Warm air boilers and heating systems...
....were very trendy in the large housing estates built in the 1970s and
1980s, but have been largely superceded by water-filled radiator systems these
days. A surprising number of warm air systems still exist though, and a
significant proportion of my work is servicing and repairing them. (I hold the
right certificates to competently and legally work on domestic gas-fired warm
Most people who live with warm air heating seem to actually like it. A well
designed system is quiet, warms the house very quickly and the absence of
radiators makes room layout much easier. On the other hand, some people report
problems with draughts, dust being wafted around the house, and problems with
temperature control. Draughts are caused by incorrect system design or
incorrectly set air outlet grills, dust should be removed by an air filter
(sometimes missing) and poor temperature control can be improved by cleaning the
filter(!) or having proportional control instead of an on/off room thermostat.
Most people with well-designed warm air systems really like them. Most other
people have already taken their warm air out and replaced it with a 'proper'
Maintenance of warm air...
... can be a problem. Faults are generally easy to trace and fix but few Gas
Safe Register bods
are inclined to try as most do not have the (compulsory) 'warm air' qualification.
Sadly when faced with a warm air system there is a tendency for some technicians
to condemn it and advise the
customer to replace it with a conventional radiator system rather than admit it
is an appliance for which they are not qualified. This is appallingly unprofessional in my opinion but I encounter customers
from time to time who have had this problem. For example I was contacted a while
ago by a new customer whose warm air heater had been (incorrectly) disconnected
from the gas supply and labeled as dangerous by another gas engineer. This chap
then helpfully provided a quotation of £3,000 to fit a new boiler
with radiators. I re-connected the gas supply, serviced and and tested the warm
air system and found nothing wrong with it. The water heater attached to it was
dangerously sooted up but this was repaired and both appliances are now
back in use and operating safely.
The customer was so pleased he wrote this
letter thanking me.
There IS a problem with warm air heating in general though. It relates to
'ventilation'. Most warm air heaters are 'open flued'. This means they have a
'chimney' type flue to remove the burned gas, or products of combustion. For the
open flue to work correctly and safely it is of critical importance to have a
ventilation grill to outside air that lets fresh air IN to replace the flue gas
going OUT (up the chimney flue), and to provide oxygen for combustion. If a
correctly sized ventilator to outside does not exist then the warm air heater is
potentially dangerous to use, and many installations I encounter do NOT have
such a ventilator.
A ventilator was almost always fitted as part of the original installation
but users fail to appreciate the significance or importance of the ventilators
and have them blocked up to get rid of the draught they sometimes create. The
original ventilators are often installed through windows or door panels and are
lost when double glazing is installed and/or new doors replaced. This is
If you invite me to service or repair your warm air heater I am obliged by
law to carry out a set of four safety checks and one of them is checking for
adequate ventilation. If I find your installation has inadequate ventilation I
am obliged to issue a Warning Notice and take action to ensure the installation
is made safe. This can come as a bit of a shock when you were expecting me to
can service or repair your appliance, and protestations from householders that
'it's been like that for years' carry no weight. I have no choice. It is a legal
obligation on all gas installers to carry out the safety checks and 'make safe'
any installation that presents a risk to an occupier.
A further drawback to warm air heating is inflexibility. The system is
designed as a whole and cannot be easily extended at a later date if, for
example, the house is extended. When a house with warm air heating is extended
it usually makes sense to abandon the warm air heating and fit a conventional
wet radiator system.
Hot water with warm air heating...
...is not the same as with a water-filled central heating boiler. Either a
hot water cylinder with electric immersion heater or an 'ascot' style gas-fired
instantaneous water heater can be used, but the best option is the 'Janus' water
heater accessory which fits inside the warm air heater casing. This is a
miniature gas boiler designed to heat just a hot water cylinder (and bathroom
towel rail if needed), and they work very well.
Specific warm air boilers...
The warm air market was once populated by many big names but since wet
central heating achieved dominance they all went to the wall except one, which
now dominates. Johnson and Starley Ltd have the market to themselves*, and they
do a pretty good job of looking after it! They have a huge factory in
Northampton where they make new warm air boilers in every imaginable
configuration as direct replacements for many of the long obsolete boiler
models. They also manufacture spares for not only their own boilers but also
several of their dead competitors' warm air heaters including Balmforth and
Myson. They now have an interface to their parts
database so you can search to see which parts are still available for YOUR
warm air unit, here.) So if you've been told 'can't get the parts for it mate, it'll have
to come out', do some checking first before accepting that advice :-) It
would also be worth contacting a chap called Dave Knowles. Dave runs a business
supplying parts for a wide variety of warm air appliances and he includes many
obsolete names in the list of boilers he claims to be able to supply parts for.
