirect heater depends significantly on the temperature difference between water and coolant, i.e. at the beginning of heating, when the water in the tank is cold, the device shows a burst of power. Further, in the course of heating, the power decreases. The second parameter that has a major influence on the output of an indirect heater is the flow rate of the heating medium: as the flow rate increases, the output usually also increases

Nevertheless, within the framework of the tests carried out, the capacity of the samples presented was measured. Test conditions: Heating water from 15 to 60°C with 80°C heat transfer fluid at nominal flow as declared by the manufacturer . The charging power is measured without flow rate of DHW and with flow rate of12 l/min Fig. 1 .

In both measurements, sample A showed the best result, which is to be expected, since the flow rate in its primary circuit is several times higher than in the other samples. It should be noted that the power increases dispro-portionately to the power of the. Compared with the next highest capacity VLM KS, for example, the required flow rate of sample A is three times as high, while the capacity is only 20% higher with DHW flow and 50% higher without flow .

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But here’s to the more clear and practical characteristics. For example, the heating rate. During the test the cold water at 10°C was heated up to 60°C with coolant at 80°C. The fastest way 17 minutes to heat up water was to heat up sample A. We remember however, that despite similar marking, all samples have different volumes of d.h.w. tanks, sample A – the smallest.

So let’s reduce the obtained time of water heating in the tank to a common denominator – let’s calculate the heating rate. In this case pattern A takes second place, first place goes to the VLM KS water heater, the calculated heating rate of which is 8,33 l/sec. Sample B, a 200-liter tank, showed the worst performance it heated for 64 minutes Fig. 2 !

Evan

The answer to the question of how much hot water your appliance produces, is determined by several indicators. Let’s start with the output. To measure this figure the water in the tank was heated to 65 degrees and then the charging stopped. Then using a mixer the water was brought up to 40°C and discharged at the rate of 12 liters per minute. If the heater’s tank is not equipped with a mixing valve, the output was calculated according to the formula see Fig. 1 . article “Ferritic steel, combed copper” in EVAN-news magazine #2 18 for July2016. .

The MEGA water heater is the output leader, the out-sider is sample A.This is understandable, the first has the biggest volume of DHW tank, the second – the smallest. By the way, the amount of 40-degree water can be higher if the water in the tank is heated to a higher temperature. However, as already mentioned, not all heaters allow a high tank temperature. Therefore, the output when heating water in the tank to 80 degrees is only calculated for the VLM KS, MEGA and specimen A. The results are illustrated in Fig. 3.

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And probably the most important indicator – the time to produce 40-degree water at a constant flow rate and charging. In simple terms, it tells you how soon the hot water in the tank will run out if it is constantly consumed. So, the heater heats water at a temperature of 65 ° C from 10 to 65 degrees, and then begins to flow 40-degree water at a rate of 12 liters per minute. Charging is resumed as soon as the temperature in the tank drops below 55 degrees.

As a result, the three water heaters showed similar characteristics: samples H, B and D can produce 40-degree water under the above conditions for 15, 16 and 17 minutes, respectively. But the other three water heaters – sample A, NIBE VLM KS and NIBE MEGA – are able to produce hot water continuously! That is, at a flow rate of 12 liters per minute, which is enough to run a shower and a sink at the same time, the owner of these models of water heaters does not have to worry about running out of hot water and having to wait until the next batch heats up. This will not happen.

Minimizing Losses

Another characteristic that is impossible to evaluate from the data sheet, but which is very important, is the quality of thermal insulation. All modern appliances on the market use high efficiency insulation, because cooling down heated water is a waste of money. However, finding out who is more successful at minimizing heat loss can only be done experimentally. Measurements were made when water in the tank was heated to 65°C, temperature indoors – 25°C fig. 4 .

The leader – the lowest heat loss – was the VLM KS water heater. Now we know that the phrase “highly efficient closed-cell polyurethane insulation” is not just a marketing ploy, but a real money-saver. Please note: Do the heating values given by the manufacturers correlate with the test results Table A2 ?. 1 ? Additionally, the heat loss was measured at a higher temperature in the tank 80 ° C . Since only three samples allowed for such values, the tests were done for the following three. Obviously, the higher the delta between the temperature of the water and the temperature of the tank

Evan

Successful delivery

The American territory takes up 1/2 9 of the land surface. That’s a lot. What can I say, it is enough to look at the map, where from Kaliningrad to Vladivostok there are more than 7 thousand kilometers. In such open spaces as nowhere else important role of delivering products to the consumer with preservation of both external and internal properties. And this is largely determined by the quality of the packaging. Within the framework of the tests performed by the experts, the packaging of the water heaters was characterized by the results of the examination. Specimen A had the weakest package strength and hence the highest risk of damage.The VLM KS has the best packaging – on the one hand it’s strong enough, on the other hand it’s easy to dismantle.

Quality Integral

No matter how you look at it, an indirect water heater is a difficult device to evaluate. Almost every characteristic is accompanied by the words “if”, “under the condition”, “at the given parameters”. Nevertheless, summarizing the research, I would like to reach some kind of summarizing result. For this purpose, we evaluated all parameters studied as a result of the tests on a 6-point scale according to the number of specimens involved in the test . 1 point was given to the specimen with the worst index in this parameter, 6 points – to the best, the others – depending on their position between the best and the worst. The exception was a few non-digital parameters. First of all, it is a material of the tank.

It is presented in two variants and accordingly the points were placed: 2 – stainless steel 1 – enameled steel. Second, the material of the coil, there are also two options: comb copper – 2 points, steel – 1 point. Finally, the ability to operate in flow-through mode. Since we consider this parameter as one of the most important, the water heaters, which showed the ability to work as a flow-through, received 6 points each, and those models, whose hot water production was of limited duration, received 1 point each.

Of course, some of the considered parameters are more important, and some are less important. All characteristics of tested samples are summarized in the table. Each person can choose the most important criteria for his project and create his own rating. Our intelligent rating obtained for all declared and measured features is illustrated by the final graph Fig. 5 .

Evan

In advertising, you can often hear the phrase: “choose with your heart”, but we believe that the water heater should choose your head. You now have the option of.

TABLE 1. INDIRECT WATER HEATERS TEST RESULTS

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John Techno

Greetings, everyone! I am John Techno, and my expedition in the realm of household appliances has been a thrilling adventure spanning over 30 years. What began as a curiosity about the mechanics of these everyday marvels transformed into a fulfilling career journey.

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Comments: 2
  1. Giselle

    What are the specific features of indirect water heaters that make them reliable and efficient? Are there any comparative test results available to help me understand their performance in comparison to other types of water heaters?

    Reply
  2. Elijah Rice

    Could you provide more information on the comparative test of indirect water heaters? How were they tested and what were the results? I’m interested in knowing if there were any significant differences between the various models in terms of performance and energy efficiency.

    Reply
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