C1D1 HVAC Design

Hi All,

I’m designing a facility from scratch and I want to see if you guys can poke holes in my HVAC design. After many iterations, here’s what I’ve come up with:

Each classified room is equipped with a split unit (think window unit, but in the wall) that is rated C1D1 or C1D2 for temperature control.

Both rooms are equipped with a recirculating blower to ensure air is changed through the room filter 10-50 times per hour.

Both rooms are equipped with an outdoor supply and outdoor exhaust blower that will trip when the gas detector goes beyond its alarm limit—these will purge the room in about 10 seconds when turned to full capacity.

All components are rated for their locations.

Does this seem adequate? I can’t think of any reason it wouldn’t be, but this forum is filled with people smarter than I—any advice, critique or comment is happily welcomed.



If we could read minds my freind we would not be here. Please help us help you. How big are the rooms(sqft)? How many tons are your AC units? How many CFM are your exhaust fans? Where is this located( metal shed, container,)? Is the location of said lab in the desert or in Alaska?

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It’s likely going to depend on the determination of your local AHJ, but this seems like a pretty reasonable setup to me. I wasn’t aware that there are C1D1 and C1D2 split units available… might have to look at those for my facility.


Anything that will go into a room with volatile Solvents being used has to be C1D1 classified.

Hey there,

The rooms are each 150 sqft and 1500 ft^3.

Located in Oklahoma, hot humid summers, dry cool winters.

Cooling capacity ~12,000 BTU per room (overkill)

Located in a warehouse, typical a-frame steel construction, rooms located inside warehouse as independent construction. rooms are double-gypsum board, steel frame, typical drywall on exterior. C1D1 room has one exterior facing wall—intended to be fastened with damage limiting washers/nuts.

Exhaust/supply fans (for purge) have 4000cfm capacity, which is extreme but will flush the room in 20 seconds.

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I will look it up. 12k BTU or a 1 ton unit is over kill for a room without a constant exchange.

I’d get another opinion about the recirculation. Most setups are single pass through from outdoor air during normal operations and a second booster fan to supplement when the alarm trips.


This is true but not true. A/C does not have to be C1D1 rated as long as your primary exhaust fan is abover a certain CFM.

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Easiest thing to do is seek professional help OP.


Ya you should be outsourcing this to a consultant who purely does fire safety designs. As laws change from state to state and even county to county. Or reach out to a GC in the area to point you towards someone who can at least tell you your regulations. It sounds like your moving forward with what regs your used to, but could shoot yourself in the foot when they end up having much more strict guidelines. Or overspend when they barely have any requirements.

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There’s no regulations from the local AHJ, I’ve designed the airflow in accordance with the most stringent national and international regulations. As far as fire safety designs—I’m asking about an HVAC system. Fire suppression has been taken care of separately.

As far as consultants, I’m not gonna pay exorbitant amounts to have an engineer who is specifically geared to explosion proof HVAC clarify a few minor details. I’m an engineer myself, with experience in non-explosive HVAC—it ain’t worth the cost of the stamp.

The design is safe, and surpasses all local, state, NFPA, IEC, OSHA standards—just looking for some clarification.


Thanks for the input, the units we have will Be wall mounted, so the conditioned air will be recirculated without passing through another conditioning unit.

Do you think this would still be an issue?

EDIT: also, how would the room remained conditioned if you were passing conditioned air directly through to the exterior. With 1 air change a minute, how could you keep the room temp controlled?

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Most quotes I’ve received place split unit C1D1 units at about $5,500 per ton of cooling and about $2,500 for C1D2 units.


I would not use split system units.
You will have a problem air balancing the system. Also your air quality will be poor because the outside air will need twice the btu rating. I suggest you use a package hey pump. That way the air is processed and will be close to contaminate free. It will also provide you with enough cfm’ s to et the right air change ratio. And if there is a fire. You will provide less oxygen so the fire will burn slower. You can add make up air and 2 speed fans to evacuate your area if you don’t have mua you will cause negative pressure and the blowers won’t remove the smoke or solvent fumes.


I have a design I can show you that I’m building now as we speak

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Consider this:

The whole point to a C1D1 space is to provide a safe working environment for a process that regularly ventilates combustable gases. The C1D1 regs are setup to prevent process fumes from reaching the Lower Explosive Limit for said gas and to prevent the buildup of CO/CO2. The code mandated process is to size exhaust fans accordingly for normal and alarm modes while providing a source of makeup air (in both operating modes) that is free of contaminants/VOCs, breathable, and free of potential ignition sources.

Operator comfort, though important, takes a big backseat to airflow design. If you want conditioned air, I’d suggest ducting your primary makeup air for some distance through the rest of your space, so that the duct acts as a heat exchanger. This will minimize thermal losses and eliminate the need for a separate heating/cooling system (that’s not to say you can’t put the split unit in line to provide more precise or different thermal control, just an option). A second, larger, shorter, unconditioned makeup air source, with active or passive louvers, should be provided to provide the high flow makeup air when both exhaust fans are running.

There are 1000 ways to skin a cat and just because something is code compliant doesn’t mean it’s going to eliminate the risk. Think big picture and let the code guide you towards a solution for your particular space. Happy venting.


Yeah, so I understand your an engineer. The HVAC units have a small device inside called a contactor. Trust me when I tell you, you do not want one of these in your C1D1 space.


The HVAC split units are built to C1D1 regulations—that means any electronic components that could potentially create sparks, excessive heat, etc are designed to work with air containing explosive vapors.

I don’t know how else to say it.

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I’m in OK also. By the time you run fans, lights, switches, etc. I think it’s way more cost effective to get a C1D1 box and a.c. out the door for 23k. Just my opinion though. Fyi. If the extraction lab is in a climate controlled space, no need for extra a.c. Also, most prebuilts come with 3rd party cert. We don’t have AHJ in OK yet (depending on county) but it’s coming our way in a hurry. So, pay for it up front and you dont risk shut down time later🤔.


If this price tag sounds like a reasonable price to you. (It was the best I could get) DM me and I’ll get you the vendors info.

@Future headhunting opportunity?