HRV with bathroom exhaust fan
I asked this question in the comments of an article, but thought I would have more luck here…
I am installing a LifeBreath 155ECM HRV in a new home. It will be exhausting air from 3 bathrooms spread across 2 stories. The HVAC sub is recommending installing separate bathroom exhaust fans for local exhaust (shower moisture, odor, etc.).
An energy star consultant says this is not necessary and redundant. It certainly seems redundant to me, but the HVAC sub feels that at max speed on the Lifebreath you are pulling around 150cfm @ .2″ static pressure that means 50cfm at each bathroom location. For 2 of the 3 bathrooms, I would typically use a 110cfm PAnasonic Whisper Green fan because of the size of the room. So the HVAC sub feels like it will take too long running the HRV at full speed to adequately vent the space.
A 50cfm Panasonic has a rating of 8.5cfm/watt while the Lifebreath is 1.5 cfm/watt. So it seems that the Panasonic is much more efficient.
Any thoughts…additional bath fan or just wire the HRV to be switched to high speed for local exhaust?
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Replies
Chris,
You don't indicate where you're located, and that makes a difference.
While many here will agree with your Energy Star consultant, I would side with your HVAC contractor. Depending on your climate zone, the level of thermal insulation in the building, and the permeability of the bathroom walls and ceilings (and how well the rest of it is air-sealed), it may be unwise to rely on slow evacuation of shower vapors.
Additionally, depending on the relative locations of HRV exhaust and return ports, there may be cross flow which pulls bathroom moisture into other rooms. And, depending on the relative distance of the HRV unit from each bathroom port and the type and tortuosity of ducting, there probably won't be an identical 50 cfm from each room, with at least one bathroom evacuating at a lower rate.
The advantages of dedicated bathroom exhaust fans, in addition to the much higher electrical efficiency, are quick evacuation of moisture and smells, isolated exhaust with no cross-flow to other spaces, reduction in moisture load at the HRV and consequent reduction in inefficient defrost cycle time, and reduction in duct condensation.
But, for bathroom fans to be effective as well as efficient, they need to be controlled either by delayed-off timers or by delayed-off motion detectors (though the latter will turn on the fans when they're not necessarily needed), and they have to be used by occupants.
If you're going to use three dedicated bath exhaust fans, however, it may be wiser to eliminate the HRV and central ducting and use the bath fans as exhaust-only ventilation by wiring one or more in parallel with 24-hour programmable timers. This may also shorten duct runs and make the fans even more efficient. Depending on the tightness of the house, you many need to install passive make-up air inlets, such as the American Aldes Airlet 100.
I've had great success with such systems in my superinsulated homes.
Thanks for the response.
Zone 5
Closed cell at rim joists
Dense Pack cellulose in 2x6 walls
Closed cell foam in 2x8 roof rafters.
I'm really torn on which route to pursue. The bathrooms are spaced on opposite sides of the house from each other.
Wouldn't mind the cost savings of just going with dedicated exhaust that could be wired to run in parallel with timers. Not sure that I'm crazy about the passive inlets (more from an aesthetic standpoint, seeing additional grills on bedroom walls)...
I considered installing dedicated fans in the bathrooms that have showers, but not in the powder rm and just relying on the HRV in that location.
Opposite side bathrooms are a design error. An efficiently designed home has all plumbing consolidated and stacked.
The Airlet coverplates are hardly noticeable and can be painted to match both interior and exterior surfaces.
Using the HRV for the powder room and dedicated fans for the shower rooms may make some sense, but it still creates a redundant system and still requires additional exhaust ports to balance the intake ports.
Unless the house has zero air leakage (or something close to PH standards), a "balanced" HRV system can create an unbalanced pressure zone because of natural stack effect leakage in cold weather. This creates a positive pressure zone in the upper levels which can result in exfiltration and condensation. An exhaust-only system (at least when it's running) maintains a safer negative pressure within the entire air boundary and prevents condensation potential, and when not running the upper level passive air inlets offer a safe positive pressure escape route and allow some air exchange even when the grid is down.
