HRVs or Heat Recovery Ventilator in your bus?

[Firepuncher]

This is a topic that came up in a thread on foaming or not in your bus build. Some type of whole house ventilation is a building code requirement for new construction across Canada. Where much research has been done on keeping houses warm and healthy with the least amount of energy expended. Back in the 70s builders were encouraged to tighten up their construction practices and lots of houses were built extremely tight but no one had put much thought to the consequences of airtight homes with no ventilation. It was a hard and expensive lesson with many hundreds of cases of mould and rotting wall assemblies as well as windows. I lost 13 wood frame windows in a house I had built in 1979. The condensation was crazy. I had one under performing bathroom exhaust fan but critically, no make up air. Some government construction scientists came up with a better idea and it was mainly introduced in a program called R2000. R2000 was a prescriptive guide for building higher efficiency homes in Canada, that among many other things required an envelope as tight as could be achieved. It involved basically installing a continuous vapour barrier with every possible opening in it sealed using special tape, red stuff that can remove the skin from your lips ugh, and acoustic sealant that is a caulking tube product that never hardens and has the nickname Black Death, almost impossible to remove from bare skin. Now the methods that were developed to achieve air tightness have evolved greatly since those early days and are mostly now part of the minimum code for construction in Canada. The air tightness is actually measured by certified energy auditors using a blower door to depressurize the house and infiltration is measured by use of manometors mounted on the blower panel. There is a maximum allowable and houses fail regularly. Builders then have to find the leaks, plug them and call for another test. But my point is that obviously with a house that tight, ventilation is critical for occupant health. This actually can be done with exhaust fans and passive intake of cold air. However, and now to the real topic, almost every house has an HRV or Heat Recovery Ventilator. There are a number of different designs, but the essence is a system that exhausts moist, polluted air from rooms such as the kitchen and bathrooms, and brings outside fresh air into the living spaces such as living room and bedrooms. In the process of this to avoid introducing cold air directly into the home, the warm polluted air passes on one side of a heat exchanger tempering or warming, and giving up some of it's heat to the fresh incoming air on the other side of the heat exchanger. As far as this type of system in a bus goes there are models of HRVs that are not ducted and are small wall mounted units built for mini homes or mobile homes. So the question is, has anyone installed or considered this in their bus. I plan to use the highest performance spray foam system I can find but isolate it from my living space with a very detailed and continuous vapour barrier on the warm side or inside of it. This will eliminate any concerns re off gassing and will allow me to heat to the cold winters we experience here. And then ventilate!

Steve

ps Apologies for the rambling but I think the background is important, comments or additions Borealis?

[gs1949]

I've been thinking of designing my own ventilation system. So I am always interested in looking at things that could be components. I've never looked at HRVs, didn't really know what they were called, but I just did look on Amazon, which is the first place I look for everything. Amazon has just one HRV that had 12V in the description:

https://www.amazon.com/BAL-25110-Air...T0A/ref=sr_1_1


This product has one review: "Nice well working unit. The only problem is that the exhaust air is not perfecly sealed from the incoming fresh air so a certain percentage of the bad odor is pushed back into the rv. I also find that the warranty is pretty short (only 3 months)."
The reviewer gave it 3 stars. I think negative 1 would be more appropriate I will keep looking.

[PatrickBaptist]

Very interesting.

[Firepuncher]

Originally Posted by PatrickBaptist

Very interesting.

Very, we have a strict Healthy Home designation and ventilation is absolutely critical

Steve

[Borealis]

Great post, Steve. It certainly lays the groundwork.

There are some innovative ideas out there to be sure. The LUNOS E2 links several low power fans with a controller to balance inbound/outbound air volume. The fans have a ceramic core which stores some of the outbound heat. Periodically the fans reverse direction so the stored heat in the core can pre-warm the inbound air. It's clever, but I'm not convinced that it will give me the airflow I want such that my sleeping area, is always getting fresh air.

