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Community and Q&A

ERV with dedicated duct system

Bruce Davis | Posted in Energy Efficiency and Durability on

I would like to follow Martin’s advice about using an independently ducted ERV in my cold edge of Zone 4 new construction.  There will be two Carrier Performance line gas furnace/AC systems, one on the main level and one on the finished walk-out basement lower level. About 2300 sq ft of finished space on main level; 2000 finished plus 1000 additional conditioned on lower level, with tall ceilings on both.

The contractor thinks I only need one conventionally installed ERV (ERVXXSHB1100) tied to the furnace on the main level. He says the ERV has an intake & exhaust fan that is independent of the heating & cooling system. It will work even without running systems.  He has the same in his house and says it works great. I will have 2-stage furnace & AC with ECM fan.

One side note on using the ERV tied to main level. In the middle of the house is a very open staircase (16×9) which will probably mix air from both levels more than most houses.

The additional duct system would be $500, adding about 2-3% to the HVAC cost. I don’t mind the additional cost if it makes sense. Based on some of Martin’s numbers, seems I would make that money back in a couple of years with lower operating costs (although I may be overproducing PV electricity anyway–won’t know till it’s too late to change anything). Thanks.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Bruce,
    If you haven't seen it yet, you might want to read this article: "Ducting HRVs and ERVs."

  2. Trevor Lambert | | #2

    I don't think that model of ERV is big enough to handle a 5300 square foot house. Do you have an HVAC design specifying the design airflow requirement? Its efficiency specs are also underwhelming.
    https://hvi-1491.quickbase.com/db/bh6688vwb?a=dr&ifv=1&rid=7923&dfid=12

    It almost sounds as if the contractor's own system is relying on the the ERV fans to push air through the heating/cooling ducts without the use of the air handler. I'm pretty sure that's not right.

  3. User avatar
    Dana Dorsett | | #3

    >There will be two Carrier Performance line gas furnace/AC systems

    A pair of even the smallest 2-stage Carrier furnaces are likely be ridiculously oversized for the loads (even a 2-stage at low-fire) given the limited amount of conditioned space involved. Heat pump solutions probably make more sense.

    There are at five different lines of "Performance" furnaces, ranging from 80% efficiency to 96.7%. The smallest "Performance 96" 2-stager is 40,000 BTU/hr, which is by itself almost certainly more than the design heat load for the whole house, and a pair of them would likely be crazy-oversized for the whle-house load. The single stage Performance 90 Boost can be had as small as 26,000 BTU/hr which may be appropriate for the 2000' fully above grade portion, but still likely to be ~2-3x overkill for the walk-out basement. ASHRAE recommends holding the line at 1.4x load at the 99th percentile temperature bin. Going over that is usually less comfort and more money.

    Get an independent Manual-J from a RESNET rater or engineer (not an HVAC contractor) before going forward on ANY of it!

    >... I may be overproducing PV electricity...

    With an oversized PV array and appropriate sized heat pumps you can probably put a huge dent in the heating bill, or even wipe it out.

    If $500 is 2.5% of the total HVAC cost it means you're talking $20K for the whole shebang? I've seen houses of similar size & configuration come in at or under $15 for all-ductless multi-split or mini-split heat pump solution in locations with high single digit to very low double digit 99% outdoor temperatures, and those were as retrofits, which is usually more expensive. Even with a pair of appropriately sized ducted mini-splits it should be under $20K. Spending the $5K difference on an another ~1500 watts of panel on the PV to better offset HVAC energy use is probably the better deal.

  4. Jon R | | #4

    > independent of the heating & cooling system

    If you share ducts, the pressures and flows *aren't* independent.

  5. Bruce Davis | | #5

    The proposal is for:

    Main Level 2,300 sq ft - Carrier Performance 96 100,000 BTU gas furnace & Performance 17 4.0 ton AC.

    Lower Level 2,000 sq ft finished + 1,000 unfinished storage with just a little bit of heat/cool = 3,000 conditioned - Same models but 80,000 BTU & 3.5 ton.

    With one ERV, 2 humidifiers, 2 fancy stats, cost is about $23k. To upgrade to heat pumps is $3,500 more for two units. That would be the Performance 16 HP. My PV will be 12 kWh, generating about 16.5 kWh per year.

