How to Build — DIY Air to Air Cross-Flow Heat Exchanger HRV
Completed heat exchanger located near ceiling in office workshop. Actual exchanger is covered with shiny bubble wrap insulation, white pipe stem protruding to left is the intake for indoor air, it is elongated to prevent short circuiting in/out vents. The two variable speed fans are the black objects located at the ends. Click all images to enlarge.
My studio-office-workshop where we deal with ski touring gear and more is remodeled to be fairly airtight. It needs ventilation. During summer here in our moderate climate I can simply crack a window and prop up a box fan if I need more than normal infiltration air flow. But paying to heat the planet’s atmosphere during our mountain winters is not in our business plan. Solution: air-to-air fresh air heat exchanger, also known as a heat-recovery ventilator or “HRV”. But do I want to sell my soul for an expensive commercial unit that I’ve heard tends to quit working after just a few years? Forget it. DIY to the rescue.
I came up with this DIY design by seat-of-pants based on years of experience with plumbing and ventilation parts, as well as being familiar with the basics of air-to-air heat exchange. It is simple. Easy to over-think. My design is intended to work and last for years, it is not a temporary science experiment.
The gist: Rig something that locates air flow from outdoors next to blowing air out from indoors — you swap the two flows — and let one airflow heat/cool the other so you “recover” energy. To do this you need an “element” or “core” that conducts heat well, a way to flow air next to the core, and a shell to contain it all. Variable speed fans, insulation and wireless thermometers round out the build of this HRV.
My design does all this quite simply. The core of this heat exchanger is 3-inch aluminum ribbed expandable “dryer” duct. Aluminum is highly heat conductive, thus a good material for heat exchanger core. Shell-enclosure is 4-inch thin wall CL200 white PVC plumbing pipe. (Note, commenters suggest that rigid aluminum duct pipe would perform as well as the expandable dryer duct, and be easier to work with. I agree. If you build, use rigid duct, perhaps with stick-on foam dots for clearance spacers.)
Testing a temporary lashup of the rig run through a window to cold exterior air here in Colorado, my design worked quite well from the start. It could probably be shorter. Having too much core surface area really doesn’t hurt anything, it just throws of your efficiency observations because the incoming air continues to be ‘tempered’ beyond an even exchange of energy. This can all be controlled with air speed as well as size, so don’t obsess on size. It’s easy to shorten the exchanger, harder to lengthen it.
The pipe spec is important. Regular schedule 40 PVC has walls too thick to allow sufficient air space around the aluminum duct core. “Drainage” or “sewer” pipe PVC is thin walled enough to create an air space, but doesn’t have the outside diameter of the common schedule 40 pipe, thus limiting your options for fittings. CL200 PVC pipe has the same outside diameter as schedule 40, but a thinner wall so the core has enough space around it for air flow. Perfect. (Other types of pipe could be better, but were time consuming to source in our mountain valley, see notes below).
The Build
The length I chose is somewhat arbitrary (8 foot shell). Testing shows this size to be totally adequate with my choice in fans (see parts list below), and could perhaps handle higher air volumes. You’ll need a place you can mount something of this length without messing up your interior decorating; a location with ambient temperatures similar to your living space. In a house, a crawl space or basement might work. Attic would be too hot in summer and too cold in winter. For residential use, creativity with location might be as important as the actual engineering as you need to consider things like distributing your nice fresh air. More, locating the vent that draws indoor air near the ceiling uses stratified warmer air which otherwise just stores unused energy. Here in my 25 x 20 foot single-room shop, I simply mounted up near the ceiling on the side of a wooden beam running down the center of the room. It works so it looks pretty. If it didn’t work I’d leave it there to humble me.
Bear in mind you’ll need to do an approximately 5-inch circular exterior wall penetration, check to be sure the required hole location doesn’t cut directly through a wall framing member and of course think about cosmetics and sun heating of your venting (more on that below.) Interior inlet and outlet are separated far enough to avoid short circuiting of the venting. Exterior vents should also be separated, doing so is not as critical as indoors as outdoors air is usually breezing around a bit.