Chris,
[Re-post of my response to your question, asked elsewhere:]
I agree with your Energy Star consultant and I disagree with your HVAC contractor.
The entire point of buying an HRV is that it provides an exhaust fan and a supply fan -- balanced ventilation. It will be perfectly adequate to exhaust the air from your bathrooms.
I love how you can always (almost always) count on getting completely different answers from Robert and Martin. Keeps for a lively and balanced discussion.
Martin,
I assume you would recommend having the HRV tied to a push button switch in each bathroom to ramp up the fan when needed for local exhaust? You don't see the energy savings from the Panasonic significant enough to warrant using them.
Chris,
Go with the source point HRV, I have used this system for 25 years and never had a complaint. Remote timers in the bathrooms work nicely.
Chris,
Talk to the manufacturer of your HRV. The major manufacturers all have tech-help phone representatives.
Almost all HRVs include bathroom controls that allow occupants to boost the fan speed when necessary in the bathroom.
Lifebreath does offer a 20/40/60 timer switch sold separately for bathroom control.
Thanks for all of the input.
Which, in itself means nothing at all. Certainly not which system is better in which applications.
I've been using my exhaust-only system for 25 years and never had a complaint, but I didn't give my customers a chance to compare the two so they could determine which one to complain about. Did you?
From the posts to date there would appear to be two, differing, solutions each of which has accumulated 25 years of satisfactory user experience. As far as the users are concerned, the solution that they have been provided with is satisfactory. They may not, as Robert points out, have experienced the other solution but no solution that they have encountered is so much better than what they have that have felt compelled to complain. They may be dissatisfied at the "this thing's not ideal but it's not worth bothering about" level in which case the suppliers will never know about it unless other customers mention that they would rather not have this feature.
Have Messrs McEver and Riversong had such requests? If not, then there is, at the home-owner level, no difference between the two solutions.
You have three great ways to vent Chris. Two above and an even better third.
Panasonic makes hr style bath units. That may be the lowest cost combination solution you desire.
Aj
Personally, I like the idea of negative pressure for winter here in the adirondacks, and just open windows for vents... year round manually controlled.
So much depends on customer and climate.
RR, The question was, if an HRV is used for ventilation, can the HRV substitute adequately for bath fans. In my experience it can.
Unless we as designers and builders are more concerned with user perception (marketing) than function (effectiveness & efficiency), then lack of user complaint is not a good metric to determine the best choice for mechanical ventilation (or anything else). I have repeatedly talked my clients out of unnecessary, wasteful, excessive or counterproductive design options - that's my responsibility as an authentically green designer and builder.
Can an HRV adequately substitute for dedicated bathroom exhaust fans? Sure - IF properly designed, properly installed and properly commissioned - but minimally so. ASHRAE requires a minimum of 50 CFM of exhaust from a bathroom. The system described by Chris Harris may provide that minimum at each bathroom if there is equal equivalent duct resistance at each intake, but only if the HRV is boosted to max by the user.
HRV's are a good way to reduce ventilation costs by recovering some portion of the energy of interior air, and they are becoming more efficient in operation. It's not clear to me that they are typically more effective in operation than dedicated bath fans, if a primary function is to evacuate excess and potentially damaging and unhealthy quantities of moisture.
I do see that there is a widespread tendency, both within the building community, and in our society in general, to latch onto the latest technological development without adequately assessing both its benefits and its liabilities. The Law of Unintended Consequences is still the elephant in the room that few are willing to acknowledge.
The Precautionary Principle should make us slow to adopt new technologies, and the KISS Principle and Occam's Razor should lead us toward the simplest approaches to any problem.
And the foundational issue which almost no one in our culture, including most "green" designers and builders are willing to face is that, if we were to build essential shelter rather than lifestyle enclosures, status symbols and investment instruments, most current efficiency issues would vanish into insignificance.
