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Dehumidifier for bathroom condensation?

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 Alex Pryor 24 Nov 2022

Our bathroom is on the N of the house, with breeze block walls and a flat roof. So it's perfect for condensation and mould. It's only 150mm x 260 and has a decent radiator but still gets loads of condensation after a shower.

Being so small we don't have much room for a dehumidifier so it needs to be not too big. There doesn't seem to be much choice between 1.5 litre and 10 litre max per day extraction, which I think may be too small and too large respectively.

Any suggestions?

 wintertree 24 Nov 2022
In reply to Alex Pryor:

> Any suggestions?

Turn the heating off and open a window, or fit an “to MVHR” heat exchanging fan, e.g. a K.Air unit, to ventilate the room whilst retaining the heat.

To comment on your actual question…

> There doesn't seem to be much choice between 1.5 litre and 10 litre max per day extraction, which I think may be too small and too large respectively.

It’s not “per day” that matters though; you want to remove the humidity rapidly after a shower so that the water in the air never has a chance to condense on your walls etc.  So, that tilts towards the higher capacity units. Make sure you pick the right type for your typical room temperature - for cooler rooms, a heat pump unit doesn’t work well and you’ll want a desiccant based unit.  Think about getting it plumbed in for simplicity, and think about how much a power feed will cost, as the room is unlikely to have sockets given the dimensions and the rules.  

 

 Fiona Reid 24 Nov 2022
In reply to Alex Pryor:

We use a Karcher Window vac to get rid of most of the moisture after a shower, like this https://www.currys.co.uk/products/karcher-wv-1-window-vacuum-cleaner-yellow-and-black-10210398.html

Our old house was really badly ventilated and using the window vac after a shower sucked up most of the moisture on the tiles etc (we'd floor to ceiling tiles so you could sook up a fair bit). Using it meant we never had any mould issues as the room was always bone dry again a few hours later. 

We still use it where we live now as it means the bathroom stays nice and dry.  

In reply to Alex Pryor:

Do you own the house?

If so forget the dehumidifier and instead install a whacking great extractor fan (not just a bog standard one, you want really high air flow), ideally with an automatic damp-activated switch.  Ventilation is the key to avoiding damp and mould.  Dehumidifiers are more for when there's no other option.

As wintertree says if you want to avoid losing heat a mechanical heat recovery ventilation unit may be better but will cost a lot more.

If not, ensure the window is wide open while showering and leave it open afterwards until the air isn't damp any more.  In my place this is the difference between it getting very mouldy very quickly and basically none at all.

Post edited at 00:40
In reply to Alex Pryor:

Do you have a shower squeegee? Total game changer for us. Gets your shower bone dry almost immediately so it doesn't sit evaporating moisture all day every day. 

In reply to purplemonkeyelephant:

Interesting.  I've tried one and it made little difference to any damp.  That was mostly generated by way of the condensing steam during the shower itself - so the most important time to be well ventilated is actually during the shower rather than after.

Same in a kitchen.  I reckon a lot of people don't realise most cooker hood installations are just set up to filter and chuck back into the room, which removes oily muck but not damp (no doubt posh ones exist with a built-in condensing unit, but most aren't like that).  You need to vent it outside if you don't want to have to open a window.

Post edited at 01:21
 La benya 25 Nov 2022
In reply to Neil Williams:

Don't most cooker hoods just vent outside?

3
In reply to La benya:

> Don't most cooker hoods just vent outside?

No

1
 veteye 25 Nov 2022
In reply to La benya:

Mine vents into the garage.

 veteye 25 Nov 2022
In reply to Neil Williams:

I've not got the central heating on so far. My bathroom is also north facing. 

I have the window open moderately during the shower; and I wait until the mirror at the other end evaporates its condensation, so that I can comb my hair. Then I reduce the window opening to the lockable slit, and leave it like that for most of the day.

What I will do soon is run the fan heater for a short while before the shower ( my FH just reaches inside the door from a plug in the bedroom), then turn it off before getting in the shower. The warmer air will absorb more moisture, and again I will have the window moderately open.

 deacondeacon 25 Nov 2022

   It's only 150mm x 260 

> Any suggestions?

Get a bigger bathroom 😅

In reply to La benya:

Thus proving my point.  No, most of them don't, but people think they do.  Most just filter the air and kick it back out into the kitchen.  This is fine in terms of taking particulates and grease out of the air but does nothing for damp.

Double glazing is a good test if you have it - if that steams up the atmosphere is already VERY damp.

