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Heat Load assessment different from quuted Boiler Size

Hello there,

I made heat-load calculations (see attached excel) for the large 3-storey home pictured in the attached photo. We're planning on using the existing hot-water radiators and installing a new mod/con gas boiler.

One quote for the job suggested we get the <a href="http://www.nythermal.com/uploads/NTI_TFT_Firetube_Bro_SB_Oct0313(1).pdf">NTI TFT 155</a>, which has a heating capacity of 144 MBH.

There seems to be a huge difference between my calculation of heatloss (~62,000 BTU/ hr) and what he thought would be an appropriately sized boiler (144,000 bth/hr). Which one of us is off?

What makes me suspect that I might be closer than the contractor was, is the fact that the existing radiators tend to match (roughly) the heatloss calculations on a room by room basis.



  • JStar
    JStar Member Posts: 2,752

    What program did you use for the heat loss?

    If you need 62,000 to heat the house, then you should choose a boiler that is about 75,000 BTUH INPUT. The NET rating of the boiler should match the heat loss. I would stay far away from any contractor who wants to oversize a system by that much. They obviously do not have your comfort or wallet in mind.
  • John Mills_5
    John Mills_5 Member Posts: 935
    At a glance

    Looks like an awful lot of house for that small of a heat loss. Did you get the type of windows & insulation level right?

    Some dealers like to oversize. When my Aunt was getting quotes to replace a 335K boiler for an old 2 story, on dealer wanted to put in 2 200Ks, 1 per floor. The dealer that got the job went with my suggestion of a 210K for the whole house!
  • icesailor
    icesailor Member Posts: 7,265
    Knowing Better:

    And the guy who wanted to put the two 200,000's in, didn't know any better.

    The Slant-Fin Heat Loss Explorer is accurate for 95% of what you will run into. If that won't cover it, you'll know it.

    If you have it in a laptop, it takes longer to measure the room than to input the information. You can set any parameter you can think of. Its the easiest I have ever used. Far easier than ACCA, Manual J, ASHRAE etc. For strictly hot water heat loss, it can't be beat. IMO. ACCA, Manual J are for Scorched Air and AC. I owned a few of them. I never owned a ASHRA program. But I used one.
  • RobG
    RobG Member Posts: 1,850
    Thick & Thin

    I hate trying to do a heat loss on a home like that. There is no easy way to do it. How old is the home? Has there been added insulation? If so, what kind and where? How many square feet is it. What kind of system is in there currently and how does it perform? Why is it being replaced?

  • Gordy
    Gordy Member Posts: 9,537
    edited May 2014
    Big house

    62k looks a little light unless there were a bunch of envelope upgrades. If there were any did the heating contractor know.

    Where is your location?

    What design temp are you using to calculate the heatloss?

    What water temp are you using to determine btu output of rads?

    He may be sizing the boiler to the connected load.
  • icesailor
    icesailor Member Posts: 7,265
    Heating Assessments:

    If you do the IBR/GAMA H-22 heat loss guide program, it is easily explaines about the goals. In fact, if you really get into it, it will explain many other factors you will run into abut heat loss that you will never understand unless you understand the concept of heat loss.

    The most important thing I discovered is that you MUST have a "Constant" to work with. For example, you break up odd sized rooms into boxes and add then together. If you have a knee wall that is protected by a roof but is not outside, the wall looses 1/2 the heat that a fully exposed wall does. But when you add it all you add it all up, it might equal 3" of #30 baseboard. When you total up a room and it says that you need 7.33' of baseboard, you're going to install 8'.

    Its like a crusty friend I have who had a very smart son who was having a problem understanding number subtraction. All important things like school were left to the wife/mother. While he sat in his chair watching TV and burping from his Bud's. Mother explained how if you had 6 apples and you gave 4 away, how many would you have left. "I wouldn't give any away". If you had four oranges and you ate 2, how many would you have left? "I don't know".

    This went on for some time. Finally in desperation, she asked the husband/father to try to explain it. His response over being torn away from the TV: " Sam, if there were 6 bums sitting on a bench down at the wharf, and you came along and threw 4 of them in the water, how many would be left"? "TWO" was the immediate reply. "That's how you do it", To which the son replied that it was easy. Why wasn't it explained to me like that before. He never had a problem with math after that. He graduated near the top of his class in High School.

    It just depends on how things are explained.
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 3,504
    edited May 2014
    That calculation

    looks about right for a new 3,000 [] house; double-glazed windows, fully insulated walls, floors and ceilings. But then again, I am always floored when the numbers come in low and they always seem to do just that. Over the years, I've trusted my numbers and I've never under heated a house.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
  • HydronicRookie
    HydronicRookie Member Posts: 54
    In answer...

    Forgive me for getting back on here so many days later. I was expecting email notifications and when none came, I resigned, thinking the forum was simply inactive these days. I checked my spam folder by chance today and lo and behold I was very wrong.

    First off, thanks for lending your expertise and asking the pertinent questions, I’ll try to answer them in turn. Before I do though, I need to add an important piece of information that I neglectfully left out of my initial post – that is, the contractor’s figure included indirect domestic water heating. I’m guessing he was referring to a system that would perform space heating and DHW heating simultaneously, and so, now painfully obvious, this at least in part explains the difference in numbers.

    On to your questions:

    I used John Siegenthaler’s Heat Load Pro, mostly because I’m reading his book.

    The house was built circa the 1920’s here in Toronto. Its about 3333 square feet (3 floors + basement). I used the design dry bulb temperature (97.5%) of about 1˚F and a 170˚F average water temp to determine radiator output.

