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  1. #1

    Need solar heat differential controller that works with an outdoor reset controller.

    My question below is technical and for someone who has experience with solar heating with radiant circuits. Any help in finding an answer or giving me a push in the right direction is appreciated.

    I am a green homeowner in NJ designing a solar hot water (DHW) and solar heating system (low temperature radiant) for my home. The low-temperature radiant manifold is a secondary manifold that I would like to use an outdoor reset controller (vary heating circuit radiant temp by outdoor temp) and mixing valve or injection mixing from 2 sources: (1) Solar water storage and (2) my condensing boiler which would be a backup system and heating the remainder of the house as well.

    I can find mixing assemblies that use outdoor reset controllers, and I can find differential controllers that can handle two heat sources; however, I am not sure about a controller that does BOTH. For example, lets assume that the solar storage tank is at 80 degrees. The outdoor temp in the day is 56 degrees and the outdoor reset controller calls for 78 degree temperature for low-temp manifold. Because the solar storage tank is at 80 degrees, it should injection mix from heat source #1 - solar storage tank heat exchanger circuit. Then in the evening, the temperature falls to 45 degrees, now the outdoor reset controller requires 82 degrees to meet the heat load of the space. The differential controller should determine that the tank does not have sufficient temp and it should injection mix from heat source #2 - boiler.

    Keep in mind that this may be more complex than a simple differential controller - it would be comparing to the outdoor reset desired temperature, not a fixed temperature. I'm not sure if the standard differential controllers would work in this case.

    Please advise if there is a differential controller that can do what I need above.

    Alexander P. Brown
    The P. stands for Plans

  2. #2
    Join Date
    Apr 2010
    Location
    southern california
    Posts
    535
    Look into Tekmar or Heat-Timer. The differential controls are still needed to for the solar side of your equation.

  3. #3
    Join Date
    Aug 2009
    Posts
    2,459
    Alexander- I believe that you are looking for a "Freely Programmable" or "General Purpose Controller." Controllers in that category can run custom programs to make them do absolutely anything.

    Every controls manufacturer offers controllers that will accomplish this for you. The differentiating factors between controller options would include the language that is used to make the custom control, whether it serves web pages for control over the internet, whether it uses an open protocol which you can build on in the future, and if it can be bought off the shelf or if you have to go through a controls company to install it for you.

    I would suggest contacting a local controls company and asking them for a quote. This looks to be a very simple job, but there are pitfalls for a homeowner. Building a relationship with a local company will pay off when you decide that you want to add more controllers to your system to control your sprinklers from your cell phone, etc.


  4. #4
    Join Date
    Apr 2007
    Location
    Amarillo by mornin'
    Posts
    850
    BACnet is right. If you don't feel like getting an outside contractor involved your best bet will be a freely programable. Have you looked into something like Netduino?
    "It's not that I'm smart, it's that I stay with the problem longer”
    Albert Einstein

  5. #5
    Join Date
    Mar 2005
    Location
    Pacific Time Zone
    Posts
    4,151
    While a programmable controller would be nice, why not simply put a low limit t-stat on your solar tank with like a 5 deg + differential and a relay? Tank temp below low limit then enable boiler and close solar injection valve and pass reset value to boiler/boiler valve. Granted you'll have a reset schedule, but since it is low temperature radiant I would guess the reset to be minimal.

    Besides, why not use up the heat in the solar tank first? Eventually you'll hit the low limit even with a higher reset schedule and in turn it'll enable the boiler.

    Also will you have a circulating pump running all the time during 'heating season' or is it zoned in a manner that a zone call turns on a pump?
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