Heat Recovery Ventilators

[Trail Talk]

A recent thread (Amazing DIY Truck Camper) introduced me to the concept of HRV; heat recovery ventilation. The idea, as I understand it, is that outgoing stale air transfers its heat to incoming fresh air by crossing paths through a filtered chamber. This seems particularly useful in very cold climates to a) reduce interior frost buildup by ensuring constant air exchange, and b) reduce energy costs by capturing some of the heat that would otherwise be wasted out the exhaust. A further benefit is filtering of outside air, an improvement on simply opening windows or roof vents.

There is a related device called ERV; energy recovery ventilator, that also deals with humidity. Not an area I'm interested in but it may certainly apply to those further south.

Most of the literature I've found online is dedicated to large HRV devices for industrial or whole-house applications, but there is a subset of small "single room HRVs" that might be suitable for RV applications.

The device that was installed in the previous thread, the Lunos ego HRV System, is designed for spaces under 150 sq. ft. A deep dive into their literature indicates that operation is limited down to -15C/+5F. Although not sufficient for winter use in northern Canada, it would have application in more moderate climates.



Another device popped up in my search for something useful for winter travels: Accurasee Mechanical's MINI portable suitcase style HRV is rated to -40C and operates on 12V. They, however, have no experience with RV applications.



There are a handful of other brands of single room HRVs with a sea of data to wade through regarding air flow and efficiency ratings. Some obvious misdirection by manufacturers can be discovered in the fine print. Caveats on efficiency claims can reveal the difference between inside and outside temps was minimal. Measuring air flow without filters is another means of boosting the numbers.

This is where I'm at right now; deciding if an HRV would be a silver bullet for our interior frost issues in winter. If yes, the path ahead is which device and how to install...

Anyone out there have experience using either HRV or ERV in their rig, or professional knowledge? Please add to the conversation

 

[Victorian]

Thank you for posting this. As you know, we have the Lunos in our camper. So far it is performing/self adjusting fan speed as advertised.

 

[carleton]

There is a related device called ERV; energy recovery ventilator, that also deals with humidity. Not an area I'm interested in but it may certainly apply to those further south.

To add more context, HRVs are very common in newer construction homes in Alaska, from the humid coast to the dry interior.

One of the problems is because new buildings are built so "tight" that humidity builds up, so having air exchange helps to lower the ambient humidity and prevent mold.
Even on the coast in the rain, the HRV in our home would pull in drier air than what was inside our house.

You can see where I'm going with this.....even in your northern climate, the ambient humidity of the inside of your vehicle from cooking & breathing will be higher than outside, so exchanging that air is crucial.

Now the -15c thing is a showstopper, though. Wonder if a very small heater could be incorporated to prevent frosting of the device

 

[Trail Talk]

You can see where I'm going with this.....even in your northern climate, the ambient humidity of the inside of your vehicle from cooking & breathing will be higher than outside, so exchanging that air is crucial.

The technology is also being advanced by Yukon Territorial Govt for new home builds. I think its a issue especially relevant to super-tight composite cabins in our drier climate east of the Rockies, or anytime during the winter, when the variance between interior and exterior humidity is most extreme. This year we brought an Aranet4 CO2 monitor into the cabin during a winter trip and watched it climb into the middle "caution" range pretty quickly! We are reluctant to decrease comfort and increase heater runtime by opening windows as often as required to clear the air so this HRV, in concept at least, offers a possible answer.

 

[Trail Talk]

Thank you for posting this. As you know, we have the Lunos in our camper. So far it is performing/self adjusting fan speed as advertised.

Say, I would be most grateful if you could describe your experiences with the installation and any considerations or issues. Have you been able to measure the unit's efficiency in cold weather conditions; inside vs outside vs conditioned air temps? Really curious to know actual values for heat transfer through these units.

 

[simple]

Thanks for starting this thread. Fresh air in a sealed space is key. I've wondered about how to deal with tightly sealed boxes and CO2 build up. It totally makes sense to use one of these systems. I'd be interested in the various failure modes like fan burnout and iced up passage ways. It wouldn't be good to have the air supply compromised during the night.

I want to add a little more context about cold incoming air's drying effect even if the cold air has high humidity. Air is like a sponge. The warmer it is, the more moisture it can absorb. Cold air can only hold so much moisture. It might be fully saturated(100% humidity) at a low temperature but draw it in and heat it up and it can now absorb more moisture. Blow out the warm wet air and draw in more cold and it makes a sort of pump to remove humidity from a conditioned space.