Although Robert implies that the idea that HRVs can pull exhaust air from bathrooms is somehow suspect or inadequate, it has been the standard way to install an HRV for decades. HRVs are designed to pull exhaust air from bathrooms! That's what they do!
What spend $1,500 on an HRV designed to pull exhaust air from your bathroom, and then turn around and install an additional exhaust fan? Totally nuts.
Martin,
I didn't "imply" that HRV exhaust from a bathroom can be inadequate, I simply pointed to the actual flow rate calculated by the HVAC contractor, which barely met the minimum ASHRAE standards for bath exhaust.
Is it "totally nuts" to use an independent kitchen range exhaust hood when there's an HRV removing air from the kitchen? This is standard practice, and not just because the grease can undermine HRV function but because no one would trust a 50 cfm fan to adequately remove cooking moisture and odors. In fact, the trend these days is to use 300 cfm plus fans for this purpose.
Why then is it "totally nuts" to use a bathroom exhaust fan that's capable of more than code minimum flow rates to evacuate what has become the most dangerous indoor pollution in tight homes: excess moisture.
Where we might agree is that it is at least pretty nuts to install adequate and efficient bathroom exhaust fans and ALSO spend $1500 for an additional ventilation system when the bath fans can do double duty. You've got it backwards. Code requires bathroom mechanical exhaust, so the decision to ALSO install a whole-house HRV must be weighed against the effectiveness and efficiency of the code-required minimum system.
The purpose of an HRV is NOT to evacuate bathroom moisture, but to provide whole-house air exchange. A well-designed system may be able to do some of the job of a bathroom exhaust fan, but not as adequately because that's not its design function.
Robert,
I disagree with you that most HRV systems include exhaust from a kitchen. They generally don't -- although a few Passivhaus builders are experimenting with the idea. HRV manufacturers recommend pulling exhaust air from bathrooms and laundry rooms.
HRVs are balanced ventilation systems that pull exhaust air from bathrooms and supply fresh air to bedrooms and living rooms. Properly specified and installed, these systems easily handle bath exhaust requirements. That's what they are designed to do.
We have been using HRVs for 25 years and have experimented with a few different approaches. Early on we tried using the HRV as the only exhaust in baths and our clients felt that they did not have sufficient exhaust especially, during periods when the house was somewhat open with passive ventilation. We also tried using the HRV with dedicated zoned dampers which worked but was as expensive as adding exhaust fans which is what we routinely do these days. We also normally pick up the kitchen which tends to be the public space, we just stay away from the kitchen exhaust fan.
After all this...I think we are just going to go exhaust only with Panasonic fans. It is a big cost savings and the house is configured in such a way that it will be pretty easy to retrofit an HRV or other method for supplying makeup at a later date if we find the house to be too stuffy in the winter.
Chris,
Unless you're building a deliberately leaky house, you can't rely on exhaust-only ventilation without make up inlets. Every system needs to be balanced.
One of the primary reasons the industry has shifted from "natural" ventilation to mechanical ventilation is for control. You'll get half the control by timing the operation of the fans, but uncontrolled infiltration wherever there are leaks, and most likely not where you need fresh air (bedrooms and living spaces) and not in sufficient quantity to balance the exhaust.
Make-up air inlets are an inexpensive way to assure fresh air delivered to where it's needed and in sufficient quantities to at least partly match the fan exhaust flow (normal leakage can make up the difference but cannot be relied on for sufficient flow).
Martin,
I never said that most HRV systems exhaust from kitchens. But all new homes have dedicated kitchen exhaust hoods, which makes it just as redundant to use two exhaust systems in the kitchen as it is in the bathroom, but that's never recognized when HRV exhaust ports are installed in kitchens.
