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Summer cooling using ERV circuit?

rlangley | Posted in GBA Pro Help on

Just watched The BS* + Be”er Show: Hydronic Heating and Cooling” with great interest.  I am building a (nearly) passive house (a tear down to foundation and complete replacement) and hoped to not need an air handler with its associated ducting.  Clearly radiant (going with ceiling radiant) is the way to go for heat but the risks for cooling are significant. 

Can a low load home be cooled with a water to air exchanger in the ERV (fresh air) loop? 

I am also looking into an ERV that incorporates a dehumidifier (saw this in an article in GBA).  The source of heat and cooling is a GSHP that I am reusing – I can provide more information on that if needed but am trying to avoid over complicating this question. 

Any advice will be appreciated.  I am in Zone 5b: -26.1°C to -23.3°C
Marmora Ontario Canada.

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  1. Expert Member
    Akos | | #1

    This is unlikely. When you are cooling you have lower delta T to work with, so you need a bit more airflow to get some cooling BTUs. I would check the energy model of your place and see how much cooling you need and what airflow that translates to.

    In most places, ducting that is sized for providing cooling is big enough to also heat the place. So why bother with the radiant?

    Radiant only makes any sense if you skip cooling.

    1. rlangley | | #4

      Thanks for your response AKOS. The radiant heating solution is silent, even heat, more energy efficient no dust blown around the building and no feeling of a draft all the time. I am concerned that we will need AC in the summer with the ever warmer temps. I am just hoping to find a solution that does not require an air handler.

      1. Expert Member
        Akos | | #6

        "more energy efficient"

        Maybe. An air source heat pump with a decent install gets you the nameplate energy efficiency rating. Anything hydronic needs to be designed and comissioned properly to hit those numbers, even simple things like not adjusting pump speeds properly can tank your efficiency. I doubt any geo setup is ever running at rated efficiency and most are maybe 50% better than resistance heat.

        I live with radiant and I'm glad for the minimal amount of white noise my ventilation provides. Without that every creak and sound in the house can be heard, there is such thing as too quiet. Increasing from ventilation flow to full cooling/heat is a small change, barely noticeable in the background. Also most high efficiency units are modulating, which means outside of the coldest of days it will be running at lower speed.

        Unless you have been in a high efficiency home, it is hard to describe how unnoticeable the HVAC system is, in most cases you would have to put your hand in front of the register to even know that it on. Because of the low loss temperature is so even that all the touted benefits of heated floor simply don't matter.

        "no dust blown around " Again, living with radiant with an ERV with a HEPA filter, there is still plenty of dust in the house, unfortunately us humans are a great source of dust. A decent HVAC system will have a good air filter, the supplied air will always be cleaner than the rest of the house, so it reduces the amount of dust and pollution in the house.

        "am concerned that we will need AC in the summer" Yup. My cottage used to be no cooling, now it has at least one and half month of cooling season. I'm glad I installed an air source heat pump there, not just for the low cost of heat but also for the cooling it now provides.

        "does not require an air handler" No way around this. If you want to take out moisture, you need a coil thus an air handler.

        At the end of the day, it is your money to spend, just don't expect to be "better".

  2. Expert Member
    DCcontrarian | | #2

    This is a subject that hasn't gotten much study, but I feel that the next frontier in climate control is active management of humidity along with temperature.

    The question is, what is the maximum dehumidification (or latent cooling) load you'll ever see? The reason I say this hasn't gotten much study is that this isn't something that a Manual J calculates. It calculates the total cooling load, and assumes that dehumidification will be adequate. Where I am when it's really hot it tends not to be that humid (design day is 49% RH) and when it's really humid it's not that hot, so that assumption isn't very valid, it's common here to run a dehumidifier for much of the year.

    To start with basics, the reason they call it "air conditioning" rather than "cooling" is that there are actually two functions of AC: cooling the air, and removing humidity. The term "sensible cooling" is used to describe cooling the air and "latent cooling" is used to describe the removal of humidity. In the cooling season, heat comes into the house due to the outside being warmer, solar energy, occupant activity, and air infiltration, both deliberate and unintended. Humidity enters the house from occupant activity (cooking, bathing and breathing) as well as air infiltration. In modeling, occupant activity is assumed to be constant -- 200 BTU per occupant per hour is what is used. Air infiltration is also assumed to be constant, so the maximum dehumidification load for where you are could be calculated by finding out what is the highest dew point your area sees and calculating the load at that point.

