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Can I get away with radiant floor heat, some ducted dehumidification for summertime, and spot ERVS in a large-ish new home in Virginia?

WEem9nXKGU | Posted in Mechanicals on

I live in the foothills of the Appalachians. The house under construction has three-stories, 3600 sq’ of conditioned space, with a full bermed/walk-out basement, a main living floor, and a small top floor, including a great room with a large loft above.
I’ve tried to protect it from heat gain in the summer with shaded, low-e windows and foam insulation and sealing.
I’ll heat it with a hydronic radiant system in the floors, backed up by a gas direct-vented fireplace.

Here is what I’ve thought of doing in addition:

fight summertime humidity and bring fresh air to the basement with something like this ducted dehumidifier ( http://www.thermastor.com/Ultra-Aire-65/ ). I would duct this to the main floor upstairs as well, and use it as necessary.

gain additional control over circulation on the main floor with a few spot ERVs like this one ( http://www.efi.org/specs/whispercomfort_sheet.pdf )

resort to a ductless mini split A/C should I otherwise fail to keep the loft above the great room cool in summertime

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Neal,
    1. Low-e windows do not necessarily reduce solar heat gain. What you want is a low-solar-heat-gain window, not a low-e window. Every NFRC-rated window is rated with its solar heat gain coefficient (SHGC). The lower the SHGC, the better the window will perform in a hot climate.

    Some low-e windows have a high SHGC, while other low-e windows have a low SHGC.

    2. Whether or not your spacing heating, space cooling, and ventilation systems are effective does not depend on the equipment you choose. Simply listing the equipment choices tells us nothing. Your equipment could be grossly oversized, grossly undersized, or optimized. We have no way of telling from your question.

    If you are unsure of how to design your HVAC system, you need to hire an experienced energy consultant or engineer who can perform a Manual J calculation and design the appropriate systems to meet your needs.

  2. WEem9nXKGU | | #2

    Mr. Holladay,
    Thanks. Sorry to be so vague and sloppy with my query. The manufacturer of my windows calls their low SHGC (.18-.2) windows "lowE3"), thus making it easy for me to confuse the two issues when I describe them.

    I wrote to your forum because local HVAC installers have been giving me a very hard time with any plan that does not begin and pretty much end with central ducting of air conditioned by a heat pump. All deviations from that paradigm have been met with scorn and punitively high pricing on anything that smacks of green. Hence, my cry for help.

    In case you have patience for a slightly better specified query: I'm not so much trying to get anyone to size the equipment online (though I would be happy to get tips on that), as I am looking for a gut sense of whether this basic approach is crazy:

    ducted fresh/air and dehumidification throughout a bermed-in basement

    use of central ducting that reaches the main floor (but no higher) to circulate some of that air into a vaulted great room (dehumidified or not depending on my control)

    a few spot ERVS in parts of the house not connected to the vaulted great room or basement

    I'm hoping that with properly sized versions of such equipment, I can augment my radiant floor heat to keep air fresh and dehumidified but neither too hot or cold. But I'm having to negotiate with local contractors who have lots of experience and expertise but who appear to dislike anything that doesn't involve whole-house ducting and a heat pump. Any general thoughts, cfm specs aside? Thanks very much!

  3. GBA Editor
    Martin Holladay | | #3

    Neal,
    You're building a very big house. It sounds like you won't have much cooling -- just what you call "a ductless mini split A/C should I otherwise fail to keep the loft above the great room cool in summertime."

    If you size your ductless minisplit system to meet the cooling load of your entire house, your house will stay cool.

    If you size your ductless minisplit system to handle only part of your cooling load -- or only the cooling load in a single room -- you obviously can't expect it to keep your 3,600 square-foot home cool.

    If I'm guessing correctly -- that you don't want to have central air conditioning -- then that's fine. You probably know how hot it gets in the summertime in your corner of Virginia. People have lived without air conditioning for thousands of years, and there's no reason you can't, too.

  4. Armando Cobo | | #4

    Neal,
    You really need to follow advise in getting a Mechanical Engineer to design your HVAC system. Most HVAC contractors do not know how to do it right, and you need an independent 3rd party to design and verify installation. I would not work on any project that all these check and balances are incorporated.
    You are the one writing the check, make sure your HVAC system is design, installed and balanced properly. Also, in most cases you save money even after you pay the engineer.

  5. Armando Cobo | | #5

    Sorry, I just thought of something else. For a 3600 sf house, by the time you add 4-5 mini-ERVs and 2-3 AC mini-splits required for proper ventilation & cooling, I would consider installing a Hydro-Air system, where your boiler heats the water and the air-handler AND AC unit provide the cooling and make-up air requirements. Using high efficient equipment with variable speed appliances can be better and more cost effective. As you may be aware, you are not being presented with all, or at least better options.

