Portrait photo of Mike Bryant, AKA Mike the Boilerman

Mike the Boilerman -

Your Gas Safe Registered boiler 

and central heating repair technician in west Berkshire. Willing to travel :)



I am still carrying out house visits. Full details explained


(Last updated 21st April 2021)

Warm air boilers and heating systems...

… are a BIG subject! So big that notwithstanding all my waffling on here, I’ve now written and published a whole separate website dedicated to the subject, where I discuss in detail older warm air boilers such as the J25/32, HI-SPEC J32, Economaire, Brink, and the modern J&S condensing “Warmcair” boilers. Click here (opens in new tab):


Warm air was very trendy in the large housing estates built in the 1970s and 1980s, but have been largely superseded by water-filled radiator systems these days.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 air heaters). 

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' radiator system!


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 I cleaned this up 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 DANGEROUS! 

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 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 :)

You can keep your existing warm air heating ductwork, grills etc but power it with an ordinary gas-fired water-filled combi boiler. The old warm air boiler is replaced with an ‘air handler’ or ‘heat interface unit’ (HIU) which delivers warm air heated by hot water from the combi. 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 an old-style 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, the curiously named "Aquair HIU”. 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 ubiquitous 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 it here:

WarmCair condensing warm air by Johnson and Starley

To enquire about a repair to your warm air boiler, call or text me on 08766 766364.


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Copyright Michael Bryant 2021

Site first published 16th January 2004

Last updated 26th April 2021

Gas Safe Register 197499, CIPHE registration number 56207

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