Author: Mónika Vértesy
2011-02-07
We designed a passive house, where, for the first time, we put a so-called compact device functioning with a ground heat-air pump. Ever since, we realized that better solutions are avaiable, now we are presenting these. Have you ever tought that a wood combusting fireplace or a condensational boiler can be installed into a passive house?
Because of a misinterpreted definition, it is believed by many that heating is not even necessary in a passive house. This way, it is not true. What the word is about: due to the great thermal insulation, the heat demand of the building is reduced significantly, compared to the category „C” of the Hungarian decree (7/2006 TNM), say, to its one-fourth. The specifity of passive and low energy consumer buildings is the rise of heat loss becouse of ventillation regarding its scale, as the habitants still require fresh air. However it is easy to forget about ventillation and this could cause serious problems in the case of a well-insulated airtight teguement. (misting, mold) The constant resupply of fresh air can independently justify the spreading of artificial ventillation. The other argument siding with controlled ventillation is the opportunity to reduce ventillation-caused heat loss by installing a device with a heat revocery unit, which reduces the the otherwise not too great heat energy demand to its half.
The truth is, heating is necessary, but only as much as the after-heating of fresh air required after using the device with a heat recovery unit.
However, it is not that easy to cover a small energy demand, becouse we are settled for this. The market lacks low-performance appliances, moreover, given their novelty and the low demand they are still more expensive than their greater-performance versions. (We have found some since this article, read more about it in the continuation of the article! 26th of February, 2012) Other problem is the many false information and half-truths published in the internet. In the lines below we embark on presenting the results trough a multi-part article in a simplified manner. The results which enriched the list of trainings owned by é z s é Ltd. by a degree of facility energetic engineering within the training of the Mihály Pollack Polytechnic of Pécs.
The passive house was already presented from a structural point of view, now it’s time to see its energy balance!
1., Heating with a compact device in a passive house
The compact device includes a ground-heat – air heat pump and also a ventillation device equipped with a heat recovery unit. The heat source is the pre-heated/pre-cooled air in the ground collectors, whereof the ventillation appliance only utilize a certain amount of heat, so that the heat pump can gain heat for its proper function.
Well done, but is it logical to use a ventillation device with a heat recovery unit with slightly worse efficiency just to treat forth the unutilized heat with an electrical energy demand compressor? This is quite an irrational train of tought.
Usually it is quite hard to heat up a building just by air heating. One of the main reasons: the air can not be heated over 52 °C because of the danger of dust burn. An other obstacle in the way of air heating could be the limit of air exchange, as it can not be raised too much becouse of heat energy demands. Neither the too dry air nor the draft is tolerated well by us. To top all these reasons, a heating medium with a higher temperature is required compared to surface heat dissipation (like ground heating). This equals lower COP, or grade of wellness.
The great advantage of air heating compared to surface mounted radiant heating is that the ventillation, heating and cooling functions in the same tube system, so there is no need to consturct separate tube systems.
In case of applying a heat pump, it does not worth to design a system with radiators, with this mostly convective heat dissipation a high flow temperature would be required, which would equal a big temperature step and in consequence, a bad efficiency.
So we are staying with the approved and pleasant surface heating, which has only one disadvantage, that this way not only the ventillation ducts have to be constructed, but also the the ducts ciruclating hot water in the slab.
Trough our calculations, we found a stunning result that a compact device is only capable of covering 45% of the heat demands. To cover the rest we recommend infra panels.
2., Heating with wood combusting fireplaces
The use of compact devices has several flaws. The first problem I faced was the impossibility to heat up the whole building with air heating. This canbe changed if:
The last two tasks can be solved by an enthalpy plate exchanger. The novelty of this technology lies in utilizing even the moisture content of the exhaust air and returning it back to the airspace. (the principle of function is quite similar to the process in condensational boilers, where we regain the steam content of the flue gas) With this creative solution we are able to soften two problems at the same time. The efficiency is raised significantly and the amount of air exchange is not limited by the danger of dry air, as the moisture is retourned to the living space.
The German-manufactured Paul Focus 200 device has a thermal efficiency rate of 91%, the same time its electricity demand is also favorable.(0,31 Wh/m3) This appliance is also testified by PHI as a passive house component. The Paul Focus 200 ventillation device with a heat recovery unit has a net list price of 2800€ with dust and pollen filters, counting with 295 HUF of exchange rate it equals 826000 HUF.
The calculation justify the greater efficiency of the heat recovery device, but air heating in the whole building can not be solved even by this one. However, a good solution could be a fireplace, which dissipates its heat partly by direct radiation, and partly in the ventillation system.
A fireplace only can be operated simultaneously with a ventillation system if its flue gas drainage is solved in an independent way from the airspace of the house. Such a quality fireplace known by me is the Taranis model of the Supra firm, which has a nominal performance of only 5 kW, the reason why it is specifically recommended for passive houses. The price of the firebox is 550000 HUF (1870€). The fireplace has four heat dissipation tubes above. These can be connected to the ventillation system and this way the air heating and the ventillation is solved simultaneously, there is no need to install a separate heat dissipation system. To do this, ventillation devices with an enthalpy plate exchanger are required, this is the only way to solve the air heating of the building. With the nominal performance of 5 kW, the efficiency is 85%. Regarding that the boiler will only function with a performance lower than its nominal, I assume 75% efficiency. For domestic hot water, only fireplaces with a water storage unit can be used, however, the complete separation from the ventillation system is not ensured in their case.
part 3: Melegvíz készítés és hûtés
part 4: Árak összehasonlítása
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