Direct-Fired Make-up air IAQ concerns 1

[EnergyProfessional]

I'm working on a project requiring make-up air for garage and work spaces. We typically use direct-fired MUA. I'm in WI, so we do have quite some heating requirement.
This project also includes installation of a new boiler plant. So at just a bit higher cost I could use hydronic MUA with glycol. our OAT design temperature is -15°F, so glycol is needed.

I'm getting a bit concerned with direct-fired MUA since they introduce some levels of CO and NOx in addition to what the vehicles already produce. i also feel with old dirty vehicles that may not have been so significant since the air we displace was much dirtier. But newer combustion vehicles, and future electric vehicles would be cleaner and the addition of pollutants from an MUA would be significant.

I researched what emissions a direct-fired MUA has. i cam across this manufacturer, who downplays the emissions, but gives some numbers. Note they assume a perfectly functioning unit and no added pollution from in the space.

To my surprise there isn't a lot of information. Ironically an MUA is used to improve IAQ. i could imagine in a space that has really dirty old diesel trucks a direct-fired MUA won't make much of a difference, and people wouldn't spend much time in there. but in a space where people spend more time (i.e. work on vehicles) and the vehicles are cleaner, it would be significant.

Actually when i was new to HVAC and heard r-fired MUA the first time, i couldn't believe they are actually used for human-occupied spaces.

what is your take on this?

 

[GT-EGR]

Direct fired MUA units can only be used by code in certain occupancies and they also have to meet a specific ANSI Z83 standard which limits the emissions and likely has other safeties (never read the full standard). I think the rationale would be (that you allude to) that these just aren’t clean areas to begin with, and this is just a drop in the bucket. I think 5 PPM of CO or whatever the limit is isn’t much compared to everything else that goes on in a garage. It’s not necessarily right, but residential appliances put out acceptably higher levels every day right in people’s homes.

I think the exhaust system is much more critical as far as indoor air quality - the proper capture and removal of the real contaminants. The makeup air unit is more of a secondary necessity to bring that air back in with relatively improved quality.

But in the end if you don’t want to introduce it, then don’t do it. As some point there is a tip in the balance between energy savings, practicality, and having clean air.

 

[nglty]

For such high volume space, CO will be sufficiently diluted. Such spaces have a high level of infiltration.
What you should consider in your climate for such a space with high level of infiltration is gas-fired radiant heat, including infra-red heaters. In addition to FTR for pits and offices.
MUA can still be there controlled with combination CO/VOC sensor to flush the space say above 15 PPM.
With radiant heat, your space will recover instantly from open/close of roll-up doors. You energy consumption ca be cut in more than half, the MUA will be enabled no more than 10% of the time. I did a similar project in MN, and it worked great.

 

[EnergyProfessional]

Thanks for your replies. Another wrinkle is that this is a masonry building without rainscreen wall. So the added water from a direct-fired MUA is another issue.

Yes, legally most commercial occupancies can use a direct-fired MUA. it doesn't mean it is a good idea, though. Ultimately the only OA i can rely on is what comes from the MUA. So if that already comes with content that I don't want (CO, CO2, vapor, NOx), it raises the concentration of those gases.

 

[willard3]

>>>>>For such high volume space, CO will be sufficiently diluted.

Dilution is not the solution.

 

[EnergyProfessional]

As for dilution: if you run the MUA long enough (assuming you exhaust the same air volume) over time you would have the same CO,CO2, NOx concentration as the MUA spits out. That would add to what is produced in the space. So the CO levels would be at least 5 ppm if no CO is produced in the space. I realize code allows higher levels, but if people are in that space long enough, 5 ppm isn't great. Neither is 4,000 ppm of CO2 (in offices we freak out if it is above 1,000 ppm. And this assumes the unit combusts as designed.

Yes, infiltration will help a bit. But if we could rely on infiltration for ventilation, we didn't need exhaust and MUA to begin with. If someone told me my office would be ventilated to allow 5 ppm CO and 4,000 ppm CO2, I wouldn't be happy, even if that is legal. Ethically I have a problem to design a system with these parameters for other people. Maybe it is OK in a space that is only temporarily occupied (i.e. to park and retrieve a car). but for a shop or other occupancy where we expect people to spend a lot of the day this is an issue.

As for my current design that brought up this issue: this is a masonry building that will be barely heated. So we will already have issues drying out the masonry wall. So besides the issues of CO/CO2 for people in it, i don't think adding many pounds of water vapor will be helpful. So i decided to use a glycol system (this project will get an all new boiler plant anyway)