Repairing a Ridgeflow Conservatory Vent Fan

The Ridgeflow is a conservatory vent fan that was was manufactured by Ultraframe, a company that supplies parts for conservatories and similar extensions. An external thermostat can be connected to the terminals provided, alternatively if these terminals are connected together then the built-in electronic thermostat will turn the fan on when the ridge temperature reaches about 38C.

The Ultraframe site lists the model number as RFCB001/UF here however my particular unit was labelled RFUC001. I believe it dates from around 2002 but it is possible that it was replaced some time prior to 2016. So bear in mind that the instructions that follow may not be applicable to newer versions of the Ridgeflow.

Our vent fan failed during hot weather in 2022. This year I got around to investigating the problem. Unfortunately the unit is no longer manufactured, nor does Ultraframe offer a compatible replacement. As there appears to be little data available for this unit apart from the fitting instructions here I am providing this information in case it is useful to others.

General observations

  • The unit uses a brushed DC motor. This is a poor choice for a unit that can be expected to run for long periods.
  • The motor part number is KD5B-18150. I have not been able to find this exact part, however similar sized motors that include the -18150 part of the number are this and this. The motor in the Ridgeflow unit has a shaft length of around 20mm rather than the 13 to 15mm shaft of those units.
  • The motor brushes are not replaceable, so when the brushes wear out the whole motor must be replaced.

I was tempted to replace the whole unit by two or more 230VAC brushless blower fans in a 3D printed holder. However, when I removed the motor it appeared that the brushes had plenty of wear left in them, so I elected to repair the unit instead.

Removing the unit

  • Isolate the circuit that powers the unit
  • Unclip and remove the white plastic cover
  • Pull off the two grey plastic end caps. The mains input cable passes through a slot in one of them, so that one will remain on the cable.
  • Pull the mains inlet connector out of the terminal block on the printed circuit board (no need to undo the terminals because it is a pluggable connector)
  • Remove the two screws (one at each end) that secure the unit to the roof

Removing the electronics board

  • Disconnect the wires from the motor by pulling the connectors off the motor tags.
  • Feed enough of the motor wires along the groove in the housing to provide some slack at the transformer end.
  • Two plastic rivets secure the transformer to the housing. Drill through these from the housing side. I found it easiest to use a 1.5 mm drill bit to drill a hole centrally in the rivet, then use this as a pilot hole to drill the rivets out using a 4.5mm drill bit. The holes in the housing are between 4.5 and 5mm diameter.
  • The transformer and attached PCB can then be slid out of the housing.

If you need to remove the motor

The motor is held by two rubber grommets screwed to the front. You do not need to remove these screws because the grommets fit in a keyway. To remove the motor you need to rotate the body of the motor from the back clockwise one eighth of a turn. This will be easiest if you make a tool to fit into the two slots in the back of the motor. After rotating the motor, it can be pulled out. Once the motor is out, the rotor can also be removed and cleaned.

Repairing the electronics board

The incoming mains is fed directly to the transformer (no fuse) which has a single 12V 0.5A secondary. This is fed to a bridge rectifier follower by a 10uF tantalum smoothing capacitor. This feeds a PTC fuse, the motor and a N-channel power mosfet in series. The low value of the smoothing capacitor means that the motor is effectively running from only slightly smoothed full wave rectified DC.

A diode takes this slightly smoothed DC power and feeds another 10uF capacitor, followed by a resistor and a Zener diode. This regulated supply feeds a Schmitt trigger constructed from a NPN transistor, a PNP transistor and a few resistors. The input is connected to a thermistor and the output drives the mosfet.

In my case the PTC fuse was showing a resistance of about 240 ohms. The fuse part number is Bourns MF-R030 which should have a much lower DC resistance, so it was clearly faulty. Replacing that PTC fuse was all I needed to do to get the unit working again. The PTC fuse had been covered by some heatshrink sleeving – possibly because the surface temperature might otherwise be sufficient to melt the nearby motor wires – so I sleeved the new part too.


Reassembly is just the reverse of assembly. To secure the transformer and PCB to the housing you could epoxy a couple of Nyloc nuts to the transformer base so that you can insert screws through the housing, or use blind rivets. I used two 4.8mm aluminium blind rivets.

Please post a comment if you have repaired one of these units and have anything to add to the above.

This entry was posted in Electronics and tagged . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s