Bathroom – ERV boost vs standalone exhaust to the outside
orange_cat
| Posted in General Questions on
I am not familiar with ERV boost to exhaust the air from the bathrooms. I am wondering whether they will remove as much moisture as I would like – using as a baseline a conventional fan vented to the outside. (The idea is to have it on a timer – e.g. 15 min – after a shower)?
Is there any “common knowledge” about it?
Heating-dominating climate, but humid summers and shoulder seasons.
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orange_cat,
" I am wondering whether they will remove as much moisture as I would like"
That's where it gets complicated. What people see as satisfactory is quite individual. Mainly people judge the effectiveness by whether the mirror or windows fog up - which depends on a bunch of variables, not just the exhaust capacity. Most of the humidity in bathrooms comes from showering, and the amount of moisture produced varies by how long the shower is, the flow of the fixture, the rh in the house, and the size of the bathroom.
If the metric is keeping the space from developing moisture related problems, ERVs with boost functions work fine.
Assuming LOONG showers in a 8x5 bathroom? Think 15 minutes or longer? Possibly two in a row?
I detest mold in showers and it is a common problem here (granted in older less well ventilated bathrooms - but with exhaust fans).
The balance between moisture removal and energy efficiency makes this an important consideration for homeowners. Choosing the right system geometry dash deadlocked should depend on specific needs, bathroom size, and overall ventilation goals.
This is an interesting question and would make a good subject for a long article by somebody with an engineering degree. I don’t think I’ve seen that article anywhere. (I am assuming the question here is about adding a bath fan to a house served by an ERV, not about choosing between them..)
Here are a couple of relevant facts:
ERVs recover moisture as well as heat, so a stand-alone ERV (no bath fan) will cause the house to retain some of that unwanted moisture.
A low load house in the climate you describe may not have a high enough cooling load to keep moisture levels down if there are sources of interior moisture (showers, construction moisture etc)
ERVs in boost mode increase the whole house ventilation rate. The bathroom might then see 30CFM.
A typical bathroom exhaust rate is around 100CFM.
Installing both seem like a good idea to me in spite of the downside, which is that the bath fan will tend to unbalance the ERV and possibly put the house under temporary negative pressure. (zehnder ERVs have an automatic balance feature but I think a bath fan might push the limit of their rebalance range.)
So the question is whether the upside of good moisture removal is worth the downside of temporarily depressurizing the building. If you’ve taken care of radon and soil gas I think I’d put in the bath fan.
The alternative is to use a ERV and run a dehumidifier on a humidistat, which might also work well.
Hope that’s helpful.
Yes, thank you - exactly, adding (or not) a direct vent to the outside fan to a bathroom already provided with an ERV.
I appreciate you taking the time to answer.
No problem. If this is new construction watch out for construction moisture—it will probably take two years for all the construction moisture (mostly cement) to dissipate. After that, things tend to settle down. We’ve seen humidity spike above 70% in that time with the erv and heat pumps working properly. Some of that may be occupant behavior of course. In any case it’s good to have some way to deal with that.
Digging deeper....
These are the ERV models:
https://www.lifebreath.com/wp-content/uploads/2024/10/100-METRO-120-ERV-Spec.pdf
There are 3 altogether, and for the two heavily used bathrooms there is 1 ERV within 3 feet distance at most (by design).
I have stared at the spec sheet and have been defeated in my efforts to figure out if the boost option will provide sufficient air flow for that bathroom to remove moisture (I cannot even figure out what the CFMs would be in that case...)
The ERVs on the top floor have one bathroom and two bedrooms per ERV (so 4 rooms total, 2 bathrooms) and it is around 1000 square feet combined area (all four bedrooms with 2 bathrooms)
Any thoughts?
I am not a fan of both bathroom exhaust-only fans and ERV’s. The way I plan to tackle this is with motorized dampers on all the ERV stale indoor air pickup registers. They will have a set screw on them to keep the dampers partially open when the ERV is in regular mode (~50% capacity). Boost mode will be activated woth a smart switch in the bathroom that sends a Z-Wave signal to kick the ERV into boost mode but also via a hardwired power line sends power to the motorized damper of that room that called for the boost. The damper will open fully while all the other rooms still have mostly-closed dampers. In this way you get increased spot ventilation only to the room that called for it, instead of all the rooms. The position of the set screw for the non-boost mode will be adjusted using flow hoods to dial in the right designed CFM of fresh air under normal circumstances. This solution avoids needing extra bath fans and depressurizing the home.
Matthew,
If you don''t have combustion appliances, does a bit of depressurization really make any difference? The amount of powered controls sounds like a recipe for problems down the road.
Probably not. But this avoids needing extra bath fans. And the automation system will be in place anyways for other reasons such as the hot water recirculation pump. I also like the idea of using humidistats to turn on the boost mode automatically instead of relying on a user to hit a switch. Especially with guests who may not know the protocol.
Edited to add: the motorized dampers do not have to be at the register in the bathrooms, they could be in the same mechanical space the ERV is located at, for easy access in case they need to be replaced.
There is a big difference between continuous ventilation of an ERV and intermittent operation of a fan.
Even a long shower say 30min is nothing compared to 24h. While the ERV will recover some of moisture from the bath in use, it is still exhausting air from the other places, so overall the air it sees is a mix of house air and bath air. During bath use the RH of supply air to house will slightly increase but since this is only for a very short period of time (3om over 24h) it simply doesn't matter.
It sounds like you have a good setup there. The ERV is good for about 120CFM on boost when connected to reasonable ducting, assuming the ducting is somewhat balanced you get about 60CFM from each bath. I have about 30CFM of ERV exhaust on my bath and I find that I never need to use the bath fan when showering.
Also a typical 100CFM bath fan really ever runs at 100CFM, you might get 75 or so. Your ERV is almost there plus it runs at some level of continuous ventilation, so you are all good. One thing to watch is not to over ventilate. This is less of an issue with new construction moisture but might dry out the air too much down the road. Code min values of continuous ventilation is good enough, so in case of two beds, that works out to about 30CFM total.