Story so far, I replaced my old Helios MVHR unit in April time with a Komfovent unit. The key difference is that the method of HR; unlike the Helios unit, the Komfovent unit doesn't have a condensate drain.

At the same time I added in an in-line air filter to remove more of the crud before it got to the unit. This is an un-insulated galvanised metal box.

All the pipe work to/from the unit is the flexible insulated type. The unit itself is in a sort of wardrobe/storage room that does not have any specific ventilation (ironic).

Outside temp is around freezing and has been for a while now.

Just had a check of the unit, and noticed condensation on the outside of the filter box. Some of the flexible pipe also feels 'heavy' - my suspicion is that the insulation has got saturated.

My feeling is that the humidity in that cupboard/room is too high (I have put a sensor in there to measure it - only suggesting 50% RH currently). I suspect I either need to introduce a dehumidifier in there, or actually fit a ventilation 'extract' for that room (relatively easy to do.

My other thought is that the condensation on the outside of the box, is being wicked by the insulation - the 2 layers of tin foil, has insulation between them. Inner tinfoil tube is taped to the spigot. The end of the insulation is left exposed so damp air could be getting in there. No vapour barrier effectively.

Thoughts / suggestions?

The important bit of info is that this is happening on the inlet pipe. It is this unit http://www.komfovent.com/en/products/domekt-r-450-v-c6m-742 with a rotary heat exchanger.

Double checked the install and a vertical extract is fine.

The issue is totally being caused by the metal inline filter I fitted. Condensation forms on the outside and can work it's way into the insulation of the insulated pipe as the ends are not sealed. Unintended consequences.

I am going to need to replace the insulated pipe and better insulate the join between the pipe and the box, and seal the end of the flexible insulated pipe, so the insulation is not exposed. I had reused the pipe I had to avoid the cost of new pipe, but now it is unavoidable.

I will try and find a sheet of EPDM to cut up and stick to the outside of the filter box to insulate the shell. I have some of this 50mm wide tape that I have used on difficult to insulate pipe fittings and I could wind that round the spigots. https://insulationandlagging.co.uk/Pipe-Insulation-Lagging/K-Flex-Class-0-Sheet-Insulation/KFLEX-TAPE

Final thing would be an inline heater close to the inlet, but I'd rather not for the limited time in the year it is an issue.

Key lesson, seal the ends of the flexible insulated pipe!

Interestingly, I can't find any insulation for rigid pipes. Do folk that use that just drain the pipes? The drips off the outside of the metal box are not insignificant.