Subscribe or Renew Membership Subscribe Renew

Humidifier in a cold air return

bergsteiger1's picture

I am looking to install a Honeywell HE360A Whole House Powered Humidifier in my home furnace however there isn't enough surface area in the supply duct coming out of the furnace to fit it.  Can I install it in the cold air return instead (where there is adequate space)?  What would the drawbacks be of this type of installation?  I have a Rheem direct-vent furnace running on natural gas at an altitude of 10,000'.  Thanks

You  could install an (post #200616, reply #1 of 18)

You  could install an unpowered humidifier -- the type that has a cross-over duct between supply and return.  Such a humidifier can generally be installed on either the supply or return duct.

But if you want to install a powered humidifier -- with no cross-over duct -- on the return duct then you need to supply it with hot water and use a humidifier rated for hot water.  AFAIK, the Honeywell units aren't rated to use hot water -- only some Aprilaire models are.

(One advantage of using the hot water setup is that you can rig the humidifier to run when the furnace isn't firing, to better handle "shoulder season".  Unfortunately, this requires using a relay setup that takes some technical knowledge -- your standard HVAC guys won't know how to do it.)


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

HUmidifier in a cold air return (post #200616, reply #2 of 18)

I just mentioned the Honeywell since that is the same one I have in my other furnace. I could certainly connect to the hot water, however I definitely lack the technical knowledge for the relay setup.

I am not familiar with the other type of humidifier.  Is that what is called a bypass humidifier? Do you lose some efficiency in your furnace by have a duct connection between the supply and return?

Can you please suggest a brand and model?  If so. I will take a look at it.  Thank you

Different standards (post #200616, reply #4 of 18)

Your "standard HVAC guy" would understand the psychrometrics of trying to humidify unheated air and would also understand the fultility of trying to "Rube Goldberg" something together to attempt to humidify when the need and the benefit are so low as to be practically non-existent (i.e. the shoulder season).

IF one had the technical knowledge to provide such an installation, they most like have the practical knowledge not to do so.

And our unit is rigged with (post #200616, reply #6 of 18)

And our unit is rigged with the relay I described.  Works great.

But I'm sure I must be doing something wrong, since I'm just a know-nothing idiot with no practical knowledge.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

I can't dispute anything you (post #200616, reply #11 of 18)

I can't dispute anything you posted here.

The answer is (post #200616, reply #3 of 18)

no, you can't install a powered humidifier on the return. I never recomend even bypass types, because of some of the moisture issues listed below.

The cooler return air will simply not hold as much water. Most of the water supplied to the unit will trickle down the matrix and down the drain. Say the return is 65 and the supply is 100. At 60% rh, the supply will "hold" over three times the amount of moisture (in well controlled commercial systems, the duct humidity is limited to 60% rh for practical and wetting reasons).

You will wet the filters on a regular basis and most likely the fan and blower compartment and also the fan timer (otherwise known as the control board). The blower could tolerate a little moisture but the electronics and the filters will not do well getting wet(ted).

It would be tough to humidify as the installation appears. Looks like new construction. Humidity an after thought here? Why is the furnace so high off of the floor (looks like the top is about 5'6' high)?

Interesting.  I have a (post #200616, reply #5 of 18)

Interesting.  I have a powered humidifier on the return trunk of our furnace.  It's supplied with hot water.  Never noticed any water leaking down, the air filter has never been wet, and the unit does a good job of humidifying the house.

What did I do wrong??


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

You installed a humidifier on (post #200616, reply #7 of 18)

You installed a humidifier on the return. Because you got away with doing something illadvised and have noticed no consequences, doesn't mean it was wise.

I installed the humidifier (post #200616, reply #8 of 18)

I installed the humidifier per the manufacturer's specifications.  Aprilaire specifically specs installing the humidifer on a cold air return if supplied with hot water.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

The fact that (post #200616, reply #12 of 18)

you "Never noticed any water leaking down" or understand the the tube/hose on the bottom of the unit is a drain and that all power humidifers dump water down the drain constantly while in use, further adds to your expertise in the field. Thanks for the "advice" Dan!

I assumed that no one would (post #200616, reply #13 of 18)

I assumed that no one would be idiot enough to believe that water running through the drain hose and into the condensate pumps would be considered "leaking".  I guess I was wrong.

Using a powered humidifier with hot water on the cold air return is one of the supported configurations for several Aprilaire models, but apparently you don't believe that the folks at Aprilaire know anything about humdifiers.

I installed the humidifier on cold air return of our old furnace, and when the furnace guys replaced the furnace they reinstalled the humidifier on the cold air return of the new furnace, without batting an eyelash.  But I suppose you think they were idiots.

