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Can whole house dehumidifiers be “daisy-chained” to give higher levels of dehumidifying?

Terryssr | Posted in General Questions on

I live in the humid mid-Atlantic region. For health reasons, I have a “balanced” “fresh air” ventilation HRV integrated into my home HVAC system. We also added a “whole house” Dehumidifier system [like all such residential systems, it is a bypass system instead of acting to dehumidify the ENTIRE return air stream] consisting of a Honeywell DR90 [max. 90 pints/day]. Our local “expert” HVAC service company recommended the DR90 as being max. size appropriate for our 2k sq.ft. house–in hindsight, I wish we’d gone with my instinct which was to get the max size possible in order to dehumidify adequately with an HRV system running.
The Honeywell DR90 is ducted so that it captures part of the house central return airflow & the HRV incoming air from outdoors. Here’s the problem that generated my question about whether “daisy-chaining” 2 dehumidifiers is feasible & appropriate & capable of dehumidifying MORE pints / day than the single dehumidifier.
During humid parts of the year, with the dehumidifier set to constant “run” in conjunction with the HVAC fan set to run all the time, if the HRV is running & bringing in outdoor air, the current dehumidifier canNOT keep the house humidity under 50%–even if the HRV is set to only run at low fan speed & only 20 minutes out of every 60 minutes. In fact, on high humidity days, the house humidity would quickly soar to 60% if the HRV was operating. So on the most humid days/weeks, we’ve had to turn OFF the HRV in order to keep house humidity under 50%. For health reasons, & to avoid having interior humidity levels that may facilitate mold growth, it is essential for me to be able to run the HRV but keep the indoor humidity BELOW 50%.
Pretty thorough research & consulting with local HVAC service companies has informed me that the largest capacity residential central dehumidifier made [don’t want a portable unit] is 130pints/day. My Honeywell DR90 is very new, so I’d prefer to NOT have to dump it & replace it with only a 130 pint capacity new one [in part, because I am NOT confident that an additional 40 pints/day will be adequate for running the HRV & keeping humidity under 50% during the most humid times of year].


[While I appreciate your time & assistance: Please don’t use your time to debate whether it’s necessary for me to keep the indoor humidity below 50%–I have lots of knowledge about humidity & mold, and health reasons why it is essential for me to keep humidity below 50% when I am running the HRV. Also, for reasons I won’t go into at this time, an ERV is NOT a solution–explored that with experts & tried 1–an ERV will NOT improve the humidity situation during high humidity periods–simply not capable of removing any significant amount of humidity from incoming outdoor air.]
Thank you for your advice, insights, & info! Much appreciated.

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  1. charlie_sullivan | | #1

    You would be better off putting the two dehumidifiers in parallel rather than in series. If #2 has the output of #1 feeding into it, that air is already much drier than the air in your house, and #2 would only be able to squeeze a few more drops of moisture out of it, so you'd get very little additional moisture removal. If you put them in parallel, they are both working on the same ~55%+ humidity air and so they are both working as effectively as the one you have now is.

    I general, larger dehumidifiers are more efficient, so getting a single higher capacity model is worth considering. As much as it is frustrating to have to give up on a new lower capacity model, by the time you are done reworking the ducting and power supply to use two in parallel, it might not be any more expensive to get the higher capacity model, and you can either keep DR90 in reserve to run unducted if and when you need some additional dehumidification, or sell it on ebay and get some of your investment back.

    I 100% respect your advice not to debate the necessity of <50% humidity. This isn't a health advice forum. ERV might be a more appropriate topic to get our help with. ERV can't do the job of a dehumidifier, but it can be set up to help in conjunction with a dehumidifier. I won't get into details on that unless you express interest.

    Another thing you might consider is whether there are humidity sources such as a damp basement that are contributing to the dehumidification load, in addition to the outdoor air ventilation.

  2. GBA Editor
    Martin Holladay | | #2

    Depending on how the HRV is ducted, it's possible that (with the HVAC fan operating 24/7) you are pulling more outdoor air through the HRV than you realize (even when the HRV is set to operate at 20 minutes per hour).

    The first thing I would do is to get a home performance contractor who is skilled at making air flow measurements to measure air flow through the HRV when the air handler fan is on -- you might be pulling in way too much outdoor air.

  3. STEPHEN SHEEHY | | #3

    Terry: Following up on Charlie's last point, have you determined that the hrv is responsible for the excess humidity? Have you tried shutting off the hrv for a few days during humid weather to see whether the dehumidifier can then keep up?