His website is http://www.daveknowleswarmairsales.co.uk/
* It has come to my notice that Lennox (another famous warm air name) heaters
are now being imported from the USA (or Australia depending on which website you
read) by a Northampton company, Gasflow
Ltd. Given they are in the same town as Johnson and Starley I'm inclined to
suspect the two firms are connected in some way, but this is entirely my own
speculation and completely irrelevant!
Regardless, the Lennox G61MPVT they now import is a very interesting
product; a high efficiency condensing warm air heater which can also provide air
cooling in summer! I've only just discovered this appliance and have yet to see
one or even read the specifications, but I can imagine this being very
successful given the claimed increase in fuel efficiency and ability too
circulate cooled air through the house in hot weather.
Warm air in rental properties...
Rarely a Good Idea. An annual 'landlord gas safety inspection and
certificate' is required and few gas technicians hold the necessary
qualification to carry out an inspection on a warm air system. Servicing them is
time consuming and therefore expensive too, which new landlords sometimes find a
bit of a shock! Further, warm air systems designed in the 1970s ands 1980s tend
not to heat the bedrooms and bathroom to the temperatures expected by today's
standards leading to complaints that the heating doesn't work properly upstairs.
There is nothing that can be done about this short of installing a whole new
system. To head off these potential problems I generally recommend rented flats
and houses have the warm air systems removed and conventional radiator heating
systems fitted before tenants move in.
Warm Air flue problems...
Most warm air systems use an old style 'chimney' format flue to carry away
the combustion gases to outside. Sometimes referred to as a 'conventional flue'.
It is a simple vertical tube, sometimes with bends and offsets, going up through
the house and discharging through the roof. It is usually 4" or 5" in
diameter and made from thin steel sheet or cement encapsulated asbestos. The
terminal where it discharges through the roof is usually a 'ridge terminal',
which replaces one of the ridge tiles on the very peak of the roof and looks
very similar to an ordinary ridge tile.
These crudely constructed flues suffer from two common problems. The first is
that the joints can leak, allowing small quantities of flue gases to escape into
the house or loft. Sometimes a gas technician will (correctly) condemn a warm
air boiler due to such leaks but they can usually be easily sealed using special
adhesive aluminium tape made for the purpose, or temperature resistant silicone
sealant can be pressed into the joints.
The second fault is trickier. The steel bolts holding the ridge tile terminal
together can rust and fall apart leaving the top of the flue tube
disconnected and partly discharging into the roof space. Occasionally these
ridge terminals can be fixed and reassembled from inside the roof space but
usually not, and a new ridge terminal will need to be fitted (or at least the
bolts replaced). This requires scaffolding or a roofer willing to climb up onto
the roof ridge using ladders. Then the flue tube inside the roof space needs to
be re-connected to the ridge terminal. Cost for this can easily run well into
four figures so householders with this fault often elect to abandon the warm air
and fit a conventional radiator system.
Replacing a warm air boiler...
Although replacement warm air units equivalent to old heaters are still
available, there has been a new development. You can now replace your warm air boiler with a combi, and
still heat your house warm air. Yes you DID read that correctly!
Now you can keep your warm air heating but power it with an ordinary
gas-fired water-filled combi boiler. The advantages here are as follows:
1) You no longer have to find a warm-air-qualified gas bod to service and
maintain the gas boiler. Excellent for landlords especially, who have to have
their systems certified annually by law.
2) You no longer need a hot water cylinder as the combi will deliver hot
water. Regain storage space.
3) Annual servicing becomes quicker and cheaper. A combi is FAR quicker to
service than a warm air unit!
4) The combi will be room-sealed, meaning the outside air vent needed for the
the warm air boiler can now be blocked up and sealed, getting rid of the draft.
Johnson and Starley make this
device, called the "Aquair HIU". Search their website to find it. It uses hot central heating water
as the heat source for a warm air fan unit. This unit replaces the conventional
warm air boiler and a combi boiler is fitted to heat it. Simple and effective
alternative to replacing an old and beyond-repair warm air boiler in my opinion.
Johnson and Starley also now make a complete warm air and hot water unit
incorporating a conventional high efficiency condensing combi boiler and an Aquair
style of HIU device combined into one single appliance. Its one of the range of
boilers called the "Warmcair", and uses the same fan-powered pre-mixed
gas-and-air room sealed technology that modern water-based high-efficiency
condensing boilers use.
There are also Warmcair boilers from Johnson and Starley that don't heat the
hot water, manufactured as direct replacements for old style warm air units from
the 60s and 70s, many of which are still in service and approaching time for
replacement. The WarmCair C10D for example replaces the ubiqutous J25-32 model
fitted in tens of thousands of 70s houses. It is a high efficiency condensing
boiler boiler that contains no water, dimensionally similar to the J25-32 it
replaces. The flue is completely different though, being fan assisted and of
dual concentric construction, so a whole new flue will need to be installed. I'm
quite impressed with the WarmCair boiler actually, so I've written a review of
Page first created 27th November 2005
Last updated 16th January 2017