Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Need to contact us? Visit

One zone, do I need two pumps?

I'm building a single zone radiant heating system, 300 sq. ft. and 300' of ½" PEX, for my garage/shop. I will use an electric on-demand water heater as the heat source. Probably an EcoSmart ECO-11. The primary loop will run at 105 degrees. My question is, do I need to install two circulating pumps, one for the primary loop (Taco 003) and one for the zone (which is the way my existing radiant systems with multiple zones are plumbed), or can I use a single pump in conjunction with a mixing valve? If the latter would work, I need to size the single pump for the head loss in the entire system (i.e. a Taco 006).


  • hot_rodhot_rod Member Posts: 13,502
    Not the ideal heater for your job, it's really more of a water heater. They require a large circuit because they are designed heat water instantly. The flow rate defines how the output ramps up. Your loop at .5 gym may not allow it to ramp up properly?

    What is the load on that 300 sq ft, probably 6000 btu load.

    A small 6 or 12 gallon electric tank with a 3500W X 3.41= 11,900 btu/hr element may do the trick, needing a 20A 240V circuit.

    A single circulator should be fine in either case.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • rimrockrimrock Member Posts: 9
    The reason I picked the ECO-11 is that it is similar to what I have currently to heat the house and an outbuilding. i.e. both buildings have on-demand water heaters that supply both radiant heating and dhw.

    ECO makes a couple of smaller units that would be adequate to my needs, but the ECO-11 is priced about the same, only $5 different, so I decided to get the larger heater expecting a longer life since it will be lightly used.

    The main functional difference between the ECO units and my existing heaters is that the ECO does not have a built in pump and outdoor reset control logic for the primary loop, which is why I asked the original question about the need for two pumps for a single zone system.

    I was not able to locate an on-demand water heater that is sold specifically for space heating, except for some that are designed for a much larger heat load. The manufacturer does not state whether the ECO heaters support use with a recirculation pump, so I have sent them an email to find out. My reasoning being that if the heater can handle pre-heated water as the input for recirculation, it would also handle pre-heated water for space heating.

    I don't like tank water heaters, having used them as the heat source for the first 15 years I lived in this house. The on-demand systems have given me excellent service for the last 12 years.
  • ZmanZman Member Posts: 5,870
    If you are set on the ECO, you should go primary secondary.
    I would not live in a house with an open system like that. All that tubing is just a bacteria breeding ground of stagnant water all summer long. I have converted systems like yours and been blown away with the amount of biologic nastiness that comes out of the pipes.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • rimrockrimrock Member Posts: 9
    Zman said:

    All that tubing is just a bacteria breeding ground of stagnant water all summer long.

    All radiant heating systems have that issue. what's your point? its a closed loop.
  • GroundUpGroundUp Member Posts: 991
    edited December 2019
    The Eco is only serving a single 300ft loop of radiant and nothing else? 35,000 BTU unit for a 10,000 BTU heat load, I'd go with the tank style or boiler personally. But if you insist, a single circ will do just fine as you need very little flow and the pressure drop through the floor will be minimal. Keep it simple. You cannot find an on demand water heater for space heating because on demand water heaters are not designed for space heating. You would be looking for a boiler if stuck on the tankless variety. Electro makes a nice one for you but at a much higher cost than the Eco
  • GordyGordy Member Posts: 9,523
    Two circulators using primary secondary piping. Instantaneous water heaters have high pressure drop heat exchangers. Chances are a single pump will not produce enough flow with the pressure drop of the IWH, and the radiant.
  • GroundUpGroundUp Member Posts: 991
    edited December 2019
    A single 300ft loop of radiant will have a pressure drop of roughly 3.5 ft of head on top of the 4 feet created by the tankless at .5 GPM (per the MFG). A second circ is completely unnecessary if the single circ can produce 8 feet of head at .5 GPM. There is a guy that installs these disasters everywhere around here with an 007, on up to 10 loops that I've seen. Although a shoddy idea altogether, it will heat just as good with 1 circ as it will with two. Start adding loops and flow requirements, there will be trouble but a singe loop, nah.

    Also, why would you want a mixing valve? Set the tankless temp to 105 and let it work
Sign In or Register to comment.


It looks like you're new here. If you want to get involved, click one of these buttons!