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Katherine
01-27-2014, 12:35 PM
My Office area had new roof mounted HVAC units installed last year, this is their first full winter in operation. For some reason the cold air return vent in our area is blowing cold air back into the office instead of extracting it. I talked to the facilities manager and this is happening at a few other places in the building as well in what seems like a random manner. We'll get a cold burst, then nothing for hours. He's trying to diagnose what is causing the negative vacuum, but hasn't found a way to stop it from happening yet.

David G
01-27-2014, 12:42 PM
And your question is...

Are you morally justified in tracking down the architect, engineer, HVAC contractor, and GC'; project manager... and torturing them with ice and fire until they admit who screwed up, rectify the situation, and make restitution? I'd say yes, but you might want a second opinion.

Paul Pless
01-27-2014, 12:44 PM
negative vacuumofficial engineer geek speak? ;)

CK 17
01-27-2014, 12:45 PM
As Opposed to a positive vacuum? :)

Paul Pless
01-27-2014, 12:46 PM
just how many engineers specializing in either thermodynamics or gas flow work in that building anyway? :D

bogdog
01-27-2014, 12:49 PM
We have four vacuums, nothing negative about any of them really.

Soundbounder
01-27-2014, 12:53 PM
Does it only happen on windy days?

Katherine
01-27-2014, 01:04 PM
My question is what is causing it and is it easy to fix?

I'm not sure about the windy day thing.

bobbys
01-27-2014, 01:29 PM
My Office area had new roof mounted HVAC units installed last year, this is their first full winter in operation. For some reason the cold air return vent in our area is blowing cold air back into the office instead of extracting it. I talked to the facilities manager and this is happening at a few other places in the building as well in what seems like a random manner. We'll get a cold burst, then nothing for hours. He's trying to diagnose what is causing the negative vacuum, but hasn't found a way to stop it from happening yet.
I may be all wrong but my furnace is Having trouble, I f the relay switch trips it does not heat but the fan keeps going without the gas fire on blowing heat, So it blows cold air until it's reset. , it resets itself after a couple of hours? it could be the pressure switch.

Soundbounder
01-27-2014, 01:30 PM
It could be that you are getting a backdraft on your return air duct. There is a damper on the return line which may not be closing and it is allowing outside air to flow back in. There should also be a hood where the duct exits the building.

I suspect it's the damper. They aren't closing when the system stops blowing warm air. I doubt they are seized up if the system is new. Most likely an electrical or thermostat issue that is not signaling for the damper to close.

(I guess it's also possible that there wasn't a damper installed but that seems unlikely)

Call the HVAC guys. The whole system will need to be balanced anyway

wardd
01-27-2014, 01:32 PM
My question is what is causing it and is it easy to fix?

I'm not sure about the windy day thing.

i don't know what is causing it but it is easy to fix

Figment
01-27-2014, 01:35 PM
How old is the building?
Is it new and tight enough that the system might need to introduce fresh outside air from time to time when CO2 levels rise above acceptable limits?

Katherine
01-27-2014, 01:36 PM
This section of the building is probably 40 years old.

Soundbounder
01-27-2014, 02:02 PM
Has anyone traced the return lines that are having this problem?

Do they RETURN to the system to be recycled or do they serve as an exhaust. Either way, if the dampers aren't closing, cold air can rush in.

Michael D. Storey
01-27-2014, 02:39 PM
The roof-top unit prolly has an economizer, which lets in ambient air when the temperature is warm enough (but not too warm) outside. Sometimes the temperature sensor goes to Hell. Go look around where you live and see if there is one...never mind.

But seriously, folks, this can be hard to diagnose, because of its intermitant behaviour and the reluctance of someone to go up on the roof and look at it for a while, through several cycles. You can just unplug the motor that opens the vents. They will then stay shut.
Wish you well.

Paul Pless
01-27-2014, 02:41 PM
I talked to the facilities manager. . .bet this guy loves to see you coming :D

Katherine
01-27-2014, 03:08 PM
bet this guy loves to see you coming :D
He's the only other man I make go up on the roof for me, lol.

seanz
01-27-2014, 03:57 PM
The unit might be faulty.......or it might be a defrost cycle that's giving you the problem. Blows warm air out to keep from icing up, maybe?
The other thing is, some of these heat-pump (NZ term) units have the annoying habit (program) of blowing cool air into the building when the air temp inside reaches a certain level. As you currently have an over-abundance of cool air it might be struggling to regulate this properly.
Not only, but also, these units are designed for a particular (exterior) heat range. Maybe this one will struggle in Arctic temps? Some just don't work at all.....

Michael D. Storey
01-27-2014, 04:13 PM
The unit might be faulty.......or it might be a defrost cycle that's giving you the problem. Blows warm air out to keep from icing up, maybe?
The other thing is, some of these heat-pump (NZ term) units have the annoying habit (program) of blowing cool air into the building when the air temp inside reaches a certain level. As you currently have an over-abundance of cool air it might be struggling to regulate this properly.
Not only, but also, these units are designed for a particular (exterior) heat range. Maybe this one will struggle in Arctic temps? Some just don't work at all.....
Ice is handled by putting hot refrigerant through the frozen coils. Like an ice maker in harvest mode.
I too think that it is the economizer, as I have stated, above.

Chris Smith porter maine
01-28-2014, 06:59 PM
Not knowing how these units are ducted, controled, and what they have for extras, like economizers, variable frequency drives, variable air volume boxes, its a tough call. Does the fan/blower run all the time or does it just start and run when there is a call for heating/cooling. If you tape a piece of tissue to the supply vents you can watch them move when the blower is on, it they all stop moving at the same time and then you feel the cold air, that would be a clue, and a quick fix might be to just leave the blower on all the time, untill a service tech shows up, to adjust the units.