Bath fan and AC interconnection with HRV
I’ve done some reading on this including this excellent post and discussion comments: https://www.greenbuildingadvisor.com/article/does-a-home-with-an-hrv-also-need-bath-fans
We’re doing a major remodel, adding an HRV, and gutting/building a bath in addition to other work. Other baths already have dedicated Panasonic exhaust fans, and its not practical to change them even if we wanted to. We’re in Chicago.
My question is this: During Sumer we can go for weeks without running air conditioning, and just open the windows for cooling. So I am assuming the HRV is wired such that it does not operate when the AC is off? And if this the case, then using the HRV also for bath exhaust would require a switch to turn it on for that function. Except if it was already on, due to the heat or AC being “active”, (but not necessarily running.)
I understand that a “booster” control can be wired to kick the bath exhaust up to a higher level. But could this normally turn the HRV on as well?
Are there controls smart enough to handle all this?
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I've never heard of wiring the HRV to only be on when the A/C is on. Doesn't sound like a good idea to me, since A/C off doesn't automatically imply windows are open. There is a pretty wide range of outside and inside temperatures at which you might have the A/C off, but still want all the windows closed. In those cases, you still want fresh air.
If you are adding an HRV with its own ducting, it can be run completely independent of the A/C system, on its own schedule or just constantly. That's one of the many reasons that is superior.
If you have good bath exhaust fans, keeping them installed at least for now seems sensible. You may find you have no more need for them, and you might eventually want to remove them, but keeping them installed seems prudent in case you do want to have them available to use some of the time.
Trevor: I don't know conceptually how HRV's are typically wired or controlled so I am asking. My research so far has not produced a good answer.
If the AC is run for say 4 days in a month, and the windows are open most of the rest of the time (a very plausible scenario here) if the HRC is set up to operate continuously, it will be consuming energy without providing any benefit for most of the month. This seems like a waste, so I am wondering what is done in practice, from people with experience designing and implementing these systems.
Daniel,
The way I deal with this is to turn the ERV off if the windows are open for extended time during the shoulder season. The boos switch still works if you need to ventilate the house or bathroom.
Less manual setup is to wire up a set of windows switches used for alarm systems to disable the HRV when the windows are open. Seen this done in southern climates to automatically turn off the AC when the windows or patio doors are open.
Switching the HRV on and off manually seems like the only practical solution. We are certainly not going to be adding sensors to every door and window.
But that still leaves the question of whether the necessary control device exists to allow the HRV to substitute properly for a bathroom vent fan, as many suggest it should. The necessary functions would seem to be:
1. Recognizable to a user as a bath fan control
2. Turns the HRV on in override mode if it has been manually switched off due to the windows being open
3. Leaves it on if it is not manually shut off
4. Boosts it to high speed
5. Has a time delay to keep the HRV running for x number of minutes after being switched on
6. Contains a humidity sensor to run the HRV if the bath humidity goes beyond a certain level.
For the other baths, we have a $30 fan control made by Leviton to handle this stuff. But for the HRV, the control seems more complicated.
The HRV is not typically turned completely off, but to an away setting. So the boost switch still works. Items 3 and 5 seem to be at odds with each other; usually it operates as in 5. A humidity sensor is an optional extra for many systems. Another optional extra is a CO2 monitor. With that you can leave the HRV in the away setting most of the time, but if you have the windows closed for long enough to affect the air quality, it will ramp up.
Thanks, Trevor. What is the difference between the "Away" setting and the "Off" setting? Is this just for the purpose of allowing the bath "booster" switch to work? Or is it more than that?
And the humidity sensor is a separate device from the booster switch?
(By item 3 above, I meant if the HRV is already running in normal mode to ventilate the house, this theoretical device would just recognize that it is already on and boost it).
Away means the machine is on, and can respond to external inputs or internal programming, but the fans are set at a very low level (as low as zero, but there's a good argument for having some positive amount to prevent reverse flow in case of a breeze - reverse flow means deposits in the filters could blow back into the house).
The humidity sensors I've seen are separate items from the boost switch. I've never had a humidity sensor in a bathroom, and never felt like I was missing something.
After the boost timer times out, it returns to whatever state it would have been in had the button not been pressed.
Charlie: The two existing baths with Panasonic fans will not be changed. The only bath under consideration to be vented via HRV is the new one. But I can't do it if it can't be properly controlled.
I'll throw this out there because it seems it hasn't been covered yet.
Typically, the ventilation system runs on its own, completely independent of other systems. It is set to ventilate at whatever minimum level is needed to ensure fresh air. Any more than the required amount results in an energy penalty. Sometimes it's set to run continuously, and sometimes on a duty cycle (X minutes per hour).
Air conditioning has nothing to do with providing fresh air into the home.
Typically, the ventilation system exhausts from humid/stinky areas like bathrooms and kitchens. This helps reduce humidity and cooking smells in the house.
Sometimes the ventilation exhaust is enough to ventilate a bathroom on its own. When it is not, a "boost" feature is sometimes offered that can temporarily increase the ventilation rate, or a standalone bath fan can be used in addition to the ventilation system. Not all ventilators offer a boost function.
The responsibility to turn off the ventilation system lies with the homeowner if they open the windows.
I suppose the ventilation system could be hooked up to a CO2 monitoring device that would only operate it if/when CO2 levels are above a predetermined set point, but that level of complication may not be a robust solution.
Regarding the bathroom, you may choose to exhaust with the ventilation system and see if that's adequate, with plans to add a standalone bathroom fan later if it's not.
I'm planning to use an exhaust/light combo in our showers, which disguises the exhaust and puts it in the best possible location; in the shower:
https://www.homedepot.com/p/Panasonic-Recessed-Exhaust-or-Supply-Inlet-for-Remote-Mount-in-Line-Fans-H-ERVs-or-Light-Only-Option-with-WhisperRecessed-LED-Fan-FV-NLF46RES/303696523
I'm not sure how an HRV without a boost function could be used as a bath exhaust fan. If the windows are open, the HRV will be shut off. Therefore no bathroom exhaust.
If it works as described above, with the HRV set in an "away" setting, and the "boost" function actually turning it on as well as boosting it, then I guess it could work.
The most simple approach may be to install a redundant bathroom fan for use when your windows are open and your ventilation system is turned off. That way you are not limiting yourself to ventilators that have a boost function and you have the flexibility of using the bath fan if necessary.
I'm not a fan of the boost function in ventilators. In use, the boost function is basically over ventilating the entire house in order to add a small amount of extra ventilation to one bathroom.
The CO2 monitor is best used as a back-up system to cover for uncommon situations, in my opinion. If you are running the ventilation only when CO2 gets close to too high, it's going to be running less frequently, and at higher speed. This results in both higher energy usage, and higher noise, as power required to move the air increases exponentially. I have two CO2 monitors hooked up to the HRV, and I've used them to dial in the various HRV settings such that the CO2 monitors rarely call on the HRV to do anything different. But it covers me if, for example, someone stays home during the day when no one would normally be there and I forget to change the HRV setting before I leave home.
Agreed Trevor. Depending on CO2 level monitoring seems like a bad idea, especially when CO2 is only one small piece of a very complex Indoor Air Quality pie. What happens when only one person is home in a house normally occupied by four people? CO2 is managed at the same level but the concentration of all other pollutants increases due to reduced ventilation.
I much prefer the idea of continuous low flow ventilation, and therefore continuous dilution of all indoor pollutants.