    My insulation will be JM R-23 blown fiberglass 2x6 walls (a few spots like garage cold wall are R-15 2x4). R-49 flat ceilings, R-38 cathedral. Closed cell in some places, un-vented part of the cathedral, rim joists, etc.

    Sounds like you guys are saying HVAC is grossly oversized. Contractor assures me he knows what he's doing without Manual J. They're the biggest new home HVAC sub in area, doing several hundred homes per year. I had bids from three other contractors, their sizing was the same or slightly smaller, but nothing major. Thanks for the help on this.

    1. User avatar
      Dana Dorsett | | #8

      >. Contractor assures me he knows what he's doing without Manual J.

      SURE he knows what he's doing- 180K of furnace and 7.5 tons of cooling., perfect for 5300 square feet of conditioned space, half of which is partly below grade. Right. That's a bit over 30 BTU per square feet of heating output and about 700 square feet per ton. That would be safe oversizing for typical houses without going totally nuts.

      WHAAAAT!?!!

      OK, that IS totally nuts- it's completely ridiculous. No, seriously- it's bordering on LUDICROUS!!

      How bad is it?

      As a sanity check:

      Just ONE 100K Performance 96 could heat...

      ...TWO of my 2400' sub-code 1923 2x4 framed antique with the clear-glass single panes + clear glass storm windows + 1600' of insulated basement (about 4000' x 2= 8000' total conditioned space) at an outdoor temperature of 0F, with margin to spare.

      Or it could heat just one of my house, (and not it's twin too) down to about -80F or a bit cooler outdoors. Is your house as l0w-R as mine? (I sure hope not, if it's brand new construction!) Add on the 80K furnace and I'd be good to under -200F, a temperature not seen in New England even during the last ice age.

      Assuming yours house is at least current code minimum JUST the 100K furnace could theoretically heat your house at a temperature below -100F.

      I suppose hell might be about to freeze over, who really knows, eh? :-)

      Typical air conditioning loads are about a ton per 1500' for above grade floors, and unless the walk-out is all west-facing slider windows the basement's cooling load is likely to be about 1-ton (for the whole 3000'.)

      Even a crummy leaky 2x4 house with clear glass single panes and R11 in the walls, R19 in the attic will usually come in at about a ton per 750' or so. So 4 tons of air conditioner would cover a ~3000' slab-on-grade house built to 1940s insulation, window & air tightness standards. Hopefully the upstairs is better than that(?).

      Adding another 3.5 tons for the basement zone is off-the charts overkill, probaly 4-5x oversized.

      Allison Bailes (blogs here regularly) runs Manual-J numbers as part of his services, and compiled this graphic plotting the square feet per ton against house size for dozens of actual houses new and old, large and small:

      https://www.energyvanguard.com/sites/default/files/styles/panopoly_image_original/public/square-feet-per-ton-air-conditioner-sizing.png?itok=vsJxOobH

      Even the lousiest house in his sample set between 4000-6000 square feet of conditioned came in over 1000' per ton. Only the lousiest of the houses below 2000 square feet was as low as your ~575 square feet per ton proposed for the upper floor.

      It's possible that the cooling & heating loads upstairs are that high, assuming it's a greenhouse(?). If it's a normal house with typical window/floor ratios it's more likely that the cooling load upstairs is less than 2 tons, and the 99% heat load is less than 25,0oo BTU/hr, could even be 20,000 BTU/hr depending on your actual 99% outside design temperature. (Got a ZIP code?)

  6. Yupster | | #6

    I design hvac systems for a living and I guarantee you, there is no way to "know" without some form of calculation, usually either Manual J or fuel use calcs. Rules of thumb and guessing do not work. And yes, those are almost certainly oversized by a very large margin. I just finished a heat loss/gain calculation on a 3500 ft² 3 storey home in Ontario, Canada where the design temperature is -11°F. The load for the whole building was only 50,000 btu/h and it was just a standard code-built house. Also, without an accurate load calculation, you won't know how much heat/cool each room needs, making it impossible to properly design and balance the ductwork. From an energy and a comfort perspective it pays to look for someone who doesn't sell or install equipment to do your design.

