Heat Recovery Ventilation

The heat recovery ventilator is used to provide fresh air to the house in a controlled manner without loosing too much heat in the process. The unit consists of an insulated box with 2 blowers and a plate heat exchanger and a load of ducting. One of the blowers is extracting moist warm air from the house. The other blower is supplying air from outside to the house. The ingoing air is warmed as it passes near the outgoing air inside the heat exchanger, The blowers have 48 volt dc high efficiency motors and the power supply is a stepping power supply. The speed of each blower is controlled via pulse wave modulation (PWM) provided by an arduino or attiny chip on a circuit. Both extract and supply air ducts are filtered using automobile cabin filters.

One thing to remember about this project is that making the ventilator unit is only half the battle. Installation of the duct work is equally time consuming and difficult as making the unit. Also bungalows are much easier to add duct work to than multi-story houses.

If the unit is installed in a cold attic then all ductwork and the unit itself must be insulated.

The heat exchanger is an expensive thing to make, unless you can get 2nd hand materials. I used more than 10 tubes of mastic alone to glue it together. If you can get the heat exchanger, or the whole unit 2nd hand then go for it.

I created the heat exchanger from 2 no 8ft x 4ft textured aluminium sheeting. I got these at a metal fabricator shop and they guillotined the two large sheets into 32 2ft x 1ft sheets. I used 30 of these sheets to make my heat exchanger which measures 12"x24"x8" approx. The size of the heat exchanger will dictate the size of the box. Don't make it too big for the access hatch to the attic.

The aluminium sheets are laid one on top of the other with a pattern of mastic glue, that alternates at every other layer. Before starting, look carefully at the pictures and you should be able to see that there are alternating gaps between every second sheet, at the corners For example, layers 2 and 3 are gaps at that corner. The diagram above shows the pattern of mastic (shown in blue) for each plate; the odd layers are shown in one image and the even layers are shown in the other. Notice also the air flow direction. The airflow goes in the opposite direction at each layer.

The sheets are quite heavy so I used small beads, (shown as red dots in diagram) embedded in the mastic to prevent the sandwich squashing while the mastic set.

I also created a jig consisting of a flat piece of mdf with 4 blocks of 4"x2" sticking out of it vertically to help keep the plates aligned. Once you start using mastic, you need every bit of help you can get, and you want the finished heat exchanger to resemble a rectangular block and not a spiral staircase.

I used 3/4" marine ply for the enclosure box, but ordinary ply would've probably been ok. Don't use chipboard or mdf as the interior will get humid.

The box is approx 4ft x 2ft6" x 12" deep. The size depends on your heatexchanger and blower size and also the size of your attic hatch! I lined the box on the inside with high density extruded polyeurethane (I had a bale of this hanging around). I used a jigsaw and a plasterboard saw to shape it and glued the pieces together with a non-solvent mastic adhesive. Once the box was complete I used a 5" hole saw to create 4 holes through the back of the box. A jigsaw would also do just as well.

I cut 4 pieces of 12" long 5" electrical underground duct (usually red in colour) and glued these into the box through the holes, sealing around the edges with non solvent mastic

I added two plywood dividers and draught excluder foam, as shown in the picture, to made sure that each compartment 1 2 3 and 4 were all separated. Air can flow from compartment 1 to 2 via the heat exchanger. Air can flow from compartment 3 to 4 via the heat exchanger.

The blowers are 48v blowers similar to AVC model BNTA1769Y8U, which I got on ebay. Delta also make a corresponding one. These are centrifugal blowers as opposed to fans but I'll use both terms at will :-). This type of blower can supply a lot of air under pressure and is energy efficient and importantly, doesn't require an external enclosure. The shape of the compartment that the fan is in doesn't make too much difference but the blower inlet (internal diameter 125mm approx ) should closely match the diameter of the inlet spigot (provided by the red 5" duct). Approx 2mm clearance max around for best performance/low noise,

These fans have 4 wires each. Black for ground: red for 48volt: Yellow for PWM input: Blue for hall sensor output. To control the speed you supply full 48v supply and gnd, and supply a pwm low voltage 0 - 5v-ish from your controller to the yellow depending on the speed you want. Your controller should share a common ground. You can also connect up the sensor to control the blower more accurately, or just to monitor its speed. Because the motor itself has the electronics to deal with the PWM signal, your controller can be a low power milli amp device like an atmel attiny. ie no power transistors required and no heat sinks etc.

I mounted the blower with 5mm bolts onto a small section of ply. Make sure the bolts are the correct lenght. I then mounted the ply/blower on aluminium angle brackets using 6mm bolts with nylocks and screwed to the divider and box (through the insulation) with 70mm wood screws. I cut 6 mm x 10mm slots in the aluminium bracket to allow for adjustment which helped when centering the blowers over the spigots. Earth the blowers and drill a hole out of compartments 1 and 3 for the wiring. Push wiring out thought the holes and seal with mastic.

