Johnson and Starley J25-32 basic, and Modairflow

The J25-32 warm air heater was introduced long ago in the 1960s I think, then updated with Mk2 and then Mk3 versions in the 70s then 80s. This was probably the most successful of all the warm air heaters in the early days of central heating and many, many examples of the Mk3 version are still in use. As far as I know, all of the commonly failing parts in the Mk3 are still freely available so any breakdowns can usually be fixed. The original and Mk2 versions are much rarer and no longer fully supported with spares.

Air from the house is drawn into the boiler by a large, quiet fan and blown downwards over a heat exchanger which heats it by gas and distributes the warmed air to most parts of the house through ducts installed by the house builder under the floors and/or in the roof space. (The gas fumes are fully separated from this warm air by the heat exchanger, and the fumes leave the house via a chimney.) In the earliest installations it is not unusual to find warm air ducts not installed to the bathroom or smallest bedroom, as warm air to anywhere in the house was seen as a considerable luxury.

The heating is turned on and off by a time switch in the front panel of the appliance and an adjustable room thermostat. The earliest design of room thermostat was a bead of mercury inside a curved glass capsule, the angle of which was adjusted by a lever to adjust the temperature. A bi-metal strip also changes the angle with temperature, so the bead of mercury rolls from one end to the other as the room temperature rises and falls, and the mercury makes a circuit at one end turning the boiler ON, and breaks the circuit when it rolls to the opposite end turning the boiler OFF again. A charming device to see working, but selling products containing mercury is no longer considered safe nowadays.

The Modairflow version is structurally the same as the basic J25-32 but has electronic temperature control. The electronic room thermostat (called a “Thermista-Stat”) measures room temperature and communicating with the electronic panel inside the J25-32 Modairflow, speeds up the air circulation fan or slows it down according to how close the room temperature is to the room temperature selected by the user on the room Thermista-Stat. This proportional control system is more comfortable to live with as the inherent rise and fall in room temperature associated with the basic room thermostat turning on and off in cycles no longer occurs.

A minor point to note with the Modairflow/Thermista-Stat system is that there is no audible ‘click’ on the Thermista-Stat control knob as one turns it up and down past the actual room temperature, as happens with a conventional on/off room ‘stat. Some people find this disconcerting.

Common faults and fixes:

1) Air circulation fan failure:

As the years pass, the air circulation fan wears out. The most common failure is the motor bearings get stiff with entrained dust and the fan simply won’t spin, so the gas burners light but no heat comes out of the air vents around the house. A different and more more alarming failure is where the fan continues to work but emits the most horrendous screeching noise. Most people run to the boiler and turn it off in panic but there is actually no danger. In both failure modes a new fan is necessary, or to rebuild the fan with a new motor.

2) Pilot flame goes out or will not light:

Being designed in the days of cheap and abundant North Sea Gas, these boilers have a permanent pilot light running to light the main burner when the boiler comes on. The pilot flame occasionally goes out and needs to be re-lit by the user. Doing this involves holding down the grey button on the gas valve to start the gas flowing, and pressing another (black) button to deliver a spark which lights the pilot gas. Once the flame is established, hold the grey button down for 20 seconds or so while the safety device sensor warms up, then you can let go and the pilot flame should remain alight.

The safety device that warms up is the “thermocouple”, and once hot this produces a small electrical current that holds open the valve supplying gas to the pilot, all the time the flame is alight. Should the flame blow out, the thermocouple cools, starves the electromagnetic gas valve of electricity, the valve closes and shut off the gas for safety.

Occasionally this lighting process will not work and the pilot flame goes out no matter how long you hold the grey button in. This is caused by failure of the thermocouple and a new thermocouple usually fixes it. You’ll need a gas technician to visit and fit it, which takes perhaps half an hour once in front of the boiler, plus some further time to complete the safety checks. Sometimes the coil inside the gas valve fails giving identical symptoms, in which case a whole new gas valve will be required.

3) Randomly poor response to the room Thermista-Stat:

This is a fault that can only happen on the Modairflow version as the basic 25-32 does not use the proportional control system with a Thermista-Stat. The cause in my experience is difficult to diagnose other than by replacing parts. Sometimes simply replacing the Thermista-Stat fixes the problem, other times I find a new electronic control panel and fan speed controller are necessary. (Johnson and Starley recommend replacing both these items as a pair, as do I.)

4) Noisy or failed time controller:

The time switch/clock on the front panel makes a quiet ticking noise, almost imperceptible normally. But as they grow older the ticking noise gets exponentially louder until people find it drives them demented. Ok a slight exaggeration but a new time switch is still available, and fixes the problem for another 40 years… hopefully

5) Blocked return air filter:

Not everyone realises there is a gauze air filter which catches particles in the air returning to the boiler for re-heating. The user instructions say this filter should be cleaned every fortnight, but if you don’t know it is there you’re unlikely to be cleaning it! As the months and years pass a thick layer of what we call ‘lint’ builds up on the filter and slows down nearly stops warm air circulation through the house. As this fault builds up imperceptibly slowly, the performance can degrade considerably before anyone realises. I get the occasional call from people saying their warm air heating just doesn’t seem to work as well as it used to, and sometimes my suggestion to find the filter and clean it solves the problem without me even needing to visit :)))

P.S. The filter draws out right from the top. Pull the grey plastic decor strip along the top front edge of the appliance, the filter is attached to it. Once I find a photo, I’ll add it here.

6) Blocked air supply vent:

This one is not really a boiler fault at all, but makes the boiler potentially unsafe. This boiler has an ‘open flue’, which means the combustion gas discharges to outside up an approximately vertical chimney by natural convection. For the chimney to work, the combustion gas flowing out of the house needs to be replaced with fresh air flowing into the house. So there will be an air vent or grill to outside supplying this fresh air to the interior of the house or flat. If the location of this vent grill has been thoughtlessly located, the incoming flow of fresh air creates a cold draught so users will sometimes block it off, preventing it supplying combustion air to the boiler and chimney. Checking for the existence of a fresh air supply is one of the mandatory safety checks we gas technicians are legally obliged to carry out and if we find a blocked-off air supply vent, we are obliged to classify the boiler as “AT RISK”, or unblock the air vent.

7) Intermittently chattering gas valve:

This is usually expensive to fix as the cause is not the gas valve, but the electronic control panel. Unfortunately the electronic control panel and the fan speed controller both need replacing at the same time as a pair, and the astronomical cost of these two parts will have wondering about replacing the whole boiler instead.

If you are within driving distance of me near Hungerford and need your warm air boiler fixing, call or text me on 07866 766364.

Mike