    1. User avatar
      Dana Dorsett | | #9

      Why waste your time on Manual-Js when you can run online load calculators like this one in less than 30 seconds?

      https://www.inchcalculator.com/calculate-many-btus-needed-heat-home/

      ( That's probably how this contractor ball-parked it, at ONLY ~4-5 x oversizing "...just in case...". :-) )

      1. Yupster | | #10

        Haha, this is my favorite advice on that page "Homes that are poorly insulated may require 10% more BTUs to heat due to heat loss. Very well insulated and tight homes will require 10% less BTUs to heat since very little heat is lost." Who knew you could only swing things by 10% with insulation?

      2. Trevor Lambert | | #15

        That calculator overestimated my heating requirement by almost 15x.

  7. Yupster | | #7

    Per your original question, you can connect an ERV to your furnace ductwork but you will have no way to guarantee which rooms receive fresh air. It will probably be the ones closest to the furnace. Fully ducted is superior and if $500 is all it costs, that's money well spent. The hrv ductwork needs to be designed the same way furnace ductwork needs to.

  8. Bruce Davis | | #11

    Guess I know why it only costs $500 for the independent ductwork. It will only have one supply & one exhaust on each level. I guess 2 or 3 supply/exhaust on each level would be preferable. Is that correct? Thanks.

    1. User avatar
      Dana Dorsett | | #13

      At +5F outdoors, 70F indoors, with 2x6 /R23 walls and U0.30 windows even if it's 12' ceilings and a 15% window /floor area glazing fraction I doubt the 2300' above grade part of the house has a heat load of more than 30,000 BTU/hr, 35K tops, and it could be as low as 25K. That would make the 100K furnace "only" 3x to 3.5x oversized.

      Just for grins, run a loadcalc.net load freebie Manual-J tool calculation for the upstairs, using absolute tightest zero infiltration & ventilation assumptions (otherwise it'll oversize it by more than 50% compared to most pro tools using aggressive assumptions). Take your time. I'm sure it'll be an eye opener.

      With a wet finger to the wind and a WAG it'll probably spit out something between 35-45,000 BTU/hr of heat load, and 25,000-30,000 BTU/hr of cooling, which would not be a disaster if sized EXACTLY to whatever it comes up with. Oversizing by very much from there would probably be a mistake, and reality (or a professional Manual-J) will always be less than that.

      Basements are a bit tricker to get right, and you haven't specified the foundation insulation, but the load per square foot is going to be a LOT lower than upstairs, due to less window area, less above-grade wall exposure, etc.

      But seriously, hire somebody who makes their living & reputation on the accuracy of their numbers to do the full on Manual-J /D /S treatment on specifying the loads & equipment. And DO consider a heat pump solution. Most 2500' houses would do just fine at +5F with a 2-stage 3 tonner for both heating & cooling, (with perhaps a bit of heat strip for backup for Polar Vortex events.)

  9. Bruce Davis | | #12

    Looking back on some earlier notes with the HVAC contractor, I stated 575 sq ft/ton seemed high. He said they use 550-650 or 1,000 sq ft/ton for Energy Star homes. I just reviewed the Energy Star qualifications. It's the same as the 2009 IECC, which I meet or exceed. So, it would seem even by their own rule of thumb, they should be nearly halving the size of my AC equipment. Does this same rule of thumb somehow apply to heating BTUs?

    BTW, 1% Cooling Limit 93°F; 99% Heating Limit 5°F. Thanks.

    1. User avatar
      Dana Dorsett | | #14

      Some heating BTU rules of thumb:

      For a tight 2x4/R13 type of house with U0.35 windows would be about 15 BTU/hr per foot @ 70F indoors, 0F outdoors.

      For a tight 2x6/R20 house use 12BTU/hr per foot @ 0F.

      Scale accordingly up/down with your actual 99% outside outside design temp. eg: At +5F it's only a 65F temperature difference, so for a 2x6 house it's only 65F/70F x 12= 11 BTU/hr per foot of fully above-grade conditioned space. That would put your 2300' upper level at about 25,000 BTU/hr.