You need to decide which is your extraction and which is your supply. So lets choose Compartment 1 is the supply of fresh air from outside. The blower in compartment 1 on upper left sucks in air and forces it through the heat exchanger into compartment 2 on lower right of picture. The 5" pipe out of the back of compartment 2 goes to the supply duct work in the house and delivers warmed fresh air to the bedrooms and living rooms.

Compartment 3 sucks air from the house (bathrooms showers and kitchen) and forces it through the heat exchanger to compartment 4 which is the outlet and exits the building through roof or out the gable for instance.

A drain must be added to the bottom of compartment 4 because condensation will be produced in this compartment. I drilled a 16mm hole an added a copper pipe hot gluing around it to seal. I added a swan neck to this copper and added a garden hose and allowed this to slope downwards and exit the building (throught the gable of the attic) If you don't add this drain, the box will fill with condensation as the outside temperature gets colder.

Add an insulated back and attach to the box. I added a few layers of draught excluder around the perimeter and also across the boundaries that separate compartments 1 - 4. At this point things were getting rough so I simply screwed the back on with 70mm screws. No hinges and clasps. Later perhaps.

The inlet to compartment 3, which is the extract from the building is laden with moisture and must be filtered.

I got some new automobile cabin pollen filters from a 5 series bmw. These were rectangular 150mm x 300mm approx, which seemed big enough. I created an enclosure box from some more of the blue insulation and added a lid that had another piece of the 5" duct. The lid seals around the box with more draught excluder and is held in place with galvanised band and screws. (gettting rougher as we go but it works.) Only issue is that you need a screwgun to change the filters. I added a filter to the supply inlet compartment 1 also

I suspended the box from the rafters using some old rope and some loop brackets attached to the box on both sides. They recommend not resting these things on the joists to stop noise transmission.

I got a new 48v power supply through ebay. something like a MeanWell, RS-100-48. This supplies 48volts and ground to both blowers.

Separately, I have a 5v mobile phone charger to power a circuit with an attiny85 chip. Note that the ground for this circuit should be connected directly to the ground of the output of 48v supply. The advantage of the attiny is that it costs about 2 euro.

see other instructable on programming one of these using an arduino uno https://www.instructables.com/id/Program-an-ATtiny-...

A few additional notes on programming attiny here ::

-You must add hardware support for attiny chips by downloading and saving support files in the correct file location

-You then program your arduino to be a programmer by loading a sketch ArduinoISP to your arduino. The target board selected at this stage is your arduino uno

-Next step is to connect the arduino to your attiny via breadboard, add the capacitor and choose the target board now to be the attiny model. Depending on what version IDE you are using , you may need to choose separate options like board processor and clock

This sketch will output two different pwm outputs to pin 0 and pin1 of the attiny. Also if a button is pressed, it will output two different pwms (higher speed ) and start a timer, and revert back to low speed when timer runs out.

See attached arduino sketch. The blowers that I used were very both high speed blowers but I ended up with two different models. Both needed a small fraction of full speed 75/254 and 35/254 as shown in the sketch.

I arrived at these numbers based on the spec sheets of the fans and also by putting my hand up to the vents in the ceilings. Typically I think these will output 400cfm at full speed, and I calculated that the house could do with 100cfm. If the fans are on full speed they are too intrusive noise wise, and they'll suck the heat out of the house. I just allow it to run at this slow speed all the time and based on calculations it should use about as much elec as a 60w bulb and also loose a small percentage of heat. I haven't wired up the speed up button either.

Results after running for 1 year. The system completely eliminates any condensation on the bedroom windows overnight and from kitchen windows. The house is warmer cause no windows are open and manual hole in the wall vents have been blocked. Smells disappear from the house also

Additionally I took temperature readings with an infrared thermometer and compared to readings at a friends house. They have a commercial HRV sytem which is Sap Appendix Q as best of the best. This produced identical results for efficiency but won't win in the beauty stages :-(

http://www.engineeringtoolbox.com/heat-recovery-ef...

-all ducting was insulated in the cold attic. Riigid 6" pipe was used for both supply and extract main runs with 4" duct, T'd off close to the outlet. Remember that the duct work is easily more than half of the work and probably half of the total cost.

-If you are using rigid ducting, you should use small lengths of flexible ducting to connect the unit to ductwork to prevent vibration and noise transmission.

-The extracted air from compartment 4 must be vented to the outside. I diamond core-drilled through the gable of the house and stuck the pipe through. If you vent the extract into the attic, your attic will become foggy and damp. You'll need to put a grill over all ducts that exit the building otherwise all kinds of creatures will invade. I used stainless a fine stainless steel mesh and a large zip tie. The mesh, I got from a frying pan anti-splatter guard.

-I didn't put in any cutoff in the event of a fire in the house or the unit. Perhaps a temperature probe might be a good idea, arduino can take an input sensor.