Begin with an 8 foot chunk of the 4-inch PVC pipe, hopefully on a workbench rather than working on your knees.
1. Grab your 4-inch PVC pipe T fittings. Make end caps by inserting a 5-inch chunk of 4-inch PVC into one side of your 4-inch T fittings. Pound in the PVC pipe with a plastic or rubber hammer until the joint is snug. Don’t overdo it (you may need to reverse later), and don’t glue anything. More, take care not to blemish or otherwise damage anything, so you can return most of the parts to your big-box store if you don’t like the results. Your rubber 3×4 couplings will mount on the 5-inch pieces of 4-inch PVC, but don’t install the 3x4s couplings yet.
Your ‘end caps’ will end up looking like this. The 3×4 rubber flex coupling centers the 3-inch pipe core inside the 4-inch shell so air can flow around the core.
2. Stretch out your aluminum duct to about 7 feet. Attach a 3-foot chunk of 3-inch PVC to one end of the alu (this is your indoor side), and an 18-inch chunk of 3-inch to the other end of the alu. I made some connector sleeves out of aluminum dryer duct connectors, and taped the joints with duct tape. You won’t be able to access these joints for upkeep and if they fail the system won’t work, so consider cinching some wire ties over the duct tape or otherwise adding insurance.
Stretching out the dryer duct vent used as core. Be careful not to crimp or compress, keep it nice and round.
3. Insert this resulting core assembly inside the 4-inch PVC shell.
4. Slip your end caps (from step 1) over the ends of the core and press fit the 4-inch T fittings to the ends of the 4-inch shell.
5. Spray some water on the protruding 3-inch pipe and slip the 3×4 rubber couplings to the point where they mate your the 3-inch PVC with the 4-inch.
Core joints are made with some aluminum sheet and Gorilla Tape. Add plenty of tape for a good seal. I did not use silicon as I wanted things to be reversible if I disassemble to check for mold and seal issues.
6. Critical step: you need something to keep an airspace open between the core and shell. Some builds I’ve seen on Youtube and elsewhere use chunks of sticky foam and things like that to separate one surface from an other. I wanted something more stable and mechanical so I installed a few dozen machine screws in the PVC pipe shell at carefully figured depth to act as spacer for the core component. At each end of the shell, be sure three of these screws support the 3-inch PVC pipe. Thus, once the 3×4 fitting is tightened up the 3-inch PVC is supported and stable. See parts list for machine screw size I used, but due to exact choices in materials be sure to evaluate your own rig and choose the correct size screws. I placed washers under the screw heads to tune exact penetration depth.
Note that you’re using ‘machine screws’ because they have a flat end that won’t pierce the aluminum core if you’re careful about depth and turn the shell so you’re inserting the screws from the top, allowing the core to settle away from the screw as you insert. I took my prototype rig apart and inspected, the screws had caused no damage but I was quite careful while inserting.
To place machine screws for centering core, draw a triad of straight lines on the shell, using workbench as a guide simply slide marker along on a spacer, in this case I set the marker on my roll of tape.
Measuring to space three lines of screws equidistante, so internal core is held nice and evenly away from shell, creating an air space for flow.
Machine screw with washers for precise insertion distance. It’s important these screws do not make holes in the core.
Placing screws in pilot holes, they thread easily into the plastic.
7. Now you should have a long chunk of 4-inch pipe with 3-inch stubs protruding from either end. The longer stub goes inside your living space, shorter to daylight.
