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Aquastat for Hydronic Loop using Wood Boiler and Oil Boiler as Secondary

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AK907_2
AK907_2 Member Posts: 9
I have a wood gasifier which circulates (non-pressurized) through a plate exchanger with pressurized hydronic loop circulating on the other side of the exchanger (see attached drawing). To supplement the wood heat I installed an oil boiler and tied it into the hydronic heating loop. I planned to use a pipe-mounted aquastat to start the oil boiler when the wood wasn't keeping up or not fired, i.e. when on vacation. Can anyone recommend what aquastat I need to start my oil boiler on a decreasing temp using the boiler's 24v wiring? The boiler has its own circulator, shut-offs, etc.

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  • hot_rod
    hot_rod Member Posts: 22,244
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    It looks like you have the start of a primary secondary piping.
    You have a pump on the wood boiler, pump on the primary, What about a pump on the oil boiler, and the two loads? Where are the returns from the loads.

    Just want to make sure you have the piping correct.

    Is there a buffer tank connected to the wood boiler? they can be tough to manage without some buffer or a large volume in the boiler.

    Also some sort of return temperature protection for the wood boiler?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • hot_rod
    hot_rod Member Posts: 22,244
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    If all the loads are high temperature, no mixed down required for radiant?, a simple parallel piping would work.

    With P/S you always have a blending of temperatures based on the various flows.

    A differential control watches temperature in two places instead of an aquastat watching only one.

    So when ever the wood boiler (sensor 1) is hotter than the system return (sensor 2) it's is powering the wood boiler control and pump. Set it for a minimum 140F that give you additional return temperature protection, I like a thermostatic return valve also, shown at the boiler. Never run the wood gasification boilers with return temperature below 130 ish.

    If the wood boiler drops below the system return it powers off and starts the oil fired. That differential is adjustable.

    With an indirect tank for DHW, depending on the tank and your DHW needs, you may need to run the wood boiler fairly hot, that minimum may need to be 160. Same for your heat emitters if they are sized for design at say 180F.

    Another option is with a buffer on the wood boiler, then you could pull the heat loads off the tank with outdoor reset and better utilize the BTU, and reduce boiler cycling.

    What you will find with wood boilers when the loads turn off, the boiler cannot ramp down quickly, so a buffer tank, or dump zone may be required. Or tend the fire constantly to match the load(s) Easier said than done.


    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Solid_Fuel_Man
  • AK907_2
    AK907_2 Member Posts: 9
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    Yes, circ pumps on the hydronic loops and for the main loop (which runs continually), all controlled with a separate relay box. Wood boiler (crown royal pristine 1000) is not pressurized and has its own circ pump to/from the plate exchanger and temp limit switches.
  • AK907_2
    AK907_2 Member Posts: 9
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    The gasifier combustion air shuts off when boiler hits 140 F. High limit is adjustable to 180 F.
  • hot_rod
    hot_rod Member Posts: 22,244
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    That is the attempt to control the boiler output. They don't, can't shut the air completely of the fire goes out.

    You would also get a huge flashback when you open the door on an O2 starved fire. Always stand to the side when you open the door on a wood boiler is some singed advice :)

    So a small amount of air is always entering the stove, and overtime can cause over-heating even with the fan off. It's best to have a plan for that shut down condition, buffer or dump. Or carefully monitor the burn.

    Nobody likes to reach in and pull burning logs out of the firebox, so unless you can stoke the fire exactly to the ever-changing loads, you need to plan for that condition.

    Pellet or chip boilers are easier to control as you stop the fuel source and shut down the air, you don't have a belly full of fuel like a cordwood boiler typically does.

