FHT Stoves Frequently Asked Questions
How many radiators can an FHT Stove supply?
This is always a difficult question to answer as it depends on specific manufacturers'
stove designs. The simple answer from current demonstrations of the FHT Stove technology
is that around half of the stove net output can be sent to the radiators.
Taking a 5kW nominal (net) stove output, around 2.5kW can be 'switched' to the
radiators as required. A typical living room radiator (600mm tall, 1200mm wide,
single panel) will output 1200 watts if supplied with water at around 75C, so the
FHT Stove can supply two of these radiators in this case.
The complex answer is that the amount of heat that can be supplied to the water depends
on many factors including the following:
Stoves can be optimised to provide more or less heat via the convective surfaces by the
shape of the stove, the surface area of the door glass and the type of bricks used; some
bricks reflect more heat as radiative heat through the front of the stove, whereas other
designs absorb and disperse more heat via the sides and back surfaces of the stove.
The temperature of the water passing through the FHT heat exchanger has some influence on
how much heat can be transferred; if operating with low temperature systems such as
under-floor heating, a greater proportion of the total stove heat output can be transferred
to the heating system than if the FHT Stove is supplying conventional radiators at higher
To some extent the heat output of any stove can be varied by the user through the choice
of fuel type (dryness and quality of wood for example) and through the use of air controls.
However, most stoves have a practical limit to their controllability as a 'damped' fire tends
to cause tar deposits in the chimney and reduced efficiency. When the customer allows a fire
to burn out, there is usually considerable stored heat in the metalwork and bricks of the
stove which can continue to provide heat for the heat exchanger for a few hours - this may
for example provide a background heat throughout the house at the end of the evening.
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In a well insulated home, the output from a FHT Stove will of course go much further and
the ability to switch heat from the stove will help avoid overheating in room in which it
Is the FHT Stove noisy?
The FHT Stove incorporates a small electric fan to direct air heated by the stove surfaces
towards a heat exchanger. As relatively low air flow rates are involved, the fan is optimised
to be quieter than most refrigerators and consumes only around 10W of power - about the same
as a medium sized energy efficient light bulb.
Are there any restrictions to the location of radiators connected to the FHT Stove?
No! Unlike 'back boiler' systems, the FHT Stove utilises a low power pumped circuit where
hot water can be sent to radiators in any location, even on floors below the room in which
the stove is located. Conversely, back boiler systems must be 'gravity fed' to ensure safe
operation, so the majority of the heat must be dispersed in a close-by room above the stove
to ensure natural circulation and must always be 'on' when the stove is lit.
When the FHT Stove is connected to an existing central heating system, heat is distributed
throughout the home via the heating system, supplying any radiator that is turned on.
Do I need big pipes to connect the FHT Stove?
No! The FHT Stove can be connected in any preferred pipe bore, including 10mm (~3/8")
pipe and flexible connections if required. Connection via small bore piping allows neat,
hidden pipe-work to be used so that the appearance of your installation is not affected.
Can the FHT Stove be connected to pressurised central heating systems?
Yes! FHT Stove components are designed for operation in excess or 10Bar (~145 PSI) in line with testing
requirements for modern boilers, although they can also be connected to open vented systems
if required. Many back boiler systems can only be connected to open vented heating systems
with the additional hassle and costs associated with such systems.
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What happens in the event of a power supply failure?
The FHT Stove simply reverts to 'off' mode and all the stove heat is released into the room
in which the stove is located. The design is fail-safe and cannot result in dangerous water
boiling in the vicinity of the fire.
Can I have TRVs on my radiators?
Yes! This is a unique feature of the FHT Stove. Whether the stove is sending heat to the
central heating system or a dedicated radiator loop, the radiators can be turned 'on' and
'off', or can be controlled by thermostatic valves. The FHT Stove incorporates simple
controls that detect whether there is sufficient heat demand elsewhere to distribute heat,
and if necessary it can cycle like a central heating boiler.
What happens if the radiators are off?
See answer above - if there is not a heat demand the FHT Stove simply reverts to sending
all the heat to the room as hot air.
How does the FHT Stove compare to back boiler systems with safety cold water loops?
A number of stove manufacturers are now offering back boiler systems which incorporate a
safety feature which dumps cold water through the back boiler to prevent boiling if the water
overheats. Whereas such technology is clearly an important safety advance over a simple back
boiler system, it only offers emergency protection to the customer. The FHT Stove offers fail
safe operation which does not rely on additional cold water 'dump' circuits (and the plumbing
costs associated), but also offers the considerable benefit of being able to choose whether to
transfer heat or to send all the heat to the room.
Is the FHT Stove only a replacement for back boilers?
No - although it does offer considerably lower installed costs for customers who specifically
want a 'back boiler' system. The FHT Stove is aimed at all stove users, primarily to give
additional comfort control, improved efficiency, and a reduced Carbon 'footprint'; as the FHT
Stove can be turned 'on' and 'off', customers can choose to have all the heat passing to the
room, or if it becomes too warm, to direct around half of the heat to other locations via the
existing central heating system or a dedicated radiator loop.
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Can FHT Stoves be used in Smoke Controlled Regions?
In addition to the safety and controllability benefits, the FHT Stove maintains high internal
firebox temperatures that can be utilised to achieve 'clean burn' conditions and may allow
exemptions to operate in smoke controlled regions. To achieve exemptions for operation in
smoke controlled regions, the firebox and air controls must be designed to achieve clean burning
and be certified by an accredited test agency. With only one exception to-date, back boiler
stoves cannot maintain combustion conditions for clean burning, so cannot be exempted for
operation in smoke controlled regions, even if the basic stove model (without a back boiler)
Most stove models that meet the standards required for smoke controlled region exemption require
limits to their burn control to ensure that the user cannot 'damp' the fire to reduce the output
as this often results in inefficient, 'dirty' burning and unacceptable smoke emissions.
In these circumstances the FHT Stove offers a unique level of controllability through being able
to significantly reduce the heat output of a stove, even if the air controls have to maintain
full stove output.
Not answered your question?
Please email FHT Stoves (
) with any further questions or comments.
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