8. Install your exchanger so the outdoor end (with shorter 3-inch pipe) goes to daylight. In my case, I cut a fairly neat hole in my exterior building siding, removed the T-fitting from the outdoor end of my exchanger, slid the 4-inch PVC through the hole, then replaced the T-fitting on the outside so it acted as a collar snugged up to the building siding, to help neaten the look of things. Slant the whole heat exchanger assembly at least 1/4 inch a foot to outdoors, so any condensate drains quickly to the outdoors. You’ll need some kind of support system indoors. I mounted on the side of a ceiling beam, which required simply using one pipe brackets and screws. You could hang from a floor joist in a crawl space with some plumbing strapping. Anything that works, just remember the whole thing needs to be pitched, and you need to think about how you’ll get both the ingress and egress venting into your living space with minimal pipe bends.
This is a good place to mention “short circuiting,” meaning the situation where your ingress vent air ends up being caught in your egress flow without mixing into your residential air volume. Indoors, prevent by putting some thought into locating your vents at least 3 feet apart. In my case, I wanted to utilize warmer stratified air up near my ceiling so I put my egress vent up high, and put my ingress vent lower.
9. Outdoor finish is easy.
A) Seal around the pipe where it comes through your wall, using something reversable in case you have to remove your rig for upkeep. If you anticipate much moisture, perhaps add a chunk of sheet metal flashing to act as a rain shield above your wall opening.
B) If you’ve not already done so, cut the end of the 3-inch pipe to make a slanted opening that faces downward. C) Cover 3-inch opening with insect screen. D) Place an approximately 24 inch stub of 4-inch PVC into the outdoors T fitting.
C) Add some sort of “bell” to the outdoor ingress vent. I used an expensive 4×6 PVC upsize coupling, something from the sheet metal venting world would be much cheaper and probably do fine. Idea here is to create a dust filter holder with lots of surface area. Cut out a circular peice of cheapo furnace filter and press-fit it into the 6-inch side of your “bell.”
D) Run a few sheet metal screws into your exterior pipe press-fit joints so they don’t work apart during expansion and contraction. Again, do not use glue, keep everything reversable and friendly to your air suppply.
10. Install fans indoors. Install a short length of 4-inch to exposed 4-inch side of the indoor T fitting, cut your 4-inch flange to fit a fan, and install the fan so it draws air into your living space. Likewise, install your 3-inch flange to the exposed 3-inch pipe protruding from the end of your assembly. This fan pulls air from indoors and blows it to the outdoors through your heat exchanger core. Use fairly small diameter machine screws to attach the 120 mm fans, and you can get diagonal holes in the PVC flanges to line up with those in the fans. I used small knurled nuts so I can remove and replace the fans with no tools.
PVC ‘closet’ flange makes a perfect 4-inch fan mount. When sourcing, be sure flange mounts to pipe in such a way as to restrict airflow as little as possible possible. See parts list for suggestions.
11. Install two thermometer sensors in small holes you drill in the PVC pipe. One sensor outdoors in the end of the vent that provides indoor air (the one with the dust filter). This will be your “outdoor ingress” air temperature — generally the same temperature as your outdoor ambient temp, though located your heat exchanger outdoor components in a sunny area can cause temperature variations. Install sensor number two just behind your ingress air fan.
Speaking of outdoor vent locations, in my case I’m only running this heat exchanger when it’s cold outside, so I figured why not install where the outdoor vent gets cooked by the sun, for a tiny bit of added solar heating of my ingress air? Likewise, if you’re concerned about the sun messing with your heat exchanger performance, locate your outside venting in the shade.
12. It is important to insulate the do-it-yourself heat exchanger shell so you don’t get a false exchange when your input air sucks heat from your room’s ambient through the exchanger exterior pipe wall. In my opinion a thin layer of insulation is enough. I made a jacket out of that foil faced bubble wrap stuff from Lowe’s, seamed with duct tape. I like that stuff because it’s fire resistant (I think about fire safety with all my DIY projects since they’re usually so far outside the parameters of any building code standards.) For budget insulation, just wrap with bubble wrap. Note we use our regular plastic pipe for the exterior shell which slows down parasitic heat transfer. But you do need a layer of insulation, especially during extremely hot or cold outdoor temperatures. Since our exchanger is mostly for use during cold weather, I mounted it at ceiling height so any parasitic heat transfer is taken from warmer stratified room air, probably at nearly zero net monetary loss on the heating bill. If in doubt, just add another layer of insulation wrap.
