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ThunderGuts
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PostPosted: 15:46 - 17 Nov 2021    Post subject: Positive pressure ventilation Reply with quote

As the knowledge bounds of the BCF populace seem limitless, I'll throw this one in here to see what people's thoughts are.

We have a fairly common issue in that a period house has double glazing (with no trickle vents) so therefore is pretty well sealed, it's not practical (for security reasons) to have the windows either open or on "vent/night mode" 24/7. As a result, we get issues, particularly on the north side of the house and downstairs (where it's coolest) with high humidity, resulting in a bit of mould in corners, on the back of the sofa etc..

We have experimented with dehumidifiers and they do draw out plenty of moisture, but they're also noisy, require emptying fairly frequently and I suspect draw a noticeable collective power drain when ran as a permanent solution.

So my thoughts at present for a relatively economical but hopefully effective solution is to get trickle vents installed retrospectively, then get a positive pressure ventilation system installed in the loft (I saw installed, I think it's a DIY job as it looks very simple). The theory is they draw cool air from the loft void (ours is "roughly" converted, but still a hatch-access area and isn't sealed from the outside very well, so should be no issue in getting outside air and if necessary I could locate the unit behind the stud walls), this air is then pumped into the house where it warms up, it's relative humidity drops and at the same time the increased pressure in the house encourages a gradual flow of air out of the trickle vents (and any other openings in the building fabric). Yes, there's an overhead with heating as there's some cool air which will need warming, but it's better than having mouldy walls.

Has anyone experience of these positive pressure ventilation systems?

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steve the grease
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PostPosted: 16:09 - 17 Nov 2021    Post subject: Reply with quote

We have 2 fires- ( a Rayburn and a woodburner) , by September when we havent had a fire for months it starts to get difficult to close the internal doors because of the damp.
Ignoring the heat, the fires pull quite a lot of air through the house (before venting it up the chimneys of course) and dry the whole house out a treat . Positive ventilation sounds like a great idea and would do a similar job - moving air through the house, in a different way of course, but would cost a lot less than de- humidifiers would do, they can be expensive to run, and only affect one room rather than the whole house.
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tatters
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PostPosted: 16:18 - 17 Nov 2021    Post subject: Reply with quote

A older house even with double glazing will not be very well sealed therefore the additional of a HRV (heat recovery ventilator) which can also dehumidify would not be ideal, you would also have to run ductwork and can not vent into or from an loft/attic space. There are ductless options such as below but they are designed for single zones.

https://foursevenfive.com/lunos-e/

For low cost dehumidification/air exchange the installation of a 150-200 cfm thru-wall exhaust fan controlled by a humidistat would work. With the house not being as airtight as a modern build this shouldn't created noticeable negative static pressure when it is running.
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stinkwheel
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PostPosted: 16:41 - 17 Nov 2021    Post subject: Reply with quote

High ceilings? Sometimes even a ceiling fan on low can move the hot air back down in larger living rooms.

Humidity in my house reduced massively after I fitted a proper, externally vented cooker hood.

Also worth making sure you don't have a leak somewhere under the floor. Lift a couple of boards and stick your head in and have a sniff and a good look with a torch. Mould on the back of the sofa sounds like a bit more that just condensation. That would tend to form on a cold surface when the surface temperature reaches the dew point. If you have any sort of wall insulation, I wouldn't expect the walls to get that cold. I get that in my porch but I grow plants in there and it's only a single skin of blocks. I put up insulated wallpaper in there which helped reduce condensation by keeping the surface temperature higher.

Mrs stinkwheel lived in a house at her last work that was chronically damp. A few years later we found out there had been a burst water pipe under the house and it was effectively sitting on top of a swamp. They had to take up the floor and dig a hole in the soil under the house which kept filling with water which they then pumped out. Took weeks to dry it all out.
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ThunderGuts
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PostPosted: 16:44 - 17 Nov 2021    Post subject: Reply with quote

steve the grease wrote:
We have 2 fires- ( a Rayburn and a woodburner) , by September when we havent had a fire for months it starts to get difficult to close the internal doors because of the damp.
Ignoring the heat, the fires pull quite a lot of air through the house (before venting it up the chimneys of course) and dry the whole house out a treat . Positive ventilation sounds like a great idea and would do a similar job - moving air through the house, in a different way of course, but would cost a lot less than de- humidifiers would do, they can be expensive to run, and only affect one room rather than the whole house.