    The hydronic ceiling panels can only provide sensible cooling, so the question is whether the ERV ducting can provide enough latent cooling on the highest load days. Here's the thing: you can't get latent cooling without sensible cooling. The only way to get latent cooling is to cool the air below its dew point; in doing so you lower its temperature (which is sensible cooling). And the latent cooling and sensible cooling move in lockstep depending upon the temperature of your cooling equipment, although the colder you go the more latent you get relatively.

    If the amount of latent cooling you require delivers too much sensible cooling, you need to heat the air after you're done cooling it. That's what a dehumidifier does, it adds back all of the heat that was removed from the air -- both sensible and latent -- as sensible heat. It also adds the heat from the electricity used to run it. So a dehumidifier is also an efficient little space heater.

    Just to throw out some numbers, if your interior air is at 78F and 55% RH and you cool it to 40F, 58% of the cooling will be sensible and 42% latent. The rule of thumb in the HVAC business is you get 30 Btu/hr per CFM. So if you:
    1. Figure out what your maximum latent load is.
    2. Figure out how many Btu/hr of cooling you'd need to get that much latent cooling assuming a 60/40 split.
    3. Figure out how many CFM you'd need to get that much cooling.

    I think you'd have the answer to your question.

    Looking at my Manual J, it says 1700 BTU of latent on the design day. So that's about 4250 Btu, which equates to 142 CFM. Which is kind of in the ballpark.

    1. rlangley | | #5

      I'm going to show my ignorance here...I am not sure what you are basing your numbers on. Does 142 CFM mean that an ERV can provide sensible cooling with a water to air radiator in the ducting? I obviously need to do more homework but is a variable speed ERV an option or would the optimal (for fresh air ventilation) ducting size be to big if it supported cooling and too small if just for ventilation? Thanks for your detailed response.

      1. Expert Member
        DCcontrarian | | #7

        The ERV is trying to replace indoor air with outdoor air that has been conditioned to the same temperature and humidity. It won't be completely successful, so if it's hot and humid outside the replacement air will be somewhat hotter and more humid than the air it replaces.

        In order to dehumidify the interior, you need to be introducing air that is quite a bit drier than the interior air. My example of 142 CFM assumes that the air you're introducing has a dewpoint of 40F. The air coming out of the ERV isn't going to have a dewpoint that low. The simplest way to achieve that dewpoint is to cool the air to 40F.

        Now, what if introducing air at 40F makes the house colder than you want? That means you don't need any cooling, and the reason you are trying to dehumidify the house is so that you can run the hydronic cooling without condensation, and if you don't need cooling that need goes away.

        So basically you need some sort of cooling coil in the ERV. More important, you need some sort of control mechanism that looks at how much cooling you need. If you need just a little it fires up the fan in the ERV and the cooling coil. If you need a lot it checks the interior humidity, makes sure it's low enough, and then sends cold water to your hydronic emitters.

        A couple of points:
        1. This control mechanism does not exist as far as I know as an off-the-shelf product. There are multi-stage, humidity-sensing thermostats available, and you might be able to cobble something together with one of them and some relays. In my own home I built such a controller from a microprocessor, temperature and humidity sensors and some relays, but I'm a skilled programmer and it's a work in progress. I'm also a bit of a mad scientist.

        2. The cooling coil in the ERV duct needs to be colder than the water in the hydronic system. Probably by about 15 degrees. If you're thinking of using water in the ERV coil you need to have some way of splitting the circuit into two different temperature loops.

        3. There might be a failure scenario where the ERV duct is never able to dehumidify the house enough so the rest of the cooling system never kicks on. Even though you have ample cooling capacity it never gets used and you're hot and uncomfortable.

        In the BS&Beer video, Siegenthaler recommends using an air handler unit like one of these for primary cooling and dehumidification:

        (That same CXI unit is sold under a number of brands, and there are other similar models available)

        That's what I have in my house.

  3. Expert Member
    DCcontrarian | | #3

    I watched the BS & Beer episode. A couple of interesting take-aways. First, according to Siegenthaler air-to-water heat pumps are now outselling boilers in Germany and in China over two million a year are being installed. Maybe we'll be seeing some of that technology on our shores some day.

    Siggy lost me though when he started talking about the "thermal mass" in concrete floors. Do the math, John!

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