    Burning wood for heat is a very interactive hobby, when you add water and steam flash potential, it gets more interesting.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    SWEI
  • AK907_2
    AK907_2 Member Posts: 9
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    I'm a little confused about what aquastat (brand/model?) I should use to start the oil boiler when the loop falls to 140F. I don't think there's any way to effectively shut down the wood boiler beyond it's own control limit switches, besides, if the hydronic loop has dropped to 140F, the wood fire has already stopped producing adequate heat.
  • njtommy
    njtommy Member Posts: 1,105
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    You could add a Taco pc 700 control and zone/pump panel to control both boilers. It will control two boilers no problem.
    http://www.taco-hvac.com/products/zone_controls_main/zone_controls/pc700_series_add-on_power_controls/index.html

    Looking at the boiler it looks to have very little mass to the boiler from what i can tell online.

    I will highly agree with Hot Rod and recommend a large buffer tank to store all your btus. It will allow long burn times and get the efficiency out of it your suppose to. It will also help the oil boiler from short cycling as well another added bonus.

    http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_10_0.pdf

    http://forum.heatinghelp.com/discussion/155927/econoburn-biomass-system-completed#latest
  • hot_rod
    hot_rod Member Posts: 22,244
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    AK907 said:

    I'm a little confused about what aquastat (brand/model?) I should use to start the oil boiler when the loop falls to 140F. I don't think there's any way to effectively shut down the wood boiler beyond it's own control limit switches, besides, if the hydronic loop has dropped to 140F, the wood fire has already stopped producing adequate heat.

    The oil boiler should already have a control and limit onboard, just send power to the boiler when it is needed, it will run itself. basically you are trying to run either, or. When the wood drops to 140, or whatever you decide the oil takes over. The oil boiler temperature will set below the operating temperature of the wood. so when the wood catches up, the oil drops off.


    The idronics 10 that Tommy linked shows and explains control logics with ∆T controllers. It may not have your exact piping, but the controls work the same.

    Idronics 17 on Thermal Storage has more piping options
    Idronics 14 Controlling Hydronic System steps you through control selection and wiring options, also with piping and wiring schematics.

    I assume the oil boiler is already in place, wired and working correctly? No need to rewire all that, just add the control that "makes the choice"

    It helps to determine the control logic before you start piping the system, so you end up on the same page.

    Decide on how you want to pipe, pros and cons to all the various methods P/S, parallel , series, hydro sep, or buffer tank.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • AK907_2
    AK907_2 Member Posts: 9
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    Thank you, I'll study the attachments.
  • Zman
    Zman Member Posts: 7,572
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    I agree with what Hot Rod and NJTommy are saying. Mass is super important with OWB's.
    Another consideration when determining how to pipe and control these relates to the heat that is wasted when you circulate through a boiler that is turned off. Boilers, particularly ones that don't have dampers become really good emitters when they are not fired.
    In your case, if you are not running glycol in your OWB, it may be necessary to circ through it to prevent freezing, even though you will burn more oil to do so.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • AK907_2
    AK907_2 Member Posts: 9
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    Sorry I wasn't clear - I'm past the "engineering stage" - what I have is basically a "HYBRID OPEN/CLOSED SYSTEM" as described on page 20 of the Idronics 10 brochure, except that my wood boiler is an indoor model gasification boiler, so no underground piping (its actually attached to the exterior of the house in its own boiler room). I misspoke in my earlier post - the wood boiler combustion fan shuts off at an adjustable set point of about 170 F (boiler reservoir temp) and it's circ pump shuts off at 140 F.

    The wood boiler is plumbed as per my original attachment and I've run it for about 4 years and it's done great (3500+ sqft house in Alaska). However, when going on vacation our house sitters aren't as reliable as we'd like so I added an oil boiler and tied it into the pressurized side of the plate exchanger. The oil boiler is plumbed as shown also - supply and return are 4" apart on the piping exit the exchanger (pressurized side).

    I'm looking for the right aquastat to make the oil boiler come on when the water temp leaving the exchanger (pressurized/hydronic loop-side) falls too low (140 F ?). I tried a single function Honeywell L6006 from Home Depot but then realized it only activates on temperature rise to set point not temperature fall.
  • hot_rod
    hot_rod Member Posts: 22,244
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    Some models of the Honeywell allow you to chose NO or NC, so you can use it either way.

