question about the direc fired make up air unit 3

[lzh007]

where can we use direc fired make up air unit? Somebody told me it is for any ventilation purpose as long as no one sleep in the conditioned area, such as restaurant and office. Direct fired unit makes me a little uneasy, is it really safe for people inside?

 

[walkes]

The direct fired units are safe for occupants, the burn is cleaner and more complete than an indirect-fired unit (from what I've read). As long as the air is not recirculated you should be fine. There are 80/20 direct fired units out there but I believe you would require CO detectors in the space.

 

[tombmech]

Walkes is correct, as most people that have a gas log fireplace can attest. There are fire protection issues, though - they can't be used around volatiles (paint, for instance) or anywhere near fueling operations. It doesn't sound like you're contemplating that, though.

 

[ChrisConley]

Check your local gas codes though. In Canada direct-fired make-up air unit need to be interlocked with an exhaust fan. In the area where I live an air-proving switch in the exhaust air stream is required to make before the direct MUA can fire.

We regularily use direct-fired in occupied spaces (not sleeping areas). The advantages are efficiency (~100%), turndown (30:1 as standard) and cost.

If energy is a concern, you can also arrange for a heat recovery system with the MUA. I have done plate, and heat pipe heat recovery with direct fired MUA.

 

[GMcD]

While codes may allow it, do you really want to the guy who designed/selected a piece of equipment that dumps flue gas on people? A direct fired burner is exactly that- raw flue gas mixed with air that you are breathing. In my opinion it's not good practice. As engineers we should be striving to higher standards than that- remeber the "Code" is a minimum standard, surely we can do better than that?

 

[ChrisConley]

I disagree GMcD, I don't use direct fired in every application but the idea that we are dumping 'raw-flue' gas is misleading.

The products of combustion of natural gas, burned in a high excess of combustion air, are CO2 and H20. What exactly is harmfull about that?

Dumping flue gas on people occurs when you don't make-up air that is being exhausted and chimneys from gas fired equipment start downdrafting from the negative in the building. I

 

[GMcD]

Real life: these low cost direct fired units are dumped on a building, seldom commissioned or set up properly, poorly maintained, and in fact dump a good CO% and are never a "perfect combustion" delivery. Not to mention the various contaminants in the natural gas or propane as the source fuel- high sulfur content etc. An open flame is products of combustion=flue gas, and it's being delivered, albeit in a diluted method, onto people, even if that make-up air is 100% exhausted. Show me a CFD model of a kitchen exhaust and make-up air terminal setup where the make up air actually gets exhausted without substantial mixing in the space. Likely only with a specialized exhaust hood with the make-up air terminals integrated into the hood edges, not a common set-up.

 

[cme]

I'm hesitant on direct fired equipment for occupied spaces simply because people are sue happy.

 

[willard3]

snip
I'm hesitant on direct fired equipment for occupied spaces simply because people are sue happy.
snip

Not to mention the carbon monoxide........

Not to mention the moisture problems.......

Not to mention the increase in carbon dioxide.....

 

[lzh007]

Thx for the discussion. At first I can only feel very uncomfortable with the direct fired unit, now I know why.

 

[imok2]

Question: If direct fired units were even reasonably safe ..Why would anyone want to spend money on heat exchangers? Gee think of all the money mfg. could save! :>)

 

[ChrisConley]

I'm going to stop after this post, because I don't manufacturer, sell or market direct-fired equipment. In fact I rarely specify it, and generally only in industrial applications and parking garages.

What I would like to know is why the misinformation about the product? There are many applications when direct fired make-up air products are both acceptable, economical and safe.

Is there some history in the US of these things blowing up or causing huge IAQ problems?

From a safety point of view direct fired burners are always burning in a large excess of combustion air - meaning if you've selected the unit properly the only products of combustion are CO2 and H20. With an indirect burner the burner burns with very little excess air - meaning that if the HX leaks the products that escape into the airstream are much more dangerous, in fact the only time I've seen a piece of gas-fired equipment dumping high levels of CO into a space was an indirect MUA.

If anyone has seen a bad installation, or has heard of a bad experience please post.

 

[SAK9]

I fully support Chrisconley's views on this issue.Many on this forum seem to be prejudiced without knowing the facts of a direct fired unit.Direct fired u nits work well in a number of applications such as kitchens(where you have huge make up air requirements),small shops etc.Who knows may be you have higher CO2 levels if you were to stand on the sidewalk in a busy city such as New York

 

[tombmech]

Ditto ChrisConley and SAK9. My experience is almost parallel to Chris's, except that I've used them in aircraft production applications and warehouses. I've also experienced the CO problem - on a cracked, indirect heat exchanger.

The only caveat to that is what I've stated above and on another thread - they cannot be used in a flammable environment. Residential/Commercial applications are not a good choice either, because they simply don't have precision control.

 

[62hog]

As an engineer who has designed around the installation of hundreds of direct fired MUAUs over the past 15 years, I have experienced no problems with these units. The only installations for these designs, in my opinion, is industrial settings that will afford no possibilities of recirculation or the possibilities of flammable gasses entering the unit.

The units for my designs go into food processing plants in which there can be no recirculation for one reason or another.

For what it is worth, my firm has just finished a design for a building that included about 400,000 CFM in 15 direct fired make-up air units heating for ventilation and clean-up operations.

I don't know much about statistics, but I do know that if something has a 50-50 chance of going wrong, 9 times out of 10 it will. Author unknown

 

[friartuck]

Go to Reznor for indirect heaters. These are very efficient and you will not get the smell sometimes associated with Direct units. (reznor can do units at 90% efficiency)

Direst units in the UK ARE used, but I would say that discharging the products of combustion into a habitable space is not ideal. You would need a large quantity of fresh air to prevent or reduce condensation (if you are in a cold climate).

I personally use indirect to reduce any risk (however small) of CO poisoning.



Friar Tuck of Sherwood