PIV

[swill453]

Positive Input Ventilation

Why aren't more people talking about this?

We have a 70s bungalow in the north part of central Scotland, at about 100m above sea level. Being a bungalow it has a lot of external surface area in relation to the internal space.

The insulation on the house wasn't really up to scratch when we moved in, and in the cold spell of December 2022 we had a lot of problems with condensation and dampness leading to black mould. The most northerly corner of our bedroom was noted at its worst as 9 degrees C in the morning and over 90% humidity. The double glazed unit had masses of condensation, mould formed on the external wall, behind the headboard of the bed, and also inside a chest of drawers there.

In the spring we got the insulation sorted out - loft topped up, cavity walls done, and under the suspended floor too. This has made a big difference to the temperature, the house retains heat far better now.

But humidity was still a problem. We got a bunch of humidity meters (https://www.amazon.co.uk/gp/product/B094N2KM7B) and spread them around the house. Hardly any room was below 70%, and 80%+ was typical in the bedroom we use.

After a bit of research I came across the concept of PIV. This is a fan unit that filters air from the loft and blows it down through a vent in the hall ceiling. The idea is that it creates positive pressure in the house so there is a constant small flow of air to the outside. A vital point is that the trickle vents above all double glazed windows are opened (ours were already).

We got this one https://www.amazon.co.uk/gp/product/B00NIV51RU though cheaper models are available starting about £300 or so.

I fitted it myself. All you have to do is cut a hole in the ceiling, fit the vent, hang the unit from a rafter in the loft with a nylon string and connect to power.

We have the fan on the 2nd lowest level (of 6) after having it at a higher level for the first few days.

It's been a revelation! Since about 2 days after installation we haven't seen any humidity readings over 60%, and levels of 45% to 55% are typical. Zero condensation on the windows in the mornings, and of course no danger of black mould.

Since dry air is cheaper to heat than moist air it's probably saving on heating costs, though that's difficult to quantify given the insulation we've added as well.

We have it on 24x7, and at the setting it's on it is costing about 70p/month (3.4W). It's virtually silent unless you stand underneath it and listen carefully. It doesn't cause a noticeable draft.

Highly recommended.

Scott.

[tacpot12]

They are very good - I recommend them in situations where condenstation is a problem or neighbours cooking (or smoking) smell is coming into properties.

They have been somewhat superceeded by MHRV systems, but MHRV don't pressurise the home to the same degree so smells can still get in, unless the intake is carefully sited to avoid the problem.

Sometimes PIV systems fitted in flats get a bad press because they don't have the loft that you have, so draw their air in from outside; in the winter this means the air being pumped into the property is as cold as outside, unless a pre-heater has been installed. Pre-heaters may not be installed due to the initial cost or concerns about the cost of running the heater. The heater is thermostatically controlled so doesn't run all the time, but the cost will add up across a winter. You are lucky that you can draw in warn air from your loft to help drive moisture out of your home. They generally work very well.

[swill453]

Sometimes PIV systems fitted in flats get a bad press because they don't have the loft that you have, so draw their air in from outside; in the winter this means the air being pumped into the property is as cold as outside, unless a pre-heater has been installed. Pre-heaters may not be installed due to the initial cost or concerns about the cost of running the heater. The heater is thermostatically controlled so doesn't run all the time, but the cost will add up across a winter. You are lucky that you can draw in warn air from your loft to help drive moisture out of your home. They generally work very well.

I did buy one with a heater just in case, but I don't actually think the heating element has kicked in. I have it turned to the lowest level (5 degrees C).

Scott.

[Tedx]

My loft is freezing.

Id be worried I'd just be creating a permanent draught. I know you said yours has a heater, but I wonder if it was possible to heat the air 'passively'. Maybe by routing the ducting under your loft insulation or something?

[Urbandreamer]

Positive Input Ventilation

Why aren't more people talking about this?

Quite simply because its an answer to a problem that we don't want to recognize exists.

In simple terms you are DELIBERATELY, and for good reason, increasing heat leaks.

The "correct" method would be to both add air, and remove air. Exchanging the heat between the two.
By increasing the ventilation (forcing it) you have simply undone the purpose of the insulation.
As you are forcing "hot" air out of the building and replacing it with cold air that needs heating.

PLEASE don't take this the wrong way. Damp is a SERIOUS problem. Ventilation is the traditional answer. The heat exchange that I mentioned either has to be very carefully designed to limit how well it works, or have a drain.