Panasonic has the FV-04VE1 ERV unit. I know #TinyLab was using this at one point, but they modified it heavily so it didn't have input and output flow mere inches apart. I believe they are also using a Broan HRV for their two-room tiny house. I'm guessing they discovered the core in the Panasonic unit was too small but retained it as part of their moisture control. #TinyLab has really "scienced the ****" out of their tiny home and has a ton of sensors and displays in their demonstration home.

At the other extreme, we have some horrific offerings like the one gs1949 linked. I have looked at that unit in the past. One user described it as "essentially just two 12 volt computer fans housed in a metal box with a poorly constructed plastic exchange unit". It's not surprising. The manufacturer makes miscellaneous RV products from jacks to hitches and has no expertise in HVAC.

Here in Canada, I like what Venmar is doing. They have a lot of experience with harsh winters (Zone 2 Northern Ontario). The orange big box store carries their bottom of the line consumer-installable units. The unit is not as large as many, but it's still 36"x18"x12" and you need to mount it somewhere accessible to do filter and core maintenance. You also need to periodically inspect the condensation drain. Looking at efficiencies at low temperatures, it's using about 60W of power and is 60% efficient at 48 CFM at -25C. The efficiency climbs significantly in warmer weather and at lower CFM. I suspect you could run it in "Eco" mode all the time on a bus and it should pull significantly less than the rated 60W. Putting this in perspective, a Dometic 13,500 BTU AC unit will draw 2800W. A charger for a MacBook laptop is rated at 60W. I suspect a significant part of the power equation, assuming you're on solar, would be having the inefficiency of your inverter in use 7x24. There's also the small issue of the $900 CDN price tag.

The alternative would be to build something from scratch, perhaps using one of the smaller Venmar ERV or HRV cores. It would require a couple of good 12V fans, electric air flow dampers, an Arduino microcontroller and some temperature/humidity sensors. For RV/bus use, you may also want to be able to close both intake and output while you're driving. Doing some quick math, let's assume the living space on an average bus is 30'x8'x6'. As a guideline, building codes in my part of the world say you need to exchange the air once per hour, so that's moving about 25 CFM. That's half of what the base Venmar HRV is quoting in their efficiency tests. It would be an interesting project and eat up a ton of time to design and fine-tune. I'm not sure if the difference in power consumption would warrant the effort, but I'm certain you could source the components for a fraction of what the commercial unit retails for.

Thoughts?

[cadillackid]

I have played with such a system in my house... I have an Economiser that opens for cooling cycles when the humidity is real low outside.. it is a modulating 6 inch damper (there is a blower on it also..



I located that 6 inch pipe inside of an 8 inch duct.. the 8 inch duct is the exhaust air that I have played with but only use now and then.. in fact In winter i am able to recover some heat as it does raise the incoming air temperature a few degrees across that 20 foot pipe run... the exhaust air source is a topic of discussion.. right now I pull it from only one area of the house.. and its a low down return duct in my main greatroom.. the thought process being that its some of the coldest air in the house already (excludingthe basement which I consider semi-conditioned space..



in summer when I want to exhaust air I give uo the heat exchanger and simply activate the bathroom fans and open the economiser damper.. the thouight process here is that i asm removing air from the ceiling where some of the warmest air would be..



its probably over the top even for a modern home like mine.. which is standard 2x4 wall construction.. OSB on the 2x4's Insulating board and then vinyl siding on the outside.. every stud cavity has standard fiberglass insulation with a vapor barrier and inside the walls are 5/8 drywall..



the windows and doors are Andersen 400 series Woodwrights. which are a high performance window.. although not as High R value as some of the vinyl products.. however I really detest the plasticy white-generic look of vinyl windows..



while condensation isnt a problem in my home as its not tight enough for that to occur.. in fact my humidifier operates when its cold and dry out.. stale odors are an issue.. I cook dinner and smell it for 2 days.. unless i build a fire in the fireplace. which of course is going to create some definite air-loss in the house.. the fireplace does have its own make-up air damper and when the fireplace is burening you can definitely feel a draw from that vent outside.. it defiinitely still creates a negative static on the house.. (fireplaces are purely aesthetics and not practical.. but it helps my mental health which goes in the crapper in winter due to all the cloudy cold gray days ohio gets)...


