Exhaust gas condenser, return flow boost and buffer efficiency

The following table shows the different fuels and heat generator efficiencies:

At first glance, it is surprising that the heating oil CHP unit has an efficiency rating that is just below that of a natural gas condensing boiler and the fact that a wood condensing boiler beats a heating oil condensing boiler by one percentage point in terms of efficiency.

In our view, there are many reasons to believe that this technology will continue to grow. So be there from the start and gain experience where others are still turning a blind eye!

Condensing boiler technology still means falling below the dew point

However, the art of condensing boiler technology still means falling below the dew point, because only then does the water vapour contained in the exhaust gas condense, and there is no way around this physical fact. On the one hand, this means that your heat distribution must be operated at the lowest possible return temperatures. On the other hand, it also means that, for reasons of buffer efficiency, you must be very careful with the limited supply of cold water when loading, because once it is exhausted, the buffer can no longer be loaded. The cold water supply of a buffer is as valuable when loading as its hot water supply when unloading! The cold water supply of a buffer tank is as valuable during charging as its hot water supply is during discharging!

Condensing boiler efficiency versus buffer tank efficiency

Renewable heat generators, such as wood boilers or CHP units, usually require a regulated minimum return temperature. This is often around 60°C and is achieved by adding flow water to the cold return from the lower buffer connection.

This so-called return flow increase (RLA) can be used with a rendeMIX 2×3 for particularly efficient two-zone charging of a buffer storage tank.

With two-zone charging, the upper part of the buffer is first heated as quickly as possible by drawing warm water from the middle connection of the buffer. In two-zone charging, the upper part of the buffer is first heated as quickly as possible by drawing warm water from the middle connection of the buffer.

Single-zone charging causes a large amount of boiler flow water to enter the boiler return and only a small amount to enter the buffer.

Although the temperature at the lower buffer connection and thus the proportion of boiler flow water entering the buffer increases as the buffer is loaded, the rise in temperature at the lower buffer connection usually indicates that the buffer’s heat absorption capacity is about to reach its limit, as loading must be stopped when the temperature at the lower buffer connection exceeds the required return temperature to the boiler. be stopped when the temperature at the lower buffer connection exceeds the required return temperature to the boiler.

Due to the two-zone loading of the rendeMIX 2×3, less boiler flow water enters the boiler return and more enters the buffer. Only shortly before the end of the single-zone loading do rising temperatures at the lower buffer connection lead to a higher loading speed.

Only shortly before the end of single-zone loading do rising temperatures at the lower buffer connection lead to a higher loading speed.

As soon as the buffer water temperature at the middle connection is above the required return temperature, the two-zone loading of the rendeMIX 2×3 applies to the cold water at the lower connection.

The two-zone loading of the rendeMIX 2×3 ensures more economical use of the scarce resource “cold water” ensures significantly more convenient and efficient loading of the buffer.

With the same power input, the buffer heats up faster at the top and stays cold longer at the bottom. This means that more convenience (faster hot water) is not at odds with greater efficiency (longer cold water).

Comparisons between the two systems have shown that the time between heating a wood boiler and the availability of hot water can be halved while doubling the heat absorption capacity of the buffer!

How does the rendeMIX 3×3 Condenser work and why does it need the VTB valve?

The two-zone loading of the rendeMIX 2×3 with integrated return flow elevation (RLA) loads the buffer tank so efficiently because it heats the warm water from the middle before the cold water from the lower part of the buffer. However, if , if an exhaust gas condensing heat exchanger (condenser) is also operated on the wood boiler or CHP unit, it must, of course, be supplied with the coldest water from the lowest buffer connection in order to achieve the greatest benefit – in other words, to fall below the dew point as much as possible. The thermostatic flow temperature limiter (VTB valve) with adjustable minimum temperature takes on the task of not allowing more cold water to be drawn from the buffer than is necessary to achieve the desired flow temperature.

The thermostatic flow temperature limiter (VTB valve) with adjustable minimum temperature takes on the task of ensuring that no more cold water is drawn from the buffer than is needed to achieve the full calorific value effect. It therefore optimises system efficiency by drawing as much as necessary, but as little as possible, from the buffer. To do this, it monitors the temperature of the heating water leaving the condenser and controls the amount of water accordingly: if too much cold water flows through the condenser, the system automatically reduces the flow rate. as possible. To do this, it monitors the temperature of the heating water leaving the condenser and controls the water volume accordingly: if too much cold water flows through the condenser, the heating water exits the condenser well below the dew point temperature of the exhaust gas (natural gas ~56°C, wood: ~49°C, heating oil ~47°C) from the condenser – the water volume is reduced; however, if too little cold water flows through the condenser, it exits well above the dew point temperature – the water volume is increased. A too small water volume reduces the combustion efficiency, as the exhaust gas does not condense over the entire surface of the condenser. Too little water reduces the combustion value yield, as the exhaust gas does not condense over the entire surface of the condenser. Too much water, on the other hand, is not economical enough with the scarce resource of “cold water”, because the condenser hardly increases the combustion value yield.

However, the rendeMIX 3×3 condenser does not require an additional pump to change the amount of water flowing through the flue gas condensing heat exchanger in the manner described: the necessary differential pressure is simply generated by an overflow valve at the outlet of the assembly to the return flow of the wood boiler or CHP unit. This means that, as before, the entire boiler circuit only requires one circulation pump. This can either be flanged to the rendeMIX 3×3 condenser or installed in the return flow of the wood boiler or CHP unit. This means that the entire boiler circuit only requires one circulation pump, as before. This can either be flanged to the rendeMIX 3×3 Condenser or integrated into the heat generator. The only prerequisite for its use is that the pipe between the condenser’s flow and the heat generator’s return is connected to the rendeMIX 3×3 Condenser via an additional connection (red pipe with T-piece to connection 5 of the rendeMIX 3×3 Condenser). The servomotor of the rendeMIX 3×3 Condenser is connected to the heat generator’s flow (red pipe with T-piece to connection 5 of the rendeMIX 3×3 Condenser). The servomotor of the rendeMIX 3×3 Condenser is connected to the heat generator’s flow (red pipe with T-piece to connection 5 of the rendeMIX 3×3 Condenser). The servomotor of the rendeMIX 3×3 Condenser is connected to the heat generator’s flow (red pipe with T-piece to connection 5 of the rendeMIX 3×3 Condenser). The servomotor of the rendeMIX 3×3 Condenser is connected to the heat generator’s flow (red pipe with T-piece to connection 5 of the rendeMIX 3×3 rendeMIX 3×3 Condenser (red pipe with T-piece to connection 5 of the rendeMIX 3×3 Condenser).

The servomotor of the rendeMIX 3×3 Condenser is controlled via the 230V~ three-point signal of an external controller , which keeps the return temperature to the heat generator constant. Alternatively, the rendeMIX 3×3 Condenser FWR has an integrated fixed-value controller with adjustable setpoint temperature and 230V~ mains connection.

Only with the rendeMIX 3×3 Condenser can you get the most out of your exhaust gas condenser and your buffer storage tank. Since stratification has a significant influence on the efficiency of the buffer and this simply cannot be generated or stabilised or prevented from being destroyed by loading alone , it is obvious that the best result is achieved by simultaneously using rendeMIX charging and discharging, as we already explained in our rendePOST 4 under “Why do you actually need both: two-zone charging and two-zone discharging?”.