ERV vs fresh air ventilation using fan coil units
I am continuing with the construction of my 6500 sq’ home, in Central Texas. I recently had a blower door test done which came in at below 0.5. I was pleasantly surprised, but of course the inspector brought up the question of fresh air. When designing the house, I asked the mechanical engineer to include an ERV – he told me that it is a waste of money for residential construction. Instead our house is designed to bring in outside air to the 3 fan coil units to meet the required ventilation rate of 230 CFM. This obviously does not balance with our exhaust air created by 3 bathroom fans and large vent hood and creates an over pressurized envelope.
I am now at a crossroads and would like to solicit some advice. My options are:
1. continue and finish the current system: use fan coil units with motorized dampers to bring in fresh air, use bathroom and kitchen exhausts to get rid of unwanted air, have the system be largely unbalanced (I am not sure if the balancing procedure that is done at end of installation actually balances everything).
2. Bring in an ERV (I looked at Zender 550 which would satisfy my requirements), get rid of the fresh air connection to the fan coil units and have them only become air conditioners, add make up air to the kitchen exhaust fans, use bathroom fans only as supplements.
The latter option seems to be preferred in theory according to my reading, but there are some down sides: cost, increased system complexity. I would like to ask what the thoughts of others on this forum are, will this system pay off in the long term financially, is there a significant improvement in air quality. Is there really a significant benefit to a balanced vs net positive home?
Thank you all for advice
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Replies
I think some details are missing. What are "the three fan coil units"? Are these one half of a mini split system? Are they dedicated for fresh air intake?
I don't know enough about your climate to know whether an ERV makes economic sense, but what definitely doesn't make any sense to me is having variable amounts of incoming and outgoing air in a tight envelope. In a 6500 square foot palatial mansion, wouldn't the cost of an ERV amount to a rounding error?
The fan coil units are part of a ducted VRV system. In the design, outside air is connected to the return side of the FCUs via a mechanical damper system. According to the mechanical engineer, the house is designed to be 230CFM positive. I hope this helps.
I was really hoping to get some advice...
Another question which has come up...do I need a dehumidifier? Apparently the VRV system is good at dehumidifying due to low flow rates. Can a dehumidifier be connected to both the return and supply ducts from the blowers?
I do appreciate everyones input, please avoid condescending remarks without intent to help... Thank you.
I'm surprised no one else has weighed in. I don't know what a VRV is, so it's above my pay grade.
@ Deerefan - everything I've read about Texas builds says that you need a dehumidifier, even in good (air sealed, insulated, ventilated) new builds. Matt Risinger (Texas builder) has some videos on this topic, I kinda remember at least one where he incorporates a whole house dehumidifier into the system. Also kinda think that dehumidifiers are put on the supply side, but I'm not a subject matter expert.
@ Deerefan How did you make out with your situation? I’m in a similar predicament and I stumbled upon your question and saw that there really wasn’t any useful answers given.
Em12345,
I ended up installing a Fields Control Ventilation system, including fresh air dampers, current switches to compensate for bathroom fans and kitchen exhaust fan and using the fan coil units and duct network to bring in fresh air. I just didn't see much sense in installing a whole new duct system to bring in fresh air, with all of the potential issues it entails (I think it probably makes more sense for european homes retrofitted with mini splits and no ducts present). The system is fairly inexpensive and straight forward to install. The only part I wish I had installed was a dehumidifier, as the humidity does go up when heating and cooling requirements are low.
Thanks for the response. Are you happy with how the fan coils work for conditioning? I saw you are in Texas so you most likely don’t use them for heating also but I would be interested to hear how they perform