HRVs or Heat Recovery Ventilator in your bus?

[ted@campbycanoe.com]

Funny that this thread popped up now; I've been staring at components sitting in my shop and been "s'posin'" along the same lines.

I found a free truck camper in the want ads so got an RV fridge, a furnace, and a stove out of it. (All work!)

I fired up the furnace and noticed there was more heat cranking out of the exhaust (which was vented right out the side of the camper) than from the heat plate. So I got to thinking, how I could build a heat exchanger?

The furnace vents out of a 3" pipe. What if I extended 3" stove pipe through the roof. Wrapped the stove pipe with copper tubing, then covered all that with 6" PVC pipe. Then ran the copper piping (filled with coolant) through one of the heaters I took out of the bus, through the heater pump removed from the bus, through some sort of expansion tank (I have a radiator reservoir from an old subaru sitting in the shed).

When I turned the furnace on I could hit a switch that would turn on the pump and the fan from the heater.

Could this work?

 

[gs1949]

I'm not totally sure I am visualizing that correctly, but how hot is the PVC pipe going to get? It sounds to me like it may get too hot. Maybe try to find some aluminum irrigation pipe to put it in.

 

[ted@campbycanoe.com]

I wondered about that, too. Good idea about the irrigation pipe...

 

[gs1949]

Yeah, go for it. I think it can be made to work.

 

[Borealis-SKO]

I don't think the Vtronic unit is worth any further investigation. Look at the exploded diagram where they show the components.

Ceramic is an insulator. They use it on spacecraft for re-entry shields. They use it here because it can be fashioned into many small channels and it will absorb and retain the heat. The Vtronic only moves air in one direction at any point in time. It does not have separate fresh air feed and exhaust.

Let's assume we're talking about winter. The Vtronic exhausts air for some percentage of the time, warming the ceramic core with the interior exhaust air. Then it reverses direction and the core pre-warms the inbound air. Not sure where you saw the 90% efficiency claim, but I'm very skeptical. As the core cools after the flow reverses it's going to get less and less efficient if what we're measuring is the temperature of the incoming air versus the interior temperature. Finally, it creates negative pressure when it's exhaling and positive when it's pulling air in. There is no way to balance, even if you had two of these units. Nor does it allow you to create consistent airflow from the living area to your bath and kitchen.

 

[gs1949]

I am skeptical of the 90 percent claim too. I got if from the Amazon product page linked by wrenchtech.

"【Advantages】High Energy Recovery Efficiency Up to 90% Reduce Pollen,Dust Particles,Radon,CO2,Condensation and Moisture."

And this is a low capacity unit: "【Energy Saving】Minimum 2W Working Power and Max 6W, 28CFM to 35CFM Suitable for Bedroom"

 

[Borealis-SKO]

I am skeptical of the 90 percent claim too. I got if from the Amazon product page linked by wrenchtech.

"【Advantages】High Energy Recovery Efficiency Up to 90% Reduce Pollen,Dust Particles,Radon,CO2,Condensation and Moisture."

Ah. The lack of punctuation makes that really an ambiguous statement. I read that as two things.

1. High energy recovery efficiency.
2. Up to 90% reduce pollen, dust...

If you look at the tiny size of that exchange core and the fact that it gets lower efficiency the longer it blows any given direction, I'm pretty sure it doesn't get anywhere near 90% heat xfer efficiency. If it was that easy the larger commercial units would be doing much better. I'd want to see efficiency graph at different temperatures or the number is just marketing hype.

 

[gs1949]

I agree about the punctuation. When I was a grad student in History, I marked hundreds of undergrad papers every year, and taught classes on writing papers. Punctuation is always the first thing I notice when I am reading something, and punctuation is usually a very good indicator of how much, or how little, effort the author put into writing the paper.

And there is nothing on that Amazon product page that makes me even slightly interested in that device. Even if it works half as well in real life as their figures imply, it's still too small to do much.

 

[gs1949]

And there is something else about that product description, and actually most of the other product descriptions I've read on Amazon, that might not be obvious to most readers who have not taught English to Chinese university students. After several years of that experience I recognize Chinglish syntax whenever I see or hear it.

 

[Doktari]

I have a new HRV core and case I found at a Habitat store. The fans and controller are missing. I don’t want a computer in my vents. Ive been waiting to hook it up in my new vehicle whenever I get the insulation done.

 

[gs1949]

I need to find one of those stores. Not sure we have any close around here. Might have to drive 100 miles or even more.

 

[kidharris]

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?

I don't mind the rambling, i do it too. But, a little white space, especially when you change subjects would make it easier to read. LOL



Looking at Amazon, I guess if you call an air to air heat exchanger an EVR/HRV, you can double or triple the price.

 

[Firepuncher]

Yeah I agree, I was visiting family in Ontario trying to get out from under the flu. All I can figure is I was a little delirious LoL

When I search Amazon for an air to air heat exchanger, most of what comes up are actually HRVs. Some are a single duct unit calling itself an HRV, and a few just the heat exchanger . The heat exchanger is only a component of an HRV. This is is maximized for the application The rest are the controls, fans and sensors etc.

None of the heat exchangers I saw were for an HRV so I suspect using one in a DIY application would disappoint. I'm going to try and find a used or broken down residential HRV and modify it for 12v use. I expect it may end up less efficient, but I'm okay with it in a bus. will have all of the ability to remove moisture, provide fresh, tempered air and balance the intake and exhaust so I am not pressurizing or depressurizing my envelope and insulation cavity.