We have a woodburner in the living room, but in practice it isn't lit that frequently or for long enough to be a real benefit (if we worked from home it might be a different story of course).

tatters wrote:
A older house even with double glazing will not be very well sealed therefore the additional of a HRV (heat recovery ventilator) which can also dehumidify would not be ideal, you would also have to run ductwork and can not vent into or from an loft/attic space. There are ductless options such as below but they are designed for single zones.

https://foursevenfive.com/lunos-e/

For low cost dehumidification/air exchange the installation of a 150-200 cfm thru-wall exhaust fan controlled by a humidistat would work. With the house not being as airtight as a modern build this shouldn't created noticeable negative static pressure when it is running.


The type I'm thinking of doesn't operate heat recovery, instead it literally just pumps air into the house from the roof void. It is intended to just generally (slightly) pressurise in the inside of the house, creating a flow of air from the input of the positive ventilation fan out to the trickle vents and any other gaps in the building fabric.
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ThunderGuts
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PostPosted: 16:55 - 17 Nov 2021    Post subject: Reply with quote

stinkwheel wrote:
High ceilings? Sometimes even a ceiling fan on low can move the hot air back down in larger living rooms.

Humidity in my house reduced massively after I fitted a proper, externally vented cooker hood.

Also worth making sure you don't have a leak somewhere under the floor. Lift a couple of boards and stick your head in and have a sniff and a good look with a torch. Mould on the back of the sofa sounds like a bit more that just condensation. That would tend to form on a cold surface when the surface temperature reaches the dew point. If you have any sort of wall insulation, I wouldn't expect the walls to get that cold. I get that in my porch but I grow plants in there and it's only a single skin of blocks. I put up insulated wallpaper in there which helped reduce condensation by keeping the surface temperature higher.

Mrs stinkwheel lived in a house at her last work that was chronically damp. A few years later we found out there had been a burst water pipe under the house and it was effectively sitting on top of a swamp. They had to take up the floor and dig a hole in the soil under the house which kept filling with water which they then pumped out. Took weeks to dry it all out.


So, yes we do have high ceilings (circa 10ft downstairs). One of the first things I did when I got the house was fit an externally vented cooker hood, so that's ticked too.

In terms of damp underneath, well yes there is an issue there (as you may or may not recall from a post a few years ago). Unfortunately, no easy fix for that one - high water table means there's often standing water under the house but I have explored many options to deal with this and haven't arrived at a suitable solution; there are three "solutions";

1) Simplest, dig a pit, fit a sump pump and backfill with gravel. There are issues with the water table being lowered locally (which can upset the foundations as the house is sitting on clay) but the bigger issue is there's going to be a flow of water through the brick footings into the void, eroding both the lime mortar and potentially the soil too

2) Tank the floor void; line the void floor with concrete, tank the walls and floor and have a sump pump. Definitely the best for humidity control, but again concerns over the water table and erosion

3) French drains (deep ones!) around the perimeter of the house; partial effectiveness unless the neighbour agrees their house is included in the scheme. Massive upheaval and still the issues with changed ground conditions for the foundations

Essentially, all of these have risks which are significant, albeit ones which might not manifest themselves (either in the short term or possibly ever). The thing is, if they do manifest themselves, we're talking structural movement . . . not good. Options 2 and 3 are properly expensive too.

My plan with the floor void is to increase the ventilation with a combination of upgraded existing airbricks and some new ones.
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Kawasaki Jimbo
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PostPosted: 19:58 - 17 Nov 2021    Post subject: Reply with quote

An interesting point which I doubt ‘Insulate Britain’ have even considered is that in some circumstances cavity wall insulation promotes damp.
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JackButler
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PostPosted: 20:55 - 17 Nov 2021    Post subject: Reply with quote

Kawasaki Jimbo wrote:
An interesting point which I doubt ‘Insulate Britain’ have even considered is that in some circumstances cavity wall insulation promotes damp.


