I don't entirely follow some of what you are saying. I'll throw a coupe ideas out there...do the units have economisers?
If so do they have a power exhaust, if so functioning properly?That could be your issue.
Also why would you not want the building positive? Take a measurement at one of the doors.

 

The bms system should tell you building pressure. If its reading low, find the sensor and tee into the hoses to check sensor. If its reading high you'll have to find which part of the system isn't doing what it should.

 

See that's what I find to be lacking, it doesn't actually use building pressure at all. It just takes supply fan flow, subtracts the exhaust fan flow of exhaust fans throughout the building, then speeds up or slows down return fan based on this. I would prefer if it did modulate the return fan based on average building static pressure compared to outdoor static pressure.

 

Sorry, wrote that blurb quickly, it is definitely unclear, let me try again.
My combustion air duct for my mechanical room is blowing air from inside the building to outside the building, even when the boilers are running (9 million btu's). I'm assuming my building is overpressurized? I've gone through the outdoor air dampers, return dampers, exhaust dampers, supply fan, return fan, and all of the exhaust fans in the building, everything seems normal. It's fair to say the building is excessively pressurized? Any idea how much air flow imbalance between supply and return could cause this? The doors on this building open frequently, many doors throughout. The AHU's have airflow stations, and vfd's that control fan speed so I'm thinking the building overpressurization is caused by an imbalance between what the computer has determined that the supply and return fan speeds should run. Dampers are always 100% open.

So, dampers are all good, fans are all running properly. Most if not all exhaust fans in the building are running. For a measurement at the doors you mean air flow measurement or an actual static pressure reading compared to outside? You can feel air rushing out, it's subtle, but you can feel it.

What would be my best plan of attack? Shut down each air handler individually to see if i can determine if one or the other seems to be the culprit?
There's no other reason air should be exiting combustion duct while boilers are running, correct?
If an exhaust fan weren't functioning that were 1000 L/s, is that enough to throw my air balance off? Many doors in the building are frequently opening, and the problem persists.
I'm thinking it's unlikely to be air flow restriction, since the supply and return vfds have flow stations and the control system calculates the flow of each, so even an airside obstruction should just speed my exhaust fan up to compensate?
I might have to verify that the flow stations are reading properly?
I find the control scheme is poor because the computer takes the flow from the supply fan, then it assumes that exhaust fans 1-7 are always running, and that these exhaust fans remove a certain amount of air from the building. It then determines return fan speed based on the difference of the supply fan, and exhaust fans through the building. Perhaps poor assumptions, wrong flow readings for supply/return fan?

 

Forgive me I am not an air balancer but I have seen our sub contracted balancers take measurement at doors. I believe it's differential pressure from inside to outside. I would get this measurement first to confirm if you are over pressurized as you are assuming.
Are there any economizers on the equipment? When taking in 100% outside air you would need power exhaust on the units to remove air or you would over pressurize the space.
I can ask my balancer for correct terminology and procedure to measure the pressure after the holiday. Email me after the holiday if you want me to ask.
After that I would look at whatever sensors are used to set fan speeds. A faulty one could throw everything off.

 

Is there any problems being caused by over pressurization, such as doors not closing? Are the boilers working properly? It's normal for buildings to be slightly positive. If the combustion air duct is exhausting air from the building, then the boilers must be getting their combustion air from somewhere else or the flames would be rolling out or the exhaust would be spilling. A combustion analysis should be done and they should be serviced just to be safe. If there's no problems caused by building pressure, I would just note the issue but would not recommend trying to correct it without redoing/replacing the entire system.

 

Thanks for the responses. No, no other real problems are being caused by this, it's just something I noticed recently that seems wrong. Boilers appear to be working fine. We don't have a combustion analyzer, but my thought is that with excess pressure in the building the boilers are still happy, and are using some of this excess air for combustion. Still seems wrong, why even have a combustion air duct if air exits it?

 

If you aren't having trouble with air rushing out of the doors of your building then I wouldn't think it is a building wide air balance issue. I would focus on your mechanical room which may have an exhaust problem. Where does the boiler room get the outside air from? Does it have a dedicated AC unit, a makeup air fan, etc. Is it supposed to be exhausting some of this air and isn't. The blueprints should include a lot of this information.

My WAG is that the AC unit serving this space is suddenly economizing and there is no planned way to exhaust all this air. Maybe this was a problem when the building opened and someone disabled the economizer for this unit but a new service tech "fixed" a disable economizer last Summer.

Anyway, I know that is a story, but I would not think it is a building wide problem if you aren't noticing it building wide.

