What kind of heating is the best in a passive house? part 2

Fireplace, boiler or heat pump?

Author: Mónika Vértesy
2011-02-21

WWe have already „tested” the fireplace and the compact device in the passive house designed by us, now it is time to present an extremely low-performance condesational boiler and a system called „Energiakulcs” (Energy-key). Why is it important that the boiler has a great modulation? Also what can an Energy-key offer with a ground heat/water heat pump as a central element?

The preludes can be found here.

3., Heating with a condensational gas boiler

Why do condensational boilers function with a good efficiency?

In theory there is no obstacle in the way to have a gas-combustoning boiler in a passive house. The modern boilers are made in a way that the condensation of the steam content of flue gas is somewhat desirable. This was something to avoid in older boilers as it caused corrosion, so it was a necessity to keep the retourning water at a high temperature. In case of a low-temperature heating, a mixer valve was also needed. In theory, the efficiency of condensational gas boilers can be 111%, and it differs only a few percents in practice. The efficiency over 100% does not mean violating the law of energy persistency, it comes from the fact that we insisted on the usual benchmark. This benchmark is the heating value (also called as the lower heating value and marked with Ha), which grants the total heat content of a unit of combusting material. The heat content of the steam in the flue gas is also contained by this mentioned value, wich causes 11% of surplus compared to the heating values, so now it is comprehensible how it is possible to reach the efficiency of 111%. Just becouse of condensation, these boilers have the great advantage in case of part load use: the efficiency is better, as the temperature of the retourning water is lower and the flue gas can condensate in a greater proportion. However, this is not valid in all cases of a really small part load.

Large range of functioning

The mondulation of a condensational gas boiler is usually between 30% and 100%, which means that it is capable of functionig with a good efficiency in this interval of performance by modulating the flame and changing the temperature of the water. The condensational boiler with the lowest performance distributed in Hungary has 12kW of performance, due to my knowledge. It has a performance of 3.6 kW at a 30% part load, compared to the 1.5 kW of maximum performance demand of the building. I can not recommend using such a boiler, as the efficiency is reduced in case of functioning below the modulation zone. What is even more important is the fact, that the life of the boiler is reduced greatly by frequent turn-ons and turn-offs.

We do not have a solution (in Hungary), but the Greman ELCO has

West from Hungary, the manufacturers and the distributors are keeping in mind the owners of passive houses. Not a lot, but several firms manufacture low-performance condensational boilers. The speciality of the boiler called Elco Thision S 9,1 is its modulation which have a proportion of 1:10, so it functions at a performance of 0.9 kW without any problems. Its efficiency is 109% at a load of 30%, its price is 2500€ (737500 HUF), altough the transportation have to be solved too if someone would like such a boiler.

Unfortunately this is not a combi boiler, so like cooling, DHW production has to be dealt with also in a separate way.

Elco Thision S 9,1 low performance condesing gas boiler

Until then we have found a boiler with a sufficiently low performance distributed in Hungary: the boiler called Unical Alkon R12 is like that. Moreover, it is quite possible that other opportunities have already appeared in the market. Like the presented ELCO appliance, the Alco R12 device is not a combi boiler neither, but control of an exterior DHW container is built in, allowing to have an indirect boiler joined. A weather-following controller and an exterior probe are accessories too, the modulation interval is between 1.9 – 12 kW. The boiler body has a warrantee of 5 years. It is a AlSiMg alloy, top quality. Its recommended distirbutor price is 281000 HUF (950€)+VAT. (Modified on the 6th of February, 2012)

4., Compex soluiton with the system called Energy-key

The new conception, chartered with the name „Energy-key”, developed by Kardos Labor Ltd. was ironed out during the detailed examination and performance measure of the heat pumps applied in Hungarian passive houses (and other buildings), which were not always used successfully. The system does not contain any stunning new elements compared to those that are already avaiable on the market. The only aim of the creators was to select and synchronize these elements in a more reasonable manner. The chartered system have already been applied in three passive houses ever since.

What were the problems?

As I have pointed it out before, the ground heat/air heat exchanger applied in the case of compact devices is not always capable of covering the performance demand with the contitions in Hungary. However, with gorund heat /water heat pump systems, the „passive cooling” (free-cooling) mode functions quite well. In these cases the heat pump does not even turn on, only the cooling effect of the ground collectors is utilized. I will explain this in a more detailed manner soon.

Instead of air/ground heat exchanger, it is worth using a ground collector or a probe in which an antifreeze liquid is circulated with a greater heat capacty. Propylene glycol, for example. This way we can deprive more heat from the ground and we are dealing eith a closed system, so it is not polluted with any kind of contamination. (leafs fallen from trees, bugs, etc.)

How deep we should install the heat collector in the soil?

The heating energy demand of the examined passive house exceeds its cooling energy demand by five-six times. As we are dealing with a heat exchanger, it is obvioues that when the building is being heated, the heat gainig side, so the soil is being cooled and vice versa: during cooling in the summer the soil is being heated. Still, we also produce heat during the summer but we do not have to worry about overheating the soil during the summer, as the „regeneration” is assured. Keeping in mind tough that the heating energy is determinative and has the same presage as DHW producing (also needed in winter), it is easily forseeable that the ground collector or the probe should not be installed too deeply into the ground. By installing the the tube system close to the surface, we can also utilize the energy of the Sun, this way the soil will not cool down, or in other words, it will not be depleted.

Due to valid licensing processes, the probes and ground collectors installed shallower than 20m are not obligated to be licensed, so it is a pure profit!

Where can be heat energy saved, is it sure that the conventional puffer storage needed?

There is no puffer storage in the system of Energy-key, the concrete takes its place, were the tube system of the underfloor and ceiling heating is embedded. This solution is based on the fact, that the concrete has a quite good specific heat capacity (c= 0.88 kJ/kgK for concrete, c=4.187 kJ/kgK for 20 °C water). So in 10 tons of concrete, the same amount of heat can be stored as in a water resevoir of around 2100 litres.

Leaving out the puffer storage opens up an other opportunity: the ground collector and the heating system of the building does not have to be separated, a 25% propylene glycol antifreeze fluid can be circulated in the whole tube system. This causes some raise in the expenses, but the savings exceed this as the pump and heat exchanger is not needed anymore and the performance of cooling raises. The heat pump is not operational during free-cooling, only one pump transports the cooling energy from the ground collectors for the surface heat dissipators and for the heat exchanger of the air handling unit.

Main components of Energy-key:

1.      Energy-key hydraulic system controller unit,

2.      horizontal ground collector or vertical soil probe,

3.      ground heat/water type heat pump,

4.      ventillation device with a heat recovery unit and a bypass branch,

5.      pre-heater/cooler calorifer,

6.      heat dissipator surfaces for mounted radiation heating and cooling, integrated to the structure of the floor or the ceiling,

7.      DHW storage unit with indirect heating,

8.      controller unit (PLC).

details soon on the own website of Energiakulcs...

The article will continue soon!

part 3: DHW and cooling