Most folks accomplish this effect by cracking a window and a vent and running a heater. The HRV is a step up because the heat exchanger passes a percentage of heat from the outgoing air to incoming air which saves heating energy. How much energy actually gets passed is a good question.

I'm excited to learn more about HRV's that can be used for RV's.

 

[Trail Talk]

Had correspondence with the Canadian manufacturer of a small space HRV unit and posed this question:

"This technology is new to us so I have a simple question: Assuming an outside ambient temp of -30C and inside temp of 20C, what temp would the fresh air be once circulated through the HRV into the interior space? Can you share the calculation?"

They responded quickly. Unfortunately, I don't really know what to do with the information:

"HRV manufacturers rate products at standard conditions which is 0C as per AHRI and CSA standards. It is impossible to accurately predict the indoor temperature in a space without understanding several variables, which include, but are not limited to the following: insulation of the mounting surface, surface heat flux, in-filtration / ex-filtration, radiant heat sources, solar irradiation angles and intensity, etc.

You may predict the performance of the unit for energy savings calculation by using the quoted effectiveness in our spec sheet but this would not be applicable to every scenario - as it intended for design. In its simplest form, as per ASHRAE Handbook, the sensible energy saved for heating would be approximately ~ HRV efficiency (%) x 1.23 x flow rate (L/s) x (Temp out - Temp in, C). Again, design load calculations are generally prescriptive and would not be exact except detailed models / experiments of a specific space are conducted which is outside the scope of what we can provide."

Can anyone translate? As far as I can glean from their website, the quoted efficiency is up to 80% at 0C/32F. What that means at -30C was my question as I suspect that efficiency drops as the difference between inside and outside air increases.

 

[Victorian]

Say, I would be most grateful if you could describe your experiences with the installation and any considerations or issues. Have you been able to measure the unit's efficiency in cold weather conditions; inside vs outside vs conditioned air temps? Really curious to know actual values for heat transfer through these units.

The install was pretty simple with the exception that you need roughly 16" of depth to house the ceramic core. I also payed extra attention to "weatherproof" it on a moving vehicle. The electrical also made me scratch my head a bit as I'm not an electrician.... At then I figured it out and everything is running fine. You barely hear the fan in the default setting. Durring the night with two people in the 6.5 flatbed camper I could hear the fan speed changing to the next higher setting due to humitity. Therefore I believe you never have to worry about it. You can also manually control the fan speed. In the highest setting, this unit is moving a lot of air but also makes much more noise. No idea about the actual real world numbers when it comes to power consumption and efficiency. But so far so good. At the end, it's much better than having a window cracked with the resulting draft and condensation on it.

 

[wfv56]

The install was pretty simple with the exception that you need roughly 16" of depth to house the ceramic core. I also payed extra attention to "weatherproof" it on a moving vehicle. The electrical also made me scratch my head a bit as I'm not an electrician.... At then I figured it out and everything is running fine. You barely hear the fan in the default setting. Durring the night with two people in the 6.5 flatbed camper I could hear the fan speed changing to the next higher setting due to humitity. Therefore I believe you never have to worry about it. You can also manually control the fan speed. In the highest setting, this unit is moving a lot of air but also makes much more noise. No idea about the actual real world numbers when it comes to power consumption and efficiency. But so far so good. At the end, it's much better than having a window cracked with the resulting draft and condensation on it.

more details on your install. I can see a 16 inch length would be a challenge to hide in a small camper.

 

[Trail Talk]

Also a picture or two of the unit in situ would be appreciated

 

[Victorian]

Also a picture or two of the unit in situ would be appreciated

Will do. I’m away on business but try to remember when I’m back.

 

[ITTOG]

Had correspondence with the Canadian manufacturer of a small space HRV unit and posed this question:

"This technology is new to us so I have a simple question: Assuming an outside ambient temp of -30C and inside temp of 20C, what temp would the fresh air be once circulated through the HRV into the interior space? Can you share the calculation?"

They responded quickly. Unfortunately, I don't really know what to do with the information:

"HRV manufacturers rate products at standard conditions which is 0C as per AHRI and CSA standards. It is impossible to accurately predict the indoor temperature in a space without understanding several variables, which include, but are not limited to the following: insulation of the mounting surface, surface heat flux, in-filtration / ex-filtration, radiant heat sources, solar irradiation angles and intensity, etc.

