Comparison of Old and New Heating System

6.4.1 Space Heating Distribution System

To compare the space heating system in absolute numbers it is important to remember that as described in chapter 6.2 (page 104) the old heating system did not heat the basement. The new heating system supplied about 28 m² gross area more (out of 172 m² total gross area) than the old heating system because the room in the basement was occupied since summer 2006.

A clear difference can be observed in the average temperature difference of the space heating circuit at similar space heating loads. In the new installed solar combisystem the space heating temperature difference (column 5 in Table 6–3) was all four months in a very small range around 12 K. In the old heating system the space heating temperature difference in average was 7.7 K for the whole year 2005. The maximum was around 10 K in the core heating season and around 5 to 7 K (or less) in spring and autumn. In 2004 the space heating controller was faulty, see Fig. 6–16 (page 111) with the explanation for that.

The explanation is the use of the thermostat valves at the radiators. During the period measured with the new solar combisystem after intensive explanations the thermostat valves were used almost correct and therefore the temperature difference in average was almost the double compared to the old system. At least in autumn 2006 it was possible to get return temperatures most of the time clear below 30°C. This is shown for example in Fig. 5–25 (page 93) where the space heating return temperature (Tc11) end of October was about 26°C. This was possible even with a forward temperature of 45°C, which is relative high for that period. In comparison Fig. 6–17 (on page 113) is an example of the old installation where at 08:00 with a forward temperature of about 45°C the return temperature was about 40°C. In the same graph also at 17:00 with a forward temperature of about 35°C the return temperature was almost 30°C.

This shows that even with an extremely bad equipped space heating distributing system the correct use of the thermostat valve can have a quite large effect. Bad equipped space heating system in this case means that:

• The radiators are old types (cast iron) designed for high temperatures.

• None of the radiators has a pre-adjusting valve; therefore the hydraulic circuit in this house is not adjusted.

• The newest and best radiator of the house unfortunately is installed in the technical room in the basement and switched off since this room is heated enough by the heat losses of the pipes and the heating system itself.

• One of the two radiators in the living room in the first floor is totally covered by a stone plate at the top and the back side of the couch in the front and therefore not able to deliver significant amount of heat to the room.

• The sleeping room in the second floor has no radiator.

• The working room in the second floor has one relative good radiator, but the thermostat valve is blocked and totally closed, therefore this radiator is not delivering any heat as well. The second radiator is a very old and heavy type, where the pipes are connected wrong: the forward pipe (with high temperature) is connected at the bottom and the return pipe is connected at the top. This radiator therefore is internally totally mixed and the temperature difference between forward and return temperature is very little.

Therefore the first floor is heated by two radiators, the second floor (except the bathroom) is heated by only one old, small radiator which also is connected wrong.

This is in total quite a reduced heat transfer possibility for the whole house, why it is

consequently and natural that the remaining radiators have to take over the heat load, what is only possible at relative high average temperature level.

Based on these facts the system behavior relating to space heating return temperature in the new installed heating system is acceptable.

6.4.2 Hot Water Preparation

Comparing the hot water consumption of the same period (October until January) shows that the hot water consumption in general changed quite strong (see Table 6–

4). In the old heating system obviously the hot water tank is heated to a quite high temperature of about 70°C. This is the reason why the temperature difference in the periods 1 and 2 in average is around 59 K. In the new installed solar combisystem in period 3 a temperature difference of about 38 K in average is measured, this is about 35% less. The hot water set point temperature in period 3 in the new system is 50°C.

Therefore the hot water consumption in terms of liters between period 2 and period 3 is increasing even that the hot water consumption in terms of energy is decreasing.

Table 6–4 Domestic hot water consumption of three four-months periods as average daily values, each from October – January.

DHW DHW Temperature difference

Period Year [kWh/day] [ltr/day] [K]

1: 2004/05 10.2 148 59.8

2: 2005/06 9.1 134 58.8

3: 2006/07 7.7 174 37.9

The hot water consumption of these three periods constantly decreased from year to year.

The reduction from period 1 to period 2 obviously is based on reduced hot water consumption in terms of liters since the temperature difference is almost the same.

One main reason for that is that after summer 2005 due to illness the average number of bathes of one person decreased. No technical changes are known which could explain the difference between period 1 and period 2.

The reduction from period 2 to period 3 has several impacts. First of all the new solar combisystem was installed with the hot water flat plate heat exchanger unit for hot water preparation. This leads to the following effects:

• The flat plate heat exchanger has a relative high pressure drop, which has a remarkable effect especially at high flow rates. Therefore the flow rates are reduced leading to less hot water consumption in terms of liter and following up also in terms of energy.

• The advanced circulation strategy “circulation on demand” is introduced in the new system (see chapter 3.1.2 on page 34). Circulation pumps in general have the goal to reduce hot water consumption in terms of liter since they keep the hot water pipes hot and therefore avoid wasting of water when the user is waiting for the hot water. But this wasted water during the waiting time is measured as consumption in systems without circulation pump. Therefore in systems with circulation pump this wasted energy of the waiting time is not measured as consumption but as heat loss. In terms of energy in total, circulation pumps of course increase the energy demand for hot water preparation but reduce the hot

water energy consumption itself. This topic was studied very detailed in a master thesis project at the Solar Energy Research Center in Sweden (Apel 2005).

