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Wiring Questions - FVM4X Fan Coil with HydroAir - Old Honeywell L4006B Aquastat

DanMc85 Member Posts: 1
Hi guys,
I am in the process of having my air handler (fan coil) in my attic and my outdoor AC unit replaced. However, the heat is hydronic from a wall mounted IBC boiler in my basement. The old Carrier Air Handler has a L4006B Aquastat on the pipe which looks to be interrupting the red power wire to the old Carrier fan motor (This is an old air handler, it does not have a circuit board or microcontroller).

The new air handler is an ICP - Heil FVM4X Variable Speed Air Handler matched with a HCA724GKA 2 stage air conditioner.

The question I have is this existing Honeywell L4006B Aquastat compatible to interface with this newer air handler (that has a microcontroller and board) so that the fan only turns on during a call for heat once hot water is flowing in the pipes without affecting other components such as fan, air conditioning, etc during summer months?

I was also looking at a Taco HAFC201-4 Fan controller as another alternative, but wasn't sure if it was compatible with a variable speed air handler OR a 2 stage air conditioner since it only has one Y input.

Thanks for any guidance!


  • Big Ed_4
    Big Ed_4 Member Posts: 2,134
    The L4006B is used to turn on the fan once the coil is hot . Normally wired though the control from R the power source to G the fan terminal . The B aquastat makes on temperature rise .

    I am not sure what board you have , posting a wiring schematic can give more options .

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    HVACNUT Member Posts: 4,950
    You could simply use an isolation relay to activate the hydro coil zone (the ICP might even have dry contacts on their board for a hydro coil) and have the thermostat control the fan on a heat demand, which will power G. Maybe you can set a time on delay on the ICP board, otherwise there might be cool air blowing before hot water gets to the coil. No biggie IMO.
    Or you can wire W on the thermostat to W on the isolation relay and White on the 24 volt motor lead. This is a lower fan speed than G. 
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,365
    edited March 12
    Variable speed air handler with an add on hydro coil needs some serious redesigning to get the fan to operate for heating mode. You see if you just use the G circuit on the low voltage wiring side to operate the fan for the heating circuit, the Fan may only operate at 10% or 15% of the needed speed for moving hot air thru the system. That is because just connecting R to the G terminal with no call for cooling is like changing the Fan switch on the thermostat from AUTO to ON. If your air handler is like most variable speed blowers, the R to G will operate the fan at a very low speed in order the allow the air filter to operate better and not cause drafts in the home.

    Now if you add a heat relay to the mix (like there might be on an electric heat kit), then the circuit board on the air handler will operate at the higher speed because the G and the W are both connected to the R for a call for heat. But that will cause the fan to blow cold air until the hot water from the boiler gets to the coil. that can take some time depending on the condition of the boiler at the call for heat from the thermostat.

    That is where the L4006 will come in. you can keep the blower from getting the G signal until the coil gets hot enough so the air from the ducts will be warm when the fan operates.

    I'm actually working on solving that problem for my son right now. but my project involves zone dampers also. I'll let you know what I discover. Your best bet is to get an air handler with the optional Hydro Fan Coil in the software of the PCBoard, if you did not already get one. Carrier might have something like that in their product line.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,365
    edited March 12
    In this Goodman air handler with variable speed air blower, the addition of a HEAT RELAY (like the R8222) will offer a Normal Open (NO) set of contacts to activate the boiler (or zone valve or circulator relay). In this case I am using the air handler 24V transformer to power a zone valve. The other set of NO contacts will activate a HEATING FAN RELAY only after the aquastat senses the coil has hot water sufficient to blow heated air thru the ductwork. The HEATING FAN RELAY (another R8222) will then send 24 volts to the W and the G terminals on the air handler Control. You must set up the fan speed based on the CFM required by selecting the electric heat kit dip switches. When the call for heat is satisfied the HEAT RELAY drops out making the fan and the boiler stop operating. This wiring and relay setup will allow the micro processor that controls the fan speed to operate as if there is an electric heat kit installed. When W1 and G get a call for heat the fan will operate at a speed that will distribute sufficient air flow to the zone.

    Look at the ScreenShot file for the diagram that operates the boiler with Dry Contact (No Power) to the boiler room control.

    On the old air handler, all you needed was to connect the heating speed of the PSC Fan Motor to the aquastat and when the coil gets hot, the fan would operate. Not as easy with these new fangled electronics.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • clammy
    clammy Member Posts: 2,896
    If you want some thing a little more accurate w a wider differential setting try using a ranco etc , I haven’t use a Honeywell aquastat for hydro or indirects for years . I use a ranco 110/208 voltage etc dead accurate always they have been used in the refrigeration field for ions . If need be you can use a taco hydroair relay and do what Ed has mentioned . I personally worry about hydro air in attics freezing potential but nobody seems to worry . I would also make sure that you install a check valve to prevent thermal migration durning the summer especially if using the boiler to produce domestic hot water . Seen many a case of thermal migration in hydro air once corrected not only did the electric bill go down but hot water production was restored and gas bill reduced . It is these small things that contractor are unconcerned about mainly because they don’t understand and figure it not a issue . Until they get a not enough cooling call and enter the attic to see the hot water coil hot and the ac running blowing cold air across the hydro coil and the indirect calls and shaboom the migration begins and sometimes it will be a constant migration until the ac shuts off and the tank reaches temp ,the other downfall of no check valves is sometimes it will migrate 12 months out of the year and just cycle the indirect every hour while it should be off .
    Again not a big hydro attic fan unless proper precautions are done like check valves and freeze stats and a safety pan under the unit w pan switch better yet a electronic moisture detector cheap insurance but costly most just roll the dice and hope there number gets lost should it freeze cause then who left holding the bag ? The plumber or the Hvac or maybe the contractor none of them cause they state it never happened before . Think about it water in a coil in a attic yeah it won’t freeze until it freezes . Peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • Big Ed_4
    Big Ed_4 Member Posts: 2,134
    Depends , on the locations of the registers and air handler to pick the volume ..

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