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short cycles, low diffs, oh my!

bananapeal Member Posts: 4
edited December 2018 in Oil Heating

please pardon the tone. i am angry with the state of manufacture of this equipment. seemingly the things i am allowed to adjust on the system are not allowing me to have desired effect.

i have an oil fire 2-zone boiler with domestic coil. it is a crown shorty something. whenever it turns on the flue runs it seems like a big process to start the fire... loud flue fan starts for a few secs then you hear the roar of the flame below. outside, you can smell the aerosoled oil that is certainly not getting burned each time a start up cycle occurs.

i'd like to stop throwing money out of the window. i want it to turn on once an hour maximum. even on cold days. and stay running until call for heat is exhausted. not on and off several times an hour.

currently with both zones running it can start at its cold low limit of 160 and run to my programmed high limit 205 in about 10 mins. a decent cycle, though ideally it wouldn't stop during the call for heat. i don't understand why the fuel pump inside can't just back off a little once it gets up to max temp and stay there but whatever. fixed nozzles and things i'm sure. (2018, we can land on moon but wow home heating fixed temp)

the truly baffling thing is that my honeywell L7224u has but a 10 degree differential. so once the boiler hits high limit, it shuts off a good 2 mins before turning on again. you know, kerosene out the window... etc. (boiler temp bounces 195-205).

sometimes the thermostats are out of sync, the bottom zone will call for heat --and this whole process happens much faster. This is infuriating knowing all the thermal cycles going on, probably the (extra) soot formation and localized heating among other bad effects when the machine turns on and off this often. let alone any spurious domestic water demands.

why are these systems designed like this? 10 degree differential? surely this is a bad joke played on me by the installer!

i noticed if i want "heat scavenging feature" --aka normal operation for me not a "green feature" -- i must pay $ or so for a new aquastat board from the incompetents at honeywell. how is this a thing!?

are there other levers i can pull on this machine or do i have to deal with some idiotic board from honeywell designed in 1980 with ridiculous hard-programmed differentials? are there alternatives to running this boiler as intended?

please help me understand... i've read of folks putting relays so the circulators run separate from the boiler and can "scavenge" the heat (once again, this should be normal operation not extra feature) is this what i must resort to?


  • ch4man
    ch4man Member Posts: 245
    are you sure its the engineering of the controls thats causing your pain, or maybe, just maybe your boiler needs some tender loving care?

    pay a good oil man to give it a look see.

    or buy a LP tank and a mod/con boiler. a quality installation of a quality mod/con boiler should give you time to worry about something else
  • Zman
    Zman Member Posts: 6,566
    I agree that you need a qualified tech to dial in your combustion.
    Some things you can work on:

    Check with the manufacture to determine the smallest nozzle that can be installed.
    Perform a heat loss on the home and determine your required BTU output.
    Measure your baseboard heaters and multiply by 550 per foot to determine the max output your system can support.
    An adjustable differential on the aquastat would be a good idea, they should not be that expensive.

    In a perfect world, NASA would help design boilers and you would be able to dial in your system for optimal efficiency.
    Two issue with that:
    Most installers cannot even perform a heat loss and size a boiler accordingly. Most can't install a simple "dumb" boiler per manufactures instructions, they can't handle anything more complicated.
    The American people are cheap and generally short sited when it comes to home heating. Many of the technologies you would like to see are readily available overseas. Every time I have seen a manufacture attempt to market them in the US, they flop.

    Companies like Honeywell make some great, smart, efficient products. You see them all the time in the commercial market. The problem is, the residential techs and customers are not ready.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Jellis
    Jellis Member Posts: 226
    The equipment is fine. its doing what it is told. You have a system that uses old technology. It is still very good and does a nice job keeping people comfortable when designed, installed and maintained properly. However it sounds as if your system may have been over sized and now is not running well.