One of my rental properties is now suffering damp problems in the party wall area thanks to the twat next door paying well over the odds for cavity wall insulation to be installed by neanderthals.

Will these imbeciles ever learn that the glossy brochure is designed to take your £money off you, not save the Dolphins.
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ThunderGuts
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PostPosted: 21:31 - 17 Nov 2021    Post subject: Reply with quote

No cavity wall insulation here; I’m well aware of the incompatibility of such things with a period house! It has been offered to me (repeatedly) but I’m not interested.
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ThunderGuts
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PostPosted: 09:13 - 18 Nov 2021    Post subject: Reply with quote

Edit: little diagram; essentially my idea is there should be a slight movement of air from the house into the floor void in addition to out through the trickle vents, but then good airbrick ventilation to the floor void. This should prevent moist air moving from the void into the house, but also allow the void to be well ventilated.

Edit edit: sorry if stating the obvious, but R/H is relative humidity

https://www.bikechatforums.com/files/ppv.jpg
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A100man
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PostPosted: 12:47 - 19 Nov 2021    Post subject: Reply with quote

You say period house but how old?

Has the DPC failed or been breached somehow causing damp to enter the house form the ground? I assume all rainwater goods are in sound working condition

I've not heard of venting a house in this way. I mean trying to force enough damp air through tiny trickle vents seems implausible. More (under floor) vents/airbricks - yes and also extractor fan in damp room, but I think if possible try and address the cause.
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stinkwheel
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PostPosted: 18:19 - 19 Nov 2021    Post subject: Reply with quote

My worry would be that the air will take the path of least resistance. So it'll come down out of the attic then make a bee-line for the nearest vent.

So I'd imagine you'll get a cold draught upstairs and slamming doors into the stairwell. Because how is it going to get through the floor to move this air downstairs? The floors in my house are pretty much draught-proof because they have carpet and underlay on top and plaster underneath.

That said, maybe it does work, I do know I wouldn't want to leave my loft hatch open! It's bloody freezing cold in there in the winter, and draughty. As it should be with a cold roof.

Any reason you couldn't fit a radiator behind the sofa against the North wall?
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F18
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PostPosted: 16:56 - 20 Nov 2021    Post subject: Reply with quote

Well this will upset IB members but raising the internal temperature by 1°C will do a great deal to reducing excess humidity (and any mold). What's good for one must mean 2°C will see an even better improvement.

But back to PPV, a great idea I think - but I haven't actually done it (yet). I (we) like a deal of ventilation but would rather have windows open less (esp. winter).

But I think you need any heat-exchangers first and then pressure and displace (and waste). For us, an effective charcoal filter system would be required as some effing neigbours have taken back to the dark ages and are burning their sh*tty wood for heating and creating a right smog.

I've not calculated running costs but I expect similar to running a small dehumidifier (80-140W) which ought to feature in your sums.
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stinkwheel
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PostPosted: 21:13 - 20 Nov 2021    Post subject: Reply with quote

F18 wrote:
Well this will upset IB members but raising the internal temperature by 1°C will do a great deal to reducing excess humidity (and any mold). What's good for one must mean 2°C will see an even better improvement.


It's all about the dew point of the surface. Once the temperature of that wall is over the dew point, condensation won't form on it.

Dew point is a function of temperature and relative humidity. However it's not a linear function so small changes in temperature make big differences to dew point.
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DJS
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PostPosted: 12:04 - 21 Nov 2021    Post subject: Reply with quote

I was a bit slow in seeing the OP but my simple answer is I fitted one of these systems and it solved my problems perfectly.

The longer answer is as follows:

I have a detached, 1960s built house. It has high levels of loft insulation, UPVC double glazing, cavity wall insulation and no chimney. When I moved in in 2008 it still had its original ducted Electricaire heating systems which I replaced with a gas wet radiator system.

Kitchen extractor hood is ducted to the outside and bathrooms all have extract fans fitted.

