Residential Mechanical Ventilation Question

[steveski]

A contractor friend of mine is trying to get a permit for a large custom home including a media room (approximately 600 SF). The media room is completely enclosed, with no windows. The plan checker made the following comment: “Provide a mechanical ventilation system designed by a licensed mechanical engineer (not a mechanical contractor) for the media room controlled by an occupancy sensor. Provide wet stamped and signed plans and engineering calculations with the re-submittal.”

Are you aware of any off-the-shelf systems that are made for this purpose? If not, do you have any advice on how to handle this? So far I’m considering:

1) A dedicated air handler (possibly 100% outside air) interlocked with a CO2 sensor in the media room.
2) An air-to-air heat exchanger serving the room only (exchanging stale air with outside air), interlocked with a CO2 sensor in the media room.

I haven’t asked the building department yet whether they will accept the CO2 sensor in lieu of the occupancy sensor they called for. However, I’m a little concerned about wasting energy by over-ventilating the room when occupancies are low.

I am not designing the HVAC for the rest of the house, so I am hoping to keep my design separate for simplicity’s sake.

Any advice you can offer will be greatly appreciated.

 

[KenRad]

What is the design occupancy of this room? That will dictate your design outside air requirement. I can't imagine needing 100% outside air in any event.

A dedicated air handling system for this room would probably be a good idea for climate control alone. Here's my cheap and easy route - Install a small split system or packaged unit for this room, and install an outside air intake with a manual damper, balanced to bring in your required outside air whenever the fan is running. An occupancy sensor could be wired to turn on the indoor fan, which would satisfy the ventliation requirement. Assuming an occupancy of maybe 15, and an outside air ventilation rate of 15 CFM, we're talking 225 CFM through the OA intake. Heating and cooling would cycle on as needed from the zone t-stat, but the indoor fan would remain on whenever the room is occupied.

A modulating OA damper controlled by a CO2 sensor would be operationally more ecomomical, but obviously would cost more up front and be more complex.

The other benefit of the first option is that it would allow the owner to do whatever he or she wants with the outside air damper after getting the certificate of occupancy.

Your local HVAC PE should be able to crank out a sketch with some calculations in a few hours.

---KenRad

 

[HVAC68]

What's the application ? Large custom home - how many people on an average ? how big is the building ?

HVAC68

 

[garnerengr]

I would shy away from using a split system with a manual O/A damper set to a fixed volume of air. The problem is that when the compressor cycles off with the fan running, the O/A is brought in untreated. In more humid climes such as coastal areas, the amount of humidity introduced when the compressor is cycled off can far exceed the ability of the evaporator to remove it when the compressor is on. Unless, of course, you use some sort of reheat system to override the thermostat upon a rise in space humidity. Though not as inexpensive, I would opt for a split system using a hot gas reheat coil for dehumidification. No energy penalty as there would be with electric reheat and much better humidity control. AAON make such a system with a modulating hot gas reheat option that offers excellent space temperature and humidity control. I have used this system several times with good results.

 

[Engineer6512]

Your consideration of using a small Heat recovery ventilator is a good one and well suited for this purpose. They are inexpensive, will provide you with 100% outdoor air, but will also recover the heat so when the climate outside is significantly different than indoors, it will temper it before introducing it to the room. I'd recommend sizing the unit for 15 cfm per person that will be occupying the room. CO2 is an acceptable method of sensing occupancy, but the sensors can be a bit pricy. I'd recommend just using an optical sensor and a timer instead.

Steamguy2

http://www.SteamPlantEngineering.com

- Discussion & resources for professionals in the Power Generation Industry