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Passive HRV - pressure balance ..a few questions

I would need help or pointers to understand a few things about air in a tight home
and HRV/exhausts/ pressure balancing.

Situation is the following :

Planned to run a DIY central AC at first using some GEO + AC pump..i scrapped that
and will be installing mini splits for AC next year .
Heating is all handled by electrical underfloor heating ...

Still have the 4" PVC ducts going through the house that will be used with 4" fans
in the machine room to "mix" and redistribute air at a moderate level
( the house is faily large and hight, i want air to move around a little to be sure we get fresh air everywhere .. living part is 60' X 45' and almost 40' high )

We are using often ( my wife sells homemade food ) kitchen fans ( 2X 6" ~450cfm each )
and i had bought ( only isntalled 1 yet ) 2 panasonic whisper fans for both bathrooms
( both bath rooms have showers, only the one on the first floor is read and being used for now )

Then i bought a Venmar 3100 HRV back then, never got to install it yet
( still so many leaks..no need yet )

Ducting in my house is very hard, all ICF walls with granite stones, machine room is almost "remote" so i need to try and use what i've got unless absolutely necessary.

Was planning to send both bathroom exhausts on the rooftop " greenhouse".
Not to sure if i want to do this anymore..what do you think ?
using warm humid air to help heat the greenhouse during winter?

The house will be pretty tight when i'll be finished ( all ICF , and will do a PERSIST type walls on the roof exit room ..should be good numbers )

Should i be using the 3100HRV ? i don't mind not to use it,
i have many houses to build in the next few years and i'll be able to reuse it fairly easily.

Should i consider installing passive HRV on both bathroom outputs ?

The current HRV was going to be installed in the mechanical room, and fed in the redistribution system ... but then i won't be able to use it with any bathroom vents , will need to use only from the recirculation system.

Then what about pressure balance ?
When we turn on the 2 kitchen fans, it must depress the hole house ??
do the HRV automatically input some air to balance ?? what happends ?
also the performance of output fans must be very limited in tight house if not much input ??

Need to learn more about all this,
please do not point to central systems, i do not believe in those and will never use any,
but i'd like to read about HRV, bathroom exhaust, pressure balancing etc... as much as possible!

And please tell me what you think would be best for my house ?
BTW ...Zone 6-7 ( quebec )

thanks all for your time :)

Asked by Jin Kazama
Posted Jan 13, 2013 1:45 AM ET


7 Answers

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I will summarize:
1. You built a relatively tight house, and moved in, but you haven't yet figured out (or finished installing) a ventilation system.

2. You have no exhaust fans in your bathrooms yet.

3. You have written to this forum in an earlier question complaining that your windows are dripping wet.

4. You aren't sure whether or not you really want to install an HRV, and you are thinking of installing a system you call a "passive HRV" even though I advised you, the last time you asked (in a question you posted on this blog page), that your suggested method of installing the Venmar product was not recommended by Venmar.

5. You don't really want to install any new ducts. The ducts you have are PVC, a material that is not recommended for ventilation ductwork because of possible offgassing issues.

In short, there are a lot of issues here. I recommend:

1. You get in touch with an experienced ventilation contractor.

2. You install an HRV system following the manufacturer's recommendations, even if that requires you to install new ductwork.

3. You turn on the system once it is installed.

Answered by Martin Holladay
Posted Jan 13, 2013 7:03 AM ET
Edited Jan 13, 2013 8:41 AM ET.


MArtin, vous etes la raison majeure pourquoi j'aime bien venir ici! Franc et directe!!
Par contre je vais devoir corrier quelques points.

1- we i did built a house, which is far from anything tight for now ( but it will be ), and i've ran into many problems ( health, business, kids, defective products etc.. ) that delayed everything other than
assuring that water and cold stay out of the house .. can't bypass priorities unfortunately, and i haven't find a way to make time yet :p

2- I do have 1 working install bathroom fan ( pannasonic whisper ) that has been there since the day we moved in ( july 2011 )

3- i do not have any problems with humidity this year, since we are fully heating and humidity has stabilized down to ~ 30-35% now ..been stable for 2-3 weeks even with 20-30 person holidays partys ( we did 4 partys this year ....enough :p )

4- i am considering all options, i initially purchased a venmar 3100 almost 4 years back
because i knew i needed one , but also then i was planning to use a kind of diy central AC system,
which has been dropped in favor of multiple mini splits for AC
so now connection of the said HRV unit is a little more complexe unless i use it with the recirculation system, but then i am loosing 100% of both bathroom fans energy

and THAT IS WHAT I AM TRYING TO SOLVE ...getting back this energy from the bathroom fans,
and or use it as the main air exchanger ( be it that i'd connect the HRV on the bathroom fan or not )

i am sorry if it is hard to understand, everything is DIY on my side and i'm learning as i go
so obviously many assumptions and many errors get into the lot ..
what is important is that i want to learn !!

