Draft Inducing Fan Control

[Dan1978]

I have been reading other peoples threads on here for quite awhile so now I have one of my very own.

The company I work for was involved in church project where some roof top units were installed. The local gas authority has an issue with the flue gas exhaust and wants it re-routed. All Lennox would certify was a small extension kit that basically was a 90 deg. elbow fitting type. We need to get the flue gases up around fifteen feet away from the unit.

I am thinking of using a Tjernlund inline draft inducing fan and create a shroud around the flue exhaust termination. Question I have is to control the fan I am proposing to interlock the fan with the gas control valve of the roof top unit. So when it opened the fan came on and vice versa. I had problems getting this OK'd with the local gas authority and am wondering if anyone has ever tun into a similar situation or has any suggestions on another way to control the fan?
We considered installing a gas flow switch on the gas line before it entered the unit but figured it was more reliable to monitor the gas valve on the unit itself since the flow switch could be faulty at some point.

I would appreciate any thoughts or comments any of you may have.

Thanks

 

[RossABQ]

If you really need to use mechanical exhaust to get another 15', I would interlock it via the burner controls, just like any other burner-related device. I believe Tjernlund has some go-by's.

I'd be surprised if you couldn't just enlarge the flue diameter to reduce backpressure, and run the extra 15'. Even that may not be needed, but you haven't given many details on size of unit.

 

[trashcanman]

This is puzzling. You say roof top units were used. If so, what is the problem with venting? Are the rooftop units not installed on the roof?

 

[Dan1978]

RossABQ/Trashcanman;

The manufacturer would not allow us to extend the vent any further. There is an existing vent extension kit installed there already. The unit is 250 mbh Lennox LGA and is mounted under an architectual feature that acts as an enclosure, so we have to get above the enclosure about 15' up.

Tjernlund says there control unit can interlock with the gas valve on the unit without any problem so think were OK.
Not sure if I need to get the OK with Lennox for the interlock? Reason I ask is in the event the fan fails to operate Tjernlund claims it can close the gas valve on the roof top unit, so now it is interacting with the unit rather then simply monitoring the status uf the valve. So am wondering if need to get manufacturer approval or if it is normal practice to do this?

 

[RossABQ]

I'd be careful with the powered unit; you need to make sure you don't upset the flame stability of the unit. I would expect the extended vent will be a collar over the existing, so that it "assists" the natural venting but doesn't put the firebox in a negative pressure condition.

Caveat: I've never used the Tjernlund units, they look like trouble to me.

 

[Compositepro]

I would think that you need an air gap between the exhaust outlet and the enlarged flue extension to avoid affecting burner performance (like out gas fired water heaters). Then a fan should not be required. But you will certainly need to get "qualified" advice that everyone will accept. You won't find that here.

 

[Dan1978]

RossABQ/Compositepro;

This was a first time for me using a Tjernlund product - they seem to be a big name in draft inducing fans from what I see. We did put an airgap as Compositepro mentioned. Created a draft induction hood above the termination of the existing flue exhaust vent. Just waiting to get the OK from Lennox, our local rep. had to get in touch with H.Q. in Dallas for this one so we'll see what they have to say.

Thanks for advice guys - will keep posted if anything interesting happens.

 

[lownox]

You will not get Lennox to approve much of anything. The gas portion of the unit is likely tested to ANSI 83.8. There are certain wind tests performed which include ignition, etc...

They probably won't recommend or approve any configuration that was not tested and approved.

If it were my equipment and I wanted to add a vertical stack to it... I would do the following. The exhaust looks to be vented horizontally. Measure the CO2 level in the exhaust and record it. Get some 6" dia. HeatFab SS vent pipe with an elbow and a vent cap. As long as the CO2 doesn't increase more than .1 or .2 %... you should be fine.

Of course all of this will render warranty support from Lennox unlikely, in my opinion.

 

[DrRTU]

I doubt you will get Lennox to bless the design, this would open them unknown liability. The draft fan should be interlocked with the burner. I would hate to have a future mechanic by pass the draft fan and the FAI takes in high CO levels.

You should look into exhausto

http://us.exhausto.com/com_sys_casi.aspx

they may build a custom system that will meet you needs. It would be pricy, but they would shoulder the liability for proper draft control.

Tjernlund products are very good. A draft fan with the proper wiring, draft hood or barometric and you could have a simple system.

http://www.fieldcontrols.com/draftcontrol.php

 

[Dan1978]

Thanks for the information guys. If I had more time to design it I probally would have gone with one of the Exhausto systems as DrRTU suggested. This was for a church and local gas authority was threatening to shut off gas until a fix was installed.

 

[cry22]

I think, the authorities are concerned that if for some reasons the draft inducer fails, the system is back to square one.
In addition to some interlcoks mentioned here, you may consider suggesting additional safety interlock to the authorities.
If the inducer fails, the RTU is shut down automatically.

Good luck.