  4. Tim C | | #4

    Generally speaking, no, running two dehumidifiers in series will not be effective. At the appropriate air flows, the dehumidifier is removing as much moisture from the air as it is feasible to remove. Excessive air flow would leave some moisture for a second unit to remove, but it would operate at a significantly reduced efficiency. It would be far more sensible to run two units in parallel. Or, in your case, where the source of humidity is an HRV, run a dehumidifier directly inline in the air intake, rather than rely on on the furnace blower to slowly circulate air to the bypass dehumidifier.

    At 90f, 60% humidity, the air contains 128 grains of water per lb dry air. This would roughly be an extreme high humidity for Roanoke, VA, or 0.4% design condition for Washington DC. 70f, 50% humidity air contains 55 grains water.

    Each CFM of ventilation will bring in about 108 lbs of air over the course of a day, which would come out to about 7,900 grains of water, or half a liter. So for dehumidifying 120CFM of ventilation, you would require a 60 liter/day capacity, or 80 liter/day at 160CFM.

    It would initially appear that your humidifier capacity is adequate unless you have significant ventilation rates. However, the dehumidifier capacity is determined at 80f/60% humidity conditions; it will be less if the air coming into it is already drier than that, so you may need a significantly higher capacity if not operating directly on the HRV incoming air.

    Eidt: of course, I forgot to at all consider the impact of air conditioning - if the air conditioner is sized perfectly, running at 100% capacity at that peak load, it will provide a significant amount of additional dehumidification. If it is significantly oversized, and the fan is running constantly at a high speed, it may provide little to no dehumidification.

  5. Terryssr | | #5

    Charlie, Martin & Stephen: THANK YOU ALL for your responses. To respond first to Martin & Stephen's q re whether excess humidity is coming from HRV intake of outdoor air or other sources: (1) I have NOT had measurements done with HRV off & HVAC air handler on to see if it pulls outside air in even when HRV off--may be good idea; (2) BUT, the HRV intake duct has a damper that auto-closes when HRV is off & it does close when it's supposed to, altho I'd guess it's not 100% tight; (2) basement is very dry as measured with hi quality Tramex meter & due to having had foundation excavated & footing drainpipes installed, plus other drainage to divert surface & ground water away from house foundation as much as possible; (3) when HRV is shut off during high humidity periods, the Honeywell DR90 does fine keeping house humidity well under 50% [40% or lower if I desire it]; (4) we have good kitchen & bath ventilation fans that exhaust to exterior whenever cooking or bathing. So it appears that it's high humidity outside air brought in by the HRV that is source of problem.
    Charlie: thank you for clarifying con of 2 dehumidifiers in series! As it appears that largest capacity residential units are only @ 130 pints, my concern is that if I buy 1 of those, it will still be inadequate for running the HRV during the high humidity periods of year [in our region becoming more frequent & higher humidity now that effects of global climate change are settling in!]. Running 2 in parallel is worth considering, but may not be feasible because of: (1) limited available space in house exposed main return air duct to fit 2 out-take ducts to 2 dehumidifiers, & even more restricted available space to fit 2 dehumidifier return ducts into the duct plenum BEFORE air goes into the HVAC for cooling or heating.
    I should have mentioned as well, that for medical reasons, I have a full-return airstream chemical filtration & particulate filtration unit [95%-not HEPA due to HEPA filter creating too hi static pressure for HVAC]. It filters all return air just before the return air goes into HVAC heat pump electric heat & AC unit. The dehumidifier is ducted to take out return air BEFORE the filter unit & dump the dehumidified air into the return air plenum on top of the filter unit.
    We have considered installing a "commercial" dehumidifier unit, as they have much larger capacity than the residential units [e.g., 150>200 pints/day commercial]. However, their operating specs are much poorer than the Honeywells regarding AIR INTAKE OPERATING TEMPERATURE RANGES. For instance, the Ultra-Aire commercial units' operating temperature range for air intake is ONLY 56>95 degrees, while the Honeywell units have an air intake operating range of 34>135 degrees! I have been told by my HVAC service/IAQ tech that this means the Ultra-Aire [& other comparable commercial units] would NOT operate well if the HRV intake air that is ducted into the dehumidifier is colder than 56 degrees or hotter than 95 degrees. If that's valid, then IT WOULD BE LUDICROUS TO INSTALL SUCH A DEHUMIDIFIER BECAUSE IT WOULD NOT DO THE JOB DURING THE HOTTEST OR HIGH HUMIDITY CONDITIONS which we frequently encounter here with either cold seasons with lots of rain, or hot season when--like last year--we had many weeks of very hot weather with lots of rain & high humidity.
    Charlie: re ERV vs HRV. We tried an ERV. It had 2 problems: (1) reduced humidity insignificantly; (2) the "paper" type cores grew mold, thus contaminating our indoor air with mold spores. I consulted with the ERV manufacturer's tech/engineering staff about why it wasn't helping much to reduce humidity of incoming outdoor air. They told me: (1) no ERV will significantly reduce humidity of incoming outdoor air when outdoor humidity is high; (2) the only way to avoid potential for mold growth on ERV cores is either to install ERV cores treated with antimicrobial chemicals, or switch to an HRV with aluminum cores. For medical reasons, I am unable to tolerate any materials treated with antimicrobial chemicals. Thus: we switched to HRV aluminum cores & have had no subsequent mold problems from mold spores in incoming outdoor air.
    I deeply appreciate & am thankful for this service provided by GreenBldgAdvisor & for the time & effort spent by you expert pros in sharing your knowledge & advice--deeply appreciative. Any other feedback & advice is welcome!