-Christopher

[Borealis]

That's interesting, Christopher.

It would appear your primary function for the system is humidity control. There are three broad categories of air exchange units on the market. Straight air exchangers are simply fans which balance the airflow. HRV units exchange heat as well as air. ERV does heat exchange, but the core unit is permeable to moisture. ERV helps you stay closer to your target humidity level through different seasons.

These are not like the small heat exchangers that you see in split AC units or refrigerators. With ERV/HRV you are moderating inbound air temperature with outbound air. The two airflows can't openly mix, so they pass them at right angles to each other through a sort of matrix of small channels. With HRV this is often a plastic material, similar to a big sandwich of that corrugated plastic used for signs. With ERV the exchange core needs to be somewhat permeable to moisture, but not allow the air to mix. There is a natural tendency for the cooler airflow to steal moisture from the warmer side. In a large home, you want to save moisture, especially in the winter. I suspect that HRV will be better for a bus since we're usually dealing with too much moisture in a confined volume of air. Do any of you have a humidifier on your bus?

These systems work best when you have a perfect air seal on the skin of your home. This is likely easier to achieve on a coach-style bus than with a skoolie. It is difficult to get a perfect air seal on those old bi-fold bus doors. Skoolie windows, at least on older buses often leak water, much less air.

New house construction in Canada has been using 6" studs for quite a few years. We need the extra depth to get enough R-value between the studs. Some homes have even gone to 8" studs. Something kind of exciting is a company (in Minnesota?) which is producing a sort of composite 2X6 stud to reduce thermal bridging. It looks like a 2X4 split down the middle into two 2X2's with dowel struts and a tasty injected foam filling. Their tests show these studs have better R-value than the filled 15" cavity between them, essentially eliminating thermal bridging.

You raise an interesting issue with your fireplace. When designing and balancing the airflow, you need to take into account other systems which rely on outside air. A traditional fireplace is not practical for the reasons you stated. They do make fireplace inserts which effectively turn the fireplace into an airtight woodstove. I prefer a woodstove as it radiates heat 360 degrees into your interior. For this discussion, you want the "optional" external air supply line. Pulling draft air from inside your heated space is not a good design.

These exchange units have two fans so they can effectively balance the inside and outside air pressure. If you just have traditional bathroom fans, range hoods or RV ceiling vents, you are creating negative pressure inside your bus. With negative pressure, you are coaxing smells out of your plumbing stack through your sinks, your toilet, etc... Positive versus negative pressure balance is a huge deal in an RV/bus. Imagine if your composting toilet pushed air from the intake rather than pulling it at the outtake! With homes, the issue is more complicated as it is much harder to achieve a tight vapour barrier. For that reason, some contractors recommend running negative pressure in the winter and positive in the summer. They don't want to push warm moist air into cold insulation. But you are going to do a MUCH better job with the vapour barrier on your bus, so you won't have this problem. Your house also doesn't have a metal skin. Any air leaks you have are most likely going to be around uninsulated elements like doors and windows.

Normally you want to pull air from the wet areas of your bus, traditionally the bathroom and kitchen. Essentially you create relative low pressure in your outfeed zone and positive pressure at your infeed. This creates a slow natural flow of fresh air between the feeds. Separate them as far apart as is practical so you're not just sucking your fresh air out before it has a chance to distribute evenly in the room. This also discourages dead air spaces.