    If you smell oil when your burner starts then it is not running properly. It sounds like you have a power vent of some sort? which should start before your burner to ensure you have proper draft to get flue gas out of your house safely. However while this runs your burner should not be releasing oil into the chamber.
    As others have stated, have a qualified oil tech take a look at your system. Explain your problems and your expectations clearly and they will give you a recommendation based on their expertise.
    If modulating, energy saving things make you happy than you will likely need a new condensing boiler with outdoor reset and other goodies.
  • STEVEusaPA
    STEVEusaPA Member Posts: 4,971
    The actual boiler model number would help as well as a heat loss.
    I think you're 'over' angry for the wrong reasons.
    We don't know how old the boiler is or when it was installed.
    We don't know if it's the right size or if it's piped correctly.
    In general, zoned systems tend to short cycle as if only one zone is calling, the boiler is now extremely oversized.
    Proper sizing, less zoning, buffer tanks all exist to help with the issue.
    You should never smell oil, or hear a boom, so something is wrong with the way the burner is set up and/or the venting.

    Yes their are modern controls that do pre/post purge, and yes you have to pay for them. If they came with the boiler, the price would be higher. A modern aquastat could probably pay for itself over 3-4 years in fuel savings. So if you just installed this boiler and the installer didn't offer those products, that's between you and him. If this is existing, and who knows how old the boiler is, maybe those controls weren't available, or maybe the previous homeowner wanted the cheapest install they could get, or afford.

    If you buy a stripped down car, no options, do you complain to the manufacturer about all the options they could've add? No, if you want them, you pay for them.
  • STEVEusaPA
    STEVEusaPA Member Posts: 4,971
    ch4man said:

    ...a quality installation of a quality mod/con boiler should give you time to worry about something else

    So would a quality installation of an oil boiler.

    HVACNUT Member Posts: 3,918
    It's almost 2019 and many things have advanced in the world of oil heat.

    The aquastat should be set no higher than 180-160, and it's a reasonable bet you dont need temps anywhere near that. There is absolutely no reason to have 210° water, especially with a tankless coil. I sure hope theres a domestic mixing valve.

    Be rid of the tankless coil and install an indirect water heater. Its way more efficient in the ways of DHW production and allows a much wider differential on temp.

    Even more so with outdoor reset and an aquastat that offers a wider diff and still provide condensate protection.

    I get it. Boiler is always coming on to make the diff. 1 zone calls, burner and zone run. Zone satisfies then 3 seconds later zone 2 calls and the burner starts again. It's a viscous cycle but it can be resolved.

    Like the other guys said, you need a qualified tech to address the combustion issues and help you decide on (2019) upgrades to make your system more efficient.
  • bananapeal
    bananapeal Member Posts: 4
    thank you for all the comments. you've reassured me i'm not stuck with the bad plumbing. i'm sad i didn't know the kerosene smell outside was a bad thing. seems very wasteful.

    i will tell prospective repairmen my performance specs. if any of them tell me "impossible" i will move to the next guy.

    my last questions: if a guy comes to "dial in combustion" and make the thing balanced like it should, if i later add or re-arrange the hydronic, or if i have a guest who thinks the guest room is too hot and i close all the baseboards in that room, do i have to call another guy in again to dial it in a second time? there need to be adjustable knobs on these things to deal with degradation or changes in configuration!

    what follows is just more description. i think i suffer from a combination of minimal install specification and minimal margin in the house plumbing. i think domestic hot water follows a serpentine path through the house to be easy for the installers not robust performance for me.

    last guy that came told me to my face "it's impossible to have modulating oil" ... yikes a google search shows me there are such things.

    i need to call a guy that isn't from the oil company perhaps!

    an energy star feature was added to the HON L7224u (in 2015!) for heat scavenge but i'd rather not encourage them.

    it's not that old. 2005 construction. i don't think anyone's watched the system perform more than 5 minutes otherwise they wouldn't have left it in this state when they handed it over.

    we built the house and immediately noticed a problem with where they put the hydronic upstairs. one small room has the same level of radiator installed as the double size master. Builder said "yea, we thought it looked strange too."

    nothing about the plumbing makes sense to me but we were first time owners/maintainers of things like this and it took us awhile to realize how badly everything is installed. we surely didnt feel like we had the experience to sue to get the **** back here to fix everything.

    i need high temps: the boiler cannot go below 140 (Bt on the boiler) without you feeling this as cold water in the shower. so the low limit is 170... so it dips to around 150 or so as it does its turn-on procedure. you feel this as lukewarm in shower, bearable but not ideal. at 205 we don't have circulator lockout and the baseboards seem to be working at max capacity to heat the house as fast as possible.