  6. Terryssr | | #6

    Tim C re your point about ducting the HRV intake duct directly into a dehumidifier. It's not exactly set up that way currently, but close to it. The HRV intake duct from the foundation wall runs into the HRV. Then it exits the HRV & is ducted into a part of the house return air duct system--DIRECTLY OPPOSITE the dehumidifier's intake duct. So it's almost the same as ducting directly into the dehumidifier. DO YOU THINK IT WOULD MAKE A BIG DIFFERENCE IF IT DUCTED DIRECTLY INTO THE HRV [the HRV does have a collar for doing so] instead of currently dumping directly opposite the dehumidifier's intake duct?
    I think I understand your example calculations. I am in VA suburbs of DC. When possible, I do ventilate at high rates [dependent on activity in house, whether outside air is polluted by fumes from neighbors' toxic chemical scented laundry exhaust or gas mowers, etc.]. My Lifebreath HRV is a model 195 [rated as 203cfm with external static pressure 0.1" (25Pa) & 155cfm @ 0.5" ESP (125Pa)]. I usually run it at an HRV fan speed of 2-3 [scale of 5]; and prefer to run it full on instead of 20 mins out of 60 minutes. HRV port diameters are 6" on the intake/exhaust side & 7" exiting the HRV & going into the house ducts. FYI: my 2K sq.ft. house has a 3 ton heat pump AC unit, which i was advised is proper for this house size.
    When I have the problem of too high humidity when running the HRV, it's typically days when outdoor humidity is 80%>100%.

  7. Tim C | | #7

    While the air handler is running at full speed, air is typically travelling down the return plenum at 1000 to 1500 feet per minute. 100 cfm from the HRV's 7" duct (setting the HRV to 3, and assuming the return is not drawing in significant additional airflow) is traveling at about 400 feet per minute. In these conditions, I would expect that the supply from the HRV is very quickly swept down the return plenum with most mixing occurring down stream. The two ducts may be just across the street from each other, so to speak, but the "street" is a busy interstate; it is not almost the same as directly ducting into the humidifier but in fact about as far from it as you can get.

    In other words, yes, I do think it would make a significant difference if ducted directly from the HRV into the dehumidifier.

    Regarding the AC sizing, even when the unit is perfectly sized on paper, it is still not uncommon to be 1/2 to 1 ton oversized in reality. The best way to determine the actual sizing is to configure a data logger to record actual run time/duty cycle and evaluate from that. I doubt that replacing a working 3 ton unit with a 2 1/2 ton unit would make enough of a difference in dehumidification to be worthwhile, but having the data available makes it much easier to make the best decision when it is time to replace. Your comment that the problem is on the days with the highest *relative* humidity (90f 60% humidity air contains more water than 80f, 80% humidity air) suggests to me that on hot & humid days, the AC is running enough to be able to adequately assist with dehumidification, but on milder days the part load dehumidification is inadequate (with the fan on, AC dehumidification generally drops to zero below 40-50% load). If you have a variable speed blower, reducing air flow across the AC below the standard 400 CFM/ton to 300-350 CFM/ton will increase the dehumidification from the AC which may help on those cooler but humid days (note that it will also make it more susceptible to freeze ups if it does not have the correct refrigerant charge).

