Heat Recovery Ventilator (Double Flow Controlled Mechanical Ventilation)

I built a DIY HRV Heat Recovery Ventilator (Double Flow Controlled Mechanical Ventilation) that works pretty well. A small contribution to save planet's resources ;-)

You can get 15% saving on your heating invoice and it brings you clean air inside the house.

The air to air heat exchanger is made of aluminium plates assembled with special glue.

It has a measured efficiency of 65% : with an outside temperature of 10°C, renewed air is heated at 16-17°C by inside air rejected at 20°C.

The total cost of the installation with the ducts is less than 1000 Eur.

The hot fouled air from inside the house is extracted outside through the exchanger.

The cold clean air from outside is injected inside the house through the exchanger.

Both flows are not mixed together, they just transfer their heat through aluminum plates.

Because the hot fouled air is most of time humid, it condenses at the contact of cold plates. So a condensate evacuation must be planned.

Air is extracted and injected by two fans.To prevent the fans and the exchanger getting dirty, I add two filters on both side.

Finally a temperature regulator is used to control fan speed to avoid frozen condensats in the exchanger in the winter.

There are two main types of air to air heat exchanger in ventilation units : crossflow and counterflow. Counterflow exchanger are more efficient but not easy to do it yourself. To keep it simple, I built a crossflow model. Hot floued air goes between one plate out of two and the clean fresh air that goes perpendicularly the other plate out of two is heated.

The exchanger is built from 80 aluminium plates of 350x350 mm. So it has 10 m2 exchange surface. There is a space of 3 mm between each plate (two aluminium strips of 3 mm are glued between two plates with alternate axes)

Plates are glued with polymer glue (Tec7) ant put under pressure during 24h.

The exchanger is place in wooden box.

The box is sealed and waterproof thanks to a rubber tarpaulin and rubber bands.

There are several openings provided for input and output flow ducts.

Two boxes made of wood with isolation inside contains the filters (50cmx50cm).

Big filters have many advantages : lower pressure drops and fewer maintenance.

One of the filter also has a role of flow dispatcher (see Pipe network step)

I used two duct fans of 45 W which have a theoretical flows of 260 m3/h at a pressure of 0 pa.
You have to be sure that they deliver enough rate of flow for your estimated pressure drop (see next step)

The fans are inserted between the filters and the exchanger.

All pipes are flex isolated ducts. Condensate were going in a bucket at first but it is not linked to wastewater evacuation.

You first have to estimate the number of m3 of air to renew per hour. It depends on the volumes of your rooms and the humidity level of the room but also on the number of people living in the house.

Input and output streams should get closer to balance.
Please note that if you have a wood stove or extractor hood, you have to take it into account.

Following some recommendations that I can find on the internet, I decided to renew the air as following :

Input

• Bedrooms and office : 3 x 45 m3/h
• Livingroom : +75 m3/h

Output

• Badroom : -75 m3/h
• Toilet : -30 m3/h
• Kitchen : -75 m3/h

This is unbalanced streams but I had to let 30 m3/h input overage to get enough wood stove drawing.

For a complete study, you should also calculate the pressure drops of your installation including the pipes, filters ..

Keep air speeds under 3 m/s to avoid noise.

You also have to choose a pipe network.

You have the choice between several topologies :

• A/ traditional distributed network with T connection (A+ first image)
• B/ star network with dispatch boxes (B+second image)
• C/ network with forks. (C + third image) The flow is naturally split like a "reversed" river stream. Pressure drops are optimized.

The choice of the pipe network is very important to avoid noise and pressure drops.

The best network for me is the C one in the diagram. (It is the one I set up in my second VCM installation.)

In this first installation I went for a star network (B) with big dispatch boxes that also have a filter role (see previous step)

The (A) network is the most common, the simplest and cheapest to install but results in noise nuisance. That is very annoying for a ventilation system running days and nights !

1/ Anti-frost system :

To prevent the condensates from freezing inside the exchanger in the winter you have to add a fan speed regulator controlled by temperature. That way you decrease injected flow rate and get a different energy balance. I bought it already built but you could easily build one yourself with an arduino. Have a look at my Arduino Wood stove regulator project, it should be similar.

2/ Isolation

To loose even less calories it is recommended to isolate the exchanger and the ducts if they are not in the heated part of the house. (Everything is in the attic here)

3/ Balance

To balance the flows, you can use rate flow regulators inside the ducts as the one in the picture.

Flows rate can be measured with an anemometer.

The system has been running for 8 years now without any problem !

I we consider a 15% heating saving, it is already a 250% return on investment and a sensible comfort improvement !!