1 Damper 2 Furnaces?

[morepower7701]

My boss ask if two furnaces can control one damper independantly. I have not seen the job but was told there is one fresh air inlet that is piped into two seperate furnaces via the return duct. This air damper is to open, only when at least one of the two furnaces are running, does not matter which, and to be open when both are running. The furnaces are not twined, each have a seperate T-Stat and control different rooms. How can I control this damper with two furnaces, but not to back-feed to the furnace that is off?

Thanks, Keith


I'm an HVAC/R service Tech! Just have not got enough post to be pro yet.

[jeremyhall.tech.sc]

ive never seen it done that way Keith , but hey i have never piloted a plane either........

im interested in knowing WHY this is done in the first place?

seems to me that something isn't kosher.
why would one expect two separate furnaces to share a duct system , especially if they aren't twinned?

keep us posted.

[GOVHVAC]

Relay!

[y7turbo]

Use a 24v open, spring return damper actuator.

Use a current switch on the blower common wire for each furnace and wire the switches in parallel using 24 volts from somewhere. Just make sure the transformer has enough va to stroke the damper actuator..

When one of the two blowers, or both are on, it closes the current switch(s) and commands the damper to open..

nice and easy.

Im sure more people will chime in with different solutions, theres more than one way to skin a cat.

[maxster]

power the OA motor drive direct then add one relay per boiler 24V coil N.O. contacts in stacked series with the hot wire motor voltage either or will drive it on a heating call..if the boilers are commercial size you might have dry N.O. contacts within the control panel there

[morepower7701]

I could use two current sensing relays on the G wire of each furnace. When ever the sensor picks up current the relay would close and power the damper. This would allow for a seperate transformer to power the damper correct? I know there are avalible items from electronic suppliers, but has anyone seen one at the HVAC supplier? Would one of these sensor pick up the slight current going through the G wire?

Thanks again.

Would something like this work?
http://www.johnstonesupply.com/store...odb12-020.html

[jeremyhall.tech.sc]

i still would like to know why this set-up is being used?

[*TEEKAY*]

(2) relays, (1) actuator- preferably powered from a seperate power source than the furnaces. Relay coils enegized from G to C on each furnace, parallel n.o. contacts to actuator. Done

[Epicfail]

i still would like to know why this set-up is being used?

It is only for fresh air make up if I read the OP correctly. We don't the the age of the building, but this could have been installed to bring the building up to code (%) of make up air.

Could have only been enough room for (X) size make up air, so 1 duct 2 units, this is not at all uncommon in older buildings.

Reasons can be endless, that one just makes the most sense to me.

[suBcools]

I've only seen two furnaces on the same duct system if they were twinned. If they aren't twinned, wouldn't there need to be some sort of barometric damper, just in case only one of the furnaces are running? If only one was running, couldn't the supply air blow backwards through the furnace that isn't running? I could be wrong, but I could imagine that type of scenario happening, if there wasn't a damper to stop the air from going back through the furnace that's off.

[morepower7701]

(2) relays, (1) actuator- preferably powered from a seperate power source than the furnaces. Relay coils enegized from G to C on each furnace, parallel n.o. contacts to actuator. Done

That's it!!! I can't believe I did not think of that, I feel like a dumb

I was told the damper opens and lets fresh combustion air for each furnace, still have never seen the job.

Thanks

[y7turbo]

....

[GREGHVACGUY]

That's it!!! I can't believe I did not think of that, I feel like a dumb

I was told the damper opens and lets fresh combustion air for each furnace, still have never seen the job.

Thanks

If you are using it to bring in fresh air when the heat is on you need to wire it in to w-c not g-c it will only open when you have the fan turned on or the ac is on. I would think you need it for heat. thinking it for freash air to the furnace room.
you could put it on a curnant sensing relay on the blower motor to a relay for the damper.

[*TEEKAY*]

Yep, W and C, I misunderstood the intent of the damper

[lucky6man]

Can someone draw a diagram of this? and where s a good place to get said relays... link???

[ats890]

If the damper is for combustion air.What if the damper fails to open. Don't think dampers are allowed for combustion air by code

[syndicated]

There is a company that makes the product you need right out of the box.
The company is called Hoyme.
The part number is F2. You can get them in 4-8" round duct.
Very easy to install and prewired.
Wired in series with W on both furnaces, motor fails open to allow heat if something goes wrong.

Made in Canada, so it's bullet proof ;-)

[Tecman1]

Im a bit confused the previous threat of wiring it to the w-c would be correct to have it open for heating. It sounds like you are talking about combustion air and dampers if this is the case this is a life safty issue and you would really have to look at manufacturers suggestions. For makeup air in the boiler it is not unusual to have a tie in to the burner contact point on each boiler to open the dampers.

[Chris_Worthington]

My boss ask if two furnaces can control one damper independantly. I have not seen the job but was told there is one fresh air inlet that is piped into two seperate furnaces via the return duct. This air damper is to open, only when at least one of the two furnaces are running, does not matter which, and to be open when both are running. The furnaces are not twined, each have a seperate T-Stat and control different rooms. How can I control this damper with two furnaces, but not to back-feed to the furnace that is off?

Thanks, Keith


I'm an HVAC/R service Tech! Just have not got enough post to be pro yet.

As previously suggested last year when this thread was created (2) current sensing relays in series with actuator. An independent power source would be preferred in this application due to one or the other being shut down for what ever reason.

For all those that have suggested that it might be a combustion air damper and that you cannot use an actuator with a combustion damper, well you are wrong on both counts.

The combustion air wither provided from an outside air damper or a combustion air fan must be proven before the pre-purge takes place, during the run cycle and during any post purge cycle.

CSD-1 and others provide the guide lines here.

[air1]

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