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Published: 29/11/2008
In the previous two issues, we dealt with heat pumps. The main conclusion was that with today's tariffs, even if the cost of oil is reduced, their use is beneficial both economically and ecologically. Of course, I had given importance to the air-air units, that is, to the so widespread split units due to their very wide distribution and the ease of installation in existing constructions.
But if you pay attention to the product presentations of various companies, you will see that most of them have and advertise hot water production units. With a basic difference from common boilers. They produce hot water at a low temperature of 40-50°C. This fact has two main effects. If we want to heat our house with common bodies (ACAN, PANELS) we will have to install very large bodies since their power decreases dramatically with the decrease in the temperature of its supply water. The second is that in line with the heat pump, it is easy to use other sources to reduce energy consumption and mainly solar panels. In the summer we have sun for 16 to 18 hours. Those of us with solar water heaters see the water in the water heater even reach 100°C. In winter, however, with the sun lower and the sun only 8 or even 6 hours, on days without clouds, we hardly get above 50°C.
So it is not possible to use this source to help central heating where the water starts from the boiler at least 80°C and returns to 60°C. However, if we study our heating so that it works at low temperatures, i.e. to have a supply at 45°C - 50°C and a return below 40°C, we can first lead this return to a solar water heater and then to the pump heat. But even 1 or 2 points to gain with the existing automation will reduce the consumption of the pump accordingly.
If the day happens to be rainy and the solar panel does not collect any energy, we do not lose anything. Our modern insulations ensure this. So the problem lies in how to heat our house with low-temperature water.
This problem has been solved for at least 30 years. After all, then the "low temperature" boilers were released. A somewhat misunderstood term in Greece. A low-temperature boiler is one that, if it works with water at 40-50°C, does not show liquefaction on the side of the flue gas that reduces its life span due to rusting. What are the solutions? It is the underfloor heating and the fan coil units.
I installed underfloor heating for the first time thirty years ago and have been a fanatical fan of it ever since. It ensures great comfort with losses reduced by more than 20%. There are even cases such as churches, museums, shops, industries, train or bus stations, etc. where this reduction can reach 90%. Disadvantage; It takes a long time to heat a cold building, so its ideal operation is on a 24-hour basis.
Fan coil units are heating elements. I have written several times that they consist of copper pipes with (usually) aluminum fins. Why; But water can transfer more than 50 times more heat to the copper than the metal surface to the air? With the fins, we greatly increase the heat transfer surface from the pipe to the air, since we can very easily get it from the water circulating inside the pipe. The high air velocity made possible by the FCU fan further increases heat transfer.
In this way, in limited dimensions, lightweight, etc. we get so much heat that with common bodies we would need many tetraton meters of wall to install them. We also have the luxury of working with low-temperature water. With inlet - outlet temperature 50/40 in a common body, that is for D.T. - (50+40)/2-20 = 25K the power in kW of a body is maybe 25% of the power shown in the catalogs and valid for ΔT = 60K.
Are there any downsides to FCUs? They have problems not disadvantages, which today's technology successfully combats.
First of all, let me emphasize that "radio arbyla" claims that FCUs dry out the atmosphere in winter. Lie. They do not dry it more than common bodies.
Second complaint. I get it, especially in the summer. Possibly if you are sitting in the air current generated by the FCU. Adjust the FCU's air-directional fins so that they are not directed toward your usual sitting position.
Third complaint. I'm not breathing so well. Perhaps this is because FCUs with their high velocities make the existing dust more suspended than ordinary bodies. On the other hand, FCUs have filters that clean the air as long as we wash them often. Of course, the proverb that says that the hens eat the cheap bran also applies here. Good quality FCUs have very good filters and sometimes a dust ionization system to completely retain the dust in the filters and create a Parnitha atmosphere in our room before the fire.
Fourth complaint. The fan noise bothers me. It is true that this noise is multiplied if the FCU is running at high speed (each FCU has three fans) or medium rather than low. Nothing can be heard at low volume. If we correctly select an FCU based on its performance at medium speed, then we will very rarely and for a short period of time run it at high speed. On the contrary, it will work on low for too many hours.
One last fact that needs attention is the circulation of water. FCUs have a high resistance to the passage of water that multiplies strongly the higher the power of the FCU. Therefore, the design of the water network must be done by an experienced mechanical engineer who also foresees regulating (throttling) valves in the small power FCUs or any common bodies in order to maintain a sufficient supply-return pressure difference to circulate the required water in the large FCUs. The usual in Greece casual treatment of the network is not allowed.
FCUs exist in various types: Floor, wall (like the indoor units of splits), for installation in false ceilings, or even in air ducts. The last two types are of course not visible in a room.
Automation has progressed so much that the use of FCUs results in a perfect environment in winter and summer, as long as their installation has been studied by experienced engineers who are willing to ask a fully qualified colleague.
The greatest philosopher Socrates said "One thing I know is that I know nothing" and "I always grow old learning something". So it is not a shame if a colleague of mine asks for the advice of someone else who works in an air conditioner dealership and deals every day with similar problems, and has daily contact with the manufacturer, who provides him with all possible help.
I personally listen not only to my colleagues but also to ordinary workers from the words of even unskilled workers I have learned a lot.
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