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What are folks using for flue gas thermostat?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by sgschwend, Nov 17, 2009.

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  1. sgschwend

    sgschwend New Member

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    I am looking to add one on my system and have found only two suppliers. I receive the impression that they were not very common in the USA.

    I did find an industrial model made by Honeywell and a I think the other was made in the UK.

    Has anybody used a Dwyer Instruments unit?

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  2. Singed Eyebrows

    Singed Eyebrows New Member

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    My Atmos has a built in flue gas thermostat for the purpose of controlling a Laddomat 21. Kotly sells them, unfortunatly they say they don't ship to the USA anymore. Maybe you can get the brand off the website & find it elsewhere. The UK dealers should have these & shipping should be reasonable on this lighweight item, Randy
  3. dirttracker

    dirttracker Member

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    I use a PI temp controller like this one http://www.auberins.com/index.php?main_page=product_info&cPath=1&products_id=106 I use this with a K type thermocouple and use one of the alarms as a programable means to open/close the relay that powers the cirulator on my Tarm. I use 2 of them for the circ control, 1 for flue gas and 1 for water jacket temperature. I have the normally open alarm outputs wired in series on the controllers so the pump is only commanded on when the water temp is above 160F and the flue gas is above 230F. For the flue gas measurement I use a jacketed thermocouple inserted approximately in the center of the flue pipe right at the boiler connecction to the chimney. If you boiler idles, you may need to play with the setpoint temperatures to make sure you are shutting down when the boiler is out of fuel, not just when it's at idle. On the few ocasions my Tarm goes into idle, the flue gas temperature drops fairly quick, say from 500 F to 230 F within 4-5 min. Since I run the Tarm into storage, it does not idle for long and I am able to find a set point that will keep the pump on through the idle period. It's not quite a perfect system, when I find more time to play I plan on working out a system based on the water outlet - inlet temps to determine out of fuel.
  4. sgschwend

    sgschwend New Member

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    Thanks DT,

    I purchased a similar product from Grainer, Dwyer Instruments. I am glad to hear that it will give me some control over the shutdown.

    It sounds like your second temperature switch controller replaces the mixing valve? It would be less expensive to use one, almost half the cost of a Taco mixing valve. Thats a good idea, perhaps it will make its way into the "simplified pressurized storage system design". It would be a good addition to any Greenwood, they need a hydrostat to protect against overshoot, the temperature switch controller would provide better set-ability.

    Too bad there isn't a multi prob versions.
  5. dirttracker

    dirttracker Member

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    I atill have a mixing valve at the inlet to the boiler, the 2nd temperature switch is used to command the circulator pump. The way my ssystem is set up, the stack temp needs to be above 230F and the bolier warter temperature needs to be above 160 F for the pump to run. I figured there was no point in circulating water until the boiler was up to temperature. I think you could use a controller to command either a zone valve or a small pump to use for boiler inlet protection. I wish I had thought of this when I set up my system. The Termovar mixing valve I have on the inlet is the only part of the system I am not completely happy with. I think a samll pump commandes using a temp controller woupld have been a better choice, but I don't have a good enough reason to change thisgs out and find out for sure.

    I also have an aquastat on the boiler that can turn on the pump in case the temperature controllers fail. I figured the controllers were cheap and it may be a good idea to have a backup. So far it has not been necessary.
  6. sgschwend

    sgschwend New Member

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    I have installed the Dwyer Instruments thermocouple switch and find that it works to shut the machine down but I need a hold off period because the controller has the output in the OFF position. I thought this controller had a hold off setting but now I find the setting is only applicable for the alarms and not the controller.

    The thermostat/controller is setup for a cooling cycle. When the set point is exceeded the relay is closed. The thermocouple is in the flue and the set point is set to 250 degrees F. My normal flue temperature runs between 350 and 400 F. With a fire the set point is exceeded and the relay is closed allowing the boiler draft fan and circulator to run. If the fire goes out the temperature will drop and the relay will open and shut down the system.

    So I find that I need to get the flue temperature up before the system will run. I could just drop the set point for five minutes. Any ideas?
  7. Gooserider

    Gooserider Mod Emeritus

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    Maybe rig something up with a bypass circuit and a NO countdown timer, good for say 60 minutes - build a fire and set the timer, which closes and sends power to the boiler controls for 60 minutes, by which time the flue is (hopefully) up to temp, and the controller circuit is on... At the end of the 60 minutes the timer opens, leaving the controller in charge, so that it shut things off when the temp drops back down.