Final install before wrapping shell with two layers of ‘foil bubble’ insulation.
13. Test. Fire up your fans when your indoor and outdoor temperatures are significantly different. Watch your readings on your thermometers. Hopefully you’ll be surprised how good this works. I was.
Outdoor venting, on the sunny side of my studio-shop-office. Sun heat in winter adds to efficiency and prevents mold. Vent from indoor to out is screened (upper) to keep out insects or small human beings, input to indoors is filtered with a furnace filter in the “bell” made from plumbing fitting. This odd looking configuration is due to the intake and exit needing to be separated to prevent short circuiting and mixing of input and output air. Unfortunately this configuration on the street facing side of my shop, but needs to be on the sunny side for added efficiency and mitigation of any condensation problems. To pretty it up I’ll probably build a wood valance over the whole thing, so it looks less like I’m doing what we politely call “Colorado indoor gardening.”
PARTS LIST
Thermometer, multiple sensor from Amazon, one. $56.00
3″ x 8-0 Semi-Rigid Flexible Aluminum Duct, Product #L301 from Lowe’s (used for core that’s key to making this project function), $10, one.
4″ A-2000 PVC (thinner wall than schedule 40), 12 feet, $22.00 (from plumbing supply house).
3″ A-2000 PVC (thinner wall than schedule 40), 6 feet, $10.00 (from plumbing supply house).
4″ schedule 40 PVC T fittings, 2, couldn’t find at Lowe’s, $11 each from plumbing supply house.
6″ x 4″ Sch 40 Reducing Coupling (used for filter at outdoors input end of unit) $11.00
(It is important that the two flanges below, used for mounting fans, fit OVER your pipe so you don’t get an airflow restriction from the thickness of an interior coupling. All fitups in this project are friction fit, no glue is used, so if a fitting needs to be stabilized run in a sheet metal screw through a pilot hole. Leave most fittings as friction fit so you can easily take the exchanger apart for later cleaning, upkeep, or modifications.)
PVC flange (toilet connector, closet flange) for mounting OVER 3-inch pipe for mounting fan on 3″ PVC, Lowe’s item 253221, $4.00, one
PVC flange as above, for mounting OVER 4-inch pipe, Lowe’s item 253231, $5.00, one
(These rubber connectors work very nicely, but are a bit pricy but necessary for easy assembly of the project.)
Rubber “no hub” 4-in x 3-in Dia Flexible PVC Coupling Fittings with hose clamps, Lowe’s item 23478, $9.30 each, twoSmall chunk of furnace filter element, cut circle to press fit into outdoors end of unit.
This is the Cooltron fan model I ended up using, claimed 56 CFM at max speed.
And this is the fan speed control.
Drill bit for installing core centering screws, 9/64 allows self tapping of machine screws used as centering supports for core. Do not use sharp ended screws at they’ll penetrate the core.
3/4 inch 10/24 Phillips head machine screws <> 20 3/16 inch flat washers to prevent machine screws from protruding too far inside, use two per screw. <> 40
Mold warning: Any air-to-air heat exchanger introduces the possibility of mold growing in your ducts, whichever part produces condensation (in our case, the duct moving air from indoors to outdoors is where condensate may occur.) While we have little worry about this, since the air in the our heat exchanger exhaust space is blowing to the outdoors, preventing mold is always a good idea. Testing will reveal the reality of this, but at the least we’re thinking that simply keeping a spray bottle of humidifier mold preventative and spritzing some of this into the fans now and then will take care of the problem, as well as letting the sun bake our exterior venting. Speaking of contamination, don’t forget to eventually install a fabric type filter on the input (to indoors) end of your venting, as well as placing screen wire over the other outdoor vent (outdoor air to indoors). Luckily our design begins with a nice larger 4-inch input; I stepped this up to a 6-inch diameter fitting that holds a circular chunk of furnace filter.
http://www.engineeringtoolbox.com/ventilation-heat-recovery-d_244.html
AC Infinity AI-120SCX Speed Control Fan Kit for Cabinet Cooling, Single 120mm
NOTES
The way I understand it an exchanger that’s efficient will result in input air being close to room temperature. Apparently, this is easy to accomplish with cold outside air and warm, humidified indoors air, if you slow down your air movement enough to allow a leisurely exchange of heat-energy between the two air volumes.