    All the electronic versions that I have used have the NO and NC terminal.


    You get a lot more function and adjustability with the digital stepping type.

    Honeywell, Ranco, Penn, Johnson Controls are a few brand names. Download a manual to learn how they work and program.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • AK907_2
    AK907_2 Member Posts: 9
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    If I need the boiler to shutdown then the output should open the contacts, so I should set to "NO"? How does that work if I need that to happen on a descending temp - do I have to use two switches? I've read several manuals and I guess the terminology is getting me confused. Also, the wires to the boiler are 24v T-stat wires. Does the Johnson Controls A419 you show allow for that? I've had trouble figuring that out from the manuals as well.
  • Ironman
    Ironman Member Posts: 7,377
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    The A419, as well as the other ETCs all come in either line or low voltage to operate the logic of the control. The contacts (NO, NC, C) are isolated from the power terminals, so it doesn't matter which voltage is applied to them.

    Place the sensor where it can sense the water temp in the wood boiler. Set the ETC to about 150* to open the contacts to the oil boiler and close the contacts to bring on the wood boiler circulator.

    This will make it an "either or" setup. If the wood boiler is hot, it will be selected, if not, the oil boiler will be. You made need to add an isolation relay for the ETC to control the wood boiler circulator.

    'Normally" means with no power or action applied to the device. Hence, "normally open" contacts would close when the device reached its set point and vise versa.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • hot_rod
    hot_rod Member Posts: 22,244
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    Use a light switch on the wall as an example. If the light is off, the switch is open.

    When you move the lever up, it becomes a closed contact. So this could be considered a normally open switch.

    An aquastat is just a switch, it uses temperature to open and close the contact instead of human power. It can either open or close the contact on temperature rise for your application.

    Add a RIB double throw relay and it could do both, open one circuit, as it closes another.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • AK907_2
    AK907_2 Member Posts: 9
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    Therefore the "light" is the boiler and is also NO? That doesn't answer my question about temp fall vs temp rise. If I set the aquastat to NO how do I make it activate on a falling temp rather than a temp rise? What switch/aquastat do I use?
  • hot_rod
    hot_rod Member Posts: 22,244
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    Did you have a chance to read through this Idronics 14? It has a good explanation along with graphics of how switches operate, and explains NO and NC. It starts out with basic control wiring and examples, entry level explanations.

    Maybe buy one of the controls and wire it to a bulb on your bench or desk and try different connections to get a feel for the functions.

    Wire power to C terminal, and the bulb to NO. No power will go to the bulb you connect until it reaches the temperature you set.

    Warm the sensor with a match, carefully, watch the temperature increase, when it hits the setting you dialed in, the bulb will come on. Bulb stays lit until it drops the differential range then turns off.

    If you write to C and NC, power is on to the load until it reaches the temperature you set, then it powers off.

    Or find a local HVAC tech to help you through the wiring, this is fairly basic control logic and wiring for someone that works with control wiring. I think once you see it demonstrated, or try it yourself it will be clear.

    Be aware of the dangers when working with electricity.



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    http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_14_0.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • ratio
    ratio Member Posts: 3,649
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    Normally closed means just that: sitting there with nothing special going on the contacts are closed. However, you have to know what the designer decided was normal. Unpowered? Good guess, but far from guaranteed. It's usually, for relays and contactors, it's unpowered state.

    Then there's close on temperature rise vs close on temperature fall; reverse acting vs direct acting; and don't get me started on fail safe vs fail secure or maintained contacts!

    I've been in the trades for nearly twenty years, and I still have to stop and think about what's going on, sometimes even put a meter on the contacts to see just what they're doing. With a Form C contact, that is, normally open and normally closed terminals, you're covered for whatever you need, but talk it out with yourself to figure out which contact you need. If the temperature is below this setpoint, I want this contact closed, if the temperature is above that setpoint, I want that contact closed.