Dehumidifier, trickle vents and empty property

Can't really answer the main question but some thoughts occur to me:

Is the dehumidifier powerful enough to keep the whole property dry? Even rooms far away from it?

Does the dehumidifier have a drain or just fill up its own container? In which case you'll have to visit regularly in order to empty it, otherwise it'll just switch itself off.

The level of humidity isnt a problem.

Its where you have differences in humidity which cause issues.
Damp and mould manifest in the areas where those differences interact.

The vents allow the internal/external factors to equalise, which reduces the issue.
Open the vents

EDIT: to add, you also need the air to circulate around the property. The vents will also help this.

It's a small 2 bedroom flat.

The dehumidifier is specced to be suitable for a 4 bed home.

All internal doors are open and the dehumidifier is stood in the shower with a drain hose.

Keep the vents closed if theres a source of incoming damp (rising damp or whatever) that ventilation alone hasn't been enough to overcome - if you ventilate whilst running the dehumidifier then you'd just trying to dehumidify the whole postcode 🙂 Ventilate while the property is occupied.

Unoccupied you won't be getting the usual sources of moistures (breathing, cooking, showers, laundry) so you've be counteracting damp issues that are inherent in the building

Something to keep an eye on - if theres a lot of humidity to deal with - is how quickly the dehumidifier will fill its tank. If it fills up and shuts off it's achieving nothing.

Some types have a little nozzle you can attach a pipe to (that of course isn't in the box when you buy it) to bypass the built in tank and drain into something larger

I assume this is after the property had a leak or a mould problem or something?

In that case if you have a humidistat on the dehumidifier then personally I would leave the trickle vents all closed and leave the dehumidifier running constantly with the stat set to 40% or something for a while to ensure it's fully dried out.

If it's an ongoing concern, then I'd be more inclined to use the dehumidifier to bring the level down, then switch it off and leave the trickle vents open.

We did similar in MIL's 4 bed house - doors open, didn't touch the vents, and left the dehumidifier running on the kitchen top, with the drain into the sink. Did the job in preventing any issues in a long term empty home (three years).

if you ventilate whilst running the dehumidifier then you’d just trying to dehumidify the whole postcode

I would have thought this - the dehumifier will create the circulation and add a little heat.  I presume it's one you can put on a timer (or use a timer plug) and that it will shut down when a pre-set level of humidity is reached (i.e. 55%)

As per others, close the vents and I assume the dehumidifier has a humidistat built in so it'll regulate itself.

Chew is half right, the issue isn't differences in humidity, it's where you have a difference in dew points.  e.g. warm moist air form the kitchen with a dew point of ~20C circulating through the house and contacting a 15C wall elsewhere will condense and cause a problem.  But if there's no source of moisture (breathing, cooking, laundry, showers, etc) then that shouldn't happen.

The only issue I can see is that if the house is cold then it's hard to dry it out.  If it's just leaving the house dry and empty then the dehumidifier will probably be enough on it's own. If you're trying to dry out an existing damp problem then you'll need the heating on to evaporate that moisture out of the materials and into the air so the dehumidifier can do it's work.

I run a meaco arete and am constantly amazed how much water it takes out of the air.  I darent look at the hepa filter as got builders in so it's cleaning the air too.

Mines set at 70 so once it achieves that it turns off, costs c 1 quid a day max.

Personally I'd leave the vents open. Even on a damp day good ventilation can bring humidity down.

It's also worth asking what type of dehumidifier it is. Your average compressor dehumidifier works by cooling warm air. If the property is empty, is it being heated? If not, it's not going to be very effective.

Out dehumidifier is currently sat around 35 to 40% RH. At this time of year, it's unlikely to pull much out.

Low level heat and dehumidifier. If the building fabric temp drops in a cold snap followed by warmer damp few days you can’t help but have walls below dee point and condensation form. A bit more info would be helpful. Is the flat getting damp from a leaking roof or high ground levels or purely from being cold and uninhabited

Where is any moisture actually coming from in an empty flat? If it is damp dry it out with vents closed but it shouldn't be an on going problem?

Had to look after a friends empty house through 2 winters, visited once a week (insurance required once a fortnight), turned heating on when frosty and never had any problems with humidity. Despite there being an undetectable water leak somewhere in the house.

The property is a shared family holiday property in a very exposed coastal location, basically over looking the sea.

Any nasty weather from the NE round to the SE and it gets a battering.

The flat roof was basically ripped off the full block during Storm Arwen.

The 1st floor flats were messed up, we escaped the worst as it's on the ground floor.

It gets used only occasionally in the winter.

It's basically in continuous use from April to September. So the trickle vents will be used then.

From reading all above, I will keep it was sealed as possible and run the dehumidifier during the winter.

its probably worth getting some little digital temp / humidity meters so you can see what the demumidifier is actually achieving. You can see the water collected by the machine and believe that to be progress but without knowing the rate at which moisture is getting into the property you don't really know to what degree you're actually changing the moisture level in the building. You could be hoovering loads of moisture out and the building might not be any less damp.

You can get quite cheap ones on amazon. Getting them in packs of three if they are cheap enough can be worthwhile - sit them together for a while so you can see if they are all reading in roughly the same way - they are never going to be perfectly calibrated -  then spread them out around the property, over time that might inform the best place to site the dehumidifier.

@maccruiskeen - Got a £10 humidity meter just yesterday for that very reason.

It records temperature and humidity for up to 1 year at 1 minute intervals.

Connects to app via Bluetooth and then it emails the data, at your selected interval, as a CSV to you.

I love messing with stuff like this.

Regards location, I really have a choice of the kitchen sink or the shower. I could use a longer drain hose but don't want to risk a leak as the thing will be left for a few weeks at a time and that could make a big mess.

I will probably get a USB powered fan and leave it running just to keep the circulation going.

ah - cool if you're using a drain hose. It means you've no real idea of how much moisture is being collected but at least it keeps running, otherwise it can just shut off after a day or two