In real-world use, you want your heat exchanger to be somewhat efficient, but spending a fortune and taking up space for something super efficient may not be practical. Perhaps the best rule of thumb is so long as your air coming in from outdoors is fairly close in temperature to your indoor ambient you’re doing fine. If the difference becomes too great, either the exterior-interior temperature differential is extreme, or you need to slow down your fans, or build an exchanger with more core surface area (or both). Also, as the the difference in outside and indoors temperatures increases, your performance may degrade. My rig works incredibly well at differentials of around 30 degrees F, but I’m certain I’ll see a fall-off in performance when it’s 10 degrees outdoors and 68 indoors.
In the case of this project, testing revealed astounding efficiency with indoor temperature around 67 degrees and outdoor around 38 degrees. Incoming air was 66.4 degrees with the shell well insulated to prevent parasitic heating of the shell from ambient indoor air. Turned out my first choice of 45 CFM fans was a bit too limited at times for the ventilation I needed while overcoming airflow friction resistance, so my final build uses variable speed fans with that claim to do 56 CFM (links for those below). I don’t usually run the fans at max speed and they seem to move enough air, so perhaps in the end I could have used the 45 CFM fans. Whatever, not a big deal to experiment with different fans (mine are attached to the unit with thumb screws, so I can swap in minutes).
I also paid close attention to performance during cold Colorado winter mornings, sometimes around zero farenheit. Performance was fine.
IMPORTANT: Locate your fan controls for easy access. Remember you are the brains of this rig, not a microprocessor like those of the commercial heat exchangers. For example, say you’ve had your heat turned off all night, it’s now chilly in your living space, and it happens to be warmer outside at your ingress air vent due to it being a sunny morning? Just turn off your egress fan (the one pushing air out of your living space) and turn your ingress fan up full bore to suck that free heating indoors. Also, rather than running this thing 24/7, consider hooking your fans to a timer that turns your exchanger totally off during the coldest (or hottest) part of the day. For example, I’ve set mine to shut down around 11:00 pm and wake up in the morning an hour or so before I usually sit down at my desk.
One might ask “can an engineer figure all this heat recovery ventilator stuff out with math so I’d know what length, what CFM fans, that sort of stuff?” Perhaps with sophisticated computer modeling and field measurements this could be done. But in a practical sense, no. The engineer would have to know exact CFM of air movement inside the ducts, along with exact surface area of your core. Even then, they would have no exact way to account for turbulence in the air flow. Parasitic cooling or heating of the unit by indoor air would also be difficult to calculate. Probably a better way to refine these units is simply to crowd source the experiments.
One measurement you’ll probably want is the CFM you get when everything is running and your temperatures look good. Roughly measuring CFM can be done by placing a plastic garbage bag of known volume over your indoors inlet, counting how many seconds it takes to fill, then doing the math.
I’d imagine a person with sufficient time could source my fresh air heat exchanger design using all “drainage/sewer” PVC known as thin wall DWV. Doing so would be excellent. Crux is acquiring fittings such as the fan mount flanges. Next build I do I’m giving DWV a try — doing so would probably save at least $50 over the build I did using local over-the-counter materials. See http://www.pvcfittingsonline.com/fittings/dwv.html
Fan speed controls are essential for tuning performance and noise.
Timer is also essential in my opinion, no reason to move too much air.
Multi-sensor indoor-outdoor thermometer is also essential, otherwise you’ll just be guessing at performance.
