Can I remove the inner trickle vent covers?

Tried this they just sellotape them up 🙄. Then blame you for condensation mould
Have even had air bricks taped over with cardboard.. you can’t win

There really is no point. They will tape over them, as they would with airbricks. Perhaps supplying a non electric dehumidifier might be better? They use no power so more likely to be utilised. You could also supply any refill packs so you can keep an eye on the condensation situation.

Trickle vents without their front cover would look unsightly.
What I have done, for exactly this problem, is superglue a thin spacer at either end of the flap (inside the vent), so that it cannot close fully. This cannot be readily seen and ensures at least some ventilation.

If they're at home all the time and not heating, open trickle vents may not give sufficient airflow. I used to find bathroom extractor fans were usually off at the maintenance isolator switch.

I now fit powerful humidistat extractor fans - as moisture rises the fan runs faster - remove the pull cord and tamperproof isolators.
If there's a problem in a house, I'd look to fit one in the kitchen too - to combat cooking without a lid.

North East Somerset Council are recommending landlords give tenants a "Mini Indoor Thermometer, Hygrometer Temperature Humidity Monitor Gauge" available via Amazon https://amzn.eu/d/71e3m52 ). All tenants have to do is keep the temp/humidity gauges in the green! It gives them a visual indication of when heating needs to be increased on windows opened etc. For £5.99, it's worth a try.

Fit a Positive Pressure Ventilation system with a built in thermostatic heater ( Nuaire or similar ) core 100mm fresh air ventilation holes in 2 of the rooms and fit a humidistat extractor in the bathroom as well as a 100mm fresh air vent in the bathroom (do not use hit and miss vents that the tenant can shut off ) make sure your electrician wires it into the lighting circuit and puts any protection fuses out of sight ( loft space or above a cupboard ) so its difficult for the tenant to find them. If they can't find them they can't switch it off !!! unless they want to pull the lighting circuit breaker in the fusebox and sit in the dark !!
We had terrible condensation in 4 fully modernised flats ( originally a bad 1970 conversion ) Tenants are too tight to put the heating on and too lazy to dry washing in the garden we had streaming windows and mould even complaints that their toilet rolls were damp and wouldn't unspool !!!
I had our local enviromental officer out for advice and he said lovelly flats " tell them to open a window !!! " in desperation i fitted one flat with a Nuaire the difference was night and day within days one flat had completely clear windows the other three flats were streaming.
We have used this as a template for all our rentals ever since even the three bed houses..... basically removing moisture laiden air removes the chances of condensation related mould issues, it is written into all our AST that the drying of wet washing is to be confined to the bathroom on an airer with the door shut and no wet washing is to be dried on the radiators. We find tenants stay longer as there's no mould issues and when they do move out decorating is quicker as we have no mould to deal with !!!! ......HTH Simon

The tenant(s) will tape over them.

I had ecostat extractors fitted in the bathroom and kitchen - tenant turned them off; fitted passive air vents in each room - got reported to Enviromental Health for causing a draught.

Condensation was running down the inside of double glazed windows, polling on the window sills and running down the walls underneath. My tenant put it writing that if I wanted the Windows wiped each day in the autumn/winter/spring then I would have to send someone round as it wasn't her job to do this.

As Judith suggests, I've found that the only good solution to this issue is a positive air system. This pushes fresh, low humidity air into the house slowly but constantly an. In the process this pushes humid air out of any ventilation holes that exist (so tricklevents will help).

https://www.screwfix.com/p/vent-axia-pure-air-home-with-heater-positive-input-ventilation-240v/830ht

I tend to fit them with a timer so that they're not on overnight. They're also really cheap to run and very effective.

Reply to the comment left by Paul at 30/03/2023 - 11:18
What is a non electric dehumidifier?

Are Humidity Sensing trickle vents of any use?
https://www.bpdstore.co.uk/glidevale-energy-saver-humidity-sensitive-trickle-ventilator/p/182
Glidevale aren't the only producer. A company called "Aereco" do them too (apparently since 1984) and explain on their website how theirs works. Aereco also do humidity sensing air brick type ventilators too.