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Use ERV ducting for possible future forced air furnace?

Jim Sweazey | Posted in General Questions on

Climate zone 6B.
I have gotten great advice here pertaining to heating my new construction high performance house. I am confident that mini-splits will provide enough heat for us when used with the masonry stove. I think I will plan on additional electric back up for an office and a bathroom for comfort. Many, respected, local HVAC people think I’m wrong.
My current dilemma is that I might be building a house that, when the time comes, would be difficult to sell. We have a climate with more heating degree days than almost anywhere else in the world. This makes building a high-performance house a logical choice. It also makes decisions of all systems more demanding.
This brings me to the need for a heating back up plan. I do not have a problem with starting a fire and maybe using some electric room heat in the coldest weather, but most home buyers would expect to set the thermostat and forget about it.
I have two Panasonic Intelli-Balance 100, Cold Climate ERV 50-100 CFM – FV-10VEC1. The house is slightly larger than one unit can handle, and I thought I could just run one of them most of the time. Everything we would need is on one level with guest bedrooms upstairs and an unfinished basement. (basement and 2nd floor on one ERV and main level on the other). Fully dedicated ducted installation.
The current backup plan would be to oversize the ERV ductwork, then if the mini-splits prove to be inadequate or the next home owner wanted a furnace for ease of operation, a 30,000 BTU forced air furnace could use the existing ductwork.
On the surface this seems like a logical option but the more I think about, the more I question if I’m not creating more problems than I’m solving. 1. Would oversized ductwork be difficult to control flow to each floor and room? 2. Would the stack effect create problems with the ERV supplying the basement and second floor? (I thought the second-floor air could come out at the floor and the basement at the ceiling, but it would still be a difference in height of around 12 feet. 3.Can two ERVs provide fresh air to one furnace? They would cycle independently and if run constantly would need to run the furnace fan all the time, so the fresh air would flow the correct way. Right?
Thanks for the continued support. These are important decisions.

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Replies

  1. User avatar
    Dana Dorsett | | #1

    You're overthinking this (by a quite a bit!) Designing YOUR house for what you think a subsequent owner MIGHT want to do is just silly.

    But first, your heating degree days even on the cold edge of zone 6B are hardly "...more heating degree days than almost anywhere else in the world." By definition it's no more than 9000HDD Fahrenheit (5000HDD Celcius). More than half the population of Russia, and ALL the population of Mongolia, as well as many people in Scandanavia live in colder climates than yours. See:

    http://cib.knmi.nl/mediawiki/images/a/aa/Europeantemperature_HD17_19611990_v6_0_nc_baselayers_world_polygons_HD17_baselayers_overlay.png

    By designing a high-performance building envelope the capacity of the heating systems required for keeping the place drops considerably, but that doesn't mean using the same ducts for heating & cooling as those used for ventilation makes sense. One of the fundamental design goals of PassivHaus is to be able to do exactly that, using modest amounts of resistance electricity, but if you have ductless heat pumps of adequate capacity and some resistance heaters to cover 100% of the design load of the doored-off rooms there's no point to a separate ducted heating system, or running the HRVs ducts to supply 100% of the design heat load.

    It MAY be worth adding resistance heating coils into the duct systems sized to deliver air <=140F at the registers, even if at the maximum cfm of the HRV it doesn't add up to 100% of the design heating load for the ventilation zones covered. A flow of 100cfm at a 70F rise (70F in, 140F out) is about 7500 BTU/hr (2,200 watts) , x 2 = 15,000 BTU/hr, which is probably a large fraction of your design heat load, maybe even the whole thing. A 30K furnace might heat the place quicker, but not necessarily better, and if only used as backup for extreme days when the heat pumps might not have sufficient capacity, 4000-5000 watts of resistance heating in the ventilation system would still deliver half what the 30K furnace would, without creating extra problems and expense for yourself.

    Balancing two or more HRVs into a shared, oversized heating system ducts is a fools errand, not worth contemplating.

