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Radiant Floors and Air Conditioning / Filtration

AHTTAR | Posted in General Questions on

I’m sure this topic has been beat to death but figured I’d post it anyway in case somebody wants to smack it some more.  Whole house slab w/ radiant heat.  Wood stove is main heating source.  I need to add A/C.  This is a single pitch sealed roof assembly so not a ton of room for ducting but I can design in soffits. I’m just trying to find the least redundant system.  I was looking at Zehnder but the price tag seems out of whack compared to Panasonic and Fantech.  One of the biggest motivations for the system is to be able to run A/C and filter outside air during fire season.  In the winter we ventilate the house a lot opening windows/doors, but it would be nice to have fresh air automated.  Still not sure if HRV is better than ERV in Northern Idaho Climate zone 5b-ish.

As of now I’m leaning towards the following separate systems but I’d love to hear feedback from people that have gone through this:
–  HEAT: Radiant floors/wood stove
– A/C back up HEAT: 3 zone mini split
– HRV/ERV: Separate ducted system w/ carbon filter for fire season.
– KITCHEN: High CFM fan w/ makeup air.
– Bathroom fans?  Maybe HRV/ERV deletes these.

Thanks guys


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  1. Expert Member
    Akos | | #1

    Typical bedroom fresh air feed needs around a 3" duct. To condition and cool the same bedroom, you need somewhere between a 4" to 5" duct run.

    If you look at the cost of mini splits, each zone seems to come out to around $5k. So by going with the 3 zone setup, you are adding about $10k to your budget and saving about $200 in ducting.

    The best setup is to use a slim ducted air handler, run the fresh air supply to it and used it to distribute it to the house. In the winter you can run the unit in fan only on low speed which might add 20W to 30W to your baseline load. This will no only be much more efficient for cooling, it will be way quieter and much less maintenance down the road. The extra air filtering will also help with dust in the house. Make sure to select an autobalance ERV as these work best in this type of setup as they automatically compensate for pressure changes as the air handler flow changes.

    For filtering on your ERV intake, look for a 3 stage filter box (merv8+carbon+hepa) and install it on the outdoor fresh air supply to the unit. You want it on the negative pressure side of the setup as air leaks around it won't bring extra contaminants into the house. It does mean that it needs to be insulated, easy to wrap it in some rigid insulation.

    You don't need to have all 3 filters in there all the time, carbon filters especially have a limited life. Simplest to have a merv8/merv11 and maybe the hepa in most of the time and install the carbon filter only during fire season.

  2. AHTTAR | | #2

    Thanks very much for the reply, this is all great info and falls in line to what I was hoping to install. I guess the purists would say that the fresh air system needs to be its own separate ducting. My system would be used the vast majority of the time for clean air and at the most 2 months a year combined for A/C and heat.

    I've done 3 zone Fujitsu mini splits and they always cost about $4-5K with linsets then another $500 to get an HVAC guy to do final charge. Not sure if you're assuming an HVAC guy is doing everything for $10k. I have no experience with ducted splits, should I assume Fujitsu and Mitsubishi will be the best systems for cold weather also? Maybe Panasonic has one designed to tie into a system just as you described, I've heard their mini splits have gotten much better and their HRV's seem to be well regarded. Its an open floor plan with two bedrooms, I feel like I can have one handler in the main room and one in each bedroom. I assume with this setup you'd still do bathroom fans. I'm assuming the air handlers have supply and returns but I'll definitely dig into the tech this week.

    I feel like I should just save the money and go HRV if they're cheaper and easier to install. Seem to be a lot of opinions on this.

    1. Expert Member
      Akos | | #3

      Cost is definitely less if you DIY but still significant. Wall mounts are spendy and ducting is cheap.

      A wall mount in a bedroom for cooling is very bad idea as the cooling load might only be around 1500BTU which is way less than the smallest unit you can get. An oversized unit will tend to cycle a lot with blasts of super freeze followed by long waits as the temperature recovers. It also create comfort issues as there would be too much air flow.

      Most mini split manufactures (Fujitsu, Mitsubishi, Midea, LG) offer a slim ducted unit. If you are looking to use it for heat as well, make sure the outdoor unit has a base pan heater, some Fujitsu units do not.

      Except for the shape, these are not any different than any standard air handler. Some can be mounted vertically as well for a pretty compact install in the back of a closet.

      Generally there is no need to do a two zone setup with a simple layout like that. One unit can easily handle the whole place. If you want some extra temperature control over the bedrooms, you can do a bit of simple zoning. The idea is to over provision the bedroom zones say 120% of the flow required but adjust the closed position on the zone damper to deliver say 80%. Install a zone damper driven by a thermostat in the bedroom which will simply modulate the airflow a bit, no need for any fancy controls.

      Any auto balance ERV/HRV unit can be connected to the return on the ducted unit, they will all work well for this.

      Even if the fresh air supply is ducted to the air handler, you can duct the ERV stale air pickups to the bathrooms, kitchen area and laundry room. This would allow you to skip the bathroom exhaust fan for a bit of cost save. Make sure to install boost switches in the bathroom and in the kitchen area.