 

Boiler room gets outdoor air through combustion air duct that is terminated on the roof. There is also the relief duct in the boiler room terminated just above the combustion air duct on the roof. The room is served by a fan coil that draws from the space and supplies tempered air back into the space. No exhaust fans in this room.

You are right though, it doesn't seem to be a building wide phenomenon, just seems that the mechanical room pulls a lot of air from the basement and sends it out the combustion duct. You got me thinking, there is a damper in the pressure relief duct in that mech room, perhaps it is shut. I'll have to wait now to check it, I'm not back until the new year. Ill post my findings. Thanks.

 

Mech room ventilation duct damper is open. This duct and combustion air duct are the only two ducts in this room. I think I'm going to leave this, pressure blowing into this room is better than room in a vacuum.

 

Just a question about your vfd calculating the speed it needs. Every vfd I have installed just gets a signal and it runs at a speed based on that signal. How does your vfd Calculate speed? Do you mean there is an offset in its programming? Or do you just mean the BMS calculates speed and sends it to the vfd? I'll be the first to admit I don't know everything in this trade just wanting some clarification.

 

I'll be the second to admit that then. Ha. Sorry, yes, the bms calculates the speed and sends a signal to the vfd.

 

At least you are doing the right thing by trying to understand the building as a whole. My experience has been that people change set points in bms without knowing the problems that can result. Any chance you have access to any air balance or commission report to get an idea on all the required settings?I know it's a long shot. Keep us updated.

 

Fans at 100% on a vav system in the middle of winter? This is kind of a red flag to me. Usually the supply fans are maxed out on the hottest days in the summer. I would check vav boxes to see what the heating cfm's are. Return fan tracking isn't the best way to do building pressure control. Tracking may work properly at 30 to 75 or80% of capacity but may not be completely effective at fan running full out. I suspect some vav boxes with issues first. I would st a bunch of them to 50% manually then check building static. Rebalance as some people have already mentioned is a good idea but could get pricey. Fan energy savings could make up for it. Old balance report would be of value also. What is your current duct static setpoint currently? Was it raised for some reason? You may want to lower it if it is set to high (>1.5") good luck.

 

Yes, 100% right now, and struggling to make our static pressure setpoint of .5"wc (has been this way since I've been here), but as I say, bag filters are plugged and starving the supply fan. I've never seen it go under 90% though, since I got the static pressure sensor reading properly. This would make sense then if we have VAV issues, come to think of it, I know some of the VAV's at the end of the ductwork are always 100% open, like they are starving. Perhaps they aren't sensing properly... they open and close based on space CO, and usually the space CO is high enough that they should be 100% though.. hmm

Thanks for the input, I'll try overriding a bunch of vav's, particularly those open at the end of the duct work, and see what I get. I'll try to find an old balance report too, might be tricky though.

 

Try closing 20-30% open a lot of the boxes close to the AHU to push air to the far reaches. Those areas should catch up and back down to their set points , hopefully the main static will the come up and fans back down. This might help find some open duct above ceiling.


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There is a whole science in this, and every building is different.

Download the "Honeywell Gray Manual"

 

I want to step back and clarify this again.

OP, are you having air rushing out of the building anywhere else but the combustion intake duct?

You state there are only two ducts in the boiler room, the burner exhaust and the combustion air intake. If this is true, then you have to be exhausting out the intake and bringing in combustion air through the burner exhaust ducts. I doubt the boiler would work if this were true. But I think there is something very wrong if the rest of the building isn't experiencing effects of overpressurization, but the boiler room is...

 

I tested a more wide variety of areas after opening this thread, and it is not the entire building. It seems to be mostLy a problem with the basement. There are enough open spaces and door gaps though that I would think it would just equalize throughout the whole building. Strangely, I opened an exterior door that is connected to the basement wide open and left it open for a while, and air was still rushing into the mech room! What?

Yes there are only two "ducts" in the mech room, but to clarify, one is for the combustion air, and the other is for mech room ventilation, I believe it is code here and is to be sized at 10% of the combustion air duct.

So there are these two ducts, but there is also boiler flue venting. These vents are not sucking air, the boilers fire and operate normally. Again, the combustion air duct and ventilation duct are exhausting air, even when the boilers fire. That's a lot of excess air (10million btus in that room). It's definitely rushing in from under the door, not from the venting.

 

Is coming under the boiler room door at the same time when basement door to outside is open?


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It sure did seem to be when I tried this. I only left the outside door open a couple of minutes but yup, it was still doing it. Maybe I'm missing something dumb but it was certainly blowing my mind..

