I used to like the simplicity of PIV but I'm not a supporter nowadays. The problem is that it supplies fresh filtered air, yes, but where does the humid stale air go? The answer is usually into all the cracks and crevices in the walls and ceiling and maybe floor. And they are cooler so the humidity condenses and rots anything organic that may be there. Such as timber, or insulation. So it might solve the problem of condensation within the house, but it just creates the worse problem of condensation in the structure of the house.

Dehumidifiers might work. I don't know anything about their costs or effectiveness. Somebody else will have to comment.

One thing I know works is increasing the ventilation. When we lived for a while in an old damp house we opened the windows wide even in winter for a while every day. That added to the heating bill of course. But heating bills are bad anyway in old, poorly insulated, single glazed buildings.

The windows can be improved fairly cheaply by adding secondary glazing, and/or shutters at night.

Posted By: Mike1running the cold water into the bath before adding the hot

On that topic, it's surprising how good oil is at reducing evaporation. Because oil floats on the surface.

A shower uses a whole lot less water than a bath of course!

As a standard bath is about 100 litres and the standard showers flow rate seems to be around the 10 litres per minute that would suggest a 5 minute shower uses half the water.
On the OPs original topic, I would suggest buying a dehumidifier and using it in one room which is effected by humidity to see how it works.

I have an Ecoair DD1 simple desiccant type, which is good for domestic use. I've used it to good effect for keeping down the humidity in rooms until I got to the bottom of the root cause. Its also very effective for drying clothes indoors in winter.

In a room which has high humidity and if you have electric heating, a dehumidifier is more energy efficient than heating and ventilating, because at all of the energy being used by the dehumidifier is converted to heat so it is at least as energy efficient has an electric heater, and if you don't then need to ventilate you are not losing any heat to the outside. In fact I believe a dehumidifier is more energy efficient than an electric heater because of the latent heat effect from the desiccant. If your heating is of a cheaper type, such as GCH, then the dehumidifier will be more expensive to run, but probably not much if you take into account the fact that you are not losing any heat by ventilating.

If its just condensation on single glazed windows there might not actually be a humidity problem. If its cold enough outside you will still get condensation on windows with normal indoor RH. Those window vac cleaners are good for condensation on window panes.

Edit: corrected to 'domestic use' in first line, not 'indoor use'. Would have to be some dehumidifier for outdoor use! :)

> The problem is that it supplies fresh filtered air, yes, but where does the humid stale air go?

Precisely the reason I opted against it too. Installed a dcMEV instead - Aereco. It means you have to run some ducting, but I think it's safer long term and you don't necessarily have to run it _everywhere_.

I hate trickle ventilators with a vengeance, the vast majority of the ones I look at are draughty when closed!

Blimey, adjacent to this little spat, someone has just revived http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9383&page=1 - what debates we had back then!

Posted By: gravelld> The problem is that it supplies fresh filtered air, yes, but where does the humid stale air go?

Precisely the reason I opted against it too. Installed a dcMEV instead - Aereco. It means you have to run some ducting, but I think it's safer long term and you don't necessarily have to run it _everywhere_.

I will be using PIV and will be extracting from the bathrooms and kitchen using a continuous running fan.

https://www.epicair.co.uk/products/greenwood-unity-cv2gip-dmev-fan

Min 1.1W to Max 4.1W shouldn't be too bad.

@fostertom where are you seeing these auto vents? Or a product you can recommend?

Are these decent? Reliable?

Or just fit a MVHR unit and skip all these extra air leakage points?

This is quite a useful link comparing MVHR against the other options:

https://www.passivhaustrust.org.uk/UserFiles/File/research%20papers/MVHR/2020.04.27-The%20Case%20for%20MVHR-v7%20new%20cover.pdf

Posted By: fostertomAt a quick look, the report only really compares MVHR with 'natural' ventilation, hardly with MEV, PIV or other alternatives.

There's a paragraph and table on page 3 which compares Nat Vent, MVHR & MEV

"Mechanical Extract Ventilation (MEV), whilst able to provide a good level of fresh air, draws this air from the outside, through gaps in the fabric and trickle vents. Thus, the air is not filtered, and external pollutants are actually sucked into the building. There is also no real control over where air enters the building, so good levels of air exchange cannot be guaranteed in all rooms or areas. The stack effect means that vents or open windows in upstairs bedrooms may become an exhaust route for stale air rather than inlets for fresh."

and more on page 8:

"Lowe concludes that MEV ventilation will result in lower emissions than MVHR systems of intermediate levels of performance at air permeabilities of greater than 3 m³/m².hr. However, he also notes that MVHR equipment which is ‘efficient by current standards’ is likely to outperform MEV at all levels of air permeability.

Lowe also notes that dwellings using natural ventilation with an air permeability of less than 8 m³/m².hr are almost certainly likely to lead to underventilation as occupants will not open windows and, as a result he does not evaluate the relative performance of natural ventilation below 5 m³/m².hour in comparison to MVHR or MEV. In effect, he considers that natural ventilation is inappropriate below 5 m³/m².hour."

Hi Tom, Do you know whether any MEV units have the features you advocate, particularly inlet resistance-controlled 'suction'? I'm thinking that would require a speed-controlled 'sucker' (i.e. constant volume), and they're not universal even in MVHR, plus measuring the 'suction' would I think require two pressure meters and calibration.

.

Posted By: Victorianeco@Gravelld Where did you buy your unit from and what is your experience and current setup?

Direct from Aereco. They were extremely good with support - although it didn't really need much handholding.