Greetings everyone, 

    I did an energy audit on this new home that is too tight and needs mechanical ventilation, so I thought to tell them to get an ERV installation, but then I realized that they are heating with radiant floor heating, so there is no ductwork at all and to make things more complicated, there is no attic.  To make things even more complicated, the bathrooms does not have fans, and the kitchen fan does not exhaust to outside... so I was thinking that because there is no attic, I would had no choice but offer to install the bathroom fans independently from any ERV install since there is no attic that would allow some kind of duct work installation, I going to recommend to vent that kitchen exhaust outside, but then, how would the ERV be install? 

   Your advice is greatly appreciated, 


Views: 715

Reply to This

Replies to This Discussion

Simple is best - pull from one area & deliver to another. I prefer supply by sleeping area & pulling from living area but that is with things open. Now it gets down to what is available, etc... For example you may have a closet where it goes with the supply in one room & exhaust from another.

As for kitchen & baths - that is the biggest concern I see and really needs to be corrected first.

For more on ventilation strategies for different climates:

Thanks Sean for your reply!  

   The space is pretty much open like a studio but there I am limited on how to do anything by not having an attic. So whatever I do is going to be exposed, so what would be the minimum I could run this ducts (supply and return)?


Check with the manufacturer - You probably should have at least 5' of space in between as a rough rule of thumb

Hey Chris,

  This looks amazing, now it seems that they do not have a filter at all... Is that correct? I know there is plenty of discussion about filters been an issue since now the homeowners needs to clean them, but I think the burden of cleaning them is worth while for the extra benefit of stopping pollen and such from entering the home... any thoughts?   Thanks

We don't recommend ERV's anymore.

If you are in a green grass climate ventilating dehumidifier helps solve problems without creating them.

Data log before and after to understand what has been accomplished.

Grass climate? Not sure what that is, but I just realize I should had mentioned this house is in the Northeast area.

Any suggestions on what to use for data logging? I just suggested that to the homeowners to do now (before winter arrives) in order to get a better understanding of where this excess water vapor is coming from ... to make things even more complicated, there is no AC in this house!

Green Grass (wet) climates - not Arizona.

A ventilating dehumidifier and positive pressure strategy will provide drying potential to homes in moist climates, helping avoid mold and other long term moisture related problems. VOC off gassing is lower with lower RH. Dust mites can't survive - some people are highly susceptible to dust mite feces. 

Here's the device we use for tracking - - if you want 3 for $499 just put 3 in your cart, and at the payment page enter 3FBT499 in the discount code box.​ You can also get them on Amazon.

We are finding out a lot of interesting things. In summertime, for example, running below 42%rh starts being too dry. At that RH temperature below 75f tends to be too cold. We suspect higher thermostat settings may save back a fair amount of the energy used to run the fresh air/dehumidifier. 

We used to use Ecobee thermostats. But it cost the clients a lot more, and it cost us a lot of un billable time. 

These are pretty expensive but are really sharp looking. I have an architect in Va. Beach using them.

Results coming in about a year.

My general recommendation would be 4" duct branches running to each bathroom. From a main trunk to the ERV unit. Each exhaust register should run continuously at 25 cfm or 50 cfm intermittently. Fresh air would be directed in a similar duct layout with registers in all living spaces (family room, bedrooms). If you do not have an attic for ducting, you will need to use a soffit to rout the required branches. Or use a basement/crawl space for these. Rectangle and oval duct can be used to help fit ERV ducting between wall joists. Exterior penetrations should be located at least 6ft apart.

Feel free to ask any questions.

Sorry but I got to disagree - exhaust from a bath (and kitchens) should go out through a dedicated exhaust fan & definitely not an ERV which utilizes a shared core. I still feel the same about an HRV even with separate air streams just for the simple fact of who wants to add a heater to their unit to keep it from icing up during the winter.

As for sizing, there is no sense to over ventilate - for more on this & other thoughts from ASHRAE / Lstiburek / & others

To give my argument for Exhaust Only vs ERV:

The exhaust only fans generally installed in bathrooms/kitchen are going to depressurize the home, remove conditioned air, and provide no filtration on entering air. By depressurizing the residence, the fans actively pull unconditioned air through every crack in the home. Leading to cold, humid, hot, etc air pulled across insulation, tape, paint, drywall, siding, etc. I see this as a major IAQ concern....even if you have a location for fresh air makeup during depressurization; it leads to 100% unconditioned air entering your home and another exterior envelope penetration.