Whose expertise is really under question here??


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Humidifier in cold air return (post #200616, reply #9 of 18)

That picture was from 2003 when the house was built.  I suppose the furnace, water heater, etc are on the platform since it is in a garage.  It passed all inspections, so I hope it meets code.  If I had it to do over again, I'd figure a way to put it on the house side of the garage, since the garage stays pretty cold.  Thanks for your input.

An option (post #200616, reply #10 of 18)

to consider if space is really limited is an electric steam generator type of humidifier. Not usually found in residential installations and most are very expensive. Nortec, DriSteem, Carnes and Greenheck might be worth looking into. I rep Armstrong and the smallest/cheapest is too expensive to consider for a house, in my opinion. The other manufacturers might have something to fill the bill. Good luck.

Powered humidifiers and best practices (post #200616, reply #14 of 18)

I have engineered home HVAC systems for eight years in Chicago, where humidifiers are the norm on forced-air HVAC systems.  Not once in literally hundreds of furnace installations did I specify a fan-powered humidifier be installed on the return air side of the system.  That is what by-pass humidifiers are for.  Better than 90% of those humidfiers were from Aprilaire.  I know that the instructions indicate that hot water can be supplied to the unit when it is on the return air duct, but I would never specify that configuration unless there was virtually no other way to install a humidifier.  That situation would exist where the supply air plenum was crowded with takeoffs for ductwork or the furnace was in a very tight location.

Bypass humidifiers do have a more limited capacity than their fan-assisted counterparts, so that could be a determining factor in opting for a fan-powered unit, even on the return side.  The problems I see would perhaps be moisture being drawn through the filter and the furnace blower and over the heat exchanger.  As one poster above mentioned, you could generate humidity without the furnace firing, but that would be of very marginal effectiveness.

A friend of mine just had a complete system changeout and his contractor put an Aprilaire model 700 (fan-powered) humidifier on the return air drop even though there was a brand new supply air plenum on top of the furnace.  They hooked the water supply to the hot side of his water heater.  He called me later and asked my opinion and I responded that I would request that it be changed.  He did so, and was advised by his contractor that Aprilaire approves of that configuration.  Why they did this without a reason I can't even guess.  The result will be increased water useage in getting the house humidified next winter.  Water useage by humidifiers is a significant factor in spite of what you might think.  It is very similar to leaving a faucet slightly open for months at a time.

In summary, I would suggest that most heating contractors would follow the above-described procedure of using a fan powered humidifier only on the supply air side of the furnace.  The only possible downside to this is the worst case scenario of the humidifier becoming clogged and the water might overflow and literally run down into the funace and cause rusting, or ruin the circuit board in the furnace.  This can happen, and I have seen it in cases of owner neglect or ignorance.  That slim chance of problems would not change my opinion of following the concensus best-practice.

Return side of duct (post #200616, reply #15 of 18)

I installed my wholehouse humidifier to the cold return side with a bypass duct to my supply.  I didnt have any room to install it on my supply side.  Is this going to work at all?  The bypass duct is getting air forced through it from the supple side which in turn forces warm air  to pass through the filter, is there any way to correct that my humity level wont go above 20%

Any time you use a bypass (post #200616, reply #16 of 18)

Any time you use a bypass humidifier you draw warm air from the warm air side and redirect it to the cold air side.  The unit itself can be installed on either plenum with no change in function.

It is normal for a bypass unit to have less humidifying capacity than a powered unit, as they typically move less air.  If the function of the unit seems  poor, however, double-check that the bypass duct is not kinked or otherwise blocked.  And, of course, make sure that water is properly flowing, that the "filter" inside the unit is getting wet from the water, and that your humidistat is properly set.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Return side of duct - two units (post #200616, reply #17 of 18)

We too had no room to install a humidifer on the supply side, so were told by our HVAC installer that we had to use units with no fan on the return side. We had two Goodman HE17 bypass units (rated 17 GPD each) installed  on the return side two days ago, where we did not have a unit installed on the furnace previously. Prior to this, I was refilling room units in each bedroom twice a day, plus console units once a day, for a total of 16GPD. We are measuring 24% humidity in most rooms, 16% in others, same as we did with the room units. 100+ year old house with icynene foam blown into the walls, and other insulation. Is there a way to increase the humidity?

Electrical engineer at a water treament plant in Chicago, IL

If  you are using 16 GPD, (post #200616, reply #18 of 18)

If  you are using 16 GPD, you have either a very large house, or you are leaking a lot of air. 

Do you notice drafts in the house?  Ever had a blower test done?  Are your heating bills high?