And an HRV is not designed to be a bathroom moisture evacuator. An HRV is simply a box with two fans and some kind of exchange core designed for balanced inflow and outflow of air, with some degree of heat recovery. It's the installation and the duct system which determines where exhaust and return air ports are located and how much air flow each port will achieve.
So the engineering design goal for an HRV is simple air exchange with heat recovery. That they are used to evacuate bathroom moisture does not mean either the HRV or the ducting and porting were adequately designed for that purpose.
But no HRV system is going to function as well as a dedicated bath exhaust fan of equal quality for the purpose of evacuating point source moisture.
The design program for indoor moisture control requires:
1) point source evacuation
2) whole house dilution
These are two very different and not necessarily compatible goals.
Supposing there were both an HRV and a dedicated exhaust fan in a bathroom, and the exhaust fan happened to run when the HRV was not running, would that tend to draw makeup air from the HRV duct (i.e. the reverse of its usual flow direction)? The makeup air would come from the path of least resistance, and that could be it.
It could certainly create a short circuit. Ideally, an HRV would be sized to run continuously, but if it's drawing 50 cfm out of the bathroom and the bath exhaust fan is pushing 100 cfm out, then the bath fan still might reverse the HRV air flow. It would unnecessarily complicate what should be a simple system.
As a Passive House Consultant-in-Training and a design/build contractor I recall there was significant discussion over this particular subject during my consultant training courses. Both side of the argument were explored. Adding an exhaust fan to the building envelope that utilized an HRV to perform the ventilation would unbalance the interior air pressure. I have yet to put much of this into practice but my thoughts were always along the line tying a minimum required CFM exhaust fan right into the return air to the HRV and possibly including a timer on it to ensure that the exhaust fan was not left on. This same principle and discussion concerning the HRV efficiency of meeting requirement was addressed in not only the bathrooms but also in the laundry room, the kitchen and so forth. The same idea applies. If you exhaust air seperately out of an airtight balanced system then it defeats the whole principle of the balanced system. My only other thought with something like this is to utilize something like what is used on many gas fireplaces where the unit draws its one fresh air from an outside source so it is replacing whatever air is being removed. Again many ideas that I have yet had the opportunity to apply.
I once lived in a mid-rise building (6 stories) that had a continuous bath exhaust. There must have been a central fan at the roof, and there was a small vent with no discernible fan noise. The air flow was certainly less than a typical exhaust fan. It did take a while to clear condensation from the mirror, although I tend to doubt that this was especially a source of risk. After all, any moisture that might begin to get into the walls in the first half hour or so would have plenty of time to work its way back out, assuming there weren't people showering one after another all day.
Based on this anecdote I'd suggest it makes more sense to omit the exhaust fan when installing an HRV, and understand the difference in airflow. It will not perform as well at immediately clearing the air as a dedicated exhaust fan, because it's designed to operate continuously at low speed.
It may be helpful to leave the bathroom door mostly closed when not in use to contain the air while the HRV draws it out, because walking out of the shower and leaving the door wide open would let out damp air that the HRV hasn't had time to reach.
I read through the whole thing (and the article on HRV and ERVs). I am building a new home on a sealed, insulated crawl. R10 under slab, R20 walls, R 30 house walls with an R50 ceilings. I want to be tight, but as I have no track record, I am not sure how tight it will be. I am looking at Mitsubitsi Mini Splits for heat. What is the best way to condition the crawl space? How about the dryer? How about the stove....how important is it that the stove be vented?
A question for Riversong (is that your last name or your company name?) you advocated venting bathrooms individually. Is this in addition to an HRV or ERV. An what is the difference between the two?
Thanks - this is so helpful.
Charley Bob
Charley Bob,
Q. "What is the best way to condition the crawl space?"
A. If you don't have a furnace, most building codes require you to install an exhaust fan in your sealed crawl space and a grille in the floor to allow conditioned makeup air to flow from the conditioned space above into the crawl space. More information here: Building an Unvented Crawl Space.
