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Adding a fan coil to a hydronic system

I have a small staple-up hydronic radiant installation I'm very happy with. There is an electric boiler (Argo) with a loop and circulator pump. There is a circulator pump driving the floor loops. There is a variable speed injection pump between the two.

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I also have a hydronic fan coil in my heat recovery ventilation (HRV) duct. This is currently plumbed into my domestic hot water system. I have extra boiler capacity in the Argo and would like to get the fan coil out of my DHW system and into my home heating system. I have two ideas on this.

The first is to put another set of Ts in my boiler loop

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and use another pump to circulate water through the fan coil and return it to the boiler loop. This costs me a pump, a relay to control it, and space on my wall to mount it all. It also seems a shame to insert a couple hundred dollars in pumping and control to move the water through twelve extra feet of pipe.

The second is to enlarge the boiler loop (by those twelve feet) to include the fan-coil, placing the it after the injector which feeds the radiant floor. This takes up less space on my wall, only costs me a few valves (to enable bypass of the fan coil), but gives me less control over the heat output.

Control over the heat output isn't a major concern to me. My house can consume any heat the fan coil can provide, and my Argo is currently operating at about 50% capacity. My concern is introducing complication into my short, simple boiler loop.

What do y'all think about either of these approaches? Are both of them bad ideas? Is one worse than the other?




  • SWEI
    SWEI Member Posts: 7,356

    What temperature does your staple-up system run at?  Is there an outdoor reset?

    What fluid temp does the fan coil run at?  Is it providing enough (or too little/much) heat?
  • ColoradoDave
    ColoradoDave Member Posts: 54
    Another Option

    SWEI asks some great questions to consider for you system in making the final decision.

    If there is an outdoor reset, I'd assume (I know.. I know... assume...).... but, I'd assume it would be used to control the injection system to your floor heat.  IF that is the case, and you don't limit the boiler temperature with the ODR, then you could run the fan coil as a wild loop off of your primary (i.e., if your primary loop is 1-1/4", then install a pair of 1-1/4 x 1/2" x 1-1/4 Tees so that the fan coil is servicing and returning on the runs of the Tees.)   MORE ASSUMPTIONS... assuming the fan coil uses a 1/2" Supply & Return like the Beacon Morris Kickspace heaters, if not, size your Tees appropriately.

    Then whenever there is a call for heat, your fan coil gets the hottest water available.  You stated that the heat output doesn't need to be controlled because your space will use it.  The 12' of piping you stated shouldn't provide enough pressure drop to limit the flow through the fan coil loop regardless of the pressure drop in the coil.
  • JohnInJuneau
    JohnInJuneau Member Posts: 3

    The staple up runs at varying temperatures. It pulls water from the boiler loop with a variable speed pump. I think the controller is a Tekmar 356.

    But I begin to think the floor loops are of virtually no concern. As long

    as the boiler loop is providing water above the requested injection

    temperature, it should continue to work just fine.

    The boiler loop runs between 110 and 140. How hot I set it depends on the time of year.

    The fan loop currently runs at whatever temperature the domestic hot water is set to. There's no control or logic in it at the moment. It just runs. This means it's swiping heat from my DHW at all hours of the day, even when there is high demand for DHW.

    Is it providing enough heat? No, but that doesn't really matter. My house can consume all of the BTUs it can transfer.
  • JohnInJuneau
    JohnInJuneau Member Posts: 3
    Pump choice

    If there is an outdoor reset, I'd assume ... it would be used to control the injection system to your floor heat.

    That's all it does. The boiler loop runs at whatever temperature I set on the boiler.

    The 12' of piping you stated shouldn't provide enough pressure drop to limit the flow through the fan coil loop regardless of the pressure drop in the coil.

    So I guess I'll pursue a little pump driven loop through the fan coil.

    When choosing equipment, I wonder about the pump choice. The other pumps are Grundfos (3-Spd). They are solid, quiet, and I have no complaints. I'd normally reach for another of these to drive the new loop just so I have consistent parts and behaviors. But it seems like a lot of pump for such a little loop. Is there a good reason not to use another Grundfos here?
  • SWEI
    SWEI Member Posts: 7,356
    pump choice

    Really does need a bit of math.  Do you have any data on the reheat coil?
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