In the HVAC design for your bus, keep in mind that stand-alone propane heaters have two unwanted byproducts that your air exchanger needs to address. Burning propane creates a significant amount of water vapour. Assuming 100% combustion, 20 pounds of propane will manufacture about a gallon and a half of water in vapour form (16.2 lbs to be precise). We also have to acknowledge that 100% of combustion is impossible to achieve so you are always pushing out a small amount of carbon monoxide. If you suspect you might be under-designing on your heating and end up using stand-alone propane heaters, you had better over-design on your air exchange.

You typically want fresh air to feed into your primary living space. A good interior airflow design based on many skoolie floor plans would have two intakes and two exhausts. You want to push air into the rear (bedroom) and front (living room) and pull from the middle (kitchen and bath).

Your basic bus layout should discourage dead air points, such as a space behind a couch. Dead air space is effectively unheated and becomes a point for condensation and mold to occur. A closed cupboard isn't a concern since the door discourages the ingress of moisture. Cold is OK so long as it's dry. On the topic of mold, pay careful attention to the airflow around and insulation beneath your mattress. Moldy mattresses have been featured in several van/buslife videos.

Of course, you can't ignore the three-dimensional aspect of your air duct placement. The exhausts should probably be on the ceiling. This accounts for the warm wet air we produce when showering or cooking. It will also help exhaust the hottest air in the room during warmer months. Placement of the inbound ducts is somewhat more complicated. With conventional home HVAC, you want the ducts low when you are providing heat and high when you are providing cooler air. You also don't want the input blowing directly on you when you're sitting around or sleeping. We're somewhat confined on a bus, so I'd go with putting them close to the floor. That maximizes the distance between the in and out feeds. You want the air to naturally drift from input to outtake and flow through as much of the bus airspace as possible. If you have in-floor heating, that's going to warm the inbound air quickly. If you are relying mostly on propane or an airtight stove, you probably need some fans and likely some separate ducting to move air from the warmest to the cooler zones in your living space.

Slow even air circulation throughout your bus is your goal with slightly positive pressure compared to outdoors. This doesn't account for changes in barometric pressure outside, so your positive pressure balance needs to be high enough to accommodate normal weather in your area. Also, a strong wind on one side of your bus will create localized pressure zones. Place your infeed near the front of the bus and the outfeed on the same side near the rear. Putting them both on the same side of the bus will minimize the impact of wind on your balance. They need to be high enough off the ground that they won't get covered with snow. I wouldn't go with roof vents. If you're in a heavy snow area, you're going to be constantly on the roof keeping them clear.

Just like your woodstove, you probably want to cap off or close the dampers for your HRV while you are driving. Who knows what the pressure envelope looks like around a moving bus? Some experimentation might be in order here.

Thoughts?

[gs1949]

There is a lot to think about here. I have been thinking a fair bit about ventilation, but my thinking has included concentrating on outflow and letting the inflow take care of itself.

And also my thinking on ventilation tended to strongly focus on being able to minimize air conditioner use by using reversible vent fans so that I can both blow out warm air during the day and suck in cool air at night.

But now I can see I have tended to oversimplify things, so this thread has given me lots of new things to think about. Thanks to all the posters for that.

[gs1949]

Some links:
https://www.builditsolar.com/Experim...urcliffHRV.htm


https://www.builditsolar.com/Experim...HRV/DIYHRV.htm


https://www.wildsnow.com/17884/how-t...hanger-budget/


https://handmadematt.blogspot.com/20...tion-heat.html

[gs1949]

A few short pieces of something like this would be very interesting to play with:

Finned Tube Manufacturer|Extruded Finned Tube|Embedded G Finned Tube

[Borealis]

Here are a few thoughts on the links you posted.

1. Avoid sharp 90 degree bends in your ductwork when possible. If you must go around a corner, try to do it with two 45's with a little gap between them. There is a huge difference in the vortexing that occurs on 90 versus 45 bends.

2. I would use rigid metal HVAC ducting where possible and insulate it. The collapsible flex hose introduces a lot of drag over the length of a run. Flex is fine for your home dryer, but if we're going to try to build a low energy unit with small fans, that drag is going to hurt your CFM.

