Connecting bathroom exhaust fans to ERV and Fantec model?
Hi,
First, this is a great site… thank you!
I read this article (and others) about using an HRV/ERV with bathroom exhaust fans. https://www.greenbuildingadvisor.com/article/does-a-home-with-an-hrv-also-need-bath-fans
The articles mention using “boost switches” with the exhaust fans. Can someone explain this in detail to me? Does the boost switch increase the flow rate on the ERV unit to accommodate the additional flow from the exhaust fan? Is the boost switch external to the exhaust fan switch, so the user needs to turn on both… or does the exhaust fan power trigger the boost? What happens if the exhaust fan simply tries to push the additional air through the ERV?
Also, a contractor suggested a Fantec MNSER150, but I cannot find any specs online.
And lastly, we have been dealing with months of smoke from epic forest fires the last few years. Is there an inline filter we can install on the “fresh” air side of the unit to keep it (and our air) cleaner? How might that affect the operation of the unit? Would I need to increase cfm of the unit to overcome any resistence from the filter… or install a boost (assuming I learn what those are!)?
Thanks
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Replies
The boost switch increases the ERV to a higher flow rate for a preset time.
You can't hook up the ERV in line with bathroom exhaust fans. If you have bath fans, they would operate independently. If you have an ERV, bath fans are not really necessary. However, make sure the ERV you get has boost functionality (many don't). The rest of your questions after that are not applicable, in light of this.
The fact that a contractor suggested it is almost enough to disqualify it. I'm guessing he means this one:
http://www.fantech.net/products/residential-fresh-air-systems/fresh-air-appliances/horizontal-appliances/ser/ser-150-fresh-air-appliance/
The specs on that are frankly awful. I wouldn't even consider it. I'm guessing the features are going to be pretty sparse as well.
Any ERV will have its own filters (incoming and outgoing). You shouldn't need to install another one. Having said that, you'll need a MERV 17 filter to filter out smoke. That's a pretty high end filter that will probably be pretty pricey.
Why do you say the specs are awful? (honest question)
At the medium setting, 115cfm, the SRE is either 59% or 53%, depending on how you interpret their data sheet. (It's not reported in a way consistent with the listings I've seen before, such as on the HVI product directory.) Either one is pretty poor.
Thanks
[quote]You can't hook up the ERV in line with bathroom exhaust fans. If you have bath fans, they would operate independently.[/quote]
That was not my impression based on the discussion I posted a link to.
[quote]The fact that a contractor suggested it is almost enough to disqualify it.[/quote]
Any suggestions or recommendations on units? I find the product info from the company websites nearly inpenetrable.
wkitlasten,
I can't find anything in the discussion you linked to that would suggest you can link the two systems.
I think you are confusing HRVs (or ERVs) with exhaust intakes in bathrooms, with dedicated bath exhaust fans. The former are all powered by a central fan located at the HRV that pulls air toward it. The latter generally have a fan located in the bathroom ceiling that pushes air to the exterior. As Trevor wrote, the boost function is simply a way for the HRV to circulate higher volumes of air.
As I understand it, the main problems with ducting dedicated bathroom exhaust into your HRV are:
- Manufacturers won't allow it.
- Dealing with the back-flow that the exhausts cause in a system designed to pull not push air.
- Compensating for the unbalanced flows through the HRV when the exhausts are operating.
It depends a lot on your needs and your budget. Some details of the house would be helpful.
Malcolm, you are correct I was confusing the two. So by boost they essentially mean turn the HRV/ERV to high!
Trevor, the house is about 1,200 sq ft single story with a small loft. Passive solar, 8" foam core walls, infloor hydronic heat and a fire place. The attic ventilation was horrible (2 gabel end vent to allow 10F air in during the winter so our moisture could freeze onto the roof deck). We had the mold professionally cleaned and sprayed in 4" of closed cell foam, sealing off the envelop. Our only air intake are windows and door... and of course the radon rich air seeping through the slab. We decided to do an ERV and a subslab suction system, just to be sure.
Your required cfm is probably in the range of 50-75, depending on which metrics you use (this assumes three occupants). This would be a continuous rate. Did you have a blower door test done? Where is the house located?
If your main goal is reduction of radon, making the assumption that the house naturally gets enough fresh air, I don't think adding an ERV is going to make a significant difference to that. If you haven't done a blower door test, I would do that first. That will indicate whether you need, or could even benefit from an ERV.
Here you can compare ERV performances side by side:
https://hvi.org/proddirectory/CPD_Reports/section_3/index.cfm