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Community and Q&A

Do I need a whole home dehumidifier?

Jamie B | Posted in Mechanicals on

Hey GBA,

Here’s a question that might be suited more for lennox guys. Unfortunately, asking high pressure sales reps from HVAC companies, well they just have no clue what I’m talking about.

So in my HVAC design, I am spec’ing an SLP98 Variable Gas furnace, XP14 AC and a Vanee 60H+ ERV, and the S30 Smart thermostat. I will do a 4 zone system.

I was thinking about what to do for humidity control.

I looked at the Ultra-aire split system, but that’s an extra install cost and I’d end up having two coil units on the roof. I am thinking of going with the Lennox integrated Humiditrol unit, which is technically a split system where it would integrate above the air handler unit and run off the AC lines.

However, some rep once told me that with the S30 thermostat, it can control humidity by itself using only the AC unit. However, I think I would need one of the smarter AC units to be able to integrate with the S30 Thermostat. (which increases the cost)

So here is the question: Do I need the humiditrol unit, or do I need a smarter ac unit?

If anyone has experience with this, sharing your experience with lennox products (especially the S30 thermostat) would be great.

Jamie

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Replies

  1. Jamie B | | #1

    BTW, I realized my typo in the post title. lol I can't seem to change it.

    [Editor's note: I just changed "humidifier" to "dehumidifier" -- I assume that is the typo you are talking about. It's not entirely clear, however, what type of humidity control you seek, since it's possible to buy either a whole-house humidifier or a whole-house dehumidifier. I hope I guessed correctly.]

  2. Steve Knapp CZ 3A Georgia | | #2

    Jamie,

    Where are you located? Are you planning to use the same ducts for HVAC and ventilation?

  3. Jamie B | | #3

    Hi Steve,

    I am located in Toronto Canada.

    Yes I was planning on placing the ERV inline with the return plenum in a simplified standard system. Not ideal for efficiency but space is tight and I don't have room for dedicated ERV ductwork.

  4. Steve Knapp CZ 3A Georgia | | #4

    Jamie,

    It sounds like you are trying to address a problem you may or may not have (high humidity). If your home does have periods when the humidity level is too high, the simplest solution might be to use a standalone dehumidifier. But I am curious how you arrived at the design for your HVAC system. Did you calculate a Manual J to determine heating and cooling loads?

  5. Jon Harrod | | #5

    Hi Jamie, I have some of the same questions that Steve does. Since you bring up cost, space constraints, and high-pressure sales people, it makes me wonder how you arrived at the need for four zones. Are there opportunities to simplify and right-size the system that could offset the cost of better controls?

  6. Jamie B | | #6

    Hey guys,

    I designed the HVAC system myself. I'm no HVAC engineer, but I have a good understanding of physics, mechanical systems design, interior and user experience design.

    I wasn't tricked into a zoned system, the sales guys don't even know what a zoned system is. What they try to pressure me into is to buy the same dumb system that everyone else gets and ends up having temperature and humidity issues with.

    I decided to implement a zoned system in the design so I can better manage the temp on each storey (you know, instead of duct taping over diffusers). Each storey has a significantly different heating and cooling load: different window areas, different inbound sunlight characteristics and Shadowing from the other buildings, some storeys being attached to other houses on both sides (it's a row house) and some storeys not being attached (the 3rd floor addition), physical factors, etc.

    I personally don't think the zoned system "should be" that much more of a cost, its a difference of 4 zone dampers and module. I'm already getting the variable furnace and the S30 fancy thermostat regardless.

    The manual J I did is pretty basic. Instead of trying to design the shit out of it, deterministically size the ducts to the design to later find out its still not good, I figured having a system that allows me to field adjust the parameters is a better solution for getting the comfort level I'm looking for.

    Steve, you are correct in that I am trying to find a solution to a problem that I don't know exists yet. But hey, this is what design is all about, you have to think about things in advance and decide what needs to be done now and what can be done later. This house is in Toronto, we get quite a bit of humidity here 3 out of 4 seasons. I just assume humidity will be an issue. I don't know how much a non-programmable non-variable air con like the XC14 in my particular scenario will dehumidify.