Post edited at 08:48
 deepsoup 25 Nov 2022
In reply to La benya:

> Don't most cooker hoods just vent outside?

It largely depends where the cooker is. 

They generally do if they're mounted on an exterior wall, because it's quite easy then to just drill a hole straight through.  On an internal wall it's obviously a bit more difficult, so they tend to just push the air through a filter and back out into the room

1
 jkarran 25 Nov 2022
In reply to Alex Pryor:

Get an extractor fitted or improve the one you already have. One with a timer ensures the room is well purged after you leave and flow rates vary hugely. Or open a window, ideally two to get a throughflow (cold but it works).

Also if you do have a fan ensure there is actually a fresh air supply, if the room seals too well the extractor's theoretical performance rating is pretty meaningless*, crack a window (cold) or leave the door ajar (fine if the fan is keeping the steam in and privacy isn't a problem) or put a vent in the bottom of the door/shorten it to let air in under.

If you go for one of the heat recovery fans others have mentioned you do want the room to seal well, it is inlet and outlet. I'm not entirely convinced they're worth the money unless the bathroom gets a lot of use.

*centrifugal fans tend to sustain bigger pressure differentials than axial flow types, they maintain flow better when the inflow or outflow is restricted.

jk

 David Riley 25 Nov 2022
In reply to veteye:

> What I will do soon is run the fan heater for a short while before the shower ( my FH just reaches inside the door from a plug in the bedroom), then turn it off before getting in the shower. The warmer air will absorb more moisture, and again I will have the window moderately open.

Warming the air which can then transfer more moisture onto surfaces makes it worse.  But pre-warming the walls is good and lets you have the bathroom warm for comfort.  Ventilate afterwards and it all dries quickly.  I just open the door, keep the heat, and don't find the moisture causes problems elsewhere.

 montyjohn 25 Nov 2022
In reply to wintertree:

> or fit an “to MVHR” heat exchanging fan

I've never understood how these things actually work.

I've been meaning to look into it for a while. They claim 86% efficiency (some as much as 97%). I assume with that tiny little unit they are not using a condenser with refrigerant. They claim low power usage so I assume it's just a passive ceramic heat exchanger or similar?

I would have through the theoretical maximum efficiency would then be 50%. So if it's 20 degrees inside, 10 degrees outside, best you can hope for is 15 degrees air replacing lost air through the unit using a passive heat exchanger.

But they all claim much more than this 50%. Surely this is breaking one of the entropy laws of thermodynamics. Or is it?

 jkarran 25 Nov 2022
In reply to montyjohn:

> I've never understood how these things actually work. I've been meaning to look into it for a while. They claim 86% efficiency (some as much as 97%). I assume with that tiny little unit they are not using a condenser with refrigerant. They claim low power usage so I assume it's just a passive ceramic heat exchanger or similar?

Aluminium at a guess for cost and ease of manufacture but fundamentally yes, it's just a counter flow heat exchanger with a fan pushing air out one of the ducts.

> I would have through the theoretical maximum efficiency would then be 50%. So if it's 20 degrees inside, 10 degrees outside, best you can hope for is 15 degrees air replacing lost air through the unit using a passive heat exchanger.

If the exchanger were infinitely long with zero losses to the environment (except at the ends) then the exhaust gas would be at the inlet temperature and visa versa. 80ish% efficient for something a foot so long full of nice turbulent flow doesn't seem totally absurd given the fairly modest flow rate. High 90s does!

jk

Post edited at 12:06
 mutt 25 Nov 2022
In reply to Neil Williams:

> Interesting.  I've tried one and it made little difference to any damp.  That was mostly generated by way of the condensing steam during the shower itself - so the most important time to be well ventilated is actually during the shower rather than after.

This is confusing. Whilst the shower is running there is little hope of keeping a room moisture free for obvious reasons. Regardless of how good the venting is the humidity cannot get higher than 100 percent. 

The objective after the shower is to get the humidity down fast. Your extractor fan *should* equalise the humidity with the outside world quickly, but if there is standing water in the shower on the walls or on the tray it will evaporate and the fan won't equalise the humidity until that standing water is gone. 

Therefore use a squeegee until all standing water on the tray the walls and the door is gone. 

Then see how long the fan takes to equalise the humidity. If it's more than 5minutes you need a new extractor. It's probably broken but I guess you might have a long run to the outside. This generally results in pooling water in the run. If the run is ridged then get a pipe run and fit it with a slope to the outside. This will take away any pooling. 

Obviously if it's a very humid day then the room isn't going to dry out but at least you will have removed all the water that otherwise wouldn't evaporate and just sits there.