The core of an HRV is not that expensive. Nor are replacement air filters.

Steve

 

[kidharris]

Yeah I agree, I was visiting family in Ontario trying to get out from under the flu. All I can figure is I was a little delirious LoL

When I search Amazon for an air to air heat exchanger, most of what comes up are actually HRVs. Some are a single duct unit calling itself an HRV, and a few just the heat exchanger . The heat exchanger is only a component of an HRV. This is is maximized for the application The rest are the controls, fans and sensors etc.

None of the heat exchangers I saw were for an HRV so I suspect using one in a DIY application would disappoint. I'm going to try and find a used or broken down residential HRV and modify it for 12v use. I expect it may end up less efficient, but I'm okay with it in a bus. will have all of the ability to remove moisture, provide fresh, tempered air and balance the intake and exhaust so I am not pressurizing or depressurizing my envelope and insulation cavity.

The core of an HRV is not that expensive. Nor are replacement air filters.

Steve

A lot of computer/enclosure fans are 12 volt.


For an example heat exchanger, a gas furnace is an air to air heat exchanger with the burner inside the tube/duct. with the flue gases going outside - basically a bumpy irregular shaped sheetmetal tube inside a tube/duct, with a fan, filter. switches/controls, ect. Substitute a fan for the burner and you have a big air to air heat exchanger (installed backwards for most uses).



Steel is used for a furnace because of the burner heat, but for cooler temps copper or aluminum would be a better choice. More surface area on the thin metal between the tubes and turbulence created by bumps (ball peen hammer induced) increases efficiency. You could build your own custom shaped one pretty easy if space/location is an issue. For an easily changed/installed round filter you could use a vacuum cleaner HEPA filter on the incoming air side with a manual damper to control volume or shut off circulation/flow when not in use.


It could be made to go on a wall, floor, or ceiling to fit your situation. The in and out openings, both inside and outside, should be far enough apart so that your not just recycling the same air over and over. This is a problem with the commercial units because they have to be made into a compact unit to be shipped and installed easily. Although the outside skin could be utilized as part of the heat exchanger, the outside should be insulated for efficiency in most designs.


A counter flow heat exchanger could be as simple as a 4 inch aluminum flex dryer vent hose centered inside a 6 inch galvanized duct (insulated outside) running the length/width of the bus, mounted inside or outside of the bus. Longer counter flow exchamgers would be more efficient in most simple designs.


If you build one, would you please share your design and results? A random thought just occurred to me that you could vacuum your floor and ventilate at the same time by dumping the vacuums exhaust into the exhaust of your heat exchanger. Stupid, I know.

 

[gs1949]

I want to build my own HRV System using 2 heater cores, insulated hoses to connect the heater cores under the bus and a pump to circulate the anitfreeze/water mixture between the 2 units. I assume I will probably need an expansion tank somewhere too.

The advantage to a system like this is that by placing the inlet unit and the outlet unit fairly far apart, I'm thinking a little over 20 feet in my case, a bigger area of the bus will get fresh air circulation than if the inlets were very close together.

I am sure my system as I envision it will recover some heat that would otherwise be wasted, but I have no real idea how efficient it will be and won't know that until I build it and test it.

It should be fun. Any comments? Has anyone tried anything like this?

 

[kidharris]

I want to build my own HRV System using 2 heater cores, insulated hoses to connect the heater cores under the bus and a pump to circulate the anitfreeze/water mixture between the 2 units. I assume I will probably need an expansion tank somewhere too.

The advantage to a system like this is that by placing the inlet unit and the outlet unit fairly far apart, I'm thinking a little over 20 feet in my case, a bigger area of the bus will get fresh air circulation than if the inlets were very close together.

I am sure my system as I envision it will recover some heat that would otherwise be wasted, but I have no real idea how efficient it will be and won't know that until I build it and test it.

It should be fun. Any comments? Has anyone tried anything like this?

One advantage that I can see is if your bus is well sealed then you could get by with one fan to provide both ventilation and circulation, similar to the way a whole house fan (fan blows out - negative pressure) or swamp cooler (fan blpws in - positive pressure) system works, except you could use the fan at either the inlet (positive pressure) or the exhaust (negative pressure). There are some problems associated with positive and negative pressure systems if you use large fans. Effeciency would be determined by how big (or how many) the heater cores are.


Since you are using a transfer fluid I would think that it might be possible to add additional heat/cool to the circuit.


Sounds doable and possibly cheap to me.

 

[dan-fox]

Has anyone performed a blowdown(?) test on their skoolie to find out how tight it is? I would think that finding and stopping the leaks would be the better use of time and resources.

 

[dan-fox]

And there is something else about that product description, and actually most of the other product descriptions I've read on Amazon, that might not be obvious to most readers who have not taught English to Chinese university students. After several years of that experience I recognize Chinglish syntax whenever I see or hear it.

engrish.com

 

[kidharris]

Has anyone performed a blowdown(?) test on their skoolie to find out how tight it is? I would think that finding and stopping the leaks would be the better use of time and resources.

When you have as big a percentage of outside surface area that is single pane glass as a school bus has, I would be more concerned with conductive losses/gains. Drafty leaks are easily detected and fixed, and some ventilation is a good thing. I would think that heavy, insulating curtains would be a better low cost investment of time and $$$.


BTW, this business of ERV/HRV only makes economic sense when the temperature diiffrence between indoor and outdoor air is large which is probably only in the dead of winter and summer in most areas. Changing the thermostat a few degrees makes more economic sense unless you are in extreme climates.