• The hot water temperature entering the pipes was about 70°C in the old system (period 1 and 2) but it is only 50°C in the new solar combisystem. During constant hot water tapping this makes no difference in terms of energy, if it can be assumed that the user at the tap is mixing anyway to a lower temperature. But after tapping, the hot water which stays in the pipe was measured by the heat meter as hot water consumption. This amount of water is cooling down from the high temperature level. Therefore, it is a big difference of the heat content in the pipe if the temperature is 70°C or 50°C. Compared to a room temperature of 20°C the heat content with 70°C is 67 % higher than with 50°C. This effect of course is strongly depending on the number of taps and the average durations of the tappings.

One change in the house most likely also will have some influence on the hot water consumption. In summer 2006 the basement was occupied and therefore also the bathroom with the shower in the basement since that time is in use. This is reducing the use of the bathroom in the second floor, which influences the hot water consumption because:

• The pipe lengths from the technical room to the bathroom in the basement are shorter and slightly slimmer than to the second floor, but each tap (the shower and the wash-basin) have an extra pipe coming from the technical room.

• The shower in the basement is equipped with a thermostat mixing valve and with a water saving shower head, what is not the case in the second floor.

• The tap for the wash-basin is equipped with a water saving tap, what is not the case in the second floor.

From user point of view no changes of the behavior in hot water tapping is reported beside one: since the basement is occupied by the daughter in summer 2006, her boy friend additionally was using the shower in the basement as well.

6.4.3 Hot Water Circulation

In the old heating system the hot water circulation pump was in use just for a short time of one month. The reasons for that are explained together with Fig. 6–18 (page 114). In fact during this period of 30 days in autumn 2005 the circulation heat losses of in total 297 kWh are in the same magnitude of the monthly hot water consumption at that time: 292 kWh in November and 295 kWh in December 2005.

In the new installed solar combisystem the control concept “circulation on demand” is in use (see chapter 3.1.2 on page 34). The circulation heat losses of 10 to 18 kWh per month are just a fraction compared to those before. Compared to the hot water consumption the circulation heat losses are in the magnitude of 5 to 10%.

This shows that from an energy point of view the new concept of “circulation on demand” is operating very successful. Unfortunately due to a mistake of the installer (all taps in the house are connected with parallel pipes from the technical room in the basement) only the bathroom in the second floor has advantage of the circulation pipe.

Therefore the house owner is in general not very satisfied with the hot water circulation system.

6.4.4 Electricity Consumption

The daily electricity consumption of the old conventional heating system and the new solar combisystem is in the same range. The new solar combisystem in comparison to the old heating system has in total three more pumps, five more 3-way valves, one additional frequency converter and a controller, which consists in fact of 4 controllers since it is a prototype. Based on these facts it would be expected that the consumption of parasitic electricity to operate the complete system in comparison with the conventional one would be much higher.

In fact the average daily electricity consumption of the 4-month period October – January in 2006/07 (new system) is 8% lower compared to the 4-month period January – April 2006.

The reason why this is possible can be explained as following: The 3-way valves have a nominal power of only 2 to 4 Watt and they are only in operation for very short periods, in fact just some seconds. Also the frequency converter has an efficiency of at least 95 % according to the datasheet. The electricity consumption of the controller unfortunately is not known but typically around 2 to 4 Watt.

The two additional pumps for the primary and secondary solar circuit are set to the lowest power (position 1 out of 3 results in 30 W nominal power each) and they are in this period only very little in operation. Anyway, if these pumps are in operation 2,000 hours per year, this is resulting in average in about 0.32 kWh/day. This is about 10% of the total consumption of the old system. Additionally each kWh heat gained by the solar collector also reduces the electricity consumption of the natural gas boiler because this needs to operate less time. According to the test certificate of the natural gas boiler, the electricity consumption at lowest power is about 62 W, at maximum power about 119 W.

Most likely the biggest advantage of the new solar combisystem is the speed controlled space heating pump (P4) which is set to a standard speed of 50 % during space heating. Also during hot water preparation in average the pump speed is around 50 %, which is estimated based on the monitored pump speed signal. In both the old and the new system for this pump a Grundfos UPS xx/60 is used which has a nominal electricity consumption of 90 W at full speed which was also the setting in the old system. During the heating season this pump is operating 24 hours per day. A constant power of 42 W corresponds to 1 kWh within 24 hours. Therefore this is a huge potential to reduce the electricity consumption which is used in the new system due to the standard speed of 50 %.

6.4.5 Key Efficiency Values

To evaluate a solar heating system it is necessary to deteremine different characteristics of the system. Beside a high solar gain from the solar collector a high hydraulic efficiency is required. Otherwise it would be easy to achieve high solar gain based on high heat losses of the whole system. Also the solar fraction is a very critical key figure since it is very strong depending on the definition and therefore how the heat losses are accounted for: to be covered by the auxiliary, by the solar collector or both in a specific ratio. High boiler efficiency is the “backbone” of a high overall coefficient of performance as long as the boiler is the main heat supplier in the heating system, like it is in this case.