 

Almost like no air is being pulled by the exhaust fans , or a transfer duct from basement back to AHU is closed. Are fire dampers closed ,some fire dampers are powered to stay open when fire alarm is tested power is dropped to dampers , they spring shut. When power comes back they just won't open. The little motors that drive the dampers open have failed. Time to start looking at all the dampers.
Are the exhaust fans run off drives are they turning the right way.


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Ahhh, definite food for thought here. A lot of the rooms in the basement are harder to access, but maybe even plugged returns. All our fire dampers are fusible links, but I will check for those in basement (I checked those for many ahu's). Not much exhaust in the basement as far as I know, but I will double check that along with return grills. Thanks!

 

I assume the return fan is part of the AHU?

Does the unit have a barometric exhaust damper or does it have an actuator?

Is the AHU set up as: return fan - exhaust damper - mix damper - outdoor air damper - coil(s) - supply fan

What section are your plugged filters in? Between the mixing box and coils I assume?

If so:

Is there a static sensor in the return air box between the return fan and the mix damper? If yes, what does it say? If not, check it with your manometer. If you get a negative reading in the return box and it has an actuator holding the exhaust damper open, your supply fan is pulling outdoor air in through the exhaust.

A few things that could cause this in your situation with the return fan running at a calculated (basically fixed) speed:


When a part is changed and then something goes wrong, I always assume something could be wrong with what was just changed. What made you determine that the duct static pressure sensor was bad and are you absolutely sure that the new is correct? Is there possibly an offset in the BAS for that sensor that was set to make up for a miscalibrated sensor that is causing it to read incorrectly now? If the supply fan is running faster and moving more air than the BAS thinks it is, it will pull the return box negative since the return fan won't speed up to match the calculated air flow.


I'm very suspicious of the 90-100% supply fan speed as well. Don't know your system's design, but all of my VAV systems will easily maintain 1"+ with extremely clogged filters and coils that hadn't been properly cleaned in probably 20 years before I started working here... That's what makes me wonder about the duct static sensor.


Second possibility:
One of your 7 exhaust fans isn't working.

Make sure they're all running and belts are good with no slippage. If those check good, make sure they seem to be throwing out the amount of air they are supposed to be and that they are pulling from where they are supposed to be pulling. Make sure they are rotating in the right direction. Also, a fire damper could have closed or the backdraft damper could be stuck shut. Make sure the BAS is lowering the return fan speed when each one of the exhausts is turned off. If it doesn't slow down or the OA damper doesn't close down, the building will become positive. Make sure the issue exists with all exhaust fans running.

Third possibility:
Air is being added from equipment serving another area.

We have a building (used to be 3 separate, but was frankensteined together) that is served by 18 exhaust fans, one huge AHU and three large AHU's. There are doors that are supposed to be closed at all times but are often propped open and forgotten about for long periods of time. Depending on which doors are propped open, the exhaust fans in one part of the building can cause the OA dampers in the AHU serving a completely separate area to open causing some very unusual things to happen with the building pressures in different areas due to improper zoning and sensor placements.


Fourth possibility:
Barometric damper being held open and not shutting correctly.

In the above building, we were having issues with high static pressure in a large gym. Poor placement of the AHU on a roof caused the barometric exhaust damper to be held shut from the exhaust of the huge AHU blowing directly on it. The gym unit did not have a return fan to fight against the exhaust from the other AHU's return fan. To "solve" the issue of the gravity/barometric damper blowing shut, someone decided to put a modulating actuator on it. Since the barometric damper was part of the return box, this simply held the damper open and allowed it to pull even more OA in along with the exhaust from the huge AHU, making the issue worse.




One easy way I have found to narrow down building pressure issues is to just crack doorways and stick your nose near the opening. Do this on both sides to feel the air on your face. If your outside doors are doubled, be sure to prop one set open. If your face isn't sensitive enough, try a fine piece or thread or maybe some very light tissue paper held in the cracked doorway.

Some doubled up interior doors can slow things down enough to mask issues as well, so try propping a few open and checking for air movement at other doors.

A smoke pencil would be handy as well.

 

Actuated exhaust dampers. The AHU is not set up as such, it has a heat recover wheel so as air enters: OA damper, summer filters, pre heat coil, winter filters, bag filters, supply fan, heating coil, cooling coil, supply ductwork. Then air heading back to the return goes: return damper, return filters, return fan, heat wheel, exhaust damper. The bag filters are plugged, starving the supply fan and causing it to ramp up.

