Controls for hydronic floor heating fed by drainback solar & mod/con boiler

tommcveigh

<span style="font-size:12pt">I am installing a radiant floor heating system fed from a big solar storage/drainback tank, preferably without a heat exchanger.  I need help with the controls.  This is for a rental house in Big Bear Lake, CA, only steps from the ski slopes.  It rents for over $500/night in high season, and I’m willing to give away three consecutive nights for the best answer to my control question.  I plan on replacing the old water heater and furnace for a system that is heated from a Triangle Tube Combi Mod/Con Boiler and a drainback solar system fed by about 40 evacuated tubes, as I have decent solar access.</span>

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<span style="font-size:12pt">Built in 1981, I plan on replacing all the aluminum windows and doors, and adding at least 4” of rigid insulation to all walls and the roof of this square 2000 sq ft house.  The first floor (mud room, laundry, bathroom, 2 bedrooms) will get pex inside gypcrete and the 2</span><span>nd</span><span style="font-size:12pt"> floor with cathedral ceilings (great room, kitchen, bathroom, 2 bedrooms) will get warmboard.</span>

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<span style="font-size:12pt">Although this is my first floor heating system, I am a contractor with experience in installing apartment drainback systems, building my own 200 to 400 gallon, unpressurized solar storage/drainback tanks.  DHW will be preheated in this tank through a 100’ CSST (corrugated stainless steel) heat exchanger, then goes to a boiler’s separate DHW passages and on to faucets.</span>

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<span style="font-size:12pt">I would prefer to use the same water that goes to the solar collectors for use in the floor heating system (instead of using a heat exchanger) for more performance and no need for a separate expansion tank.  The solar tank temp will get up to 150 degrees F, so I plan on using a 3 way mixing valve to reduce temps to the floor heating system.  I also want to use outdoor reset to change ‘Supply’ setpoint temperature.</span>

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<span style="font-size:12pt">For simplicity, the DHW line along with the drainback lines are not depicted in the attached diagram.  I plan on using a Grundfos Alpha (stainless, pressure regulated) for the floor heating system, feeding into manifolds with thermostat controlled actuators.  This ‘Demand’ pump (P1) must be running before either ‘Supply’ pump (P2 or P3) is allowed to start.  </span>

1)    If Demand’s return line temp is less than the tank temp, then turn on Solar Supply Pump (P2).

2)    If the return line temp is still not up to the setpoint (controlled by outdoor reset) after leaving the solar tank, then turn on mod/com boiler and Boiler Supply Pump (P3).

<span style="font-size:12pt">Any advice on what LOW COST control(s) to use for this system would be appreciated.</span>

SWEI

More info

Is the entire system built using non-ferrous components?  Every pump, valve, fitting, etc?  It's generally far simpler to add another HX coil for the space heat and keep that as a sealed system.



Control-wise, I would suggest the following:  Pipe the boiler direct to the loads.  Install an ODR-controlled mixing valve (Taco iSeries-R is generally the least expensive route) on the output of the solar tank.  Install a simple ∆T controller (under $100 from http://www.mydtcstore.com/ but plenty of options there) that controls a 3-way diverting valve to change from tank to boiler whenever the system supply temp drops below the tank temp.  When tank temp is 5-7ºF higher than the supply temp, switch back to the ODR valve fed by the tank.  Set both ODR curves to match.

tommcveigh

new diagram attached

I like simple, and was hoping to stay away from a separate expansion tank necessary for a closed loop.  However, I think a closed loop system using another heat exchanger in the solar tank is the way to go since it prevents possible static pressure issues with the boiler.

 

I don’t fully understand your control suggestions, but I have documented the piping with some of your suggestions incorporated into a system I think will work.   How close did I get to your full idea with the attached document?