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Community and Q&A

Using the supply ventilation fan to also circulate indoor air

whitenack | Posted in Green Building Techniques on
Hey folks (like the new web redesign!)

I posted a couple of months ago about needing to rethink my exhaust-only ventilation strategy due to the house being too tight for that option.

I am leaning towards getting a supply fan like an Air King QuFresh or a Panasonic Whisper Fresh and have that coordinate with my existing WhisperGreen bath exhausts.  
While I am moving air around, I am wondering if I can solve another issue I have with stale/stuffy air in a bedroom at the end of the house.  It doesn’t quite get enough airflow from the wall hung minisplit that takes care of most of the rest of the downstairs.
Could I add an exhaust duct in that bedroom and merge that with the supply air from outside (before it gets to the fan)?  That would mean I am circulating some air instead of just introducing fresh air, (so if I set the fan on say, 50 cfm, I wouldn’t truly be getting 50 cfm of fresh air), but I could play around with the cfm settings to find the right balance.  Would this be of any benefit or would it not make enough of a difference to try?

I realize I could do better with an ERV and truly exhaust that air, but I am wondering this would be a decent alternative considering that I already have the exhaust part of the ventilation in place and a supply fan is going to be cheaper than an ERV.


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  1. GBA Editor
    Martin Holladay | | #1

    First of all, if you want a balanced ventilation system -- with supply air as well as exhaust air -- the best approach is to use an ERV, HRV, or Lunos fans. But I guess you already know that (and have rejected that option).
    A supply fan will introduce cold air in winter and hot air in summer -- and that can lead to comfort complaints. If you have an exhaust-only system, the supply air comes from a thousand small cracks, and you avoid the comfort complaints.
    Finally, I'm not quite sure if I understand your plan to move air though your bedroom. Frankly, a Lunos fan (or a pair of Lunos fans) makes more sense than your idea. If you're thinking that the fan will equalize temperatures, you might want to read this article first: "Using a Bath Fan to Equalize Room Temperatures."

  2. whitenack | | #2

    Thanks for the reply, Martin.

    I initially thought an exhaust-only option would be best for us, for the reasons you mention (comfort complaints), but my house is too tight for this option (1.0 ACH50).

    I haven't totally rejected the ERV, but since I already have a few WhisperGreen exhaust fans, I'm trying to explore ways to incorporate the exhaust equipment I already have. An ERV or a pair of Lunos fans are multiples in cost over a nice supply fan, so that weighs heavily on the decision making.

    Isn't a Lunos system just a supply fan and an exhaust fan? Seems like that isn't much different than what I am suggesting, just using different equipment. I have a pair of Whispergreen fans on the 2nd floor that I set to constantly exhaust a set amount of air (still need to decide which ASHRAE guideline I want to follow -- my fresh air requirement is somewhere from 70 to 140 cfm) and I knock a hole in the basement wall and duct some fresh air to a supply fan and then duct that to one or more places on the first floor. These fans have settings on them that will shut off the fan when the temps or humidity are at a certain level, which cut down on the comfort complaints.

    Thanks for the link to the article on using fans to equalize room temperatures. I need to do more research on this particular room. I've taken some casual readings and the temperatures don't seem to be that far off (maybe a degree or two max during the extreme parts of the day)but the air feels stuffy and stale, especially when there are occupants. It's a guest bedroom, so it rarely gets used, but I'd still like to have a comfortable experience for our guests.

    So my thoughts regarding circulating some of the bedroom air was to add this bedroom air to the air that is being pulled in from outside, before it gets to the fan. Your link proves that it wouldn't do much to alter the temperature, if that is the problem. But if the problem is just stale air, this should help, right? And would it also help with some of the comfort complaints, by mixing the inside and outside air together to help dilute the outdoor air before it is dumped into the living space?

  3. this_page_left_blank | | #3

    I think you are engaging in a sunk cost fallacy in your desire to hang on to the exhaust fans. Just forget that you already bought exhaust fans, and look at the cost to implement each system. The incremental cost between a basic HRV and a supply fan is pretty small, while the performance difference is very large. You can get a decent HRV for around $600. Even if the supply fan was free, take that $600 and amortize it over 15 years. Do you really want to go with the lower comfort system for $40 a year? That's before beginning to estimate the energy savings of an HRV versus bringing in unconditioned air for the next 7000 days.

  4. whitenack | | #4

    Thanks for the reply, Trevor.

    I was under the impression than an ERV was much more expensive than $600, but your point is taken. $250 for a supply fan vs. $850 for a budget ERV still fits into your example.

    If I go with an ERV, how much of a problem is it if I don't run ventilation ducts to the 2nd floor? I have full access in the basement but nothing for the 2nd floor. It would become a much larger job if I have to run ductwork to the 2nd.

  5. GBA Editor
    Martin Holladay | | #5

    You asked, "Isn't a Lunos system just a supply fan and an exhaust fan?"

    The answer is no. A Lunos system include heat recovery. To learn more, see this article: "European Products for Building Tight Homes."

  6. GBA Editor
    Martin Holladay | | #6

    You asked, "If I go with an ERV, how much of a problem is it if I don't run ventilation ducts to the 2nd floor? I have full access in the basement but nothing for the 2nd floor. It would become a much larger job if I have to run ductwork to the 2nd."

    You haven't told us whether this is an old house or new construction. But since your air leakage rate is 1.0 ach50, I'm assuming it's a new house. That raises the question: How did you end up building a 1.0 ach50 house without designing and installing a ventilation system?

    If you have just built a 1.0 ach50 house, you need a ventilation system. And that ventilation system should provide ducted fresh air to every bedroom. For more information, see this article: "Ensuring Fresh Air in Bedrooms."

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