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Controls for hx in warm air furnace plenum

I've added a hot water coil in the plenum of a Thermo Pride furnace.  The hot water for the coil is supplied by a wood boiler, using a circulator tied to a TACO multi-zone panel.  Other zone is for radiant heat.  I have a Honeywelll 6006C strap on aquastat on the piping from the hx back to the boiler (it could go on the supply side just as easily.) Now...

what's the best way to allow the wood boiler to heat the existing house via the existing ductwork, using the coil?  I'm thinking of addding a 2nd t-stat that would activate the circulator for the hx (if the 6006C control says it's hot enough), but I need to bring on the blower without activating it through the existing fan and limit control, which will not sense a high enough plenum temp. if there's just hot water going through the coil. Has anybody done this, in a fairly simple way?  Or does one need to think about 2 stage t-stats, n/o and n/c controls, etc.  Any help would be appreciated.  Thanks


  • sasman
    sasman Member Posts: 32
    fan center

    got a fan center on furnace?

    use it to control fan

    if you've got cooling on it, you'll need an isolation relay on the cooling side so you don't make cooling run with heat
  • Wayne_16
    Wayne_16 Member Posts: 130

    2 stats are the easiest way, looks more cluttered, junky in my opinion, very simple.  I have worked with a company that installs outdoor wood boilers and the control sequence is as varied as the install.  Isolating the outdoor air conditioner may still be needed.  One inexpensive way is to turn the breaker off to the condensing unit.  Make sure the breaker is on for the required time, (24 hours) prior to starting the air conditioner.

    My biggest concern with all additional heating coils installed in existing forced air plenums is the major restriction of air flow the coil causes.  I have seen very few installs, that the new heating coil does not severely restrict the air flow through the furnace.   The water coil install needs to be sized on the output capacity of the coil, matched to the heating load of the building, rather than sizing the coil based on plenum size.

    The next component needed for the best performance is to install a 3 way valve on the coil circuit.  Most all outdoor wood stoves want the circulating pump to operate at all times.  By installing the 3 way valve in the water circuit, the thermostat controls the 3 way valve and the fan control at the same time, limiting the amount of heat in the plenum. The water bypasses the coil when there is no call for heat in the building. 

    If the customer is used to having the furnace fan on, for constant air circulation they will lose that feature if a hot water coil is installed and the control only operates the indoor blower fan and no 3 way valve.

    The temperature rise across the furnace in the gas/oil heating cycle needs to be checked/adjusted once the water coil has been installed.  The temperature rise across the heat exchanger cannot exceed the listed data on the name plate.  Ignoring this will result in premature failure of the heat exchanger and may very well void any manufacturer’s warranty.  Change the fan speed to high for heating and recheck the temperature rise.  If the temperature rise is still too high, a different heating coil will need to be installed.  I have installed 10” round duct with a manual damper blade to solve the temperature rise issue.  The same temperature drop measurements have to be made during the summer for air conditioning and solutions found to prevent poor cooling, coil freeze up, compressor failure etc.

    Minnesota Wayne

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