3. I can't see how a tube within a tube is going to handle condensation. The water is going to pool somewhere along the line. Hopefully, it's not running back to your fan.

4. Those finned pipes look interesting, but I think their sweet spot would be in boilers or other high-temperature transfers between liquid and air. From what I've seen with heat sinks, the bottleneck isn't getting the heat from one solid or liquid to another, it's getting the heat to transfer to air. I think what you want for air-to-air heat exchange is to push the two streams through many small openings to maximize the surface area to volume ratio. Pushing air through a larger round pipe actually exposes a relatively small ratio of surface contact to volume. Unlike a plumbing application, we actually want lots of surface resistance in the exchange core.

5. The DIY HRV plans look like a great start. It actually looks like someone reverse-engineered a Venmar. One difference in the coroplast core from the Harrowsmith article is they are only using the coroplast channels for the fresh air. They use spacers between the coroplast sheets and double the depth while halving the width of the stale air channels. The stale air routed through the core twice. The fresh air channel is reduced by one third, compared to a commercial core of the same size. If I'm not mistaken, a commercial core from Venmar is manufactured with a single wall between layers. (I could be wrong here. It's been 20 years since I had a Venmar in my home.) With this diamond-shaped arrangement of the channels, your condensation runs downhill at 45 degrees to the base of the unit, regardless of the direction of airflow or the season. It is easy to run a drain tube. To prevent the core from becoming a block of ice, you still need to monitor the temperature and periodically reverse the flow as a defrost cycle. One modification I would make to the Harrowsmith plans is to make the core thicker. Their unit seems quite narrow. That's going to have a big impact on your efficiency. Your optimal efficiency is, of course, a small temperature differential, low CFM and maximum core surface. If Venmar cores are not available as a purchased part to the general public, then I would look at making a similar structure from light gauge aluminum or galvanized metal.

[gs1949]

One of those plans suggests subsitituting rigid aluminum dryer vent for the flexible. And another, although that might be one I found after the links I posted, uses aluminum pop cans, with holes drilled in the bses and I think tops too, then glued together to form the inner tube.

I think aluminum is the way to go rather than the Coroplast. The structure of coroplast is perfect, but it's not an efficient conductor.

Do they still print newspapers with aluminum sheets? Of course, many small papers do not do their own printing anymore.


[edit] This is the one that uses the pop cans:
https://www.instructables.com/id/Air...s-and-pvc-pip/

[Borealis]

I'm not sure what the pop can reference is regarding, but I'm not convinced that the pipe within a pipe exchange core is the best way to go, other than it's the easiest to construct.

1. From a thermodynamic point of view, you want to maximize the amount of air which is touching the surface wall separating the two air streams. Without boring you with a bunch of math, if you take a 4" tube, the ratio of the cross-section area to the surface of the circle (circumference) is 1:1. If you reduce that to a 1" tube, the ratio increases to 4:1. A 1/4" tube is 16:1. So pushing the same volume of air through multiple smaller tubes is going to substantially increase the surface area to volume of air ratio and give you better heat transfer. You might say that the length of the long tube within a tube is going to offset this, but the one foot of multiple 1/4" tubes is equivalent to 16 feet of 4", from a heat transfer perspective. This economy of heat exchange gets better and better as the size of the passage decreases and the number of pathways increase. Coroplast is used by commercial HRV systems because the many tiny airways are more efficient thermodynamically, enough that it offsets the relatively poor thermal conductivity of the plastic wall. It also has the benefit of being compact and easy to remove and clean.

2. If you go this route, you should pull the fresh air through the inner pipe, which is the opposite of the Wildsnow model. Assuming extreme temperatures and the limited options for running the long pipe through the wall of your bus, most of the pipe/exchange is going to occur inside the shell of your bus (your temperature-controlled area). If the outbound air is in the outer pipe, it creates a shroud around the colder inbound air, reducing the amount of insulation you require around the pipe. You don't want the outer pipe full of cold air radiating into your warm living area and heavy insulation just adds to the bulk of the core.

