Archive for the ‘Heating System’ tag
Boiler Energy Efficiency
A condensing boiler is designed to have its return flow temperature at 20C below the outward flow. This is called ∆T20. Building Regs L state that boiler must be at least 88% efficient. Combi boilers on average in Britain run at 83% efficiency. The condensing mode of the boiler happens with a Return Flow temperature of 54% or less.
| Return Temp in °C | Boiler Efficiency |
| 80 | 86% |
| 54 | 88% |
| 40 | 94% |
| 20 | 98% |
So Boiler is most efficient when in condensing mode and to be in condensing mode the Return Flow temp must be less than 54°C.
Average heat loss in a house in the UK is 6kWh. 80% of properties in the UK have a heat loss below 10kWh. The most popular boiler is 30kWh which should be able to heat up 5 houses on average!!!
Heating and Hot water Energy Needs
The number of people in your new home and the size of your property are the primary determiners for the amount of heat your system needs to generate. It’s better to plan for slightly more than your maximum requirements, as this will ensure that you’re covered in the vast majority of circumstances.
A rule of thumb applies when calculating your energy requirements. The kilowatt hour (kWh) is the billing unit for energy delivered to consumers by electric utilities:
kW heating capacity (number of m² x 50 W/m²/1000) + kW hot water output (approx. 30% of the heating capacity value)
= kW rated heat output (= 6.5 + 1.95 = 8.5) (Does this apply to a combi boiler?)
According to this formula, for a low energy house with 130 m² living space the result is 8.5 to 9 kW rated heat output. The rated heat output is the maximum attainable power output of a heating system in continuous operation.
Are Boilers Overpowered?
A Vaillant ecofit 825 has power output as follows:-
| Central Heating Max. Output | 20.4 kW |
| Hot Water Max. Output | 25.4 kW |
| Flow Rate | 10.5 l/min |
The boiler brochures normally refer to the higher number which is the Hot Water Max Output number. It appears that this boiler is overpowered by approx a factor of 3 if installed in an average house/flat. This can cause problems of cycling (the boiler turning off and on because the boiler gets too hot as the Return Flow temperature is too high.
The following Heating Hub website Post covers this in more detail.
Boiler Efficiency Video
Setting the Boiler Temperatures
Setting the Heating Flow temp at 54°C will mean that the return temp will always be below 54°C and the boiler will be most efficient. However, it may take longer for the property to heat up. Vaillant historically recommended 75°C in the Winter and 65°C in the Summer. This does assume a system that operates at ∆T20 which is not always the case. New Building Regs were introduced in June 2022 which says that for replacement boilers the maximum Flow Temperature must be designed to operate at 55°C rather than the previous standard of 75-80°C. See this website here for more details on the building regulations. The Hot Water temp is normally set at 60°C. The Video below shows how to do this. In addition this Heating Hub website here discusses what temperature to set the Heating Flow Temp at but in short it will be trial and error. Older houses most likely can use 65°C with newer ones 55°C.
To access the status and diagnostic codes then call up the installer/competent person level
- Only call up the installer level if you are a competent person.
- Navigate to Installer level and confirm by pressing the two _ buttons to get to the Status Codes level. The S code will advise the current level of the boiler. Press the right _ button to enter the next level.
- Set the value 17 (code) and confirm by pressing the _ button.
- The enter the diagnostic code as required:
| D.036 | DHW flow rate | In l/min | Not adjustable | |
| D.040 | Flow temperature actual value | Actual value in °C | Not adjustable | |
| D.041 | Return temperature actual value | Actual value in °C | Not adjustable |
Diagnosing the Return Temperature on a Vaillant Ecotec Boiler Video
Heating System Design Guide
A heating system design guide may be purchased here for more details.