We experienced condensation in the corners of the four bedrooms on the first floor. If the condensation wasn’t wiped away pdq it of course turned to mould.

We had builders investigate the house construction to make sure there was no cold bridging or other construction defects and all was ok.

My builder then recommended a positive ventilation system. I was sceptical to be honest but for the sake of £340 (it was about 5 or 6 years ago now) I gave it a go. It was easy to fit and I did it myself - picked up a power supply that was already in the loft, cut a hole in the landing ceiling to fit the register to and some simple assembly work in the loft. Couple of hours work max.

The system I bought was the Nuaire Drimaster and I went for the model with a heater fitted that automatically cuts in if the supply air temp is too low.

IIRC it has five speed settings for the fan dependent on the size of your property. Mine is set on the fourth fastest setting and it is whisper quiet. You can’t feel the air flow unless you put your hand up to the register. In the summer it cuts out altogether as the supply air temp is too warm and condensation is not a problem then.

Within 24 hours of switching it on all condensation was gone and hasn’t returned.

It was an excellent investment from my point of view and I wouldn’t hesitate to fit one again.

Hope this helps.
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kramdra
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PostPosted: 16:17 - 21 Nov 2021    Post subject: Re: Positive pressure ventilation Reply with quote

ThunderGuts wrote:
We have experimented with dehumidifiers and they do draw out plenty of moisture, but they're also noisy, require emptying fairly frequently and I suspect draw a noticeable collective power drain when ran as a permanent solution.


Plumb in a drain or a large container. locate it in a hallway with a fan so the noise is not an issue. The power drain on ours is about 125watts. Where does this 125 watts go? It doesnt. Consider it a low powered heater that will suck the water out of your house and make it feel quite a bit warmer. Well worth the £150 ours cost, we get about 3-5 litres out per day on a 12l unit. We should have got a larger one.
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ThunderGuts
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PostPosted: 09:28 - 24 Nov 2021    Post subject: Reply with quote

Thanks all for the responses.

Will have to investigate the heated options. Thumbs Up
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ThunderGuts
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PostPosted: 09:34 - 24 Nov 2021    Post subject: Reply with quote

stinkwheel wrote:
My worry would be that the air will take the path of least resistance. So it'll come down out of the attic then make a bee-line for the nearest vent.

So I'd imagine you'll get a cold draught upstairs and slamming doors into the stairwell. Because how is it going to get through the floor to move this air downstairs? The floors in my house are pretty much draught-proof because they have carpet and underlay on top and plaster underneath.

That said, maybe it does work, I do know I wouldn't want to leave my loft hatch open! It's bloody freezing cold in there in the winter, and draughty. As it should be with a cold roof.

Any reason you couldn't fit a radiator behind the sofa against the North wall?


I think that'd happen if there was a window wide open, but with trickle vents and other minor holes in the building fabric, I think it'd work through all apertures (not necessarily at the same rate, but it wouldn't need to, so long as there's a gradual movement of air). Our stairwell is quite open so the airflow would move into rooms upstairs and down. There are gaps under the internal doors so no issue with airflow percolating through; the air wouldn't move through the floors themselves for the reasons you've stated.

The cold and draughty loftspace is the exact reason why I want to source the air from there - when warmed up as it enters the house it'll have a low RH. Thumbs Up
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defblade
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PostPosted: 00:38 - 25 Nov 2021    Post subject: Reply with quote

Drimaster is bloody brilliant.

Wife insisted we bought the heater extra part the first winter, but I wouldn't have bothered.

Seems kinda pricey for what is effectively a large fan with filters, but it works so well you have to forgive it.

Feels (on the landing near the vent) sort of like you've got a window open somewhere.

Yes, very easy DIY fit, so long as you can get electric into the loft.

Early 1700's stone house here, no more streaming windows since 2 days after fitting the PIV - maybe 8 or 10 years ago now.
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Ribenapigeon
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PostPosted: 14:37 - 02 Dec 2021    Post subject: Reply with quote

How about insulation on the walls that get damp? A neighbor had this issue and had secondary insulation put directly onto the inner wall amd it fixed the issue.
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