Duct static pressure sensor always sensed too low (.3" or less no matter what). I verified with a manometer on the pneumatic tubing at the sensor that I was reading .6", and verified that the sensor was reading incorrectly. The new is a 0-5" wc, 4-20 mA differential sensor and is a different manufacturer but the same thing. Used formula from new sensors literature to verify proper mA reading compared to the static pressure. The math worked out perfectly. As previously stated though, BMS varies supply fan speed depending on duct static pressure (sensor I changed). Moving more air then it thinks though is possible, it also uses the exact same differential pressure sensor hooked up to an airflow station at the inlet of the supply fan to determine the l/s (metric cfm). I see what you mean, and agree, this reading could be off and throwing off the BMS return calculation. That brings up another good point, I could do use that same math formula to determine that the fan cfm sensor is reading properly (unless the flow station itself has a problem, or the pneumatic tubing).
As ckartson said, I believe he's correct, I think honestly it's always that high because of again, the exact same sensor, reading improper flow through the air valves that feed other air handlers. 5 air valves at the end of the duct work are all %100 open all the time, most likely the culprit.

It's true that not all exhaust fans are working. 1 is used in a garage area that only comes on when the CO is high (this is at an airport, where the baggage carts drive in so the employees can rough house your breakable items).The computer still assumes it's always on in it's calculation, (Mind you, large doors open frequently in this area)Unless the BMS smart enough to know when the fans are off, but then I should see that return fan vary it's if I turn that exhaust fan on, pretty sure it won't but I'll try it tomorrow to verify.

I spent half a day lighting a lighter and watching where the flame blew through different doors... haha. I got a lot of strange looks that day, that's for sure.

The only other noteworthy thing I can think of to mention, is that I know this duct was pulling air in only a few weeks before I posted this, I actually used the combustion air duct's winter air to cool something down. This was still sucking air, even after I changed the static sensor. The one thing that did change, is that we have the heat reclaim wheel tied so it won't spin right now, because the reclaim motor brushes were f'd. The reclaim wheel normally free wheels and spins in the proper direction when not tied down. I can't imagine this could be causing any balance issues though?

Aye, sorry for the long winded posts, it's almost impossible to convey an adequate amount of information.

 

If everything is working as it should, there really should be a building static pressure sensor and a return duct static sensor. As the building static goes up, the return static should be increased to cause more air to exit the AHU's exhaust. That would usually be accomplished by increasing the return fan's speed, or a mix air damper closing, or a combination.

In our case, problems were also caused by VFD's being installed on units that didn't originally have them. The damper positions were still modulating as if the had fixed speed fans and the VFD's weren't set up properly. Might be worth looking into as well.

 

Start in the basement and look for excess air sources coming in. If there's access from each floor to the outside it would be interesting to get pressure per floor. Don't do this on a windy day, it'll throw off readings. Possibly there's VAV boxes stuck/forced open in the basement and this was another 'fix' for lack of combustion air. Also to verify you don't have Make Up Air units do you?

It's odd the rest of the building isn't pressurized but maybe there's enough holes/exhaust. Another trick vs the lighter is toilet paper. It only takes a small piece, single ply preferred, but only for this.

 

Your problem is excessive outside air. The building pressure measured at one door with the rest of the building closed ideally should be from about 0.02 to 0.05 inches of water positive. I have balanced buildings with the exhaust controlled as you stated and have never seen one operate successfully. To explain why would require a huge write-up. You are conditioning an excessive amount of outside air which is costly. The most reliable method of controlling building static pressure is to do it with static pressure sensors. Your control contractor will know how to do the install and programming. The operating cost savings will amaze you. 10,000 l/s total supply air in my opinion is not much air for a 300,000 sq ft building but the differential between your supply and return is a lot.

 

This is off the wall but is the excess air in the boiler room constant?
If not does the building have an elevator? I had this problem once where a tech had placed the ODA reset in the combustion air intake reasoning that it would get a constant reading.
The elevator while displacing the elevator shaft air pressurized the intake causing the reset to see warm air shutting down the boiler.

I said it was off the wall.

 

hvavr your comment is not off the wall. In this case the method of control is off the wall. I was called in to troubleshoot one huge building with this type of control system that was bringing in 100 percent outside air constantly. The building had a negative pressure. The airflow measuring stations were out of calibration. Their location was poor and they were not capable of tracking airflow throughout the full range of operation. There were other problems but the root problem was the inability to accurately measure, compare and control the supply, return and exhaust airflows. This method forces the operator to assume the design difference in airflows will result in the desired building pressure. They almost never do. I had one problem with an elevator relief which ended up being hilarious because the engineer showed up with a roll of insulation to stuff in the relief hood. I didn't get to see how that turned out but you probably have a good ides.

 

If it uses a VFD for the exhaust for the Building make sure the motor wasn't changed at some point and that the wrong parameters are programmed to the drive. I had this issue once, Trane unit came with wrong motor but the parameters were correct to the nameplate.