      Depending on how dry your summer/shoulder season is, an HRV might work better. I think that an ERV tends to be the best compromise in most cases as even that struggles to maintain indoor humidity in the very cold weather.

      1. AHTTAR | | #4

        Thanks. Yes, I've always used the models with the defrost mode. Though if they make cool only models that are quite a bit cheaper, I'd probably skip the heat. If its similar cost I'd just include it for multiple reasons. Conversely, not running heat through it would allow me to just run the ducts in the cavities of my unvented roof assemblies and not worry about condensation issues? Would the circulating fresh air in the winter be warm enough to cause issues? 6" Firestone ISO on the roof, R38 cavity batts.

        This is all good info on the slim ducted system. Sounds like you can pick and choose much more to tune the system properly. I like your style, simple is best. Layout attached if you feel so inclined to peek at it. I think 3 areas zoned as you described would be pretty simple though I have to figure a zone for the garage too. Garage has radiant heat and I would be too cheap to run A/C in the shop but it is my house buffer to the west afternoon sun. I think proper landscape design should keep it cool though.

        We're in a very dry climate overall. I was thinking HRV just because I've heard they're cheaper but I want to get this right for sure.

        Thanks again for your time. Definitely should fast trac my research a bit which is super helpful when trying to order everything 6 months in advance in this new world we're living in.

        1. Expert Member
          Akos | | #5

          The problem with running ducts in the attic is not necessarily about condensation, that is easy to deal with. The problem is you are poking holes through your air barrier. HRV/ERV fresh air feeds are pretty close to house air temperature, so you have to treat them the same way as HVAC ducts. Since you have exterior insulation, you can make your roof sheathing your air barrier, this is less of an issue but you are still not home free. Air leaks inside the ceiling cavity can still condense some of the time, so the best is to spray foam above the ducting in the rafter bays.

          With cold climate units, defrost by itself is not enough. Defrost produces a bunch of liquid water that can freeze in the bottom of the unit, so you need a base pan heater element to melt it. Most cold climate units do come with it, but good thing to check.

          You have a very sprawled out layout with jogs (which is not great for building efficiency) that makes ducting pretty hard without a basement or a crawlspace. I would put a ducted unit in the pantry to feed the living and bedroom and a 2nd ducted unit in the hallway ceiling for the office and bedroom.

          AC in a garage is a fantastic thing if you do work there, a budget wall mount on its own outdoor unit might be worth it.

          For most quality units HRV and ERV prices are pretty much the same. Make sure to get one with ECM blowers, these use about 1/2 the power which quickly adds up on something that runs 24/7.

          1. AHTTAR | | #6

            What do you mean make the roof sheathing the air barrier? I am using Zip on the roof for the first time which is a good air barrier but isn't the point to keep air from getting to the sheathing since that's where cold meets warm?

            Agreed on spray foam in cavities for ducting. Open or closed? Whenever I needed to put plumbing in an exterior wall I would leave the batts out and spray the cavity. This of course sandwiches the wall/roof sheathing so I wouldn't do huge areas.

            We're on the same page with defrost units, the pan is what I was talking about too.

            I'm not trying to win any passive house awards with the layout for sure but it should be very comfortable. I'm building this so my parents can come live with me for a couple of months a year, this allows us to not be on top of each other. The ceilings are tall so I should be able to create soffits and dropped areas for all my mechanicals, electrical and plumbing runs which makes it all much quicker.

            Yeah, I'll probably just rough in a line set for the garage and add it later if needed. I'd rather spend the money on a couple big shade trees, it doesn't get crazy hot.

            Thanks for the tip on the ECM blower. I'm heading to a "green" HVAC outfit in Boise tomorrow, we'll see what they have to say about all this. Thanks again

  3. RobLewisMadison | | #7

    Some manufacturers are starting to incorporate MERV8 or better filters into their ERV's (finally!), so might only need separate filter boxes for periodic HEPA/charcoal.
    I can say from personal experience that it's very important to get the duct and diffuser sizing right, especially if your ducting the ERV separately from any other forced air system. Every duct needs to be sized according to the air velocity and CFM demands for each room served. For example, ducts, diffusers and grilles that are too small could result in higher air velocities. This may be OK in cooling mode; air passing over your skin can improve evaporative cooling - and feel good up to a point. However, in heating season, higher air velocities can feel drafty and cold. Also higher velocities may increase noise in the ductwork and at diffusers/grilles.
    Conversely, ductwork and grilles that are too big may not properly distribute air throughout the entire room.
    Some thought might be given to conditioning/tempering the make-up air during winter months.

    1. AHTTAR | | #8

      Thanks. The hard part right now is just finding anybody that has any clue about anything other than conventional HVAC. They think mini splits are cutting edge. Does anyone know of a good supplier that can actually help put a system together and that will sell it to you if you're not a HVAC contractor? I know there are some online companies but I doubt they do any design.

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