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Community and Q&A

Energy retrofit: HVAC strategy?

Andrew Thompson Zone 3a | Posted in Mechanicals on

I’m planning an energy retrofit/remodel of my 2000SF single-story home in Dallas TX, involving substantial insulation upgrades and thorough air-sealing.

The existing HVAC is 20+ years old, and one or more components may be nearing the end of service. By today’s standards the components are not efficient. It’s a forced air system (natural gas furnace/AC). The 5 ton AC compressor is outside; the furnace and blower are in a conditioned mechanical closet; the ducts are not insulated and run in a vented crawlspace; return air registers merely vent to the crawlspace (no return ducting); the plenum pulls air directly from the crawlspace through a whole cut in the floor under the blower/furnace.

As part of the remodel I’d like to clear out the mechanical closet to make more room for the 2 adjoining bathrooms. There is no other conditioned space to house the mechanicals. The house roughly divides into (2) 1000sf sections: an open-plan public area, and a divided private area of 3 bedrooms and 2 baths.

The water heater will be replaced with an efficient exterior demand model. This is kind of sad, as my existing water heater, a Ruud Monel works fine and is original to the 1954 house; it’s actually quite beautiful, and deserves better than the mech-closet.

For HVAC, I’m considering a packaged unit that can be housed outside like the Trane XL16c or various arrangements of heat-pump mini-splits with multiple heads and/or limited ducting. Assuming the house will be tighter, I’m hoping to downsize the equipment, and also incorporate an HRV/ERV, if it makes sense to do so. The remodeled crawlspace will be sealed, though not necessarily insulated. There is a partial, low unconditioned attic, but much of that may be converted to cathedral ceiling.

Given the existing setup, and assuming new and efficient equipment, I’d appreciate advice/thoughts about how I might best go about improving my situation, or at least thinking about it.

Thanks.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Andrew,
    I can't believe that your forced-air system has been pulling 100% of its return air from a vented crawl space for over 20 years! What a nightmare. Your house has been continuously pressurized, at great expense to you. What an energy penalty!

    So, seal your crawl space carefully. Insulate the crawl space walls if you can afford to do it. Invest in blower-door-directed air sealing for your entire house.

    Then, go ahead and install a few ductless minisplits and an instantaneous water heater. Good luck.

  2. Expert Member
    Dana Dorsett | | #2

    I suppose that's why it took a 5 ton system to cool a 2000' house! The summertime latent loads in Dallas are gia-normous, and sucking in all of that humid crawlspace air is a total efficiency & comfort disaster.

    What Martin said, seal and even insulate the crawlspace walls to at least R5 no matter what (R10 is better). Vented crawlspaces bring more humidity INTO the house in that climate than they purge (as do vented attics, but we'll focus on the crawlspace first, since that's probably a much bigger hole in the scheme of things.) It's far easier to get good air seal at the foundation than anything you might do to the underside of a joisted floor with 1000' of subfloor seam and 100s of electrical & plumbing penetrations, and it can even take less insulation material to boot. At your deep subsoil temps leaving the crawlspace floor uninsulated is fine, but you DO want to put down an EPDM (roofing membrane) or 10mil poly vapor barrier.

    Unless you have a whole lot of west facing glass and nothing but R19 batts flung randomly around the attic (I'm assuming you'll use blown cellulose to at least code-min R30 after air-sealing the attic from the rest of the house), you would likely be able to heat and cool the place with about 2-tons of ductless, and possibly even less, believe it or not. The smallest Trane XL16c is a 3-ton, and probably going to be extreme overkill (like 2x oversized) if you do a decent job on air sealing & insulation.

    But head placement can still be an issue, and if it's not a fairly open floor plan it could get pretty expensive and run less efficiently if you need more than 3 heads to manage it. A careful room-by-room heating & cooling load calculation would tell you if that's a viable solution here or not.

    If you MUST go ducted, insulating the crawlspace walls & band joist to R10 is strongly recommended, as well as hard-piping all the duct work and mastic-sealing every seam & joint, and insulating the supply ducts. Daikin recently acquired Goodman, and there may be pretty good 1-2 ton fully variable speed ducted-split options coming out of that fusion soon. Ducted systems using Daikin's variable speed variable refrigerant volume outdoor compressor units with variable speed Daikin-nameplate/Goodman-technology air handlers are already being installed in my area (southern New England), but the jury isn't in just yet on how efficient they are compared to their super-efficient ductless cousins.

    Care to share what your construction type is (including siding & roofing), and estimated R-values/U-factors will be after the retrofit? How many square feet of window so you have on each of the N, E, W, & S sides, and shading-factors?

  3. Andrew Thompson Zone 3a | | #3

    Thanks guys for taking the time to reply. I knew my system was less than ideal, but didn't realize it was quite so far from the goal. Sadly, I suspect these types of set-ups (and their like) are not uncommon around here.

    Martin,
    This setup may very well have been pulling air from the vented crawlspace for even more than 20 years. That's just the age of the furnace and AC. The crawlspace ducting is not original to the house, but I believe it may be older than the current mechanicals. I've only lived here for 5 years so most of what I know is based on educated guesses.

    Can you explain how the house has been pressurized by the current setup? Are you saying that as the system is blowing outside air will get sucked in the crawlspace vents, and exit out the many leak points in the living space above? Since the air return vents send air back to the crawlspace, would that air not go back into the crawlspace again, only to be recirculated (and mixed with outside air) again and again, and so while inefficient, it is in a way sort of "balanced". Just trying to understand what my my system might likely be doing now.