The ERV is going to recover a percentage of the interior condition, filter the incoming air, and achieve some level of balance for incoming/outgoing. I'm not sure why you would worry about a shared energy transfer medium in a residential application? Are you concerned with smells transferring across the core? Even if some level of cross flow is present; the incoming air would dilute this at a level at 10-1....constantly during operation.

ERV defrost is handled in 3 different ways: 1. Adding a pre-heater to warm incoming air to the threshold required for operation. This is the best for IAQ because it does not sacrifice ventilation during cold periods. however, it does lead to an energy draw. 2. recirculating conditioned air in the unit itself. This is not acceptable for independent ERV systems because of humidity dumping and we are sacrificing 100% of our fresh air intake during this period. The humidity dumping would be caused if the ERV was in re-circulation mode simultaneously as someone showering. 3. Decreasing supply while keeping exhaust levels. This is a "best of both worlds" approach that sacrifices some energy abilities of the ERV and some IAQ abilities of the ERV. It depressurizes the home (not as much as exhaust only...) and it supplies less conditioning to the incoming fresh air. The best scenario is different for each project's individual needs but having a low threshold for defrost is the best option. The lowest defrost threshold is 12F by UltimateAir's ERV. However, many other manufacturers are obtaining defrost thresholds in the 20s now (not bad). If you are building in a area that would rarely see local temperatures drop below the threshold -- then the defrost requirement does not even need to be considered. My recommendation is to add a pulse modulated pre-heater that will only consume enough power to keep the incoming air above the required defrost threshold. Some manufacturers are even building the electric pre-heaters into the units themselves. I can only speak for UltimateAir on that point but maybe others are beginning to do the same.

Finally, my last point is simple comfort. If a home is being heated to 70F and the ERV is able to direct fresh/filtered air at roughly 66F into each bedroom -- we are going to have a much higher level of IAQ and very little cold draft issues. If you use a system with high heat transfer, you will minimize the issues of introducing fresh air into living spaces and maintain constant exhaust in your "dirty" air locations.


Forum Discussions

How would you insulate this roof?

Started by David Butler in General Forum. Last reply by David Butler 6 hours ago. 34 Replies

Psychrometric Chart Training

Started by John Krochmalny in Training. Last reply by Mahendra Shah on Friday. 1 Reply

Dirty solar panels

Started by Evan Mills in General Forum. Last reply by Stacie Bagnasco Oct 16. 1 Reply

Latest Activity

David Butler replied to David Butler's discussion How would you insulate this roof?
"Sean Lintow proposed rails and clips (re: first comment in this discussion), but in the context of…"
6 hours ago
Brad Cook replied to David Butler's discussion How would you insulate this roof?
"I missed where the suggestion for rails came in. The plywood would not need any rails. 4+ inches is…"
10 hours ago
David Butler replied to David Butler's discussion How would you insulate this roof?
"I'm a little confused about the necessity and purpose of using support rails if working on…"
11 hours ago
David Butler replied to David Butler's discussion How would you insulate this roof?
"If you insulate master bedroom and family room from below, you definitely need more R-value than…"
11 hours ago
David Butler replied to David Butler's discussion How would you insulate this roof?
"Brad, I was only talking about using EPS between the ridges, if necessary, to conform to bolts.…"
11 hours ago
Brad Cook replied to David Butler's discussion How would you insulate this roof?
"It kind of depends on the PIC. Not all insulation is made the same. Here in VT (and all of New…"
11 hours ago
David Butler replied to David Butler's discussion How would you insulate this roof?
"Filling those shallow valleys with rigid foam not only prevents primary foam panels from…"
11 hours ago
Brad Cook replied to David Butler's discussion How would you insulate this roof?
"Bill, you just posted a number of responses in different areas of the thread. I will try to respond…"
16 hours ago


  • Add Photos
  • View All

© 2017   Created by Home Performance Coalition (HPC)   Powered by

Badges  |  Report an Issue  |  Terms of Service