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Old 12-14-12, 02:45 PM   #1
Exeric
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Default Are HRVs really necessary?

I know it is taken as true that HRVs are the ne-plus-ultra in passivehaus design. But are they really necessary? Is some of it propaganda that all superinsulated houses need it? Before you get mad at me read this article and decide.

Are HRVs Cost-Effective? | GreenBuildingAdvisor.com

When you read it one should also interpret expense as "time to construct". Our time is worth something, even for us DIYers.

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Old 12-14-12, 06:00 PM   #2
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Hi Exeric,

That's a good question, and I will not be mad at you at all.
I browsed through the article you linked and it has several legit points, and I guess we could even add more to them if we try.

I'll keep this short though, staying close to my personal situation : -)

Fact is that an HRV does recover some energy and has the potential to improve ventilation.

Often a comercially installed HRV indeed costs $4000-7000 to have one installed and in that case you need to be patient for your ROI (That's why I decided to go DIY). Also they are somewhat complicated to maintain, if outsourced this adds an anual fee for the mechanic visiting once a year doing the maintainance.

In my case I live in a somewhat moderate climate, my house is not a passive_haus, it's decently insulated but not realy special.
Currently I have exhaust-only ventilation, the fan is in good shape, and has been since my house was build in 1984. My wife believes in fresh air, and she is right at that, so there's a window partly open at our groundfloor all day..

Sidestep:
In a newly build Passive_Haus you 'need' an HRV, and every room requires an intake and exhaust. That's a lot of ductwork, and tricky to get the proportions right. Next to impossible to implement in an existing building.

In my situation I'll keep it simpler. The only intake will be in the livingroom, air warmed by the HRV will go there. Exhaust will come mostly from the semi-open kitchen (80%-ish) and the rest via existing ductwork from the toilet and bathroom (bathroom is upstairs).

Expected results in my situation:
As I mentioned above, my wife likes to keep at least 1 window partially open on our heated groundfloor. As a result the lower 1 meter (3 feet) is cold, the floor feels cold, our legs are cold, even at table/desk level it is fresh.
If my build of my HRV succeeds the windows can stay put, ventilation increases 3 times, and we save on energy (and thus coin) for heating.

My current exhaustfan isn't the most modern, replacing just the fan would save about 60,- euros per year on electricity. The whole HRV I am building is more expensive than that, obviously, and it does cost me a lot of time, materialcost will be some 700,- euro.

Quoted energy savings on heating are 30%, but let's asume 20%. We use about 1000 m^3 gas anually for heating, costing 0,64 euro per m^3. 200 * 0,64 = 128,-. Savings on electricity for replacing the current exhaustfan with the 2 used in my HRV is 40,-. That makes a roughly estimated anual savings of 168,- while improving the Quality Of Living.
Investment is: some time, love and 700,-. Financial profit expected after 4.2 years (neglecting the fact that I pay now and enjoy lower bills from now on).

So your initial question is legit. HRV is not a magic word, it is situational. If applied correctly it will do good, but that is no proof that an HRV is suited for the generic public, like with a mortgage people would need advice wether it is the right choice for them.

Last edited by Fornax; 12-14-12 at 06:02 PM.. Reason: Oh, that wasn't short.
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Old 12-14-12, 09:46 PM   #3
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Good reply. Actually, I think the article may have said that smaller single floor homes can usually make do with simple bathroom fans. Your house, not being single story, sounds like a good candidate for an HRV. You're right - it's situational. Retrofitting can be murder though if one gets too picky about things. Good luck!
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Old 12-15-12, 04:50 AM   #4
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I had this discussion with my energy auditor. He usually doesn't suggest an HRV/ERV in existing construction but instead low-sone highly efficient fans. I asked more questions and was directed to look at the energy-star site for highly efficient bathroom exhaust fans designed for this purpose. Basically you take the existing duct that is piped from the outside into the air return of the furnace(bad idea in the winter IMHO but in the summer it brings in fresh air that gets dehumidified as it comes in while providing positive pressure to reduce infiltration) and move it into an air trap inlet(could be as simple as ducting it into a 5 gallon bucket. I fell in love with the Panasonic Whispergreen fans that are pretty much silent and have a CFM setting so you can get the exact CFM you want and when you turn the bathroom switch on, it ramps that to full speed while you are making the room stinky or taking a shower.

I haven't gotten my house tight enough to do this with one person in the house yet though and sealed off the outdoor air inlet in the winter of 2010. I still have enough infiltration for the furnace and water heater at this point but will change things if I add people or get it tight enough to need the ventilation. If I added an HRV to the duct system it wouldn't work well for me because I've only used the furnace for 11 hours so far this month or about 8 therms to where if I added the ventilation I think I'd increase not just the electricity use(they do use a bit) but also increasing the heat needed to keep it warm. Not to mention that these things often include resistance heating for the incoming air below a certain temperature which strikes me as being an inefficient and pointless waste.
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Old 12-15-12, 02:41 PM   #5
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The resistance heat on a HRV is for defrosting.
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Old 12-16-12, 03:55 PM   #6
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I've been reading the comments to the original article I mentioned and they are pretty interesting. Several people mentioned that what they most worry about is indoor air quality in the bedrooms if one is using just a bathroom fan for ventilation. After all, many multi person households like to have their bedroom doors closed at night, which would stop any fresh air from coming into the room you spend the most time in.