    Note that the configuration you have your dehumidifier in (dehumidifying the return air) is probably the worst configuration for the AC to contribute to part load dehumidification. Running the dehumidifier output into the supply may also help improve performance (particularly if you have an ECM air handler running at minimum speed).

    edit: reducing the cycle rate on your thermostat (so that it has longer on/off cycles) could also help part load performance.

  8. charlie_sullivan | | #8


    Would you be willing to sketch your ducting configuration and either scan it or snap a picture of it and upload? I think that Tim has some great suggestions about how the configuration is less than optimal for dehumidification. I think I follow what he's saying, and I have a rough idea from your description of how it's configured, but a sketch might help.

    Sorry to hear you had mold growing on the ERV core. That makes it 100% understandable why that's not a good solution for you.

  9. Terryssr | | #9

    You guys are wonderful to take so much time to try to help! Hope I'm not over-imposing, but so far, local resources haven't been as insightful as you folks are being.

    Tim: I understand your point about the central air handler cfm overwhelming the dehumidifier intake cfm & probably pulling a lot of the HRV moist incoming outdoor air into the HVAC air handler. Good point! I'll have to consult with my local HVAC service company & see if we can figure out a revised ducting in the limited available space. THEY are the ones who designed the current ducting of the dehumidifier [they're MUCH better than previous "expert" HVAC service company that ducted the dehumidifier totally wrong & dysfunctional & "balanced" my HRV wrong so that the house was constantly in negative air pressure [several thou $ wasted].

    New Question raised by your suggestion Tim: if I installed a dehumidifier on the HRV's
    incoming air duct line, should the dehumidifier be installed BEFORE the HRV's intake duct reaches the HRV [i.e., the intake duct line running from the outside to the HRV] or AFTER the outdoor intake air leaves the HRV? Am I correct in thinking that the dehumidifier CANNOT be installed in a duct line position until AFTER the HRV air balancing location? In other words, where would the dehumidifier intake air duct LEAST adversely affect the HRV air balancing? Or is it irrelevant?

    Tim re dumping dehumidified air into the supply side of HVAC air handler: the problem with doing that is that then the dehumidified air does NOT go through my chemical/ particulate air filtration system. Not sure I see a way to solve that problem.

    Charlie & Tim: I will try to take some pics that clearly show the arrangement, but it's challenging due to limited space in utility room. If I can't succeed at that, I will try a drawing [ha! drawing skill-challenged!].
    [Wish I had local HVAC/indoor air techs with expertise like you guys! I've tried numerous; conducted interviews of virtually all companies in region--FEW have any experience with balanced ventilation systems, or even how to balance airflows; most either knew less than I did or said frankly that in the limited cases they installed an ERV or HRV system, they had too much trouble trying to keep units balanced, so they no longer wanted to deal with them! I am convinced my current company is best residential expertise available in my region. Even tried to hire experts on COMMERCIAL HVAC/ventilation to design & service my system--no takers: too busy with commercial work.]
    BTW: another reason I need the HRV operating full time: when it's NOT bringing in outside air, the house frequently goes into negative interior air pressure depending on outside weather, wind etc conditions. I've had numerous IAQ & "energy efficiency" companies conduct blower door tests, etc. & they all end up unable to figure out reason for negative air pressure & unable to "balance" the HRV airflows. Current HVAC service company has done the best job of balancing HRV airflow, but with HRV off, it's nigh on impossible to avoid negative air pressure in house if we run kitchen or bath exhaust fans. I'll post pics or drawing tomorrow. THANK YOU!

  10. charlie_sullivan | | #10

    An answer to your new question...without caveat that I don't know whether I understand the whole setup. The question of dehumidifier before or after HRV: It will have the same effect on the balancing either way, unless the ducts leak. So I think the consideration is where it has an easier time working as a dehumidifer. Assuming it's cooler inside than outside, the air will be cooler and so have higher relative humidity after the HRV. So the dehumidifer has an easier job located at that point. The disadvantage is that the heat it generates will all go into the house, which will increase your A/C load, but if the goal is the increase dehumidification more than the save energy, that's not not a problem--you'll get more dehumidification from the A/C if it has to run more.

  11. Tim C | | #11

    The problem with residential techs is that if you understand the science involved, you're usually qualified for other jobs that pay much better.