There really isn't a need for anything else unless of course you can't access the fan and run.

 montyjohn 25 Nov 2022
In reply to jkarran:

> 80ish% efficient for something a foot so long full of nice turbulent flow doesn't seem totally absurd given the fairly modest flow rate. High 90s does!

I think I need to go away and have a think as something still doesn't add up to me.

So I read on the specs of one that they blow one way (presumably until the heat exchanger reaches a certain temperature), then switch flow direction, then repeat.

Going back to 20 degrees inside, 10 degrees outside. If blowing out, you're not going to try and get the HE up to 20 degrees, as it's too small a gradient, dimensioning returns and you'll be exhausting almost 100% of your heat difference when going from 19 to 20 for example.

Same when blowing in. 

So I would have thought you would vary the HE temperature between 12.5 and 17.5 degrees to keep a decent gradient most of the time.

So when blowing in, the HE starts at 17.5, and ends at 12.5. So it's averages temperature must be 15 degrees,  so you've achieved 50% efficiency max.

I guess you could try and keep the HE between 15 and 17.5 but then you have uneven gradients and you will be pumping out longer than you pump in which is cheating as the air would have to be replaced by leaking in and therefore hot recovering any heat.

In reply to mutt:

In my bathroom very little moisture builds up if I shower with the window open a decent amount, it just goes straight out.  Whether you can do this without committing indecent exposure depends on the layout of yours.

With a powerful extractor you can get close.  In brand new Premier Inns you barely even get condensation on the mirror.

Post edited at 12:59
 wintertree 25 Nov 2022
In reply to montyjohn:

> I would have through the theoretical maximum efficiency would then be 50%

Coaxial counterflow heat exchanger. In theory at least their efficiency can approach 100%.   Reality is lower.  Edit: not pulsed operation as per another post above, just continuous coaxial flows.

They can’t reach the “above 100% efficiency as a powered space heater” of powered indoor dehumidifiers, because they loose the latent heat and temperature difference in the vented humidity however, the former being reclaimed by all dehumidifiers and the later being reclaimed by a condensing dehumidifier with plumbed in water drainage.  So, if the bathroom is directly electrically heated, a dehumidifier is better overall efficiency than MVHR.  

Post edited at 13:03
In reply to jkarran:

> *centrifugal fans tend to sustain bigger pressure differentials than axial flow types, they maintain flow better when the inflow or outflow is restricted.

Good luck finding one for DIY retrofit in your bathroom, everything on the market seems to be axial.

In reply to Alex Pryor:

Dehumidifier is shutting the gate after the horse has bolted, it's far better to move the wet air straight out before it has a chance to condense. Extractor fan as close to the shower with a good flow rate blowing straight out through a 6" duct is probably the best solution.  Bear in mind fans in 'zone a' need to be fed via an RCD and have an IPx6 rating iirc*.  If the wind's blowing in such a direction that air will be sucked out the window then an open window will flow far more air than a fan will as long as replacement air can enter from the rest of the house.  If the wind's blowing in, then the damp air will simply settle elsewhere in the house unless you let it outside from other rooms.

*You won't find one in B&Q, I've looked recently.

Post edited at 13:09
 jkarran 25 Nov 2022
In reply to Toerag:

> Good luck finding one for DIY retrofit in your bathroom, everything on the market seems to be axial.

Screwfix and Toolstation both list ducted centrifugal/hybrid options, I have one in my loft. Never seen one for in-wall fitment though.

Edit: they also list a few IP6x options for wet zone installation IIRC, weren't of interest to me at the time, was looking at ducted options anyway.

jk

Post edited at 13:42
In reply to mutt:

> This is confusing. Whilst the shower is running there is little hope of keeping a room moisture free for obvious reasons. <

I've same problem as OP. I think I'll try the squeegee......but fit a top to the shower ( could even be poly sheet) so that steam stays in while showering.

 Jon Greengrass 25 Nov 2022
In reply to Alex Pryor:

Where do you hang up your towel after you've dried yourself?

I found condensation is actually worse just after the heating comes on in the morning and the damp towel from the evening before starts to dry out.

In reply to Neil Williams:

The thing is, most of the steam condenses on the shower walls. By removing it you put a significant amount of water down the drain that would otherwise have to find its way out through evaporation, increasing the humidity in your bathroom. 

 montyjohn 26 Nov 2022
In reply to wintertree:

> Coaxial counterflow heat exchanger. In theory at least their efficiency can approach 100%. Reality is lower. Edit: not pulsed operation as per another post above, just continuous coaxial flows.