Damp HAS to happen. To start with, as has been said in SciFi, we are bags of mostly water. We MAKE places damp! That ignores the hot drinks or cooking. With respect to the cooking, it ignores gas or a log fire (which has to produce water as it burns). You can't oxidize hydrocarbons to produce heat without combining hydrogen and oxygen, producing water.

But it's never acceptable to point out that it's "complicated".

[88V8]

My loft is freezing.

Likewise.
If one has an insulated roof, PIV could work OK. But probably most of us don't. We insulate our attic floors because it's a lot easier.

Two dehumidifiers pretty well 24/7 do help us a little with condensation but are not cheap to run.
What eventually diminishes the condensation for us, by this time of year at least, is two woodburners.

V8

[swill453]

My loft is freezing.

Id be worried I'd just be creating a permanent draught. I know you said yours has a heater, but I wonder if it was possible to heat the air 'passively'. Maybe by routing the ducting under your loft insulation or something?

To be honest I don't think it's really much of a problem. If I put my hand up to the vent I can feel the air coming in is fairly cold, but it's only a trickle so a metre away from it you wouldn't know. And I think heating dry, slightly cool air is probably better than heating slightly warmer moist air.

I guess you might be able to jury-rig some kind of canopy over it so it's using air that's not above 30cm of insulation, but I'm not sure it'd be worth it.

Scott.

[csearle]

I did buy one with a heater just in case, but I don't actually think the heating element has kicked in. I have it turned to the lowest level (5 degrees C).

Scott, despite his name, lives in Ciudad de la Paz, Equatorial Guinea.

Chris
(Only kidding.)

[9873210]

You're blowing warm, fairly moist air through your walls. If the outside is cold enough it will condense inside the walls. Not sure if this will, in practice, be a problem in Scotland but in someplace like Manitoba it would not only condense but would freeze, which is not good.

People like to say that houses, like people, need to breath, which is true. But people should breeze through their nose, not a sucking chest wound. Hence tight houses with balanced mechanical ventilation with designed ingress and egress.

[Mike4]

We got this one https://www.amazon.co.uk/gp/product/B00NIV51RU though cheaper models are available starting about £300 or so.

£300?! I'm shocked. I'd have thought a five quid computer fan would be capable of doing the same job.

[swill453]

You're blowing warm, fairly moist air through your walls. If the outside is cold enough it will condense inside the walls. Not sure if this will, in practice, be a problem in Scotland but in someplace like Manitoba it would not only condense but would freeze, which is not good.

It'll take a path of less resistance. Through open vents or a solid brick wall? I know what my money's on.

Scott.

[Dicky99]

Positive Input Ventilation

Why aren't more people talking about this?

We have a 70s bungalow in the north part of central Scotland, at about 100m above sea level. Being a bungalow it has a lot of external surface area in relation to the internal space.

The insulation on the house wasn't really up to scratch when we moved in, and in the cold spell of December 2022 we had a lot of problems with condensation and dampness leading to black mould. The most northerly corner of our bedroom was noted at its worst as 9 degrees C in the morning and over 90% humidity. The double glazed unit had masses of condensation, mould formed on the external wall, behind the headboard of the bed, and also inside a chest of drawers there.

In the spring we got the insulation sorted out - loft topped up, cavity walls done, and under the suspended floor too. This has made a big difference to the temperature, the house retains heat far better now.

But humidity was still a problem. We got a bunch of humidity meters (https://www.amazon.co.uk/gp/product/B094N2KM7B) and spread them around the house. Hardly any room was below 70%, and 80%+ was typical in the bedroom we use.

After a bit of research I came across the concept of PIV. This is a fan unit that filters air from the loft and blows it down through a vent in the hall ceiling. The idea is that it creates positive pressure in the house so there is a constant small flow of air to the outside. A vital point is that the trickle vents above all double glazed windows are opened (ours were already).

We got this one https://www.amazon.co.uk/gp/product/B00NIV51RU though cheaper models are available starting about £300 or so.

I fitted it myself. All you have to do is cut a hole in the ceiling, fit the vent, hang the unit from a rafter in the loft with a nylon string and connect to power.

We have the fan on the 2nd lowest level (of 6) after having it at a higher level for the first few days.

It's been a revelation! Since about 2 days after installation we haven't seen any humidity readings over 60%, and levels of 45% to 55% are typical. Zero condensation on the windows in the mornings, and of course no danger of black mould.

Since dry air is cheaper to heat than moist air it's probably saving on heating costs, though that's difficult to quantify given the insulation we've added as well.

We have it on 24x7, and at the setting it's on it is costing about 70p/month (3.4W). It's virtually silent unless you stand underneath it and listen carefully. It doesn't cause a noticeable draft.

Highly recommended.

Scott.