    The challenge with air conditioner dehumidification with continuous fan operation is the surface tension of water, and heat exchangers have a lot of surface - a typical 3-ton coil will hold about a quart of water before condensate starts running off in any meaningful amount. This is typically more than will condense in a single run cycle, so the coil need to be already wet at the beginning of a cycle to achieve any meaningful dehumidification. When you run the fan between cycles, water sitting on the coils evaporates, reducing the dehumidification, so at some point, the evaporation between cycles exceeds the condensation during cooling. If you can somehow work around that problem, you will substantially increase the effective dehumidification capacity of your system, but unfortunately, it is not an easy problem to solve.

    To answer your question, I would put the dehumidifier before the HRV core, so that some of the waste heat will be exchanged and sent back out side, increasing energy efficiency. But as Charlie notes, this does not necessarily maximize dehumidification.

  12. Terryssr | | #12

    Tim & Charlie, thanks for your continued responses--very helpful clarifications & suggestions. I will certainly understand if you've reached the point of not having the time to "donate" your expertise further on this. Charlie, I am posting sketches which i hope will clarify the ducting situation.
    Tim: re AC dehumidification with air handler fan always "on"--thanks for that explanation, but due to med stuff i have to have 24/7 fan operation & air filtration, so I'll have to live with inefficient AC dehumidification.
    Tim: re suggestion of installing a dehumidifier on the HRV supply/intake duct line BEFORE the HRV. The HRV intake insulated flex duct has 6" interior diameter as called for in HRV manual. The dehumidifier has 2 intake duct options: (1) a 6" diameter dedicated for HRV input, and (2) a 10" diameter standard duct collar for connection to house return air ducting. But the dehumidifier only has a 10" diameter EXHAUST duct collar. So if i installed a dehumidifier on the HRV intake duct line, on the exhaust end of dehumidifier I guess we'd have to install a 10" to 6" reduction duct to connect to the rest of the HRV intake duct going to the HRV. Would this create problems for HRV air balancing?

    Again, my deep appreciation for whatever time/advice you can share. You guys are terrific--wish I had you in my area.

    I know there's been lots of GBA posting & discussion about ventilation systems & their obvious necessity for high quality IAQ especially in "tight" houses. But after 10 yrs of struggling to find local expert advice & know-how on residential ventilation & air balancing in a humid climate, I have to say that at least with regard to RETROFITTING OLDER HOMES [with lots of unknowns re existing hidden moisture barriers/wraps, shell construction, etc], the theory is way ahead of the practical problems/obstacles & the know-how of most residential HVAC companies. If ventilation hadn't been essential for me for med reasons, I would have given up long ago as a home owner trying to have ventilation. I sure hope that with all the newer "tight" houses/buildings being built for energy efficiency, that there is on-the-ground real expertise on how to install & balance ERV/HRVs for residential!

  13. charlie_sullivan | | #13

    The diagrams help a lot!

    Installing the dehumidifier on the HRV intake (before or after) works pretty nicely IF you run the HRV near full speed, because the air flows are then well matched. If you run the HRV on low speed, the dehumidifier would want to pump the full ~200 cfm through anyway, making it imbalanced.

    So I think you want the intake on the dehumidifier to be a mix of HRV air and regular return air--which is what you have now--but as I think Tim said a while back and I'm just catching up to, you'd like the duct geometry to make sure 100% of the HRV supply goes into the dehumidifier, and the remainder drawn from the return ducts is only as needed, whereas now, only a small fraction of the HRV supply air goes across to the dehumidifier intake.

    Can you do that by moving the HRV supply duct to connect directly to the 6" port on the dehumidifier? Is that port unused now?

    Another thought: since the A/C dehumidification doesn't work well as it's configured now, at least not on mild days, what if you got a mini-split to provide A/C on mild days, and left the central A/C off (but with the fan running), and then only turned it on on hot days when the mini-split can't keep up? It's hard to say whether that's worth the cost, but the minisplit should dehumidify much better on mild days.

  14. Terryssr | | #14

    thanks, Charlie. Now i've got a lot of good ideas to share with my HVAC service tech. Best wishes & deep gratitude!

    1. andyfrog | | #15

      Hi Terry,

      This is an old post and hopefully you still have email notifications going on. I was interested to know what approach you took to integrating this larger amount of dehumidification capacity with your HRV.

      Additionally, I understand you mentioned skipping the ERV because it doesn't serve as a dehumidifier. However, I was wondering if you also chose an HRV over the ERV because you felt it would result in lower humidity, all other things held equal, e.g. if you had integrated multiple dehumidifiers with an ERV instead? Or if your statement about ERVs was purely to head off suggestions that you swap out the HRV and skip the additional dehumidifier, which I also agree would be ineffective.

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