    Seems like the easiest way to do things as it doesn't require messing with the controller, and is reasonably "idiot proof"

    Gooserider
  8. dirttracker

    dirttracker Member

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    Steve,

    Assuming your temp controller is similar to the ones I have, you should be able to rig up a latching relay circuit to use as a bypass for the temp controller. I have attached a simple schematic. I assumed you were using 24VAC for the control power, but this should work with any voltage as long as your temp controller, relays, and other components can handle it. The attached circuit should work if you set the alarm to come on after stack temp is above your shut down setpoint. The output from the relay should be wired to bypass your temp controller and provide power to the wood boiler. To start the boiler up, hit the pushbutton. The path from the relay output through the NC contacts in the temp controller alarm relay will latch the relay on after the pushbutton is released. Once the stack temp is above the alarm setpoint the alarm relay will activate and unlatch the bypass relay. Once the stack temp drops below the control point, the boiler will shut down as intended. This is the basic logic used by Tarm for the fan controller. The only drawback to this is the stack temp needs to come up to the alarm setpoint before the boiler will shut down with the controller. If the stack temp never reaches the setpoint (ie. a cold fire), the draft fan will run until it is manually shut off.

    Attached Files:

  9. Joegoesfishing

    Joegoesfishing New Member

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    I bought a $25 high temp t-stat from Alternative Heat. It's good up to 300 or 500C. Comes with a remote bulb on about 3ft. of capillary. I have it controlling my fresh air fan & a circulator pump. Works great, can't beat the price.

    Joe
  10. sgschwend

    sgschwend New Member

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    Joegoesfishing, do you mean to say that it does both turn the fan/cir on and off?

    DirtT, I see what you mean by setting up a latching switch.

    GR, That is a good technique most folks would know how to use it too, just twist the knob.


    I have thus far just dropped the set point to 70F to light the fire, by the time I am done feeding the fire the temp is up and I can return the SP to 180F (seems like a good place to have it based on observation).
  11. Gooserider

    Gooserider Mod Emeritus

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    Yes, and my idea isn't really that different from what DirT is suggesting - just using slightly different hardware than his approach. My timer notion has something of a fail safe, as it will shut things down when the timer runs out if the boiler hasn't reached the setpoint temp where the controller takes over, his keeps going until you reset manually.

    I would suggest doing something, for a bypass as I see needing to repeatedly reset the controller setpoint as being something it would be easy to forget to do. The other problem is that since they tend to design under the assumption that there won't be a lot of changes, many controllers may use adjustment switches or control knobs with a limited and fairly low number of designed adjustment cycle lifespans... Better to use a switch that is intended to cycle a large number of times, and leave the possibly less robust switch on the controller alone, unless there is a real need to change it....

    Gooserider
  12. Joegoesfishing

    Joegoesfishing New Member

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    I use a flue t-stat to turn on my combustion (fresh) air fan and my water circulator. When the fire dies & the flue cools, the fan & circulator are shut off. I keep the temp setting on the T-Stat low so the fan & circ. turn on quickly & shut off only when the flue really cools. Works well for my setup.
  13. sgschwend

    sgschwend New Member

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    Joe, I see what you mean. I guess if I set my set point lower it would work that way too. Something to think about.

    GR, that is a good point put the where on the easy to replace and less expensive part.

    I should spend the time to make a complete schematic of this controller. I sent a picture of the controller to the other two Essex users who tell me theirs are different. A bypass circuit is definitely needed.
  14. sgschwend

    sgschwend New Member

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    I spent last night updating the manufactured schematic. I was able to identify the uses for the relays. I found two relays that could provide the solutions we talked about: latching relay, and delay off relay. Those two relays are used for the oil burner and the smoke hood.

    I don't use either but I believe I will use the smoke hood relay; I will only need to move one wire and then the off delay relay will run the draft fan and boiler circulator pump for the program time period. There is a front panel switch (even label "start") that initiates the sequence.
  15. sgschwend

    sgschwend New Member

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    I have now completed the changes to the Essex controller.

    I took the lower limit thermostat from the oil burner relay and connected it to the circulator pump, so now it run after the set point is reached and goes off again either by the flue gas thermostat or the lower limit difference setting.

    The off delay timer now is wired to the draft fan so that when you hit the start button the draft fan will run until the end of the timer or the flue gas thermostat set point is reached to keep it running during the burn cycle.

    One thing that has amazed me is the limited amount of offering for dual set point temperature switch. I found a Honeywell which used 50 year old technology and White Rogers which I think is pretty much the same thing. These switches rely on beryllium copper springs to provide temperature related switching. Works great as long as you don't stress them, but after being stressed they will not work reliably again. The issue is using the temperature dependent spring as part of the switch mechanism, rough up the switch and you damage the temperature characteristics. The Dwyer Instrument dual stage temperature switch should be a better choice.
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