DIY Heat Recovery Ventilation. Heat Exchanger for our Yurt. How to resolve damp and mould in a Yurt.

Cheap but effective Homemade Heat Exchange Ventilation system. 
Around 50% efficient.
(Wikipedia article

Later in the article I will also touch on some other causes and solutions for damp in a yurt.

The device here essentially recovers the heat from warm, humid air that is being exhausted to outside into the cold fresh air intake.

Here is the finished product:
It is slim and fits behind a wardrobe, out of sight and out of mind.
It is totally silent, running with 12v low power computer fans.
It cost me less than £15.

The Principle is simple. A high surface area of exchange between the exhaust and intake routes.
A matrix of alternately directed pathways means a huge surface area is created in a small space.
These pictures show the concept clearly;

The ideal material for the matrix would be a good conductor. Thin aluminium sheets are most common in commercial units, it won't corrode with damp and condensation and conducts heat very well.
I used celular "corrugated" plastic sheeting. Plastic is an insulator not a conductor so it's far from ideal, the results are still impressive though. It's a material that I have had lying around, it was begging for some re-purposing. It's commonly found used in advertising boards and property "for sale" signs. It looks like this:

I cut the sheets into squares and stacked them alternatively:

The tightly packed the stack was placed inside the MDF casing made from some off cuts from a previous project. The matrix was sealed at its corners with silicone:

12v computer fans power the device. They were salvaged from scrapped PSU's. They are silent in operation and provide a perfect gentle flow to allow sufficient time for the heat to exchange through the matrix. If they were blowing too hard I think it would reduce efficiency. I actually wired them in series so that they run extra slowly. They are powered by an old 12v transformer.

Damp air coming in contact with a cold surface will most likely create condensation. For this reason the whole unit must be able to drain properly. The is why the matrix is up on it's end, so that all the cells in the material run down hill. As you can see, I simply made a "lined bucket" effect in the bottom of the unit with folded PVC sheeting. I fitted a sealed hose connector through the side to the red hose on the right. This will allow any collected water to drain to outside.

I closed the unit up, filled the gaps, sanded the corners and edges and painted it black with some paint I had left over. Here it is being tested on the bench.

I connected it to a timer to run 19 hours a day. It is off at the coldest, dampest time of night just before sunrise. Otherwise it runs every day and makes sure we have a constant supply of fresh air. I have carried out various tests and determined that the unit is about 50% efficient.
Meaning, if it's twenty degrees inside the yurt and zero outside then the fresh air coming in is ten degrees. Not bad for a project that cost me under fifteen pounds.

My thoughts for an upgrade... 
Maybe it would be better not to exhaust the warm air but simply cycle it through the matrix and then back into the room? 
This would mean that the 50% of the heat left in the air would not be wasted. The yurt would then be essentially pressurised so no other draughts would be coming in. Air would be forced out instead. Any comments on this?

More thoughts on damp and mould in yurts:

For many this is the bane of yurt life, damp and mould. 
This is a big topic and I could talk for hours on the subject.
We've had a five year journey of evolution on the matter and now have no damp issues at all.
Essentially you want to tackle it from both ends.
Firstly and most importantly minimise moisture in the air, cooking is a big problem. Drying clothes around a burner is also not a good idea. This is why we have separate kitchen cabin.
Our lifestyles easily put litres and litres into the air everyday, you'd be amazed. This then condenses on to cold surfaces or when the air temperature drops.
Houseplants don't help either, every litre of water you pour into the pot will end up in the air, fact. (Besides respiration this is our biggest issue, we have loads of plants.)

The other end of the approach is in removing the unavoidable humidity in the air.
Ventilation is the easiest. The heat exchanger above is a revolution in this front.

We also have a permanent mushroom roof vent in our crown dome that I made from a glass washing machine door bowl. 
It allows the warm air collected at the top to passively vent. It does also have a 12v low power computer fan as well that we switch on in the summer to keep things cool and occasionally on the odd nice day in the winter. Doors open and vent on for a few hours every week 

Insulation of the yurt is also another important factor, the warmer the interior of the walls are the less condensation you'll experience on the canvas.

A dehumidifier is a great tool too and almost a necessity for UK yurt living. We have a low wattage quiet "eco" model. It has a humidistat so it only comes on when the yurt is damp enough, it's also on a timer so it doesn't run at night when things are quieter and the gentle hum would disturb our sleep. This drains to outside and is basically maintenance free. It's important to get a desiccant dehumidifier as opposed to the more traditional condensing dehumidifiers because the latter require a room temperature of at least 18+ degrees, the desiccant machines work down to just a few degrees.

The only problem with a dehumidifier is that it's going to use a minimum of a few hundred watts which is really taxing on off grid PV in the winter. You'd need quite an array, easily a kilowatt or more I would have thought and that would be for very light use of the dehumidifier.

The wood burning stove is an asset as well as a nuisance. 
Mould proliferates in warm damp spaces and on natural materials… Sound like a yurt? 
The heat from the stove is only advantageous if it is evaporating damp into the air that is then being dehumidified or extracted, if the yurt is sealed and not ventilating then the heat will make the mould worse.

I hope sharing our experiences is all of some help.
Thanks for reading.