    To be honest, I'd hate to not put some sort of humidity control in place to later find out I have to retrofit a standalone dehumidifier unit in (which is energy inefficient and contributes to the total heat load), and there is no way I'm going to retrofit a split system like the ultra-aire after the walls are up. And Like hell if I'm going to get those portable wall plug-in units that you have to dump the water tank down the toilet 2 times a day.

    So question comes back to: do I get the XC20 variable air con unit to dehumidify through the S30 smart thermostat, or stick with the XC14 one stage air con and get the humiditrol module?

    Thoughts?

    Jamie

  7. User avatar GBA Editor
    Martin Holladay | | #7

    Jamie,
    I'll add my voice to the side that is skeptical of this design.

    1. Zoned forced-air systems can be problematic, because you have to maintain minimum airflow over the coil, even when only one zone is calling for conditioned air. That raises the question, "What do you do with the extra (unneeded) airflow?" Result = compromises and work-arounds.

    2. You shouldn't need a whole-house dehumidifier in Toronto. (That's what your air conditioner is for.) Whole-house dehumidifiers are energy hogs. For more information, see All About Dehumidifiers.

  8. Jamie B | | #8

    Thanks Martin,

    1. I believe the variable air handlers like the SLP98V are designed for zoned applications. At least with this model for heating it has a modulating gas valve, I will assume low air flows are properly matched with low gas. But you bring up a good point with the AC coil. Would low air flows in Cooling mode be matched with low refrigerant from a variable AC condensing unit? I'll look into it.

    2. Thank you for your advice. I think I'll just upgrade to the variable AC unit and let the "smart" Tstat manage the humidity. It makes for a simpler setup and install. If later on I decide I still need a dehumidifier, I can retrofit an Ultra-aire 70H, which at 2.4 L/kWh shouldn't be too bad on additional energy consumption.

  9. gsxr | | #9

    Hey, not sure if you went ahead with this system but from what I am told, the S30 cannot dehidify with zones. I have a similar issue as you, different levels with drastically different heat/cool demands and humidity problem. Curious as to what you landed up getting to solve your issue.

  10. jrpritchard | | #10

    Jamie - I am a mechanical contractor in Iowa and we have sold Lennox equipment for 65 years. I am currently designing my own HVAC system as well. The S30 T stat is a great option but will no truely dehumidify - it has the ability to run your equipment at various level to maintain a ‘feels like’ temperature. If paired with an outside sensor it will monitor inside temp and humidity, outside temp and humidity and will your vary your output to try to always make your house ‘feel like’ a certain temp. Their thinking is that if you can’t get humidity low enough it will overrun the set point on cooling to make it feel more comfortable. The opposite is also true - if you can maintain 45% RH, 74 may actually feel like 72. In Iowa we generally only have high humidity problems in the house during the shoulder seasons when our condensers are not running and during these times your ERV will actually increase your humidity a bit if it is set for continuous run. To compensate I like to use a floor mounted dehumidifier that is ducted thru your return similar to the way you are doing your ERV. They are very rarely needed in my zone but have done a few for customers with success.

    I would be very careful with a four zone system and a single stage AC. If you choose the Lennox Damper control module paired with the the S30 you can set zones that are not calling to ‘leak a bit’ to allow unneeded air to bypass the zone that is calling - in the event that only one or two zones are calling. If you choose a lower end zone controller or go with a different brand like Honeywell you will need a barometric bypass damper which is not advisable IMO. I would also look at the 296v drive furnace rather than the SLPv. Same V drive blower motor - much simpler, mich cheaper upfront cost and much cheaper to maintain.

    What is going to be critical your your place is to have a manual J block load done first so you can size your equipment. After that a room by room manual J and and manual D is necessary to make sure your zones are functioning properly. We do our own Manual J S D and T but in my area you could
    Have it all outsourced for approx .15 per foot - well worth it to ensure things are done correctly especially on a complicated system like yours. One more thing - if you are worried about humidity make sure the returns are sized correctly for a zone system. When we trouble shoot zones system that are not working the major problem we find is that the return is a single zone for he entire house - whenever a single zone calls for supply air - return air gets pulled from the entire house. More often than not to much air is getting pushed to the zone and it is no longer balanced putting that zone in a positive pressure which makes dehumidification even harder and the rest of the house is a negative.

    Hope his helps - Jeremy from Iowa

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