    Dana,
    One thing I can say about my crawlspace is that it is bone, bone dry. No visible moisture regardless the season or downpour, and happily, no mold, no rot. The soil is the driest I've ever seen, almost like talc when disturbed. Not saying summer air wouldn't be humid given the relative lower temps of the crawlspace, but at least it's dry. That's about all I have to feel good about, so I need to brag about it!

    I've considered laying a vapor barrier and insulating the inside of the foundation wall. This will not be easy as the house is at two offset levels, so one space has about 3' vertical clearance and the other maybe 18". I also have a metal termite barrier around the interior perimeter a few inches down from the sill plate, so I'm not sure how I would install rigid foam around that, assuming it even works as intended. I haven't seen any termites or tell-tale signs, but they do exist here.

    Interesting that my AC is so grossly oversized. (Sadly) Not surprisingly, it can barely keep my house at a conservative 80-82 in the summer, hence my need to improve things. My plan for the walls is to remove the existing, shoddy brick veneer, sheath the 2x4 walls, add 2 layers of polyiso 4-5" thick (this will be the WRB plane), rainscreen, and cement siding. Sheathing and insulation seams and pentrations to be taped or otherwise sealed. The rim joist would also covered by this same setup. The windows, which are 20% of wall area, will likely be tilt-turn and fixed Intus triple panes (low U, low SHGC). Eventually, I may also fill the now empty 2x4 cavities with dense-pack cellulose. The house is single story with a 1:12 slope roof. The current partial attic is very low, perhaps 3' at the ridge and near 0 at the low end of the roof slope. So not much volume to blow in a uniformly thick layer or cellulose. The low slope also doesn't afford great venting, so I'm planning to just seal it up. We'd like to lift the drop ceilings in many parts of the house, so my plan was to furr-out the 2x8 rafters and fill the cavities/build-out will OC and or CC spray foam, rigid insulation or a combination of all of the above. Other ideas? The roofing will likely be white EPDM membrane.

    Interesting that you feel I could cool my house with 2 tons of AC. I get your point about head placement. I'll take a look at the Daikin systems you mentioned. I can't quite figure how to make it work in the bedroom/bathroom section of the house. The smallest heads seem too big for our 225 sf bedrooms, and since it is warm through the night, privacy won't really allow sharing air by leaving bedroom doors open. I've read a little about ducting options for minisplits but I'm unsure how they impact the inherent efficiency of the system. I've got a ductless Mr Slim heatpump/ac in my garage/shop and it's really nice. I love that it is essentially a closed, factory-tuned system, and the inverter drive means it runs efficiently at partial load, and not to mention extremely quietly.

    Thanks again.

  4. GBA Editor
    Martin Holladay | | #4

    Andrew,
    In your original question, you didn't mention return-air ductwork. It sounded as if the return-air plenum on your furnace was drawing 100% of its air from your vented crawlspace. If your system includes return-air ductwork, the ducts should be a closed system that connect the return-air grilles directly to the return-air plenum of your furnace.

    If, instead, the ductwork (if there is any -- this is still unclear to me) is broken -- and your open, ventilated crawl space is somehow part of the system -- than your house is, indeed, being pressurized by your HVAC system. Using a vented crawlspace as the source of your return air is equivalent (from the perspective of pressure dynamics) of using the great outdoors as the source for your return air.

    That means that 100% of the supply air being blown into your house is being used to pressurize your indoor environment. All of that air is leaking out through cracks in your building envelope.

  5. Andrew Thompson Zone 3a | | #5

    hey Martin, regarding my return air setup, the plenum is pulling from the vented crawlspace, and some return air is also sent (pushed by pressure?) into the crawlspace through return vents that open directly to the crawlspace. No return ducting was ever installed to take that air directly back to the plenum. So, presumably the interior air that does not leak out of other cracks, gets pushed into the crawlspace and freely mixes with entering outside air and eventually gets recirculated throughout the house. This is what I meant by "balanced" above. Does this sound like an accurate description of what's likely going on?

  6. Expert Member
    Dana Dorsett | | #6

    I'd BET the crawlspace stays pretty-dry- you're drying it out with the air conditioning!

    It's keeping up with the latent load of the gross air leakage from the vents, but drying out the crawlspace (and to some extent , the "great outdoors" is not exactly what your spending your power bill, to accomplish.

    If you insulate the crawlspace but don't air seal it and install return ducts you would incur some condensation on the supply ducts during high AC season. Rain & water/groundwater seepage will also add some to the moisture loads. Any time you seal & insulate a crawlspace a ground vapor retarder mitigates against both soil moisture and to some degree, radon. It'll usually stay dry, but sometimes incorporating at least a tiny amount of air exchange with the conditioned space is necessary. I would expect very little would be required to keep it dry with a ground vapor retarder, but it would probably need something if you skipped it.

    The ducted mini-split efficiency is still quite good when splitting the flow between a pair of rooms- the duct lengths are quite short compared to central ducted AC, so the increase in blower power isn't huge over what you get with a standard mini-split head.

    I'm happy with the Mitsubishi lineup for most low load applications, and it would probably cost far less than a ducted Daikin, if you can make it work with short-ducted mini-splits.

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