Yep, I'm with you now.

My logic was either on the lines of pulses or parallel flow which by my understanding would both cap out at 50%

I came back to jkarren's analogy but imagined an infinite number of slices. Each with it's own temperature gradient varying from the inside to outside temperature.

Surprisingly single and effective.

I want one.

OP Alex Pryor 26 Nov 2022
In reply to Alex Pryor:

Thanks for all the suggestions, I'll consider them carefully.

Just a bit more detail about the bathroom. We don't have a separate shower, but a mixer shower at the end of the bath, fed by a combi boiler. The bathroom window is above the bath so for privacy we have a shower curtain on that side. We also have a shower curtain on the other side as a screen didn't stop the high pressure water spraying outside. The shower curtains are the mouldiest things in the room.

There is an extractor fan, but it's probably crap and at the other end of the bathroom on the same week as the shower, so not ideally placed. But the bathroom is very small, and shares a party wall so there is not much scope for moving it. We have used a dehumidifier in the past with some success but it's also tricky to place (between the door and radiator). Towels get hung on a hanger on the radiator. 

I did try showering with the window ajar and squeegeed the tiles afterwards yesterday and it seemed to reduce condensation. But i can't realistically imagine us doing the squeegee every time we have a shower.  

OP Alex Pryor 26 Nov 2022
In reply to Alex Pryor:

PS re kitchen extractor fans, surely a good kitchen fitter would discuss this with the owner and vent externally if feasible. Ours did but the electricity meter was in the way of the ducting so we didn't bother.

 Jon Greengrass 26 Nov 2022
In reply to Alex Pryor:

Why not squeegee  after every shower?

it takes a minute, and while you are doing it the water is dripping off you into the bath and down the plug hole. Rather than onto a towel which the sits in the room all damp.

it’ll be ugly but run ducting along  the ceiling from the existing vent to above the shower head so that the moist air is being removed at source and not filling the room.

 La benya 26 Nov 2022
In reply to Alex Pryor:

The one house I've been in where the extractor didn't go outside was a rental so I assumed the landlord was being a landlord and cutting corners. All the houses I've lived in long term have vented outside, but thinking they were all detached. Lucky I guess.

Not sure how they get away with it. Regs for bathrooms is an opening window or an extractor to outside, don't see why a kitchen would be any different. 

 gethin_allen 26 Nov 2022
In reply to jkarran:

> Also if you do have a fan ensure there is actually a fresh air supply, if the room seals too well the extractor's theoretical performance rating is pretty meaningless*, crack a window (cold) or leave the door ajar (fine if the fan is keeping the steam in and privacy isn't a problem) or put a vent in the bottom of the door/shorten it to let air in under.

Building regs state that there should be a 760 mm2 vent into a bathroom to allow for this, conveniently (on purpose) this works out as a 10 mm gap at the bottom of the door. It's a shame that many carpenters fitting bathroom doors don't know/follow this.

 gethin_allen 26 Nov 2022
In reply to Toerag:

There are loads of options, Bathroom electrical zone 1 is only up to 225 cm from the finished surface leaving 15 cm to play with in a normal modern house. If you have a loft above it's very easy to fit a ducted fan in the loft space and have the inlet directly above the shower, you can even get all in one units with a light built in so you only have to wire it to a light supply without changing the standard switch etc.

Regarding centrifugal fans, again there are many options from all the big brands, I'd probably go for a ducted unit, maybe something like a swiftair st100, just because the wall/ceiling surface mount options are mostly a bit ugly and would need to be mounted outside zone 1 although many of them contain humidity sensors and offer multi speed settings.

 Rick Graham 26 Nov 2022
In reply to gethin_allen:

7600mm2?

In reply to gethin_allen:

Do the regs cover MVHR? I would imagine you'd want no gap at all for the MVHR to work properly?  We considered one for our flat when we were renovating, but the general thinking was that they only get close to their maximum efficiencies in a very air-tight house as the incoming vent is passive. I'm surprised no-one makes an "active" version for more leaky houses...

 gethin_allen 26 Nov 2022
In reply to Rick Graham:

> 7600mm2?

That's the one.

 gethin_allen 26 Nov 2022
In reply to Alasdair Fulton:

No idea, I only happened across the numbers by chance while I was trying to find some other info about bathroom building regs.

 jkarran 26 Nov 2022
In reply to Alex Pryor:

> I did try showering with the window ajar and squeegeed the tiles afterwards yesterday and it seemed to reduce condensation. But i can't realistically imagine us doing the squeegee every time we have a shower.  

