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T-stat options that don't short cycle

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by pybyr, Nov 25, 2013.

  1. pybyr

    pybyr Minister of Fire

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    To my dismay, I am not running wood right now; I'll skip the details...

    For the time being, to my chagrin and some $ouch$, right now I need to run oil to heat this big old hard to heat 1840a Vermont house.

    It's been five years since I have run oil, and I am now aware just how much my Honeywell programmable t-stat short cycles my forced warm air ThermoPride, even though I have selected the "steam" setting on the t-stat that supposedly results in fewer, more sustained/ longer, burner firings.

    Not only are short cycles bad for efficiency of oil combustion in general, they are extra bad when you have a big heavy heat exchanger like my ThermoPride does, and then add to that a lot of long ductwork through a chilly stone basement, and I can tell that I am throwing away needless oil and dollars.

    I put in a net-controllable t-stat that gives me an option to pick the response "band" and can tell that setting a wider band results in a LOT more _net_ heat in the house per net amount of time I hear the burner run. Yes there is a bit more fluctuation, but it is still much more comfortable.

    But that net-controllable T-stat is having other issues with manufacturer server support, so I am back to "less smart" T-stat in the meantime.

    Can anyone please point me to a cheap T-stat that lets you set the band/ range of upper and lower temperatures between its switching the burner "on" and "off"?

    Many thanks!

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  2. ewdudley

    ewdudley Minister of Fire

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    Used Honeywell T87 classics are pretty easy to come by. For more hysteresis need to adjust 'anticipator' to higher amperage, but you may not have much upper range to play with if you're driving a heavy load like a Taco 555 beehive. The dip switches on modern micro-controller based T87s will lengthen cycle time independent of thermostat load.
  3. Chris Hoskin

    Chris Hoskin TarmSalesGuy

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    can you down-size the burner nozzle too? Single digits here in VT/NH this morning, if the furnace is still short cycling, it is very likely over-sized.
    BoilerMan and ewdudley like this.
  4. altmartion

    altmartion Feeling the Heat

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    are you sure it is the stat that's the problem? have you ever done a heat load to match your equipment and ductwork? is it cycling with the stat or is it cycling from the high limit?
    STIHLY DAN likes this.
  5. pybyr

    pybyr Minister of Fire

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    Thanks for the suggestions.

    The difference between the recently-tried "Cyberstat" which let me set a wider band for response cycles, and the Honeywell time programmable thermostat from about 15 years ago that I reconnected last night is immense. The Cyberstat gave me long steady runs of the burner and nice warm air coming out of the ducts, and the Honeywell gives staccato short belch sessions of the burner, with the fan running a lot but only disappointingly semi-warm air coming out of the ducts- the difference is remarkable, and more than I'd have expected.

    The fellow who put the ThermoPride in for me 16 years ago was a highly experienced warm air installer and he did size the furnace to the house. He told me that it would be undersized in the condition that I'd bought the house in/ was starting with (where wind very literally blew through some rooms) but a slight bit oversized if/after I got the house "fully tightened up." I have brought the place a long way from where it started 16 years ago, but it wouldn't yet qualify as "fully tightened up."

    I could downsize the burner nozzle but don't want to hurry in that direction.
  6. BoilerMan

    BoilerMan Minister of Fire

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    Try simply shorting the two wires together and see if the burner runs for the full time (say an hour) if it does your ductwork is not undersized for the nozzle size (but may still be oversized for the house).

    Personally with scorched air setups, I'd start with the smallest nozzle I could get way with. You'd be surprised how much oil that will save by keeping the HX cooler and more efficient at warming the air and not the flue. Find out what the spray pattern is and get a .5 GPH, and see what happens, all your out is the small cost of the nozzle.

    TS
  7. pybyr

    pybyr Minister of Fire

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    Thanks- I am pretty sure that the ductwork is not undersized and that the burner could run full out endlessly, at least back before I put the water:air HX above it for my Econoburn system (for which I use the ThermoPride's blower to push air)- but that HX is immense in cross section (bigger than the furnace outlet)(overkill is my middle name) so hopefully it does not obstruct too much.

    I do understand the difference between the thermostat ceasing a call and the high limit cutting off the burner, and am confident that I am not running into the high limit. My current/ former semi-smart time/ day thermostat has a symbol for when it is calling for heat and it is cycling at the same time/ ratio as the burner.