It's been around for decades. It never really took off in social housing where the greatest issues with condensation and mould occur. Reasons I recall were, in no particular order, people just didn't warm to a constant trickle of icy cold air into their landing area, the filters needing periodic changing would create a large ongoing maintenance cost, they were not best suited to flats as the unit had to be mounted over the kitchen wall units and then surfaces ducted to a centrally located discharge vent. These days they're probably low watts but I guess with early models having something using elec 24/7 wouldn't have gone down great with tenants.

These days constant trickle fans fitted in the kitchen and bathroom do a similar job. Trickle 24/7 and auto boost when humidity level dictates and way quiter than humidistat fans of old. Some models even come equipped with integral data loggers to collect data on temperature and humidity.

[swill453]

£300?! I'm shocked. I'd have thought a five quid computer fan would be capable of doing the same job.

Add filters, ducting, a vent, a control panel, and guarantee it'll run 24x7 for years. But if you think there's a gap in the market fill your boots.

Scott

[Mike4]

£300?! I'm shocked. I'd have thought a five quid computer fan would be capable of doing the same job.

Add filters, ducting, a vent, a control panel, and guarantee it'll run 24x7 for years. But if you think there's a gap in the market fill your boots.

Scott

Blimey, talk about extrapolation from a casual observation!

[88V8]

You're blowing warm, fairly moist air through your walls. If the outside is cold enough it will condense inside the walls. Not sure if this will, in practice, be a problem in Scotland but in someplace like Manitoba it would not only condense but would freeze, which is not good.

It'll take a path of less resistance. Through open vents or a solid brick wall? I know what my money's on.

It has a chance of working in a small/midsize modern(ish) house with plastic windows and open trickle vents. And open internal doors.
Not in our 400yo house with somewhat variable gaps around the doors 'n windows.
But if it does work then it could be a very cost-effective health improver.

There is no one-size solution to the woes of our ancient housing stock and the idiocy of (some of) its inhabitants.

V8

[swill453]

It has a chance of working in a small/midsize modern(ish) house with plastic windows and open trickle vents. And open internal doors.
Not in our 400yo house with somewhat variable gaps around the doors 'n windows.
But if it does work then it could be a very cost-effective health improver.

Yep, that's exactly how it works for me. Drier air, no condensation or mould, and pretty neutral effect on energy bill.

Scott.

[Dod101]

I have a bungalow, well if any building all on one level is called such. Mine is a biggish house with big rooms all on one level. Built around 30 years ago and well insulated. I never have any condensation but I think that is because there is a good deal of air circulation anyway.I have these trickle vents above the double glazed windows and they are always open. But I do not like cold so my central heating is on pretty much all day at this time of year. I tend to use my woodburner late afternoon and evenings and the other heating goes off then.

As the OP has discovered, air circulation seems to be the key.

Dod

[88V8]

As the OP has discovered, air circulation seems to be the key.

I think so.
The room in which I'm sitting, in what would be the attic if it had one, before I redecorated and painted with breathable paint had quite a lot of mould along the eaves plates especially at the north end. I put two 8" fans to blow along each eaves for half an hour morning and evening and I try to remember to open the window a bit at that time. So far no return of the mould.

I think what with the modern obsession with eliminating 'draughts' and the general lack of fireplaces, too many houses are sealed up in a way that totally negates healthy ventilation.

V8

[Dicky99]

As the OP has discovered, air circulation seems to be the key.

I think so.
The room in which I'm sitting, in what would be the attic if it had one, before I redecorated and painted with breathable paint had quite a lot of mould along the eaves plates especially at the north end. I put two 8" fans to blow along each eaves for half an hour morning and evening and I try to remember to open the window a bit at that time. So far no return of the mould.

I think what with the modern obsession with eliminating 'draughts' and the general lack of fireplaces, too many houses are sealed up in a way that totally negates healthy ventilation.

V8

For retrofitting old properties the mantra is "seal tight, ventilate right". In other words eliminate all uncontrolled ventilation and replace with controlled ventilation. Air quality is equal in importance to energy efficiency, not just in relation to damp and mould but also things like the VOCs emitted from paints, furniture and all manner of other manufactured products which negatively impact peoples' wellness.

[jaizan]

They have been somewhat superceeded by MHRV systems, but MHRV don't pressurise the home to the same degree so smells can still get in, unless the intake is carefully sited to avoid the problem.

MHRV is definitely the way forward. I'd like to replace my bathroom extractor with a small one covering that & some other rooms.

However the politicians are clearly trying to discourage their adoption as they require building regulations approval. Interfering policies have unintended consequences.