You may predict the performance of the unit for energy savings calculation by using the quoted effectiveness in our spec sheet but this would not be applicable to every scenario - as it intended for design. In its simplest form, as per ASHRAE Handbook, the sensible energy saved for heating would be approximately ~ HRV efficiency (%) x 1.23 x flow rate (L/s) x (Temp out - Temp in, C). Again, design load calculations are generally prescriptive and would not be exact except detailed models / experiments of a specific space are conducted which is outside the scope of what we can provide."

Can anyone translate? As far as I can glean from their website, the quoted efficiency is up to 80% at 0C/32F. What that means at -30C was my question as I suspect that efficiency drops as the difference between inside and outside air increases.

Their issue is they do not know the thermal efficiency of your camper or what heat sources you have inside the camper. So the equation they gave you is just for the unit itself. Thus the temp out is at the exit of the HRV, not the temp inside your camper, and the temp in is at the inlet of the HRV, which would be the outside air temp. So you could calculate the exit temp if they publish energy savings and flow rate ranges. Otherwise, you can calculate the energy savings with published HRV efficiency and flow rate and assuming an exit temp, but be careful on this one because you can easily surpass real energy savings if you use too high of an exit temp. Note, what you are calculating is the heat transfer rate of the unit, which they should have real world published values. The equation is basically heat transfer rate equals mass flow rate (for some reason they are using volume) times a constant times the temp delta. But if you modify the equation to solve for temp out, it would be To = (heat transfer rate)/(constant*mass flow rate)+Ti. With this equation you can calculate the temp out by varying the flow rate or temp in. Hope that helps.

 

[plh]

This is nothing new in the residential /light commercial market. I worked as an Engineer for an HRV / ERV manufacture in the mid 1980's.

Here is some relevant information: https://www.ndsu.edu/agriculture/ex...t-exchangers-healthier-energy-efficient-homes

 

[simple]

From engineeringtoolbox.com
"Cross Flow Heat Exchangers
In a cross flow heat exchanger - heat is transferred directly from the outlet air to the make-up air through the separating walls in the heat exchanger.
Both sensible and latent heat may be transferred. Latent heat when moisture in the outlet air condensates on the heat exchanger. The moisture is not transferred."

("The moisture is not transferred" is stated because some HRV's mix incoming air with outgoing air and thereby transfer moisture from one to another. Handy if the conditioned environment is too dry)

Looks like efficiency varies based on the difference in temperature between incoming and out going air. The rate of heat transfer increases as the temperature difference increases.

Efficiency also varies depending on the fan speed because it takes a certain amount of time for heat to transfer from incoming to out going air.

The latent heat of condensing water vapor to liquid state is an interesting one. When water vapor condenses it goes through a phase change and gives off heat(opposite of evaporation). Sounds like there needs to be a drain and is also where ice could potentially build up. So efficiency is also associated with how much moisture condenses and I imagine it is also relative to the humidity of the outgoing air.

So many variables.

I'm curious what the theoretical calculated efficiency is and how it relates to saved energy. It would at least be a starting point to estimate how much energy is saved under average operating conditions and if the cost of dollars and space is worth it. Would a couple of strategically placed insulated forced air ducts be good enough?

 

[Trail Talk]

Their issue is they do not know the thermal efficiency of your camper or what heat sources you have inside the camper. So the equation they gave you is just for the unit itself. Thus the temp out is at the exit of the HRV, not the temp inside your camper, and the temp in is at the inlet of the HRV, which would be the outside air temp. So you could calculate the exit temp if they publish energy savings and flow rate ranges. Otherwise, you can calculate the energy savings with published HRV efficiency and flow rate and assuming an exit temp, but be careful on this one because you can easily surpass real energy savings if you use too high of an exit temp. Note, what you are calculating is the heat transfer rate of the unit, which they should have real world published values. The equation is basically heat transfer rate equals mass flow rate (for some reason they are using volume) times a constant times the temp delta. But if you modify the equation to solve for temp out, it would be To = (heat transfer rate)/(constant*mass flow rate)+Ti. With this equation you can calculate the temp out by varying the flow rate or temp in. Hope that helps.

This helps, thanks! They published a "sensible effectiveness" of "equal to or less than 80%" and the email response indicated this number was achieved at 0C. Does that translate to your "published energy savings"? Flow rate is 3.8 L/sec (8 CFM) at low speed and 10.4 L/sec (22 CFM) on high.

Wouldn't the inside ambient temperature I provided of 20C substitute for the variables of camper thermal efficiency and internal heat sources, in that it represents the outcome/final state of both?