And later a person brought up the idea of using jumper ducts to the bedrooms from the
bathroom. That seems like a good idea but might incur a big cost in time and effort depending on circumstances. Since I'm not yet at the place where my house is even close to being superinsulated the point is moot for now. But since the house is pretty well torn apart right now for construction I'm thinking of implementing that design before things are closed back up. It's cheap to do it now.

Like others I'm pretty impressed with the Panasonic fans. Since my house is single story, 1000 ft, 2 bedroom located in California it seems crazy for me to go the HRV route, even when everything gets tight thermally. It may be a less obvious decision for others in differing circumstances, but I think people in not severely cold places with small single story homes could also benefit from this approach. If you substitute that small Panasonic $400 modular bathroom HRV instead of a fan it seems like a no brainer. But maybe my situation is coloring my judgement...

Last edited by Exeric; 12-16-12 at 03:57 PM..
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Old 12-16-12, 04:18 PM   #7
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One more thing. The FV-04VE1 HRV really wouldn't serve as a bathroom fan because it's only 40 cfm max. So scratch that idea. I'll just stick with a regular panasonic fan.

Also, to get good flow through ventilation to the bedrooms from the bathroom with all doors closed don't undercut the bathroom door and DO undercut the doors for each bedroom. That assumes direct return venting from bathroom to bedrooms. And always keep the bathroom door closed after bedtime.

edit:
Here's some numbers straight from the Panasonic whispergreen manual.
1000 sq home requires 33 cfm for house ventilation.
2000 ~ 43 cfm
3000 ~ 53 cfm

The equivalent power used on the 80 cfm model at those settings (30,40,50 cfm) is
30cfm ~ 3.8 watts
40cfm ~ 4.2
50cfm ~. 4.5

And the cost for the 80 cfm unit is 152$ at amazon, tax not included.
So I think even with the cost of putting in ducting between bathroom and bedrooms it is a good choice for a smaller home. There will be more heat content in the conditioned air for larger homes so as you get bigger volumes there are bigger returns for an HRV. That is what any person would need to weigh. But the amount of money and time you save going this route would give you a lot of options for increasing heat conservation through insulation and solar heating.

Last edited by Exeric; 12-16-12 at 06:38 PM..
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Old 12-17-12, 12:51 PM   #8
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Great post. My reading suggests that for a small house a small bathroom fan run on a timer is sufficient to exchange adequate bad air for good. Panasonic gains the nod for efficiency and sones.
that FV-04VE1 HRV though. what a nice unit. Off grid, on low/low. 20 cfm setting on low (10 cfm) it's 17 watts. x 24 hrs = 408 watts daily. That's a pretty big load.
Maybe time to open a window. And another nod to the bathroom fan on a timer.
So, to my point. Instead of undercutting your doors, transom windows above the door to bedrooms allow warmer stale air to move more easily to the HRV/fan. Allow light to filter between the rooms unobtrusively. Controls noise between rooms. And made my grandmother happy.
Now how can you beat that?
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Old 12-17-12, 02:42 PM   #9
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That transom window idea is elegant. I think it would work great. The two bedrooms in my home each share a wall with the bathroom so I'll probably just cut a hole in both walls, seal the sides, and put in vent registers. Your solution is the much more generic and more broadly useful for the average home.

As far as vent timers, the Panasonic whispergreen model I was thinking of won't require it. They're built to be run continually at user adjustable low settings. In fact, with the wall switch for it turned off you can set it to run as low as 30 cfm, and its supposedly built to run at that speed continuously.When you turn the wall switch on it goes to 80 cfm. It also has a built in timer that turns off the 80 cfm mode only and reverts it to the low speed mode. Pretty cool, all in all.

Also at 3.8 watts it only uses 91 watts in 24 hours. I can live with that.

Edit: Actually the more I think about the transom idea the more I like it even for my situation. Vents directly through the bathroom walls might end up being an invasion of privacy, if you know what I mean.

Last edited by Exeric; 12-17-12 at 03:00 PM..
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Old 12-18-12, 08:47 AM   #10
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lol.

my mennonite farmer grandmother specced transom windows in the house she and my grandfather built. My grandfather always got a look on his face when she'd go on about how wonderfully they had worked out. I think they were a pain to frame! Remember he built the windows, frames etc himself.

we, of course, can just order el cheapo's from the big box.

The posts I've read earlier in my house research indicated a timer with a strategically placed in flow vent, in the, er, event that the house was so tight you began drawing flue gases into the house, was the way to go.

with the advent of these low cfm fans though. I wonder if that's another idea that needs an update. forget the timer.

but where are you putting your fresh air vent(s)?

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