Hello, with all the talk about not needing to have heat source in every room in a superinsulated house, I was wondering: how much fresh hrv air supply should a house have? Only one in a central place if it's open design? thanks

What's your location? What kind of square footage are you looking at? What is the general construction and layout of the house? What is your primary source of heat? It does depend on certain factors, or maybe I'm misunderstanding your question, but I don't think one can necessarily get away without having some sort of heat flow into each room. I can tell you the amount of fresh air you would need based on the information I asked above, but an HRV won't exactly "heat" your house. They only help retain some of the heat that is getting exhausted and the best I've seen are ~70% efficient. But if you'll just give a little more information about your particular situation i can better help. :)

The bedrooms should get fresh air supplied and any rooms which have a high occupancy.

The Recoupaerator 200DX by UltimateAir is 96% efficient.

actually, it was more a general building science question... just wondering if you really need to supply fresh air to each bedroom and high occupancy room... seems to me that if you maintain adequate ACH and humidity level, no room, even with closed door, is tight enough that it would lack fresh air...

I think it is a good idea for smells and indoor pollutant levels (including CO2).

From a general standpoint, yes, I would strongly suggest means of bringing fresh air into the house, especially a tight house whether it's a power ventilator or just fresh air ducted to your air handling unit. Tight houses these days can achieve crazy low ACH rates from infiltration (which is good), and that is simply not enough of a fresh air rate for a home. Like jonr mentioned, it's a very good idea for indoor pollutants. Most people rely on bathroom exhaust fans to bring in fresh air via infiltration, which worked just fine 20 years ago with the "loosely" built houses. But it simply won't be enough for a tight home.

Ventilation to each bedroom and high occupancy room is ideal but not required. However since bathrooms need to be exhausted anyway for moisture reasons, why not do it via recovery ventilator to keep some of the humidity and/or heat inside? If it's a big open concept like you mentioned, you could just supply the fresh air at a central location and exhaust the bathrooms. If you leave the bedroom doors open then there wouldn't be a problem of that "stuffy" feeling from not have the fresh air ducted in there. The central location would save you on ductwork at least and it is better than nothing. A simple 150 cfm ventilator should suffice, depending on the size and occupancy of your house of course.

Mechanical ventilation is a must in a tight home. Many articles on GBA that discuss how much is needed.

Get a ventilation system that allows for a "bypass mode" as this will allow you to bypass the heat exchanger. This is vital when the interior is warm (summer) but the outside air is cold (summer night). This way you can channel in the outside cold air into the warm interior during summer to help cool off the home. I believe this is a vital feature to have and surprisingly many HRV's don't have that option.

if you maintain adequate ACH

I understand ACH to be "Air Changes per Hour". Adequate, but not excessive ACH is the goal. Excessive ACH represents major heat loss. Once you get the leaks controlled, you really do need a source of fresh air. In a tight bedroom, for example, 2 sleeping people can raise the CO2 concentration to levels that will disturb normal breathing.

Just for reference, Canadian Codes require that every bedroom and main living area be supplied with outside air.

This is based on the fact that few people will open window during cold or inclement weather.

Posted By FBBP on 02 Oct 2013 09:30 PM
Just for reference, Canadian Codes require that every bedroom and main living area be supplied with outside air.

This is based on the fact that few people will open window during cold or inclement weather.

The opposite is true here in Phoenix. Nobody opens a window during the 5-6 months of summer as outdoor daytime temps are 115F and the nighttime temps hover around 100F. People leave the A/Cs running all summer and never open a window.

Sadly they build these homes to leak air like crazy so they don't use mechanical ventilation.

"actually, it was more a general building science question... just wondering if you really need to supply fresh air to each bedroom and high occupancy room... seems to me that if you maintain adequate ACH and humidity level, no room, even with closed door, is tight enough that it would lack fresh air..."

If you are governed by the International Mechanical Code the language is as follows:
Ventilation required.
Every occupied space shall be ventilated by natural means in accordance with.......or mechanical means in accordance with......

In code speak you are referred to sections to address each.

You must then get into the language such as the definition of occupied space and adjoining spaces. Partitioned spaces (such as bedrooms) intended to be ventilated mechanically could be judged to require their own ducted outlets however if they have operable windows (at least in MI) of adequate size they would count (whether you'd open them in January or not).