Q. "How about the dryer?"
A. Vent it to the outdoors, or install a condensing dryer. More information here: Alternatives to Clothes Dryers.
Q. "How about the stove....how important is it that the stove be vented?"
A. Most building codes require the installation of a vented range hood. However, some Passivhaus builders have convinced local building officials that a recirculating range hood with a filter is adequate. More information here: Makeup Air for Range Hoods.
Q. "You advocated venting bathrooms individually. Is this in addition to an HRV or ERV?"
A. I advise that if you have an HRV or an ERV, you should install it so that it pulls exhaust air from each bathroom. You don't need an additional exhaust fan.
Q. "And what is the difference between the two?"
A. See this article: HRV or ERV?
Some designers would provide for all three showers to be used continuosly and concurrently, and recover as much heat as possible - in the frozen north. I would not. I would ask, how will people use the house, on average. Bathroom and kitchen exaust fans need timer switches. When in use, they will unballance a HRV system, but so will opening a door, for an instant. Someday, kitchen fans will have a HRV feature that is ballanced - when its really cheap, and people need to run the exaust fan alot. Till then, use a one way damper on all vents, with or without fans, up north.
Matt Fletcher: "I have yet to put much of this into practice but my thoughts were always along the line tying a minimum required CFM exhaust fan right into the return air to the HRV and possibly including a timer on it to ensure that the exhaust fan was not left on. "
As a slight variation on this, would it work to have the bath exhaust connected to both the exhaust cap on the roof (like it normally would be in exhaust-only case) and the HRV return duct. This way if the HRV is boosted and running, it will retrieve as much as it can (possibly helped by the exhaust-only fan as well) from the bathroom. If the HRV is down for some reason, the exhaust fan will simply exhaust to the outside. Has anyone tried this?
AJ - I really like your option 3: Panasonic FV-04VE1. I can see using several of these to fullfil point source and whole house ventilation requirements. Wonder if these would pass muster in the PHPP?
Matt - Were these considered in the discussions around Passive House HRV systems? I'm working through a PH design for southwest Colorado zone 6B.
Riversong - This would seem to adhere to KISS and Occam's Razor principles yet being rather new bath fan technology.
Thanks All for this very informative discussion.
Well, after looking at the Panasonic ERV bathroom fan closer I'm not convinced it would replace a whole house HRV even though they mention that as a possibility. For that matter I'm not sure how effective it is since it's sucking in exhaust air right next to intake air. In my very small steamy shower room the fan works best with the door closed and makeup air coming in near the floor.
Old post, but since I meet so many of the criteria...
I have the exact same HRV, in a 2 story house built in 2006 (London Ontario). Not passivehouse by any means, but I did do what were options at the time (low e windows, extra attic insulation etc.) Leakiness should be about average (good chain builder).
My answer is no, HRV is not sufficient to provide adequate moisture removal. I have basically closed all other exhaust ports (laudry, kitchen, cracked the powder room one, cracked the guest bathrom one) and still do not find it removes moisture fast enough. I get some condensation on the walls.
One thing that would help A LOT would be exhaust port placement. If you could somehow place the ports in the ceiling?, right in the middle of the shower, it might be adequate. Mine are on a wall to the side of the showers, and I even notice a difference withe distance from the shower (2 vs 3 feet, bathroom 1 and 2).
I have been looking for booster fans, humidity triggered, that could help out with shower humidity. I feel the trick would be to size the cfm and pressure differential just right to overcome any duct losses, but to avoid blow back. I like the whole HRV concept for bathrooms, but as designed it would need a bit more oomph to work as a standalone.
BTW at the time the 155 was the largest unit. I would consider a 300 cfm system, especially if the efficiency was good at lower speeds (and it had plenty), that might do it in a Brute Force mode. But I still feel the booster fan method would be more elegant, it it was feasible.
Mai Tai,
For a more extensive discussion of the issues raised in this Q&A thread, see Does a Home with an HRV Also Need Bath Fans?