    If I drop down nozzle sizes, do I need to start messing with different spacings/ proportions between the nozzle and the flame retention head, electrodes, etc? I do have the full Beckett tech manual around somewhere and even the gauge plate around somewhere else, and a Bacharach Wet Kit old school analyzer (I like to DIY and picked it up somewhere for a relative pittance), so I can mess with any of those things if needed, but would prefer not to get into too many moving variables all at once.

    Thank you all very much for all of the suggestions!
  8. altmartion

    altmartion Feeling the Heat

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    no you don't have to move the z demesion. you will have to adjust air. but you need to make sure you are within the factory temp rise spec so to avoid condensation in the hx. don't down fire it below the rating on the info plate of the furnace. if it soots and condensates at the same time for to long it can plug the hx like concrete. if you do down fire. you may want to pick up a few different nozzle brands and patterns because you will most likely be swapping them out till you find the best running and quietest nozzle. keep a close eye on stack temp. it realy isn't that hard, just stay within manufacture's specs for everything.
  9. pybyr

    pybyr Minister of Fire

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    Thanks everyone of the input.

    The good news is that I now have the newer 'cyberstat' running satisfactorily and it does cycle the burner WAY less often than the semi-smart Honeywell time/day unit that I'd been using in the interim, resulting in way more net warmth per amount of time I hear the furnace run.

    Re: Nozzle size, my ThermoPride is spec'd to run a 1.25 or a 1.10 gph nozzle and it has a 1.10 in it. I'd be glad some time to play around with smaller nozzles if I can really net more useful heat per gallon burned, but don't want to foul (literally) the HX.
  10. pybyr

    pybyr Minister of Fire

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    Can you please remind me the lowest stack temp that it'd be safe and sensible to run on an oil warm air unit? Thanks!
  11. altmartion

    altmartion Feeling the Heat

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    stick close to 350.

    Re: Nozzle size, my ThermoPride is spec'd to run a 1.25 or a 1.10 gph nozzle and it has a 1.10 in it. I'd be glad some time to play around with smaller nozzles if I can really net more useful heat per gallon burned, but don't want to foul (literally) the HX.[/quote]
    I don't recommend going below 1.10 if that's what the thermopride gives as a recommended minimum firing rate. there are a few different reasons for this. one being condensation. thermopride is one of the toughest oil fired resi furnaces made. most pros agree. there are ways to do it with pump pressure and other adjustments but to do it right you need a modern combustion analyzer. it can be done but it would take time and mostly trial and error type thing. it also depends on burner model.
  12. heaterman

    heaterman Minister of Fire

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    I didn't read through the whole thread here but, are you sure the furnace is not cycling off on the high limit when the oil burner is running? Sometimes various circumstances can combine to cause an overheat condition.
    Restrictive air filter + dusty blower wheel + coil in the plenum = over temp condition.

    As far as the HW T-stat is concerned....most that I use are adjustable for number of cycles per hour. If yours is not, just go by a cheapo mechanical stat with an adjustable anticipator. Crank the thing up to around 1.0 to 1.2 It might overshoot temp a bit but it won't short cycle.

    You are correct that short cycling kills efficiency in your oil furnace......just like in a wood boiler.
  13. pybyr

    pybyr Minister of Fire

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    I don't recommend going below 1.10 if that's what the thermopride gives as a recommended minimum firing rate. there are a few different reasons for this. one being condensation. thermopride is one of the toughest oil fired resi furnaces made. most pros agree. there are ways to do it with pump pressure and other adjustments but to do it right you need a modern combustion analyzer. it can be done but it would take time and mostly trial and error type thing. it also depends on burner model.[/quote]

    Thanks! When I bought the house in 97 the old furnace was a rust heap. I picked a ThermoPride 'cause when I was a teen I used to pick up supplies for my summer job at a place that dealt in heating along with farming supplies, and I used to "pick the junkpile" out back for motors and such for projects- and of all of the old furnaces that were out there, the ThermoPrides stood out as the opposite of flimsy.
  14. pybyr

    pybyr Minister of Fire

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    I am happy to report that my 'cyberstat' is not only now working, but is doing a really good job of not short-cycling my furnace. What a difference!
  15. altmartion

    altmartion Feeling the Heat

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    great!! glad to hear it. short cycling is a furnaces worst enemy.

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