What IS a BoilerMate II?
(Firstly, most Boilermates are qualified with II, III or 2000 or
similar suffixes. The first, original BoilerMate was just called a
BoilerMate without suffix. This page deals with the BoilerMate II)
How does the BoilerMate II work?
A conventional central heating boiler heats the water in the
BoilerMate II directly. The stored water is pumped to the boiler for
heating and back again by the right-hand of the two pumps on the unit
whenever the stored water temperature drops below a specific temperature
set in the factory. The left-hand pump circulates the stored hot
water around the radiator circuit(s) when the heating controls are
calling for heat.
There are two circuit boards behind the cover panel. A pump drive
board to run the central heating pump for a few seconds every forty
hours to prevent it seizing up over the summer months when it is not in
use, and a delay timer to keep the pump running for a short while after
the boiler has shut down to extract the residual heat in the boiler heat
The BoilerMate II differs from the previous BoilerMate (original) in
that it has a user-adjustable thermostat for the heat store. This
feature allows the user to control the temperature of the heat store but
was dropped from later versions, probably due to user-confusion about
how to use it. Good thing too as even I can't explain clearly and simply
how to set it! (Just follow the instructions written next to the dial.)
is transferred into the
hot tap water by passing the cold mains water through a coil of pipe
inside the unit, immersed in the store of hot water. Heat passes through
the wall of the tube and heats the cold mains water on it's
way to the hot tap. To control the temperature of the domestic tap water, a thermostatic blender valve mixes in a proportion of
cold water. The output temperature can be set by the user.
Faults known to occur in the Boilermate II:
1) Depleted water in the thermal store.
BoilerMates have a filling, or top-up tank fitted to the top of them, which
is there to fill the unit with water. This
may or may not have a float valve connected to the mains water supply to
fill it. When there is NO mains connection, there is usually provision
for manual filling by the user by means of a tap on the wall nearby.
When the user doesn't realise this, water lost from the thermal store through
evaporation and/or leaks can prevent the unit from working. If the water
level falls too low, the heat exchanger coil ceases to be immersed in
stored hot water so when a hot tap is turned
on, the unit will not deliver hot water. The answer is to check the
water level in the top-up tank and top it up to the waterline
embossed into the wall of the cistern. This problem is usually
accompanied by a noise of water trickling or flowing inside the unit.
This is caused by the flow from the boiler discharging into the unit
above the water level inside and making the 'pouring' noise. This can be
very bad for the system as accelerated corrosion usually results from
the constant aeration/oxygenation of the circulating water.
2) Blocked internal cold-fill pipe
The top-up tank fills the main water store via an internal pipe
leading down to the base of the unit. On the BoilerMate II specifically,
this pipe is prone to blockage with products of corrosion. It it usually
impossible to clear in my experience and the best solution is to install
a new, external cold fill pipe. The unit must first be fully drained. A
hole is drilled through the wall of the top-up tank and a tank connector
fitted. The drain tap at the bottom of the unit is removed and an
adaptor fitted to the hole to connect a copper tube. A copper tube is
then fitted externally from this adaptor up to the new tank connector,
with a new drain point fitted into the tube near the bottom. The unit
can then be re-filled and re-commissioned.
3) Circulating pump failure
BoilerMate IIs are all quite old now, and many of those without
corrosion inhibitor in the circulating water are suffering from advanced
radiator and system corrosion. The corrosion deposits cause the
circulating pumps to seize up and burn out. Fitting a new pump gets the
system working again but doesn't address the cause of the original
failure. A system cleanse is usually necessary (a 'powerflush').
4) Water scale-contamination of the coiled-pipe heat exchanger.
The coiled-pipe heat exchanger is extremely prone to water scaling.
This presents as maximum water temperature becoming progressively lower,
and in the final stages of scaling, the flow rate from the taps reducing
to almost zero. Chemical descaling is the only answer, using
conventional descaling chemicals and techniques.
5) Blender valve failure.
The thermostatic blender valve is prone to damage from accumulated
water scale. This results in the water taps only ever being slightly
warm. A new blender valve is necessary.
6) Thermostat failure.
The unit fails to heat up properly, leading to warm or cool
water from the hot taps and warm and cool radiators at the same time. A
new thermostat fixes it.
If you'd rather I came and fixed your BoilerMate II, contact
Page first created 21st March 2007
Last updated 19th January 2008
Copyright 2007-2008 Michael Bryant