Domestic Boilers FAQ and Problem Solving
How do I know which size of boiler I need? A flow rate of 8 l/min gives a temp rise of 42°C
A flow rate of 10 l/min gives a temp rise of 32°C
A flow rate of 15 l/min gives a temp rise of 21°C
Boiler selection should be based on a detailed survey of the property and an estimated calculation of lost heat inside the flat/house.
Once the energy requirement for the property has been known (expressed in kW or BTU/hr), the boiler model whose output most closely matches this
requirement should be selected.
How do I know which type of boiler will suit my requirements?
We strongly suggest that you seek the expert guidance from a local accredited installer such as LHS.
He will be able to suggest the most appropriate type of boiler and advise of any upgrades to the system
that may be required.
How do I get the most from my combination boiler?
In order to get the best from your combination boiler, it is important to understand the basic principles
involved. Here is the the Combi vs. Conventional System Layout.
A combination boiler works on an "instantaneous" basis. Water drawn from the rising main to the
tap is heated as it passes through the boiler’s domestic hot water calorifier.
The temperature of hot water is dependent on three factors:
1. Boiler max. output
The maximum performance of a boiler is directly related to its output (usually given in kW or BTU/hr)
2. Water flow rate
The speed at which water passes through the heat exchanger will determine how much heat is absorbed.
Simply - the slower water passes through the heat exchanger the hotter it becomes.
Boiler manufacturers will often quote a hot water flow rate for the appliance in liters/minute or galon/minute.
However, it is important when assessing the performance of a combi boiler to remember that this flow rate is
based on a specific temperature rise – usually 30°C or 35°C. If a greater temperature rise is required,
the flow rate must be reduced. Conversely, a greater flow rate can be obtained if the temperature rise required
e.g. A Combi boiler with an output of 23kW gives a flow rate of 9.3 litre per minute for a temp rise of 35°C
3. Incoming mains water temperature
We have already seen that the temperature rise depends on the flow rate, but the actual temperature
obtained will depend on the initial mains water temperature. Because this varies, the apparent performance
of the combi will change according to the season e.g. with the 23kW boiler, at standard water flow rate:
In Summer typical mains temp 16°C, hot water temp is 50°C
in Winter typical mains temp 5°C, hot water temp is 39°C
The hot water produced by a combiboiler is only available at one draw off point at any one time.
Use of two points simultaneously will result in poor performance at both.
Why do some of my house radiators become hot more quickly than others?
It is very likely that the system needs to be balanced. Your installer should be able to assist with this.
Balancing involves setting the lockshield valve on each radiator to quarter (1/4) turn from fully closed
then running the system from cold, watching the rate at which each radiator is heating up.
The lockshield valve on any radiator that is slow to heat up, should be opened slightly to encourage water
flow to that radiator. Adjustments should be made until all radiators heat up at the same rate.
In some cases where radiators are quite old, limescale and other contaminant substances that gathered
inside have to be cleared. Proper radiators flushing must be then applied.
What do the latest changes to the building regulations mean?
From 1 April 2005, Part L1 of the Building Regulations requires gas and oil boilers
installed in new and existing premises to be condensing types, with a SEDBUK efficiency in band A or B,
unless there are exceptional circumstances that make this impractical and/or too costly.
In those cases where it is considered difficult to install a condensing boiler, an assessment procedure is
used to demonstrate that a non-condensing boiler is justified.
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