3. Outside the bus you want don't want the intake too close to your exhaust. The Wildsnow model does not have enough separation. Most HRV companies recommend 10 feet between the two ports.

4. There is still the issue of how to handle the inevitable buildup of condensation, ice and mildew in the nested pipes.

[gs1949]

The interior pipe in that case is made from pop cans. The bottoms of the can have been left in place with holes drilled in them so the pipe has baffles inside it to restrict the flow, and this apparently makes it more effective at heat transfer by causing turbulence in the air that's passing through the pop cans.


[edit] @Borealis, a question re: your second point above, won't seasonal variation effect this or do you think it's best to have the hot air inside during the summer?

[joeblack5]

Hrv are pretty expensive. In our shower I experimented with two heat exchanger out of and old window Ac.. Link them together with tubing and fill with coolant and a small electric pump. One heat exchanger is letting air out. The interior heat is transferred to the coolant and then to the the other heat exchanger where it pre heats the incoming cold air.

It is hard to really measure efficiencies but it was cheap.

Later Johan

[gs1949]

That sounds like it would be fairly efficient if the heat exchangers were big enough to transfer a significant amount of heat, and in something like this lead or cadmium in the heat exchangers would not matter.

[edit] The more I think about this, the more I like it. It has several obvious advantages, Thanks for bringing it up.


And it should be easier to measure efficiencies if temp. sensors are built into the device.

[joeblack5]

You are welcome...always fun to redesign existing stuff for different application...upcycling..
Johan

[gs1949]

Yes, I agree, using things in different ways than the designers intended has been one of my pastimes since I was a child.

Lately I have been looking for heaters to replace the ones that came in the bus. I've been unable to find any new, replacement heaters with one inch hose like my bus has. So I am still looking and in the process I've noticed that replacement heater cores are relatively cheap.

I think that 2 heater cores, one on each end of the loop, would make effective heat exchangers. They could be connected under the bus by well insulated hoses. The whole apparatus could be built fairly cheaply I think. Upcycling is a good term. I like that.

I'm kind of treading water with the bus build, doing things as fast as my cash flow permits, which is not very fast. But my brothers and I are getting ready to sell our late dad's house, which is where I am now. When that happens my cash flow problems will be over, and the materials to do this project will be on my shopping list.

[wrenchtech]

https://www.amazon.com/VTRONIC-Venti...6439763&sr=8-2

I don’t understand exactly how this thing is working, with no separation between the intake and exhaust. Probably need to get off my iPad and go look at it on the bigger screen of my desktop. Any thoughts?

[gs1949]

Must be a competitve business. They don't give out any real info about how it's built.

But it's an air to air heat exchanger that works by passing air through passages in a piece of ceramic in 2 directions and heat flows through the ceramic partions between airstreams until both airstreams are at the same temperature.

That's the theory. How well it does this job is another matter, but I assume ceramic material is more efficient at conducting heat than plastic, which is also commonly used in heat exchangers as is aluminum.

[edit]
Not finished reading yet, but they claim 90 percent efficiency, which is quite high.

Farther down I see this unit is sensitive to dust and other things that might plug it. And on Amazon questions someone asks if you can wash the filter, and an Amazon user answers that is not recommended because it will effect performance.

So the 90 percent efficiency might not matter much if you have trouble with plugups.

I would want to see the results of some tests before I put any money into this, but it may be a very good idea whose time has not arrived yet.

Then in the reviews, someone points out that the minimum wall thickness for this device is 7.5 inches and notes that this is too much for a standard 2X4 framed wall. That fact would also add complications to using it in a schoolbus, but I expect that being exposed to road dust would be more serious as dust is mentioned as a plugging substance that must be avoided for the filter to function.