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Why do we use outside temperature to trigger boiler instead of inside temp?

sreja Member Posts: 175
Ok here is something I've never quite understood.

We are in a 14 unit, 5 story building.

The common practice in cases like ours seems to be to have an outside temperature sensor and have the boiler come on when the outside temperature dropped below a certain point.  That's what we have.

BUT this large brick building holds so much heat, and with the old windows in the building the effect of wind on inside apartment temperature/heatloss is HUGE.  Because of these things It seems to me that the outside temperature is often not a very good predictor of heat loss and needs.

So wouldn't it make more sense to place the temperature sensor in the coldest (non-heated) common area of the building?  Wouldn't that much better reflect the need for heat in apartment units in the building?


  • this has been on my mind too!

    tekmar have an excellent tech support, and perhaps they can answer this best. i think the initial contact has to be by internet, and then they call you back.

    ask them what the difference would be between using one of their thermostats with averaging remote indoor sensors located in the north west/windy corner room, and a 279 steam control with also averaging sensors, plus the outdoor sensor.

    i spoke to daniel at 250-545-7749 [[email protected]] maybe try that first.

    let us know what you find out!--nbc
  • clammy
    clammy Member Posts: 3,094
    Outside temp

    I believe part of the reason boilers start on outside temp is  beause it is required by most housing authorities to guarantee heat for the rent paying tenants.as for inside sensors i am a big believer of them and i have seen  in all cases which i have use them i found even though the heat timer which was there was calling for heat while the temperture average set up i had installed kept the boiler off due  to the fact that the temperture average t stat was satisfyed . the heat timer was set up and running for years before i got there except there was under and over heating issues which where straightened out later. The TAS worked fine i have used them as indoor feed back and  then installed a inexpensive ranco set point controller to sense outddor temp this set up has worked good with steam systems .If it is a hot water heating system i think i would use a outdoor reset control and use the temperture averaging thermostat  to bring the boiler on .I have used white rodgers 1F93-380 with 3 remote sensors and the stat located in the boiler room with it's sensing not being included .Hope this helps and gives you some ideas  peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • Brad White
    Brad White Member Posts: 2,398
    Common and good question

    The short and preliminary answer is, using outside temperature to

    initiate heating is an anticipatory function. A sudden ten degree drop

    in outdoor temperature starts the heating system ahead of a drop in

    indoor temperature so that the space never sees that drop, that

    potential loss in comfort.

    The challenge of using interior controls to represent interior spaces is, "which space gets it?". An interior hallway has no heat loss so should technically always be satisfied. If that is satisfied yet controls the building, Mrs. Johnson in the north wing will throw on another afghan and buy yet another cat.

    The sunny space will also satisfy some days, not on others. Properly done, outdoor based control is a good "leveler" but it also has obvious limitations.

    To explain more deeply:

    Firstly, you have to accept that heat loss is a function of indoor-outdoor temperature difference. No difference, no heat loss, so we accept that. Also the colder it is outside, the greater the heat loss. Warmer outside, the lower the heat loss. (Just nod. No, not nod off. Just nod :)

    Secondly, if we are talking about outdoor reset on hot water, (varying temperature with outdoor temperature), or steam, (varying cycles per hour with outdoor temperature), you can understand a relationship there. Hotter water or more cycles per hour during colder weather than during warmer weather.

    Thirdly, when these two concepts are married, integrated into a control system, the heat input then matches as closely as one can, the heat loss. Ahhhh.

    Lastly, here is what skews this ideal: Internal gains, external gains (solar mostly) and building mass and building insulation.

    The first two you can do little about beyond shutting off lights and power strips.

    The building mass is what it is. A storage battery, for now. Building insulation and building mass work together.

    But what all of these have in common is determining at what outdoor temperature does your building actually need heat? We call that the balance point. The point where passive or background gains to a space equal the heat loss for that hour.

    A low mass house with low insulation levels may well have a balance point of 60 to 65 degrees F. when heating to 68F. You get spotted a few degrees just for showing up.  A tight frame house with which I am familiar has a balance point in a north-facing office of 35 degrees. (2x8 walls filled with foam and a roof to match plus computers and wide screen conspire nicely here).

    Your building mass effect is tamed by adjusting your outdoor design temperature (heating on-time) to a lower outdoor temperature.

    What I think you are finding in your building is that different exposures combined with leaky windows are conspiring the other way. The sun is shining on one or two dominant sides at a time and in winter, at low direct angles. Cheery but if the radiators are on, not so much. The north sides see wind, no sun so of course that is not working out so great.

    But what this illustrates is the external gain portion, nothing to do with building mass really.

    Infiltration is usually the largest single heat loss component, the hardest to quantify and the hardest to tame. It can cause the most rapid drop in temperature and comfort. I would focus on sealing the windows better, adding storm windows is usually quite cost-effective provided the flanges are caulked.

    So back to control: If your outdoor-based temperature control is set up to start heating the building at a lower outdoor temperature than you have been, but still over-heats, then you should consider limit controls (TRVs) in those spaces prone to over-heating.

    You can also zone a building by exposure. A master control valve using outdoor temperature plus an exterior solar sensor and/or a representative central exterior space, controls that facade.

    Basically taking a proven basic concept and improving it with some fine-tuning. If one window is open, (double-hung thermostat), you have wasted energy. When all windows are closed and no complaints, you are in a good place.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • sreja
    sreja Member Posts: 175
    sensor for indoor or outdoor temperature

    Thanks everyone for the good answers -- and thanks especially Brad for such a long entertaining one :)

    I guess I can see the wisdom in saying that the Outside air is the closest thing to an "objective" temperature, and serves an important role as an anticipatory LEADING indicator of temperature to come.

    Wheras indoor  temperature is a LAGGING indicator, and might not start reflecting the need for heat until it is "too late".

    That makes some sense.  But i still think it seems overly sensitive to outside temperature and the more i think about it the more im curious about trying an indoor temperature sensor places someplace carefully chosen.

    We have a Johnson Controls controller (A99 i think) which can only accept one sensor (be it indoor or outdoor); so i might have to look into that Tekmar nicholas.

    Just for the sake of completeness:

    We actually spent a ton of money a couple of years ago replacing all the traps and putting TRVs on every radiator in the building (around 80 radiators).  And I cannot more highly recommend TRVs -- they have done wonders for regulating comfort -- and everyone is able to control their comfort level quite well.  So this really isn't a problem of controlling comfort -- it's more an issue of me thinking we are running that boiler much more than we need to.  I suspect wind is the culprit in making "warmer days" often need heat where its not needed on some "colder" days, if you catch my meaning.  Of course the better solution would be to completely replace the windows in this building with high efficiency ones but that's just not something people can afford at this point.
  • sreja
    sreja Member Posts: 175
    tekmar 279

    Just read about the Tekmar 279.. DROOOOL... my gosh it looks so wonderfully fun and interesting.. checks condensate return temperature, etc.  but its a thousand dollars just for the parts.. yikes.

    I notice in their charts you can easily set things up like alternating 15minutes on, 15 minutes off cycles, or 30-30 etc.

    Our current controller has no such ability, so when the temperature drops in the winter, the boiler is on 25hrs a day at some level, making pressure.

    Which gets us back to the age old question of whether these extended on-off cycles make sense -- or whether we are simply going to be letting the system cool and then have to re-heat up again, and that its doing more harm than good...  of course this feature is optional and we could still use the tekmar just for the multiple indoor-outdoor sensors.. 
  • sreja
    sreja Member Posts: 175
    tekmar 279 vs other things

    I think the other missing piece of the puzzle with all of these high tech devices is that its really easy to imagine why just having one of these is such a clear win.

    Having TRVs -- super clear win in terms of residents being able to control heat in their unit really well.

    Multi-stage gas boiler -- really nice way to keep the system at a good gentle desired pressure.

    But then you start adding smarter boiler on/off controller like this Tekmar 279 which can sense indoor outdoor temperature, and condensate return temperature, and regulate cycle times..

    At some point all of these different high-tech devices would seem to start fighting against each other or at the very least confusing each other.
  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604
    Indoor sensor

    What type of indoor sensor does your Johnson controller use? If it is a resistive thermistor type, you can set up a series-parallel array of sensors. This  circuit connection arrangement of 4, 9, or 16 sensors will simulate a single sensor, yet allow for averaging of multiple apartment temperature readings.

    Multiple sensor averaging reduces the problem of only a single location determining the amount of heat supplied.
  • how long does your boiler run in the winter?

    are you sure it is non-stop? maybe an hourmeter would be a good addition.--nbc
  • when is the test for this professor?

    your words will take a bit of digesting for me! i will keep on re-reading!--nbc
  • sreja
    sreja Member Posts: 175
    running 24hr a day

    i'm quite sure it runs 24 hours a day.

    i mean it does cycle between 2-3psi constantly.. but yeah its on 24hours a day in the cold winter months.

    is that not normal for a 14 unit apartment building? i thought it was.

    i guess this is kind of important for me to figure out before i go any further -- anyone else have thoughts about this? now i'm getting a little concerned..
  • jpf321
    jpf321 Member Posts: 1,568
    i'm not so sure ....

    that 24hrs on is normal. my sister's building is 100+ units on an old HeatTimer and it only runs about 15-20mins every hour or so.

    you state that your current controller is "lacking" .. what type of controller do you current have implemented?
    1-pipe Homeowner - Queens, NYC

    NEW: SlantFin Intrepid TR-30 + Tankless + Riello 40-F5 @ 0.85gph | OLD: Fitzgibbons 402 boiler + Beckett "SR" Oil Gun @ 1.75gph

    installed: 0-20oz/si gauge | vaporstat | hour-meter | gortons on all rads | 1pc G#2 + 1pc G#1 on each of 2 mains

    Connected EDR load: 371 sf venting load: 2.95cfm vent capacity: 4.62cfm
    my NEW system pics | my OLD system pics
  • sreja
    sreja Member Posts: 175
    cycle time controller

    We have a simple controller that simply turns off an on based on temperature.

    (Its a Johnson Control A419).

    And yeah im starting to think we need a better controller that can adjust cycle times..
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