# Humidifier Between Furnace and AC Coils?



## raylo32 (Nov 25, 2006)

I don't think you can post pics until you have 5 or more total DIY Chatroom posts or somesuch. So bump this a few times or post the pic to Photobucket or your fave photo sight and link the pic.


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## BGChicago (Oct 24, 2015)

raylo32 said:


> I don't think you can post pics until you have 5 or more total DIY Chatroom posts or somesuch. So bump this a few times or post the pic to Photobucket or your fave photo sight and link the pic.


I did add it as an attachment as well, do you see it now?


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## yuri (Nov 29, 2008)

Why do you not want ti install it on the return?

If the narrow side is too small I take a 8x8" piece of sheet metal or whatever it takes to block the hole at the back of the humidifier and mount it on the skinny side and then install the humidifier over it and cut the return duct full size. Seal the back of the plate with silver foil duct tape or silicone or mastic.

It is not good to have humidity going to the underside of the AC coil as any dirt on it will turn to mud/plug it and it can corrode it. Not sure if there is enough to do so but it can happen in theory.

If you put it above the furnace on the hot duct and it leaks water inside the furnace you can blow the circuit board or motor and Kaching $500-1000 repair.


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## BGChicago (Oct 24, 2015)

yuri said:


> It is not good to have humidity going to the underside of the AC coil as any dirt on it will turn to mud and it can corrode it. Not sure if there is enough to do so but it can happen in theory.


Wouldn't that be almost the same when it is installed on the return though?


yuri said:


> If you put it above the furnace on the hot duct and it leaks water inside the furnace you can blow the circuit board or motor and Kaching $500-1000 repair.


Good point, the AC coil compartment has a drip pan, right? 



yuri said:


> Why do you not want ti install it on the return?


I read that installing it on the return is not as efficient as on the supply and it totally makes sense.

I don't think I can install a steam one either as the branch is right on top of the AC coil compartment.


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## yuri (Nov 29, 2008)

If it is on the return side the high pressure air from the plenum ( hot ) passes thru it and the moisture mixes with the air before going into the fan. It is more for spray or steam humidifiers that you don't want it under the AC coil. It is harder on the plastic body if on the plenum and I have seen them warp if it gets too hot. I would be more concerned about it leaking and getting into the furnace. There is no big difference in efficiency either way. Ideally you want a flow thru AND use hot water to it to get the most moisture. If you set them properly so it just trickles out the drain they don't waste much water IMO. Keeps them a LOT cleaner. My General Aire flo thru is 10 yrs old and as clean as day one.

I never install them on the plenum. If the velocity of the air going thru is too high it could blow the water off the pad and leak out the front or end up in the return. If installed on the return then the damper slows the velocity down B4 it hits the pad. Just my opionions though. I am sure other people do it on the plenum but only if absolutely necessary.


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## BGChicago (Oct 24, 2015)

yuri said:


> If it is on the return side the high pressure air from the plenum ( hot ) passes thru it and the moisture mixes with the air before going into the fan.


Why did you use "( hot )" ? That's the cold air? Otherwise I got your point of the high pressure and mixing it with the air. 

However, max distance between humidifier and filter will be 5-6ft (again, space restrain). I am concerned that the filter will "pick" some of the moisture and possibly causing more issues.


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## BGChicago (Oct 24, 2015)

yuri said:


> I never install them on the plenum. If the velocity of the air going thru is too high it could blow the water off the pad and leak out the front or end up in the return. If installed on the return then the damper slows the velocity down B4 it hits the pad. Just my opionions though. I am sure other people do it on the plenum but only if absolutely necessary.


This confuses me as heck. You advised earlier to install it on the return [plenum]. Now you are saying you never install them on [any?] plenum? :vs_worry: Where do you install them then ?


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## yuri (Nov 29, 2008)

The air from the plenum is hot air. Return air is cold.

You have a perfect setup. Install it on the large flat side of the return duct unless there is no room and then put the bypass on and pipe it across/in front of the vent pipe to the unit. Or cut a hole in the skinny side of the return duct and add a 8x8" galvanized sheet metal plate to support the unit. Cut a hole in that plate and mount it over the hole in the skinny side first. Go to HDepot and get a 6" top takeoff and mount it on the plenum. Get some 6" galvanized pipe and elbows and use it instead of the plastic flex pipe they give you.


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## yuri (Nov 29, 2008)

We are mixing words. The plenum is the hot duct or bonnet. The return duct is just that. In the US they may use plenum for the return but where I am we don't.


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## BGChicago (Oct 24, 2015)

yuri said:


> The air from the plenum is hot air. Return air is cold.


So it sounds like you are calling the "supply plenum" just "plenum" and "return plenum" just "return".

Anyway, thanks for the info. 

Any other takers on the by-pass installed on the return duct vs steam one, if even possible - see picture attached. Again, I read that it is not a good idea to install steam wand that close to the branch.


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## BGChicago (Oct 24, 2015)

yuri said:


> We are mixing words. The plenum is the hot duct or bonnet. The return duct is just that. In the US they may use plenum for the return but where I am we don't.


