Overheat control wiring

Carl Webber Posted By Carl Webber, Nov 14, 2014 at 9:52 PM

  1. Carl Webber

    Carl Webber
    Member 2.
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    Sep 8, 2014
    121
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    Loc:
    New Ipswich, NH
    I have just installed a Tarm OT50. I have the oil side set up and running. I also have a 3 zone controller set up to run the two zones for the house and one zone for the superstor. I want to start using wood, but i need to set up the overheat control. I have a honeywell L4008 high limit aquastat. The wiring schematic in the Tarm manual shows the aquastat being set up as a jumper for thermostat for the main zone to use it as a dump zone. I want to get a little more complicated. I Have two zones for the house, one upstairs, and one downstairs. I would like to somehow set both of them to be the dump zones for the boiler so if it overheats it just dumps the extra into the house. Is there a way to wire this aquastat to two zone but still have them work independantly? If i just simply connect a thermostat wire from the aquastat to the connection on the zone controller for the two zones it will jump the zones so the thermostat for one will control both. I want them to still work independently when the boiler is not overheating. Can someone possibly explain how to do this, or point me to a diagram that would show me. I've been searching but coming up with nothing.
     
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  2. Carl Webber

    Carl Webber
    Member 2.
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    Sep 8, 2014
    121
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    Loc:
    New Ipswich, NH
    From what i can figure, I need some sort of a relay setup so that each zone is kept independent. The thing that i'm having trouble with is that all the relay setups i see are 120volt. I need a 24 volt relay setup so it will tie into the thermostat side of the existing controls and not the circulator side. Really all i need is a bank of 2 or 3 relays that will switch when i supply them with voltage. But I don't need them to supply voltage to anything. All i need them to do is switch and make the connection when i supply voltage. Then i can tie that into the appropriate thermostat connections for the zones i want to use as dump zones. I haven't found such a thing yet though. All the switching relays i have found work on 120volts and when they switch they supply 120volts. I need one that isolates the supply power for the relays and the other side of the relay so one side has the voltage and does the switching and the other side is just simply either connected or not without any voltage so it could be hooked into the thermostat side of a normal zone control setup. If anyone knows of anything like this or a way to set up a normal switching relay please let me know. I have a spare taco 3 zone switching relay witch would work for my purpose if i could figure out how to get it to function on only 24 volts and not 120volts.
     
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  3. Carl Webber

    Carl Webber
    Member 2.
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    Sep 8, 2014
    121
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    Loc:
    New Ipswich, NH
    I think after talking to a plumber friend I have answered my own question. What i need is a RIB (relay in a box). They make them that have dry contacts both before and after the relay and the supplied 120v or 24v just run the relay switch. connecting some thermostat wire to the aquastat and then to two of the RIBs should do just what i need. And, if you feel like spending more money, they also make a box that has a rack in it and you can buy rack mountable RIBs and put as many as you like in succession to run as many zones as you want.
     
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  4. polychrome

    polychrome
    Member 2.
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    Oct 19, 2010
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    Loc:
    Central MA
    This is late, but since this is the only site where anyone would discuss an OT50 and the only discussion of wiring the overheat control for multiple heat dump zones...

    The originally supplied overfire control was a Honeywell L4008B 1013.an SPST close on rise contacts fixed 5°F differential, at 120V the contacts are rated 8A, 48A locked rotor. The manual suggested wiring the overheat in parallel with the thermostat as you describe. Wiring the L4008B 1013 in parallel with the zone relay at 120V to directly energize the circulator ensures that the overfire heat dump works even when other controls have failed or will not energize the zone e.g. some L8182 configurations also lock out the circulators when the oil primary locks out.

    To energize multiple zones requires multiple sets of contacts. Without worrying about whether its assembled on site from a 24V transformer and relay(s) and a box or purchased pre-packaged, for 3 zones there have to be 3 sets of contacts.

    The Honeywell R845 relay-in-a-box is a typical one zone example, the box contains a 10VA 24V transformer to allow a single contact closure to energize a 10VA DPST relay. Of the 2 sets of contacts, one set is rated only for controls, the other for controls or power. It takes two of these one-zone relay-in-a-box devices, or a two zone relay-in-a-box to wire the overheat as thermostats, and three to energize the zones directly. The single zone boxes can't be wired to all be switched directly by the aquastat becuase that puts the transformers in parallel. Any small difference in construction causes the transformers to have a slightly different voltage and push current through each other and overheat. (if you wired the AC backward on one it would be a very rapid failure) They have to be wired so that one relay is switched by the control, and its control contacts drive the next relay.The advantage of a relay-in-a-box is convenience and a 3 zone relay-in-a-box would be even more convenient. Material cost and reliability are not advantageous.

    If assembled from available components then a 6 or 8 inch box, a self protected 24V 20VA to 40VA control transformer and a pair of DPST relays. I'm not a big fan of AC "Ice cube" style relays, buit that's just my experience. The White Rogers (now Emerson) 90-380 and similar offerings like the TE Connectivity T92P11A22-24 are compact and reliable power relays.. Older designs like the Honeywell R8222. are larger and more configurations are available but they're not actually more reliable A few physically large open construction relays have screw terminals, they tend to resemble the Potter Brumfield (now TE Connectivity) PRD-11AY0-24 have enormous contacts and can actually reliably switch high current loads for a half million cycles, otherwise 24V control relays have 1/4" quick connect terminals. The more commonly used physically smaller relays can switch large currents *or* a half million times, but not both, and especially not if the contacts don't fully cool between cycles. Miniature 24v relays intended for electronics aren't reliable enough to use for a safety control. The "best" choice of relay depends on whether it will be wired as thermostat or to directly power zone circulators, and how much use is expected.
     
  5. jebatty

    jebatty
    Minister of Fire 2.
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    Jan 1, 2008
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    Loc:
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    I haven't read through your posts in detail, but here's something to consider anyway. That is, a runaway fire in the boiler, like might happen if the firebox door is not closed tightly, or your wood is too dry, or the boiler control fails to slow down or shut off the draft fan if boiler output water is too hot because of a too hot fire from another cause. In addition to a gravity dump zone, what I also have found to be very useful is a temperature controller using a k-type temperature sensor in the flue which shuts down the draft fan resulting form a too hot fire, and then re-energizes the draft fan as the stack temperature falls to an acceptable level. With my Tarm, stack temperature falls very rapidly if the draft fan is shut down.
     
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