GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Hydronic Coil in Conjunction with ERV/HRV

Tim_O | Posted in General Questions on

With more A2W heat pumps coming on the market, I wonder if there are decent options for a hydronic coil implemented inline with ventilation air.  I don’t mean to use this entirely as conditioning, but it would help condition bedroom air, essentially the places that are hard to reach from a minisplit/fancoil unit that sits in the living room.  The system I envision uses a single A2W heat pump which provides domestic hot water (or most of it at least), feeds a fan coil to heat the main living room/kitchen area, and a hydronic coil inline with an ordinary HRV to supplement the bedrooms heating/cooling.  Radiant heating would be an easy add-on as well, but unnecessary for this concept.  It’s sort of a “magic box” concept.  But it seems as though it would be quite simple and even cost effective.  Options like the Minotair and CERV are cool, but at close to $7000 for just the ERV, a hydronic coil + Broan/Panasonic/etc seems like a good idea and doesn’t add the complication and space of another compressor system.  A ducted heat pump is an obvious option as well, but the loads are so small in bedrooms, I would think the ERV is supplying sufficient CFM.

An argument might be that having one heat pump supplying all HVAC and hot water could mean a single point of failure brings down everything.  My backup would be resistance heat would be easy for DHW and for heating.  

CERV and Zehnder sell geothermal based hydronic boxes for their units, and maybe just using one of those in conjunction with a normal ERV would be the easiest option.  Are there other options out there that people have used?

What does the community think?  

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Akos | | #1

    Sounds too complicated. Any time you say hydronic you are adding at least $15k to your build. For that kind of money there should be some benefit and I'm not seeing it.

    Connect the fresh air feed of ERV to a modulating ducted heat pump and run that to each room. This is exactly the same effort as fully ducting the ERV in your setup. Even with a fancy hyper heat unit and high efficiency ERV you are looking at an equipment cost under $8k. Hard to compete with the efficiency, simplicity and cost.

    1. Tim_O | | #4

      A simpler system (than the $15k+ options) I'm thinking comes in as such for a baseline - hydronic coil in the ERV and radiant floor being extra.

      ~$4500 - A2W Heat pump
      ~$350 - Glycol
      ~$1200 - Stainless water heater with heat exchange coil and immersed electric heater element
      ~$500 - water heater as buffer tank
      ~$1000 - 2 pumps, 3way valve

      Looking at $7500 baseline equipment, but the big advantage here is that domestic hot water is covered and you get the year round efficiency of hot water supplied via heat pump directly (not heat pump to heat pump losses associated with A2A combined with hybrid water heater). In a low load house, your water heater BTUs could be 30% of your total heat usage, so this is pretty big. I think the only way this is cost efficient is with players like Chiltrix, HydroSolar, Apollo continuing to make units that don't cost a ton more than the A2A units out there of similar size. Does my cost estimate make sense here?

      1. Expert Member
        Akos | | #9

        You are missing a lot of ancillary costs of hydronic. Heat spreaders for the floor heat, manifold, zone actuators, thermostats, isolation valves, zone controller, air separator, feed water setup and expansion tanks. Fittings. Lots and lots of fittings and valves.

        I've done an ultra budget hydronic setup before (solder up manifold, minimal zoning and controls, staple up with minimal heat spreaders) and the BOM was still over $10k.

        1. Tim_O | | #11

          This was meant to not include the costs of adding radiant floors, essentially the baseline equipment. Our intent would be to likely add radiant tubes into the concrete slab of the first floor, and that's it. Even with that, in a low load house, it might be best to only put the tubes in high traffic areas.

      2. DC_Contrarian_ | | #21

        The advantage of a heat pump water heater is that in the summer it provides free cooling.

        In winter, I'm not sure there is any efficiency advantage to heating water in one stage vs. two, but I haven't studied it. There is also the belief -- which I admit I haven't studied either -- that if you have the water heater in an unheated basement, in winter it is pulling heat out of the ground instead of the outdoors.

  2. DC_Contrarian_ | | #2

    What about something like a Chiltrix CXI-34?