Yes, that is right. I think plenum=duct (or pipe if you wish). Where are you located?


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## yuri (Nov 29, 2008)

Winnipeg, Manitoba, Canada.

Steam is nasty and unless you got a very large and wide open space for it to blow into and mix with the air it can condense and leak into the ducts. It also uses a lot of expensive electricity. Good for huge plenums in huge houses but for the average Joe a flow thru hooked to hot water if possible works the best. We sell hordes of them where I am and it gets VERY dry in our extreme cold.

TruSteem from HWell has been a big problem unless you put a water softener in front of it and it has other "issues". We stopped selling them.


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## BGChicago (Oct 24, 2015)

Ok, I am crossing off steam.

Back to the return mount - I can't install on the wide side as there is no room. The skinny side is only 7". The water panel is 10" wide. If I understand you correct, you are suggesting adding some sheet metal to be able to hold the unit but what will happen with the 3" of that water panel that will be blocked as the whole can't be big enough?


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## yuri (Nov 29, 2008)

I do it all the time. There are enough cfms of air to pick up enough moisture with a flow thru as the water panel stays wet all the time. With the slow wick action of the 400 it may not be as good but if space is not there you got to do what you have to.

Air swirls when it moves and should cross the pad even with a smaller hole. Unless you have no floor drain or condensate pump I would use a flow thru. It won't overload a condensate pump if setup properly.


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## BGChicago (Oct 24, 2015)

I think I had missed an important detail of the flow-through bypass humidifiers operation:
_ " No matter which duct the humidifier is mounted on, the airflow always goes from the supply to the return through the humidifier."_

That means that no matter if it is installed on supply or return, the moisture always passes through the filter. Am I right?


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## user_12345a (Nov 23, 2014)

yah, air in high pressure zone moves to low pressure zone


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## yuri (Nov 29, 2008)

It does but I like the idea of the air shooting into the pad and if the velocity is too high or any drops of water get lauched off the pad they don't end up inside or on top of the furnace. With it mounted to the hot air plenum that can happen. I have seen water get blown off humidifier pads and it ain't pretty what happens where it lands. With steam boilers we call that "carryover" where water drops fly off with the steam.

With the humdifier body attached to the return duct it will land at the bottom of the duct and not do much damage. Especially with your setup the highest velocity will be below the AC coil as air leaves the easiest/closest opening first. After the AC coil it slows down.


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## beenthere (Oct 11, 2008)

raylo32 said:


> I don't think you can post pics until you have 5 or more total DIY Chatroom posts or somesuch. So bump this a few times or post the pic to Photobucket or your fave photo sight and link the pic.



Do NOT tell people to bump their threads. As we often delete the bumps, since its in the rules that bumping is not allowed.


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## beenthere (Oct 11, 2008)

BGChicago said:


> Wouldn't that be almost the same when it is installed on the return though?
> 
> 
> Good point, the AC coil compartment has a drip pan, right?
> ...


A bypass humidifier works as well mounted on the return as it does on the supply. Powered humidifiers don't work as well on the return as the supply, due to lack of heat to evaporate the water.

Mount it on the return. In case of over flow, it has less chance of harming the furnace.


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## BGChicago (Oct 24, 2015)

beenthere said:


> A bypass humidifier works as well mounted on the return as it does on the supply. Powered humidifiers don't work as well on the return as the supply, due to lack of heat to evaporate the water.
> 
> Mount it on the return. In case of over flow, it has less chance of harming the furnace.


Yeap, I already got that. I was sightly mixing them up. I think I am all set with the plan, will post install pics. I will try to somehow "extend" the current 7" duct size to take the 10" wight of the water panel.


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## yuri (Nov 29, 2008)

You can go to any sheet metal shop or duct cleaner company and they can make you a small piece of metal or may have patches they use when they cut holes in ducts when cleaning. Heck any metal scrapyard should have some used sheetmetal.

Cut a 7" hole in it. Then cut the hole in the return duct and line them up and attach it with silver foil duct tape. Then mount the humidifier on both with screws. Then silicone around any places at back with silicone or mastic or foil duct tape.


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## siegwam (Feb 19, 2017)

Anybody have thoughts on installing an Aprilaire 700 on the return side? The directions say this is okay to do. However, no one that I can find anywhere is talking about the furnace filter. Won't the filter take out most of if not all any humidification that the humidifier is pumping into the return duct? This doesn't make any sense to me to place a humidifier before the furnace filter. I know its water vapor. Can that vapor make it through a furnace filter? Just thinking I don't want to install this thing and find out that I can't get the humidity levels I want because of the filter. So far I can't find any info on this. Must not be a concern I'm guessing. Thanks.


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## beenthere (Oct 11, 2008)

The filter won't absorb the moisture. You won't get much humidification from it on the return though. It will also needed to be connected to the hot water line to get any humidification mounted on the return.

You would be far better off with a 600A, than with the 700 on the return.


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