Specifications

or logs are subject to variations in the fuel, e.g. dierent woods have dierent net energy contents requiring
dierent air fuel ratios. The net energy content of the wood may therefore vary over time with dierent
batches of deliveries.
Lambda control overcomes the problem of fuel energy content uctuations. A lambda sensor measures the
oxygen content of the ue gas and makes adjustments to the air supply rate based on the readings it gets. In
this way, maximum eciency is maintained at all times in this respect. It’s a bit like having a service engineer
in constant attendance to adjust the air supply rates. Lambda control is recommended as best practice. Some
higher end boilers oer lambda control as standard. Middle range boilers often have this as an optional extra.
Heat Exchanger Cleaning
The eciency of a boiler is strongly inuenced by the ability of the ame / hot gas to transfer heat to the
water in the heat exchanger in the boiler. A build up of soot on the heat exchanger surface will impair a
boilers ability to transfer heat to the water. As a result more heat will escape through the ue and the overall
boiler eciency will reduce. Wood fuel boilers are more prone to soot build up on the heat exchanger
than oil or gas systems. With oil or gas the build up of soot is minimal and half yearly or annual servicing
is normally more than sucient to prevent eciency losses in this respect. The same approach for wood
red boilers is not sucient as soot deposits build up much quicker. There are generally two approaches to
dealing with this:
Manual Cleaning - the heat exchanger requires regular cleaning manually. To do this the end user needs
to open up the front of the boiler and brush out soot deposits on a regular basis, e.g. once per week or
less frequently when the boiler is used less. Boilers that require manual cleaning are usually available
from the less expensive end of the market.
Automatic Cleaning - the heat exchanger still requires regular cleaning but this is done automatically
by internal devices in the boiler. Generally this is achieved by springs mounted vertically within the
heat exchanger that are gently vibrated using a cam system. This causes the springs to make contact
with the heat exchanger and “knock o” the soot build up which are collected at the bottom of the
boiler. This is much preferred over manual cleaning as it is automated at regular intervals and ensures
peak heat exchanger eciency is maintained. Although end users may have good intentions initially
about cleaning a boiler that needs manually cleaning, the end user will often lessen the frequency or
sometimes forget to clean the exchanger. This will cause saw tooth dips in eciency of the boiler below
its rated eciency. Automated cleaning devices of course attract a price premium. Boilers in the middle
to top end cost brackets tend to have automated cleaning mechanisms.
In terms of best practice, it is recommended that automatic cleaning is specied as a minimum requirement.
Location of Boiler and Boiler Room Specication
Domestic wood boiler systems are bulkier than traditional oil or gas systems, so one of the main concerns is
space. Wood pellet and chip boilers require a large fuel store for pellet boilers, this is often in the same room
as the boiler, and wood log systems will more than likely have a buer tank installed. If the boiler room is
remote from the dwelling, it is important that any pipes running under the soil are insulated. Pre-insulated
piping is available (often referred to as district heating pipe) and it is recommended as best practice that this
type of piping is used for external runs.
Thermal Mixing Valve (Anti Scald Valve)
Best practise calls for the tting of a thermal mixing (anti-scald) valve on a hot water system. With the current
recommendation to store hot water at 60°C to prevent the growth of legionella bacteria it is becoming more
of a consideration to install thermal mixing valves. A thermal mixing valve ‘mixes’ cold and hot water together