It is a point in a high performance house if one wished to use an unducted heating system, then ducted ventilation may be required to areas where free communication of air could be interupted.

Posted By FBBP on 02 Oct 2013 09:30 PM
Just for reference, Canadian Codes require that every bedroom and main living area be supplied with outside air.

This is why I love Canada. Their codes are top notch. I'm sure in their climate they have to be but still. I wish the U.S. wasn't so far behind. . . .pipe dreams.

Posted By joe.ami on 03 Oct 2013 09:07 AM
If you are governed by the International Mechanical Code the language is as follows:
Ventilation required.
Every occupied space shall be ventilated by natural means in accordance with.......or mechanical means in accordance with......

That's very interesting because my current house here in OK doesn't have any outside air ventilation whatsoever. Granted OK is still on 2009 codes and perhaps bathroom exhaust counts towards ventilation, but I still think it's a stretch. I fully believe outside air ventilation is crucial to a healthy comfortable environment.

"That's very interesting because my current house here in OK doesn't have any outside air ventilation whatsoever."

No windows?

Posted By joe.ami on 03 Oct 2013 09:07 AM
"actually, it was more a general building science question... just wondering if you really need to supply fresh air to each bedroom and high occupancy room... seems to me that if you maintain adequate ACH and humidity level, no room, even with closed door, is tight enough that it would lack fresh air..."

If you are governed by the International Mechanical Code the language is as follows:
Ventilation required.
Every occupied space shall be ventilated by natural means in accordance with.......or mechanical means in accordance with......

In code speak you are referred to sections to address each.

You must then get into the language such as the definition of occupied space and adjoining spaces. Partitioned spaces (such as bedrooms) intended to be ventilated mechanically could be judged to require their own ducted outlets however if they have operable windows (at least in MI) of adequate size they would count (whether you'd open them in January or not).

It is a point in a high performance house if one wished to use an unducted heating system, then ducted ventilation may be required to areas where free communication of air could be interupted.

The Canadian Codes deal with heating as part of the Building Code. It is laid out quite clear but then says anything not cover in the building code must be design to ASHRAE etc.

It allows for natural ventilation during the non heating seasons but requires mechanical ventilation for the heating season. So if you have anything other then a forced air system, you must install a ventilation system to each bedroom, main living area and at least one outlet to each level.

Posted By joe.ami on 03 Oct 2013 09:42 AM
No windows?

Yes, there's windows of course. However we hardly ever open them, and I think it's ridiculous to rely on infiltration for outside air from a code stand point. That whole concept of "letting a house breathe".

"Yes, there's windows of course. However we hardly ever open them, and I think it's ridiculous to rely on infiltration for outside air from a code stand point."

If you are governed by the IMC, then your windows are "codeworthy" ventilation.

"The Code" is a minimum standard, not the best of everything. Some find it too intrusive already. At least it doesn't prevent me from opening my window in January (in MI) for ventilation.

Indeed we employed it (natural ventilation) on a recent renovation of a 150ish yr old opera house. Without those provisions, we would have needed a much more sophisticated HVAC system and quite a bit of structure improvements to support it on the roof (likely nixing the job).

Posted By Lbear on 02 Oct 2013 06:21 PM
Mechanical ventilation is a must in a tight home. Many articles on GBA that discuss how much is needed.

Get a ventilation system that allows for a "bypass mode" as this will allow you to bypass the heat exchanger.

Can you recommend a brand that does this?

Posted By DJV on 09 Oct 2013 12:55 PM

Posted By Lbear on 02 Oct 2013 06:21 PM
Mechanical ventilation is a must in a tight home. Many articles on GBA that discuss how much is needed.

Get a ventilation system that allows for a "bypass mode" as this will allow you to bypass the heat exchanger.

Can you recommend a brand that does this?

More and more units are now including this feature because it is very beneficial during the summer time and swing seasons, especially in a passive house that might get hot during swing seasons.

Zehnder is one and I believe Ultimate Air also has a bypass mode.

Fresh air and ventilation are vital both for your own health and to help ensure that your natural gas and other fuel-burning equipment operate safely.To know more you can check out the Artile below http://www.greenbuildingadvisor.com/blogs/dept/musings/how-much-fresh-air-does-your-home-need.