Takagi tankless heater to supply in-floor hot water

Discussion in 'Tankless Water Heater Forum' started by captainrigg, Dec 12, 2013.

  1. captainrigg

    captainrigg New Member

    Messages:
    3
    Location:
    new jersey
    i am using a takagi tankless heater to supply hot water to a in floor existant radiant heating system previously supplied by
    oil fired boiler. the remaining problem is how to wire a honeywell L6006C aquastat to the Taco recirculating pump. i know that the either of the two wires of the pump attach to the two terminals of the aquastat but then how do i supply power to the pump? do i wire in leads to the hot and neutral line together with those wires? also, the pump has a light with two small wires, red and black to control the light. where do i connect these wire to activate the light. there is no connect to the tankless heater.
  2. Dana

    Dana In the trades

    Messages:
    3,032
    Location:
    01609
    The question is so vague that I'm a little reluctant to wade in here, but without knowing what control function you are trying to achieve here (what part of the system is the aquastat even sensing the temp of?), it's hard to steer you. Got a system schematic of what you are trying to do?

    There are a lot of badly designed radiant floor systems using tankless HW heaters out there, and some sorta-OK versions. Is this something you cooked up yourself? Is this a suspended-tube, slab, staple-up, or some other type of radiant floor?
  3. captainrigg

    captainrigg New Member

    Messages:
    3
    Location:
    new jersey
    the radiant heat is from 1/2 inch copper pipes in the slab, consisting of six zones which feed into a manifold, one for hot and one for the cold return. the taco pump circulates the water and the aquastat will shut the pump off when the temperature is set to a high and a low value(otherwise the pump would run when not needed. the aquatstat is sensing the temperature coming from the tankless heater. i know which terminals on the aquastat that need to be connected but since the supplied literature with the aquastat has no information on how to wire it to a electrical source, in this case the pump which has a white and yellow wire (pump info says either can be hot). also how do i wire the pump light which has a red and black lead.
  4. Dana

    Dana In the trades

    Messages:
    3,032
    Location:
    01609
    With the oil boiler the burner would fire, and the aquastat could control the pump based on the boiler temp.

    With a tankless the burner won't fire until & unless there is flow through the tankless- there IS no temperature coming from the tankless without the flow.

    I don't understand your control scheme.
  5. rsaint

    rsaint New Member

    Messages:
    3
    Location:
    Alberta
    Put in a in line thermostat on the wall to control pump and thats all you need to do. As long as the flow is going thru the tankless it will work on the flow going thru it.
  6. houptee

    houptee Member

    Messages:
    186
    Location:
    Monmouth County, NJ
    Like rsaint said you do not need the aquastat anymore only the Tstat on the wall in the house.
    The circulator pump can be activated with a call for heat from a 24v Tstat, but would need to be connected to a Switching relay such as a Taco 501 or Honeywell RA89A1074
    You connect Tstat wires to the relay T T terminals and 120v power to the Line terminals, then the pump connects to the relay Load terminals.

    The Takagi has dip switches inside the cover to set the desired max water temp.
    It comes set for 120 F for domestic water use but radiant usually runs at lower temps so you may have to experiment with different temp settings.
  7. Raspy

    Raspy New Member

    Messages:
    11
    Location:
    Northern Nevada
    You are very likely to have problems with this setup. First will be too low of a flow rate through the loops. Second will be carbon buildup in the heat exchanger as the water heater will be throttled to the lowest fire most of the time. Third is possibly the gas line sizing. What was the BTU rating of the previous boiler and did you calc the gas pipe?

    Tankless water heaters are extremely restrictive and are not boilers. Radiant heat needs high flow rates and it runs at lower temps and lower pressures that domestic hot water.

    An injection loop with two pumps is one way to make it work better.

    The thermostat simply turns on the pump with a relay. The pump makes water flow that triggers the water heater. The water temp is set on the water heater and the flow continues until the house thermostat is satisfied. When you discover it's not working very well, work on increasing the flow rate rather than increasing the temperature at the water heater.
  8. Dana

    Dana In the trades

    Messages:
    3,032
    Location:
    01609
    Raspy: You are presuming too much about how this guy's system is plumbed, the type of radiant, and his actual flow requirements. It's simply untrue that all radiant systems require high flow rate. It's also likely (if unknown) that the radiation is already being pumped separately with a mixed down water temp, since very few radiant systems fed by oil boilers are pumped direct. Retrofitting a high delta-T/low-flow boiler or tankless onto the primary loop isn't a rocket science type of hydronic design.

    Most tankless HW heater can run at 15-20psi without any sizzle-pop flash boil, which isn't an outrageously high pressure for a heating system. IIRC most Takagi units have a low-pressure operation limit of 15psi- setting it up to pump toward the tankless can let you run it there without any issues. If pumping away from the tankless 20psi usually works.

    But without more information it's really hard to give good advice on this one.
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