Flaws in high-end makeup air design?

Jim Bassuk

for my remodel in seattle, i'm designing a new hvac system: radiant floor heat, erv, and makeup air for spot
exhaust fans (bath, laundry, kitchen). it is this makeup air design that i'm asking for comments on…

see a sketch of my idea here http://home.comcast.net/~jimbassuk/makeair.htm

each spot exhaust fan would be connected to a switch that would also control the attic duct heater, the attic inline fan, and the damper for that spot fan…

fresh air is drawn through a combination of filters. it is not clear to me if a hepa should be downwind of a
charcoal, or vice versa. gases attached to particles can detach from the particles when the particles are caught by the hepa, in which case the charcoal would go downwind. anybody know about this?

while i'll ask my installer if a 8 or 10” heater and fan would be best, anybody here have any thoughts on whether the extra 2 inches will help overall air flow? would a 10” heated duct be more efficient than a 8”? would the cost to operate be about the same? i'm not worried about the cost of electricity, as this system would only run for an hour or so at a time….

thoughts about what size prefilters i should use? i'm thinking that 4" x 12 x 24 30% prefilters would probably need to be swapped out a few times each year. housings will be a combo of sureseal and surelocks.

both options 1 & 2 contain an inline duct heater in an unconditioned attic, where downwind ducts would be
insulated, except for where a electric damper exists. loss of heat from the damper probably won’t be significant inside the conditioned space. however, the attic damper will suffer from significant heat loss. anybody have an idea to address this?

option 1 has the least drag for bathroom makeup air, but i'd have to cut a hole in my bathroom cabinet door for a vent that would permit the makeup air to pass through. imagine opening the cabinet door and seeing the opening of a 6” inch duct flush with the recessed door jam. these are high end cabinets – and i'd have to do a nice carpentry job of keeping this cabinet door pleasing to eye – as it is the 1st thing you see when you look into the bathroom. the upside here is one 90 degree turn over a short distance, downside is architectual.

option 2 uses insulated 6” flex to tie into existing “box” – this box is accessed from the basement via a cut into the subfloor under the bathroom cabinet. air then exists the box via a 4 x10 opening in the kick
plate. i don’t know how to predict how lame the air flow will be, especially at the kick plate. i suspect that
airflow at the kick plate grill will be lame. the bathroom spot fan is a 6” external unit, but comes with a pot to regulate velocity. since i can pretty much balance this bathroom system, anybody find a problem here? upside here is architectual, downside is lessened air flow due to three 90 degree turns.

perhaps option 2 could be improved upon if one 90 degree turn connection traversed the inside of 2 lower cabinet shelves and then directly pumped the makeup air down into a hole cut into the bottom of the cabinet. air would then enter the existing box and exit out the kick plate grill. upside is improved air flow, downside is loss of some shelf storage. anybody see a problem here?

i'm not a big fan of boxes, but tend to favor gradual turns before arriving at the grill to lessen noise. this is why the laundry room grill will be ultra quiet. i'd like to keep dampers away from outlet grills as much as
possible to lessen noise. i understand why a partially closed damper located at a box connection to the grill
would increase noise, but what about a damper that is either on or off?

my installer is reputable and i've used them in the past. all ducts will be sealed with enviro-friendly mastic – they are happy to slop it on as long as i provide it.

i'd appreciate comments on my design. thanks in advance.

GMcD

Make up air system

Well, you sure are building a high end MUA system alright. One thing for sure is that the HEPA should be upstream of the activated carbon filter. The HEPA will get the fine particulates and the carbon will get the molecular aerosols and odours. I think that you'd need to do a very careful air pressure drop study on this system to minimize the air pressure drops so as to keep the motor energy draw for fans down. The filter set you've shown and described will be in the area of at leat 2.2" WG clean and up to 3.8" WG dirty. You may need to have pressure controlled variable speed drives on the fans if constant airflow is a requirement, otherwise the CFM will degrade as the filters load up. By all means, go with the 10" ducted fan set, I think you'll need all the help you can get to keep the system resistance minimized. The 80-90% filters in your diagram should be a minimum 6" pleated or 12" pleated filter set to maximize change-out time and minimize air friction. Keep the face velocity to the carbon filter trays less than 200 fpm to allow maximum media absorbance time and minimum air pressure drop as well. If you are worried about carbon carry-over, install a 2" thick 30% filter downstream of the carbon set to get any particles.

DaveGateway

MUA vs HRV

Jim, I'd take a look at Heat Recovery Ventilation (HRV) design as illustrated on their website (www.nutech.com or www.lifebreath.com)We use HRV's in Seattle which would eliminate the separate requirement for MUA ducting. The HRV will use an 8" supply and return, which is oversized slightly for noise reduction, and a separate fresh air intake and exhaust. Since the unit will preheat the incoming fresh air with the reclaimed heat from stale air, an in-line duct heater is rarely required. Lifebreath also makes the Carbon Scrubber and TFP HEPA filters. Their installation and technical manuals can be downloaded. See you on Tuesday! Paul Pollets

Jim Bassuk

Flaws in high-end makeup air design?

Hi Paul --

While HRVs do a nice job of balancing ventilation, they don't address laundry or kitchen exhausts issues. For example, it is not possible to connect a clothes dryer exhaust vent to a HRV due to condensation issues. For range hoods (updraft or downdraft) there are few, if any, residential hoods that are engineered to include makeup air in their design.

Consequently, a combination of HRVs and a dedicated MUA system should do the job nicely.

Another question: where would humidifying and dehumidfying units be placed WRT the HRV connections?

Thanks,
Jim

Paul Pollets

design

we always pull a return grill from the laundry room to the HRV return main, as well as add an exhaust fan/timer. The reason you can't connect the dryer vent to the HRV has to do with lint clogging the HX. MUA should be considered if the kitchen exhaust is over 1000CFM and you keep all the windows closed. (winter)Still, the HRV will equalize pressure in the house as it's designed for continuous operation. The HRV has a built-in dehumidifier. A humidifier can be added to the airflow, if desired.

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Brad White_2

I agree with Geoff

especially his point about relative positioning of charcoal to HEPA. HEPA's generally always go last and are protectected by all upstream filters. Carbon filters are the exception but "shed" fine carbon particles that ought to be captured by a 30% pleated final filter. Kind of counter-intuitive, filtering gasses then solids... Geoff's point on pressure drop ought to be seriously considered. The airflow is not much (apparently) so you can oversize (24x24 would handle 800 cfm at 200 fpm). Rather than using a fixed-speed blower, use a belt-drive or direct drive with a micro-VFD. This way you can drive the airflow to meet the load, not degrade the airflow by imposing resistance. Puts you in control. Now, with a VFD you can set it up with a pressure differential sensor so that it maintains constant flow as the filters load up.

Personally, I would use the HRV concept where you can. Now, I have to ask, is the client immuno-compromised? Seriously, to HEPA-filter and gas absorption with 30 and 90 percent prefilters, that is practically a hospital grade system. What would make it hospital grade is to BLOW through the final filter. This way when you protect the finally filtered ductwork should air leak in after the HEPA on the suction side of the fan. Fine point but for all that work it can be defeated.

For products, I like Cambridge/FARR Glide-Pack side access filter housings. A filter is only as good as what holds it. If not essentially airtight, surrounding leakage defeats things.