    About $770. Rated for 3300 BTU/hr heating and cooling, can go as low as you want. If you're already doing hydronic it's not much to add it, it just hooks into the plumbing, doesn't need a zone valve or thermostat, which you would need with just a coil. With a coil, zone valve, thermostat and transformer you're approaching the cost of the unit anyway.

    What hydronic gives is the ability to precisely deliver as much heating or cooling as you want to any room. Delivering the BTU's with water is more flexible than delivering them with forced air or refrigerant.

    1. Tim_O | | #3

      This is what I was thinking as an option for the unit in the main room (the larger version), but adding one to each bedroom seems excessive if you could plumb just one into the ERV ducting for added distribution. The only hydronic units I've seen that are ducted are much larger however.

      Dug in a little more. Chiltrix uses FirstCo products for their air handlers. They have some options in the $1500 range on their website. Not sure if it's so much more than these because of the plenum. But $550 for the basic unit isn't bad, considering you could use it to cover 3-4 bedrooms with good distribution.

      1. DC_Contrarian_ | | #6

        The linked unit looks to be 1.25 tons cooling from the spec sheet. To me that kind of takes away from the appeal of hydronics, if you're going to have a big blower and connect 3-4 rooms with ductwork why not just use a ducted minisplit? If each room has its own emitter and thermostat then you can control the temperature more precisely.

        1. Tim_O | | #7

          This one was the only one I could find pricing on easily, but they do have smaller ones in the 2-300 cfm range. But I agree, might be worthwhile to just have an emitter in each room. Do you use the Chiltrix emitters?

          1. DC_Contrarian_ | | #12

            I have them in my own home. I haven't used them long enough to form a firm opinion. I like the way they work, they have a thermostat and you set it, it automatically adjusts the fan speed for the load. Most of the time the fan is on low or off, it's barely noticeable. In a small room it keeps the temperature very constant.

            Things I don't like: I opted for ceiling installation because I preferred the look. It was a lot of effort to get it to work properly, the unit isn't really designed for it and the instructions weren't quite right. But I did get them to work eventually. I have one unit that is kind of flakey and I'm still working with Chiltrix to figure out if is defective or just needs configuration. I haven't had them for very long and it's shoulder season here where I'm not using heating or cooling so I haven't been able to test it.

          2. Tim_O | | #13

            Sounds good, I hope to hear after you've been through a winter!

    2. Expert Member
      Akos | | #8

      It will do something but it won't be enough. Ventilation airflow is between 10 to 20 CFM per bedroom. Fan coils run around 20btu/cfm cooling and 40btu/cfm heating.

      That means you can deliver 200 to 400 BTU of cooling and 400 to 800 BTU of heating.

      The heating on the higher end is getting pretty close to a well insulated house bedroom load but still not there.

      Since two bodies put out about 400 BTU/h resting the amount of cooling is basically squat.

      The only way to get this up is bumping up the air flow, now you are talking about a larger air handler and might as well size that to cool the whole house. If it can cool the house, it can definitely heat the whole house as well so we are back to why bother with anything hydronic.

      A 1 ton ducted mini split is $1500 on the internet, not far off from a fan coil. If you look at the cost of adding cooling to an air to water setup (tank, valves, controls), you are already spending more. Any air to water setup also has two additional heat exchanges, there is no way it will ever be as efficient as a refringent to air.

      1. Tim_O | | #10

        Yeah, I see the point there. Advantage also being that commonly used mini splits would be easier to service and get parts for.

  3. paul_wiedefeld | | #5

    Ha hydronics is so interesting but it’s so much simpler to use a centrally ducted heat pump. Very few Americans mess with forced water systems.

  4. Expert Member
    Akos | | #14

    I've been noodling this problem for a while now, this is what I've come up with as a simple way to feed floor heat.

    Install a heat exchanger on the refrigerant loop of the ducted mini split, similar idea as a de-superheater coil on a geo install. Plumb this to the floor heat, no additional controls on the mini split, tap some of that heat off as needed. The issue with this setup is that the air handler will have to be also running, take off too much heat and now you are blowing colder air which can create comfort issues, not enough heat and the floor heat won't do much.