  2. Jim Sweazey | | #2

    Thanks Dana, I have read and should have said that Minneapolis is possibly the major city in the world with the most HDDs. They did list a city in Russia that could be the "winner". I think they were talking about heating and cooling.
    I did a quick search for a resistance heating coil but didn't find any plug and play offerings. Would it be added to an air handler? Do you have a link to more info. I can search more latter today as well.
    "Balancing two or more HRVs into a shared, oversized heating system ducts is a fools errand, not worth contemplating." Is this because of the concerns I asked about? Do you think it's a mistake to use two ERVs with separate ducting as described?
    And yes, I do tend to over think most of this project. It's by far the most expensive undertaking of my life, I don't think it's wrong to consider the value of the investment. So far we have had a few small setbacks but overall we have been successful. This impasse over mechanicals has us at a standstill.

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Jim,
    Heating ducts are big. A single room might require ducts that can deliver 200 cfm or 300 cfm, and a house might need a duct system that can deliver 1,000 cfm.

    By contrast, ventilation ducts are small. An entire house often needs only 60 cfm to 100 cfm of ventilation. So, no -- it doesn't makes sense to design your ventilation ducts to be big enough to be used for a non-existent forced-air heating system.

  4. User avatar
    Jon R | | #4

    IMO, it's smart to at least consider resale value when designing a home.

    Why isn't baseboard resistance heat plus mini-splits an easy "set and forget" design for future owners? And a system that mini-split skeptics would agree would always work?

    I don't think that an HRV using large ducts would generate sufficient pressure to overcome stack effect.

    You could use a hydronic system (like Chiltrix) with fan coils. It will probably cost more.

  5. Jim Sweazey | | #5

    Thanks,
    Martin, I knew about the size difference but until recently didn't think balancing the system was impossible.
    John, we considered baseboard resistance heat but feel it can collect dust, is uncomfortable, and not appropriate for the value of the house. The radiant cove heaters mentioned in another post might be a better option. It started out as small and efficient and grew to be large and expensive. Considering ROI is for my children. When the house is resold I'll be in a place where I won't worry about it any more. Could be a very HOT place. LOL

  6. User avatar
    Dana Dorsett | | #6

    Greater Minneapolis may be the coldest metro-region in the US with over a million people, but it's hardly WORLD-class in that regard. Novosibersk has over 1.5 million people, St. Petersburg over 4 million people, Moscow is on the order of 11-12 million people (dwarfing the population of greater Minneapolis/St.Paul, with twice the population of the entire state of Minnesota! ) All of those cities are in locations with more than 9000 HDD (some more than 10,000 HDD.)

    Using two HRVs on separate duct systems is fine. Having two HRVs on the SAME duct system is probably impossible to balance. Since you're dividing the systems by floor it may zone reasonably well, installing up to 2500 watts of resistance heat into a duct system running 100 cfm is fine. (I believe Zehnder has 200cfm HRVs with 2500 watt resistance heaters & controls already on board.) Put it on a wall thermostat for the next owner.

    As JON R points out, wiring the house to be "baseboard heater ready" would be a fairly inexpensive that won't screw up your ventilation function (or require higher ventilation rates during the coldest periods.)

  7. Alan B | | #7

    So design for baseboard resistance or your favoured system but don't install them until you sell. Design the locations and install the electrical and needed wires and connect them to a fuse that is kept off. If its a forced air design where the ducts could go and make it easy to open a few walls sections and install. If you sell someday buy and install and in the meantime enjoy non dust collection.
    All that said people buying houses assume the heating was already designed properly, its unlikely you will get questions unless your area is extensively populated by failed building scientists

  8. Malcolm Taylor | | #8

    Jim,

    I specced radiant cove heaters for a house I designed. Neither the owners nor I notice any appreciable difference between the heat they supply and regular baseboards. They make furniture placement easier, but in turn are very noticeable. Considering how much more they cost, I would hesitate to recommend them again.

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