    since the day the house was turned over to us, we've never been able to fill the upstairs tub in a set-it and forget-it manner. you must baby the hot water pipe to ensure it stays hot by metering it less than its flow. otherwise the on-off low limit cycle is felt in the water temp upstairs and you end up with a body temperature tub. which by the way... if it just ran to 200 when filling the tub it might be more successful (aka if low limit was just in use for showers or not at all). when i run both zones at same time to accomplish this, it fills better but it still fluctuates!

    there's a mixing valve where an "anti-scald" should sit -- last tech pointed out it should sit below the coil not as it is. idk how it was approved but it has no safety markings on it. (it's not red knob style).

    i am glad to hear i have perhaps just been unlucky with the installation. perhaps there is hope for having a hot bath one of these days without sitting next to it for the hour while it fills.
    HVACNUT Member Posts: 3,918
    You will NEVER be able to fill a bathtub with a tankless coil unless you throttle the valve way low. The coil is probably rated at 3 GPM and a wide open tub spout is not going to do it. The boiler cant heat it as fast as it's being used.
    You need an indirect water heater to solve that problem.
    The combustion issue takes priority though.
  • Jamie Hall
    Jamie Hall Member Posts: 15,858
    On the oil burner settings. Unfortunately, it takes a few specialized measuring instruments to get the ratio of fuel to air correct. It isn't all that difficult to do -- for the folks who know how to do it with the right tools. It's almost impossible to do for those who don't have the right tools. Once they are set they can easily go for a year or more -- at which time things need to be cleaned or replaced, like filters, anyway.

    I am not aware of the existence of a residential sized modulating oil burner. There are a number of gas fired modulating burners, but the technology to modulate a gas burner and maintain reasonable combustion is much simpler than modulating an oil fired system. Not that it can't be done; it can. I can think of several different ways to go about it, some of which are purely mechanical but most of which would be computer controlled. At the present time, the maintenance requirements of the former would completely defeat the average homeowner; the cost of the latter would be well beyond what could be recovered. It is much simpler -- and has very little performance penalty either in terms of fuel use or efficiency (which aren't quite the same) to simply turn the burner on or off as required to maintain a useful range of temperatures in the circulating water (for hot water systems), circulating air (for forced air) or pressures (for steam). One can easily minimize the resulting cycling either by correctly sizing the unit in the first place or by using buffer tanks and mixing controls, as the case may be.

    Once the burner is set properly, no,, changing the heating system load (such as by closing off the heat in one room) will not affect the burner performance. What it will do is affect how long the burner will have to cycle for. If you have a correctly sized system, with the proper controls and, in many cases, a buffer tank, this is not a problem. Note, however, that if you do not have the proper controls, properly adjusted, and most likely a buffer tank, if you substantially reduce the load you will have the burner turning on for only relatively short periods of time, and possibly even being forced to turn on and off fairly quickly.

    It does sound, from your various comments, that you have an uncommonly poor installation. There are things which can be done to get you enough hot water when you need it, and to get you the heat you want. They should, of course, have been specified by your architect or builder when you built the house, but they may have been working to a price constraint -- which is almost always a mistake. The fixes are not going to be free, either, though.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SuperTech
    SuperTech Member Posts: 1,640
    I have an L7224U and I can adjust up to a 20 degree differential. Whoever set up your unit doesn't know what they are doing.

    No aquastat or other device will help you save money and oil as much as an indirect tank installation. Tankless coils are like leaving your door open all winter.

    The oil smell from the power vent makes me suspicious of the cut off on your oil pump. You need that checked out as well. Where are you located? Perhaps someone here can help you
  • bananapeal
    bananapeal Member Posts: 4
    edited December 2018
    hi supertech. the diff on the falling temperature is 10 degrees. it only allows the temp to fall 10 degrees in all operating circumstances (both the low and high diffs) the diff you are talking about being user adjustable is in the upwards direction, which allows up to 25 degrees upwards and only hard-programmed 10 degrees downwards. this is what makes me nuts. why design a board this way? have they updated most recent boards to be more flexible? i haven't checked latest specs.

    i am in eastern CT 10 mins from storrs/uconn.
  • Jellis
    Jellis Member Posts: 226
    Your high limit differential is i believe 5-20 and low limit diff is 10-25. The L7224U is a good aquastat capable of a wide variety of applications.
    Having worked for an oil company myself, i would be weary of any "fuel saving" advice they give.
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