    Theoretically one can also use this setup in the cooling season to cool the slab.

    I like it for the simple fact that the only thing I'm adding is a heat exchanger which can be had with flare fittings so it is a very simple install. Since the heat exchanger is indoors, there is no issue with freezing either.

    1. Tim_O | | #15

      Are you talking about something like this? Oversized, but just one I have seen recently.

      My only fear would be you are using the minisplit without proper controls, sort of piggybacking off the air exchanger controls, and could be finicky like you say.

      It's too bad there aren't any decent options for a Multi split to use with hot water. The way I envision it is a large outdoor unit with a few indoor air exchangers and a hot water coil. Hydrosolar and Nordic do make a split air to water systems, but I don't think they are designed such that the outdoor unit could be connected to the indoor exchanger and to additional heads.

      Maybe I need to make an Arduino to make my own exchanger here! Chiltrix is apparently satisfied with Arduino control on some of there stuff.

    2. DC_Contrarian_ | | #16

      I don't have links handy, but I've seen two stories of people who have apparently been successful with something like that. In one he just put a coil of 50' of 3/8" copper tubing inside of a water tank and ran the refrigerant into the copper tubing. In the other he used a flat plate heat exchanger. My understanding is the Chiltrix CX is essentially a minisplit compressor and a heat exchanger in one package with a circulator pump.

      Like you I see the minisplits with the $1500 street price and wonder why hydronics is so expensive. I feel that fundamentally it should be simpler and thus cheaper to pump water around the house rather than freon.

      One of the advantages of a system like the Chiltrix is that it's self-contained, it uses a small amount of refrigerant and there's no need for someone with a refrigerant license to do the installation. The disadvantage is that the water flows through the outside unit, which means that all of the water has to have anti-freeze, even though only a small fraction of it is actually exposed. With propylene glycol now over $25/gallon in bulk it adds significantly to the cost and complexity of the system.

      My suggestion would be a package unit where the heat exchanger sits inside the building envelope. I don't see why you couldn't fit the piping and heat exchanger inside a cylinder with a 6" diameter. The package comes with a 6" tube maybe 3' long sticking out of the back. So you mount the compressor on an exterior wall, and you make a 6" diameter round hole and side the heat exchanger through the wall and into the inside. On the inside you hook up the water, as well as the power cable and controls, which makes for a simpler, neater installation. Seal the whole thing with spray foam and you're done.

      1. Tim_O | | #17

        I like that idea. It seems simple and odd that no one has really bothered in the US. Chiltrix and the other similar ones are decently priced, but still quite a bit more than an "off the shelf" name brand mini split.

        1. DC_Contrarian_ | | #18

          In the northeastern US there are tens of millions of houses with oil-fired hydronic heat. In the next couple of decades all of those houses are either going to have to convert to electric or something like biodiesel. I think there's going to be a huge market for air-to-water heat pumps.

          1. Expert Member
            Akos | | #19

            The problem is that most of these home will also need AC if they don't already have this. Once you go down AC route, keeping the hydronics adds too much cost.

            Around me, most renos of older homes with rads, the rads are removed and ducts/furnace+AC added in. Less cost and gain some floorspace.

            I'm still on the fence on this. There is something to be said for the simplicity of abandoning my floor heat and going with a ducted mini split.

          2. DC_Contrarian_ | | #20

            Here's my thinking on that: Depending on the house, installing ductwork can be a major job, in some cases requiring a gut rehab.

            These houses are probably going to need a little more heat that the existing radiators can provide, because heat pumps don't run as hot as oil burners. But not a whole lot, they're typically over-radiated anyway. In this climate the cooling load is typically about a quarter of the heating load. So if you were to add wall-mounts to meet the cooling load they'd probably also meet the additional heating load. By wall-mounts I mean mini-splits, or possibly hydronic fan units. For cooling you need something with a fan.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |