Heat Recovery Ventilator (Double Flow Controlled Mechanical Ventilation)

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Introduction: 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.

Step 1: How It Works

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.

Step 2: Heat Exchanger

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.

Step 3: Exchanger Box

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.

Step 4: Filter Boxes

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)

Step 5: Fans and Ducts

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.

Step 6: Flows Rate

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.

Step 7: Pipe Network

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 !

Step 8: Fine Tuning

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.

Step 9: Feedback

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 !!

2 People Made This Project!

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12 Discussions

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GTO3x2

8 months ago

Looks like a nice job. I recently was contemplating a heat exchanger for a gas-fires clothes dryer, and this gives me hope (although I am assuming lint is too much of a problem). I wish I could more readily interpret the units from SI to get a feeling of the system, but with this amount work and explanation, you must have more than enough HVAC experience. I am assuming this is in a house that has hot water heating only (not exiting S/RA ductwork. Nice

1 reply

Thanks. Yes it is in a house with hot water heating + a wood stove (that's why i unbalance the flows to all chimney drawing)

Great job! Wow, that is really impressive. I've seen before years ago someone implement something the same idea in, or just outside with the plan to go in partially, their wall stud spacing using corrugated plastic from political campaign signs carefully attached end to end with cuts on each end capped to allow a zig zag (not sure if that is correct term) longer length of flow for heat exchange.

I want to say they used geothermal also on the intake using 4" black corrugated drain pipe dug under the frost line zig zagged to with an above ground straight pipe intake with a upsidedown "U" shape pipe end. I'm betting they didn't use a condensation drain though and that's why I can't find online now or who knows.

I wonder if on eBay, as I was looking for aluminum sheet for a solar heater, if you can find "trim ends" or "scrap" that is large enough like from "carolinablades" that might save on material. I've also thought about window screen too as a cost effective way and decide aluminum window screen is more cost effective. Maybe a solar heater intake would be a great addition also? Really neat design.

1 reply

The zig-zag exchanger your are talking about might be a counterflow exchanger. More efficient but not so easy to build yourselft .. I would suggest to buy on already built

Geothermal on the intake is a must if you can do it (for an exchanger like mine at least which has 65% efficiency because some exchanger have 95% so in that case geothermal is not worth) .. Another advantage of geothermal is that you get rid of antifrost problem !

Nice idea this window screen ! I did not think about it ..You can also do it in plastic as said above.

Others have done this using corflute corrugated plastic. I wish I had time to make one.

1 reply

Indeed ! Cheaper and faster with corflute corrugated .. but maybe less durable ..

That plastic tends to age badly. If condensate are freezing inside plastic exchanger it is broken.

Awesome project, well thought out and well documented! Really get the old brain juices flowing, thanks for sharing.

Two questions.

1) Wouldn't a slower air flow increase the heat exchange efficiency?

2) Would building a counterflow heat exchanger be that much more difficult, using the same technique of creating air channels, only completely sealing off the sides, and, instead of sealing off a whole edge of a pair of plates on the other two sides, have alternating short sealed/unsealed segments along each edge to make the channels for both in- and out-flow?

I'm thinking of ways to get the exchanger's efficiency above 95%, that's why I ask.

1 reply

1) Yes slower is the aire flow better is the exchange but it is a balance between the air renewal and heat recovery .. The first aim is air renewal, and if you do not renew the air you may have humidity and even worse energy balance !

2) It was already not obvious to glue those plates and 2 pieces of aluminum .. in a counterflow you will have even more pieces .. maybe it is possible but I do not know how difficult it could be. You can also buy a pre-built counterflow exchanger. Now 95% efficiency is only worth if your house is sealed and isolated.

Have a nice DIY and gave us feedback if you go for DIY counterflow !

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katou

8 months ago

Fantastic work! There is an old design for a system like this floating around the net from Popular Mechanics, but yours is much better!

I especially like the adaptations you have included for cold climates, such as the condensate system.

J'espere que tu vas ajouter quelques mots a ton titre:

"HRV" (Heat Recovery Ventilator)

ET

Air-to-Air Heat Exchanger.

Ces sont les noms en Canada et les Etats Unis pour cette machine. Si tu utilise ces mots, on peut decouvrir ton projet vite et simple.

Encore, tres bon fait!

Katou

1 reply

Hello, Thank you very much for the exact English translation .. hard for me sometime to find the technical English words ..
I changed title and keywords following your suggestion.

That's a neat setup :)