  6. GBA Editor
    Martin Holladay | | #6

    Neal,
    By the way, what is it about your collection of equipment -- hydronic in-floor tubing, plus a boiler, plus a gas fireplace, plus a ducted dehumidifier, plus at least two ERVs, plus a ductless minisplit air conditioner -- that (in your words) "smacks of green"?

    In my mind, green means simple. You've got a hodge-podge of expensive equipment for a very large house.

  7. HDendy | | #7

    Neal,
    I'm in agreement with Martin and Armando. Since you are thinking/building "outside the box" it would be wise to have consultation from a specialist, and preferably a third party to your HVAC companies that aren't open to your ideas. I know of several (in my area of the NC Appalachians and elsewhere) that would be happy to consult long distance. For a little money and time upfront, you can feel more confident that the bigger ticket systems will work properly and maybe they'll have a less expensive/simpler idea to accomplish your goals.

  8. WEem9nXKGU | | #8

    Thanks all for the thoughts. A licenses third party will be key, I'm sure.

    My crack about "green" was meant only to indicate that I'd like to reduce consumption of electricity and loss of conditioning in long duct throws, and that this impulse has put me at odds with local contractors. The house is large, but I merely want to condition microclimates as needed rather than run a large, central system every time I want to adjust a single room. I don't mind (dehumidified) summer air in the high seventies and up, and rarely run my A/C even where I live now--in a poorly designed house full of skylights, unshaded clerestories, and lots of drafts. The house will be big, to be sure, but relatively easy to cool. My (former) way of being green was not to build at all. Now that I've given up that standard of environmental friendliness, I want to stay off of the power grid as much as possible.
    Thanks, all, for your thoughts.

  9. Jesse Thompson | | #9

    Neal,

    You've stated your house is already under construction, so there might be limited envelope changes still available to you, but for reference we're working on a house that will be built to passivehouse standard (smaller than yours, but not tiny) in southern Virginia that will be heated and cooled with a fairly simple combination of an ERV and small mini-split air source heat pump.

    The envelope will have triple glazed windows, 3" of rigid foam below the slab, R-40 ish walls, and an ~ R-60 roof. The peak heating load on this house will be under 20k BTU / hr, peak cooling much less than that, we expect dehumidification to be the dominating issue to deal with in the summer.

    In other words, if you build a high quality envelope in your climate your heating and cooling equipment will get much simpler and more affordable, and some of your complications will go away.

    Jesse Thompson
    Kaplan Thompson Architects

  10. Tim O'Brien | | #10

    Neal,

    Beware the claim that an ERV will perform all of the required dehumidification on your ventilation air. The tighter and more thermally efficient a house becomes, the more important ventilation becomes. You may be able to downsize your cooling and A/C mechanicals, but you will have to plan to mechanically ventilate your house. When you ventilate your house properly, the latent (moisture) load will be as high as a poorly insulated, loose house. Tightening and insulating do not (significantly) lower the latent load on a house.

    Now you have a high latent (moisture) load coupled with a reduced sensible (temperature) load. Your cooling equipment responds to temperature with humidity (latent/moisture) as a secondary driver. I have not seen a mini or multi-split system that does a good job removing moisture. You will likely need supplemental dehumidification to handle the ventilation moisture load as well as the moisture created by the house occupants and their activities (cooking, washing, cleaning, breathing). The Therma-Stor units are good for this.

    I am a fan of positive pressurization allowing me to condition my fresh air before it enters my home. I want to filter, dehumidify (if necessary), humidify (if necessary), and heat/cool (if necessary) my fresh air before it enters my home. Positive pressurization provides this ability. ERV and HRV only provide dilution of the indoor air since they are neutral pressure devices. Natural ventilation will still occur with a neutral device. On windy days your house will get plenty of fresh air from natural ventilation regardless of the design. On still days you will depend on your mechanical ventilation system.

    Many contractors do not share my opinions and will dispute what I have stated above. You would be well advised to seek an owner of a house similar to the one you are building and ask them about their satisfaction with the mechanical equipment and design.

  11. Riversong | | #11

    I am a fan of positive pressurization allowing me to condition my fresh air before it enters my home.

    Many contractors do not share my opinions and will dispute what I have stated above.

    Not only contractors, but building scientists as well.

    Positive pressure is a good thing during the air-conditioning season, as it will prevent uncontrolled infiltration of warm, humid air. But positive pressure is deadly to a building during the heating season, when it will result in exfiltration of warm, moist air leading to condensation, mold and rot.

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