      If you moved your house to Minnesota in a location with a 99% outside design temp were -15F, that would be an 85F temperature difference and 85/70 x 12= ~15 BTU/hr per square foot, and your upstairs load would be about 35,000 BTU/hr.

      Move your house to the Fairbanks International Airport, with an 99% design temp of -40F it's a 110F temperature difference and 110/70 x 12 = 19 BTU/hr per foot, for a heat load of 2300' x 19= ~44,000 BTU/hr.

      Real houses will hit both above and below those marks depending on the particulars, sometimes by quite a bit if there's a lot of window area (or barely any window) but that's pretty close to the mid-range. Walls taller than 10' would add a bit too, but less than you might think unless the windows grow taller as well, but you can probably move your house to Fairbanks AK and a 100K condensing furnace for the upstairs would still be 2x oversized, not 1.4x.

      For cooling a ton per 1500' of conditioned space is the middle of the road, but it's really all over the place- a lot depends on the shading factors and window orientation. A huge "sunset view" unshaded west facing window can add a LOT. But a ton per 55o-650 is NEVER close for new construction, unless it's the world's crummiest duct design with the ducts and air handler up in a blazing hot attic above the insulation, with room to room pressure imbalances of 10 pascals or more (which can double the cooling load in some houses.) The ton per 1000 BTU/hr rule of thumb would have margin on most code-min houses with the ducts & air handlers inside the insulation & pressure boundary of the house.

  10. Bruce Davis | | #16

    Although we got slightly off topic, the system sizing info has been invaluable. Let me digress back to my question in post #11 about how many vents are needed on a dedicated ERV system. Thanks.

    1. Trevor Lambert | | #17

      You need one main intake and one main exhaust. You'd typically ally have a supply vent in each bedroom, plus one in each main living area. You'd have an exhaust vent in the kitchen, each bathroom, laundry room and maybe the utility room. There are different design concepts, but that's the most common.

      1. User avatar
        Stephen Sheehy | | #20

        Trevor - good advice. In addition, we built a 2x3x2 enclosure for the cat's litter box in our mechanical room, with its own dedicated hrv exhaust.

  11. User avatar GBA Editor
    Martin Holladay | | #18

    Bruce,
    You've gotten good advice from everyone who has posted. Trevor describes an excellent ventilation system -- but you can cut corners if you need to (for example, by skipping the exhaust grille in the laundry room).

    To summarize:

    1. Your HVAC contractor is an incompetent who doesn't understand equipment sizing. As a result of his incompetence, he has suggested grossly oversized equipment. For more information, see this article: "Who Can Perform My Load Calculations?"

    2. Trying to use space heating and cooling ducts to distribute ventilation air creates all sorts of technical problems, and the result is usually overventilation, poor ventilation performance, delivery of fresh air to the wrong rooms, extraction of stale air from the wrong rooms, and high energy bills. Your ventilation system needs dedicated ventilation ductwork.

  12. Calum Wilde | | #19

    Are there any Coles notes type of books that cover HVAC building codes? I have a similar book for residential electrical, I'd like to go over my HRV system ducts and correct as much as I can and I'd like to move the ducts out of the attic.

    1. Malcolm Taylor | | #23

      Calum,
      I bet you have PS Knight's very useful "Electrical Code Simplified". There is a similar one for the plumbing code covering most provinces, but nothing for HVAC, or the wider building code in Canada.

  13. User avatar GBA Editor
    Martin Holladay | | #21

    Calum,
    You might want to consider buying "Code Check 8th Edition," which covers "building, plumbing, mechanical, and electrical." Unfortunately, the web site says that this book is "temporarily out of print" -- whatever that means.

    1. Calum Wilde | | #22

      Typically that would mean it's never coming back, but I'll look into it. Thanks, Martin.

    2. Jaccen | | #24

      I believe there are still copies of the 7th edition on Amazon. I realize that it's not current, but there is still a lot of carry over.

      Taunton shows it back November 20th:

      https://www.tauntonstore.com/building-codes/code-check-eighth-edition.html

      "AVAILABILITY: PRE-ORDER
      THIS PRODUCT IS NOT YET AVAILABLE
      PRODUCT EXPECTED: 11/20/2018"

      1. Calum Wilde | | #25

        Thanks

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