I manage and I'm a total slob, it just becomes habit and takes seconds. Oh and my wife breaks my balls if I don't squeegee, you just need the right motivation  

I'd look into getting a better fan with more flow and an over run timer.

Jk

 Wee Davie 26 Nov 2022
In reply to Alex Pryor:

You need air flow. Unless you are able to open a window, or run a fan for ages it would be leave the door open and keep the place scrubbed down. Bathrooms without windows are a nightmare.

 Tringa 27 Nov 2022
In reply to Alex Pryor:

You've said the screen on the other side from the window did not stop the water spraying outside. Could you get a larger screen and/or change its placement. I think there should be some way of using a screen that does stop the water spraying out as it would be much easy to remove water from than a curtain.

Any chance of using an opaque/translucent screen on the window side?

Dave

 Hooo 27 Nov 2022
In reply to Alex Pryor:

1. Get a better extractor. It made a massive difference to my bathroom when I realised that the existing one made a load of noise but moved very little air. Got a decent one and that pretty much cured the mould issue.

2. Get a blind for the window that can be wiped down easily. Curtains are horrible for staying wet and going mouldy.

3. Get a better screen for the other side. I've got a glass one, it wasn't very expensive and it keeps the water in. So much nicer than a curtain.

 mutt 22:48 Sun
In reply to Alex Pryor:

> Thanks for all the suggestions, I'll consider them carefully.

> Just a bit more detail about the bathroom. We don't have a separate shower, but a mixer shower at the end of the bath, fed by a combi boiler. The bathroom window is above the bath so for privacy we have a shower curtain on that side. We also have a shower curtain on the other side as a screen didn't stop the high pressure water spraying outside. The shower curtains are the mouldiest things in the room.

Fit a glass shower screen should help then. Otherwise dry the current fully drawn open and out of the bath.

> There is an extractor fan, but it's probably crap and at the other end of the bathroom on the same week as the shower, so not ideally placed. But the bathroom is very small, and shares a party wall so there is not much scope for moving it. We have used a dehumidifier in the past with some success but it's also tricky to place (between the door and radiator). Towels get hung on a hanger on the radiator. 

Try and dry the towels in another room. Evaporating into an a room that is moisture free is going to be quicker and the amount of moisture added to the room will be too small for mould to get established

Can you get an electric extractor fitted into the window? As high as possible.

> I did try showering with the window ajar and squeegeed the tiles afterwards yesterday and it seemed to reduce condensation. But i can't realistically imagine us doing the squeegee every time we have a shower.  

Try harder. I squeejee the shower twice a day or more as my kids feel like you do. 

 Shani 23:18 Sun
In reply to Alex Pryor:

Running dehumidifiers is expensive and extractor fans are ineffective in large, old bathrooms. 

I'd recommend shower pods, which contain the moisture very effectively and have the benefits of massaging jets, bluetooth speakers and mood-lighting, should you so wish to use them!

I'm eyeing up this bad-boy which you can currently get for under £1100. : https://www.insigniashowers.com/premium-shower-cabin-800x800mm-black-frame-clear-glass.html#93=4

 Uncle Derek 07:40 Mon
In reply to Alex Pryor:

What we do is turn shower on, wet body, turn shower off, soap body and scrub, turn shower on and remove soap, turn off immediately and not stand under shower for ages. 
This puts less steam in the air, and uses less water and energy. We also have a good fan and squeegee. But the game changer for us, is having the shower on less.

Just had a shower, and no condensation on mirror.

In reply to Alex Pryor:

> I did try showering with the window ajar and squeegeed the tiles afterwards yesterday and it seemed to reduce condensation. But i can't realistically imagine us doing the squeegee every time we have a shower.  

A microfibre / pertex artificial chamois works well, especially if you've lumpy tiles that a squeegee doesn't work well on.  I normally rinse the walls down with the shower to get rid of any potential mould food then wipe down with the chammy at my out-laws house where wiping down in the rules and their family bathroom looks just like it did the day it was put in in '92.

 Sans-Plan 10:17 Mon
In reply to Alex Pryor:

> But i can't realistically imagine us doing the squeegee every time we have a shower.  

Why not? It's exactly what we do, admittedly ours is a shower cubicle but it only takes a minute after a shower to get rid of most if not all the water from tiles and cubicle, makes it easier to clean too.

Post edited at 10:17
In reply to wintertree:

Just FYI, your post got me ordering am MVHR, it solves an issue I've had in the bathroom for a while that I was stumped with.


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