I had not read your reply to my question on that particular HRV post,
i just did :)
So using PASSIVE HRV is a no go in all zone 6-7 then ?? since no defrost ??

on the issues now :

1- i am not getting intouch with any pro other than u guys as this would prevent me from learning
i am in no trouble right now, everything works until i fix all of the leaks on the roof exit
and the humidity has been satisfactory with the ~2 hours of run time from the panasonic exhausting all of our showers ( kids , wife and me )
so no rush = time to learn ..and that is why i am here asking questions :)
now where can we learn about HRV and air systems ?? that reminds to be answered!

2- if i need to install new ductwork, i will do it .. but i would prefer to be able to avoid it
and i wish to consider all options before choosing the installtion type and its location
( wish to avoid doint it 2 times as i've done with many things here! )

3- the kitchen and bathroom fans + leaky windows and roof exit room is sufficiently working for this year .. but i intend to complete all i can before next winter!!

hope i am not to unpleasant with my ignorance and DIYstubborness

Sante et bonheur a tous! :)

Answered by Jin Kazama
Posted Jan 13, 2013 4:23 PM ET


I've been studying pressure balance in a simple HRV setup
for a little while now. Executive summary is unless there's
some *serious* blockage on the inbound or outbound path
somewhere, don't worry about it. Even shutting down the
fan on one of the inbound or outbound side of the HRV makes
little or no perceptible difference, so as I see it all
this guk about trying to balance flows in these things in
most residential settings is people fooling themselves.
I couldn't even begin to study this while there were any
significant leaks still in the house during construction.

My measuring instrument of choice is a small cardboard box
completely sealed up with caulk and tape, with a section of
plastic bag loosely stretched over the front and a small
tube out the back. The tube goes to my outdoor pressure
line [see the house writeup, section 03 for "whole house manometer" for
details] and the way the bag bellies shows not only the
pressure difference but the rate of trending. [The
manometer wasn't nearly sensitive enough, it turned out.]

So if I turn off one of the HRV fans I *might* get a
delta after more than a day. What this tells me is that
enough additional air would likely drift in the inbound
side of the HRV to just make up for the Panasonic without
appreciably depressurizing the house, and you still get
some of the HRV's benefit in the makeup air.


Answered by Hobbit _
Posted Jan 13, 2013 6:49 PM ET


Hobbit: i also thought about the pressure stabilizing itself automatically ,
but i don't understand how you get an energy return from the HRV if venting directly outside with the pannasonic ??
or do i get you wrong on this ??

Answered by Jin Kazama
Posted Jan 13, 2013 9:34 PM ET


Like you, I'm not sure that I fully understand Hobbit's comments. I think Hobbit is disagreeing with me about hooking up Panasonic exhaust fans to the Venmar AVS FAE 125 -- but perhaps Hobbit is saying something else.

It sounds like you already own a Venmar AVS HEPA 3100, which is a type of HRV. If you already own it, why not install it? If you choose to go this route, I recommend that you install it according to the manufacturer's recommendations.

If you prefer to have an exhaust-only ventilation system, you can just go with the Panasonic bathroom fans. It's your choice.

For more information on ventilation, you can read:

Designing a Good Ventilation System

Installing a Heat-Recovery Ventilator

Answered by Martin Holladay
Posted Jan 14, 2013 9:41 AM ET


Martin please let me clear up a few things that i might have got wrong on syntax or way to express ...

1- Yes i do own an HEPA 3100 from Venmar but i would not mind considering other options because i know i will be able to use it somewhere else in the near future, if necessary.

2- No i do not prefer to have exhaust only fans, that is the hope point of my questions, else the venmar would've been fitted on the recirc system quite some time ago :p

I wish to understand, if there is a large efficiency/$$ recup from setting up the bathroom fans into HRV , be it with the use of passive of using my Venmar 3100.

Is there ??

The manufacturer's instructions on the 3100 does not really work in my situation, except if i'd make a complete new run ... both bathrooms are already ducted, that's is also another consideration .

If there is not much $$$ to spare from going to bathroom direct outputs to using some energy recuperation method with the bathroom fans, then i should install the venmar 3100 in the recirculation run

else i wish to analyse and consider

i'll be living here for a minimum of 20 years...need to check it it is really wise to send straight out fully heated air everytime we take showers/bath etc..

i also may be out of it and may not understand ..if so please point out! :)

Answered by Jin Kazama
Posted Jan 14, 2013 8:22 PM ET


In your climate, an HRV will always cost less to operate than an exhaust-only ventilation system. You'll use more electricity with an HRV, but you will save on your heating bill.

HRVs are expensive to purchase and install. Whether the annual energy savings are worth the expense and hassle of installing the HRV is a complicated question that only you can answer. For more information on this question, see Are HRVs Cost-Effective?

If you decide to install your HRV, be sure to follow the manufacturer's instructions. It isn't a good idea to use Panasonic fans to blow air into a duct connected to your HRV. If you install your HRV, you will either install new ceiling grilles in your bathroom, and you will connect these new ceiling grilles to ductwork that connects the grilles to your HRV, or you will remove the Panasonic fans and install grilles in the hole in your ceiling where the Panasonic fan used to be -- and then you will connect this grille (using ductwork) to your HRV.

Answered by Martin Holladay
Posted Jan 15, 2013 8:44 AM ET

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