ERV and IAQ in Carolinas
Hi, I’m considering putting an ERV in a home in the Carolinas (hot humid in summers, occasional cold in winter). This has been a great site for info, but I had a couple of questions:
1. Is it OK to use an ERV with existing ductwork? I don’t think it is feasible for me to add a new duct system. My understanding is that the ERV would both pull air in from and supply fresh air to the return ducting for the HVAC. I guess if the blower in the HVAC system is on, the fresh air goes through the supply side and enters home. If the blower is not on, I suppose the fresh air enters the house through the returns? Not sure I completely have a grasp on this, so any help would be appreciated.
2. Should I be concerned with getting too much humidity in the home with an ERV? I have a two system home (upstairs and downstairs). I have toyed with the idea of putting a ventilating dehumidifier in the downstairs (ie, adds fresh and dehumdified outdoor air, but will create positive pressure because it does not exhaust stale air) and adding the ERV for the upstairs. Is there a downside to a little positive pressure? I thought it might be a good thing considering I also have bathroom fans, etc.
Thanks in advance!
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Replies
Brian,
Q. "Is it OK to use an ERV with existing ductwork?"
A. It's OK, but it's not as good as an installation with dedicated ventilation ductwork. Most furnace ductwork systems are leaky, and they are oversized for ventilation. Moreover, many HVAC contractors wire these systems so that the furnace blower is energized whenever the ERV is needed. If that is what your HVAC contractor is planning, watch out -- your energy costs are likely to be very high. Most furnace blowers are energy hogs. The second option -- using the ERV fan to distribute fresh air when the furnace isn't needed -- isn't a great idea either, since the ventilation air isn't likely to reach remote bedrooms when it is sent through leaky, oversized ductwork by the power of a relatively small, efficient fan designed for smaller ducts.
Q. "Should I be concerned with getting too much humidity in the home with an ERV?" Most ERV manufacturers overstate an ERV's ability to affect indoor humidity levels. In most homes, there isn't a big difference in performance between an HRV and an ERV. (For more information on this topic, see HRV or ERV?) If you are worried about high indoor humidity levels, choose an HRV.
Q. "I have toyed with the idea of putting a ventilating dehumidifier in the downstairs (ie, adds fresh and dehumdified outdoor air, but will create positive pressure because it does not exhaust stale air) and adding the ERV for the upstairs."
A. Dehumidifiers are energy hogs, so I advise you to avoid installing one unless absolutely necessary. Certainly an HRV (or almost any kind of ventilation system) should be all you need to control indoor humidity during the winter. And in most (but not all) climates, an air conditioner is sufficient to control indoor humidity during the summer. The exception is a very hot, humid climate like Houston.
Q. "Is there a downside to a little positive pressure?"
A. Not really, as long as it is just a little pressure -- say, in the range of that provided by a 50 to 80 cfm fan.
Thanks Martin. If I do supply the fresh air from the air exchanger into the return ducting for the HVAC, is a possible solution to use a variable speed furnance? In other words, the variable speed furnance blower is always on and more efficient that a single speed or double speed blower, and thus can accept and move the fresh air continuously?
Brian,
A variable speed furnace blower is better than a single-speed blower, as you say.
Even better is a furnace with an ECM blower -- that is, a blower powered by an electronically commutated motor. Such furnaces usually cost more than furnaces with permanent split capacitor motors, but they can save a lot of energy if the blower runs continuously for ventilation purposes.
Thanks Martin. Do you think it would make a material difference as to whether I use an HRV or ERV? I was advised that an HRV might help humidity issues in certain times of the year, even in the southern US. Also, I noticed the Max Sherman articile on the uncertainity of whether ERVs are properly venting formaldehyde from homes.
Brian,
All of my opinions on the matter are in this article: HRV or ERV?
If you are worried about the formaldehyde issue raised by Max Sherman (I'm not, particularly, until I see more data), then by all means choose an HRV.
Really, there isn't that much difference in performance between in ERV and HRV.