    I’ll have to make note of the precise age and output of the existing boiler but from what I remember it’s about 20 years old, conventional, gas fueled. The reason for doing away with it initially was the promise of higher efficiencies, but mostly because we thought the many low-hanging 2-inch asbestos-wrapped cast iron headers would be too much trouble to try and raise higher into the. We took over the house, and have no idea how it performed or what the gas consumption was like.

    Renovation, so far, has been extensive: we’ve removed the old horsehair plaster and lathe walls and ceilings, replaced all windows with double-glazed panes and the intention is to make the house as air-tight as possible with a good amount** of insulation.

    ** Unfortunately I thought I could get away with using the trial version of the software and simply screen capture the final calculations. As a result, I no longer have a record of all my insulation choices, which interferes with my ability to check my work. Should I redo the heat-load assessment?

    The contractor knew about our intentions for insulation, but looking back, seeing as he didn’t do a proper heat load assessment (mostly just made note of the output of current boiler and counted rads) its unlikely that his experience could have informed his informal assessment to any accurate degree.

    @Alan_Forbes, so far, you’re the only one to think my calculation is possible - does the fact that the house is here in the North-East change your mind?
  • Zman
    Zman Member Posts: 7,433
    Heat loss

    Your calculation seems pretty close.

    There is no reason to add for the DHW load.

    By the time you finish Siggy's book, you will know more about design than most.

    Trust the math,

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • icesailor
    icesailor Member Posts: 7,265
    Domestic water heating load:

    It matters not to me. But, disregard domestic water heating load at your peril. Especially in Canada. A properly sized heating only boiler is oversized to start with, for 90% of the time, because the OAT is always above design temperature. But of the boiler is sized precisely for that OAT, and there is a heavy DHW load, the boiler just became grossly undersized for the load.

    That big round tank next to the boiler with the pipes going too and fro, is just another heating zone with unknown requirements that just started sucking vast amounts of energy from your carefully designed system. If it's -30 out, you could be toast. If it's 20 degrees out, the boiler size just increased by double.

    Most of you are "Heaters", not Plumbers. Plumbers deal with a lack of hot water. BECAUSE THERE ISN'T ENOUGH NUTS TO HEAT THE COLD WATER!!! You're dealing with a 20 degree Delta T but the DHW load is dealing with 75 to 100 degree delta T. What did THAT just do to your careful calculations?

    That's why they have "Priority" switches on zone control boxes. That's why they have "Low Limit Circulator" controls on triple acting boiler controls. It stops the heating load draw.

    Here;s an example. In the North/Cape Cod, our groundwater was 50 to 55 degrees. In Florida, the cold water out of the tap is 80 degrees. My wife's horse developed a temperature of 104 degrees which is a serious issue in horses. She immediately took a cold hose to the horse to get the temperature down. They do the same thing with humans, especially babies that spike high temperature. In a hospital setting, they cover them with ice, 32 degrees to get the temperature down. Over 106 degrees in humans can cause brain damage and death.

    Impractical with a horse (or dog). Which will cool the horse down faster. 80 degree water out of a hose or 50 to 55 degree water out of a hose.

    Unless you've experienced almost 500,000 BTU input of DHW, drop like a stone and go cold, while a separate 600,000 BTU heating only system runs merrily away, you'll never appreciate the difference. In the "Age of Specialization" where heaters do heating and plumbers do rough waste, rough water piping and another group does the finish, and another class fix it all, never develop the skills to understand the big picture.

    Have you ever had the honor (I have) of going to a new house, that was cold in the winter, the heat won't go over 64, the indirect is running, the DHW isn't hot enough to take a shower, and the installer is no where to be found? Shut off the indirect and it "improves". You just shut off an additional heating zone that wasn't compensated for in the calculations. It happens far more than you think.
  • Hilly
    Hilly Member Posts: 418
    partially disregard indirect load

    icesailor in your opinion then... if a DWH load is on some sort of indirect priority is it safe to not include it in the sizing of the boiler?

    Well not totally, I'll rephrase, the boiler would have to be at least the same size as the the max of either of the two loads. ie Boiler has to be at minimum, the great of the two (Heat Load or DHW Load)
  • Zman
    Zman Member Posts: 7,433

    Fast forward to page 633-635 in your textbook. As long as your DHW demand is not ridiculously high, set it up as a priority and don't worry about it.It is only going to run less than an hour per day.

    I had a recent discussion with a gentleman that insisted that the boiler be upsized from 110k-175k boiler for a new 3 bath home with an 82k heat loss.

    I asked him if he thought an 60 gal. electric (about 16k/btu) or a 60 gal gas fired (32k/btu) would suffice. He said, yes we do it all the time.

    This is a largely misunderstood concept. If the boiler is to small for DHW, upsize the tank instead.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • icesailor
    icesailor Member Posts: 7,265

    30 to 60 gallon gas water heater storage tanks are all fired at between 32,000 to 35,000 BTU Input. 75 gallon gas storage tanks are fired at 75,000 to 90,000 BTU's The difference is in the amount of available stored water. The inputs are the same.  That's a different animal, storing water.

    If you have a building that looses 100,000 BTU's per hour when it is 70 degrees inside and 0 degrees outside, and the boiler is properly sized so that it maintains the design temperature, the burner never shuts off, and the circulator never stops, you have design perfection. If, however, the outside temperature rises to 35 degrees, the boiler and system are twice the size that is needed. In the summer, if the outside temperature is over 65 degrees, you do not need any heat at all.

    If every day, you spend 4 hours using hot water (bathing, washing etc.), the heat energy load remains the same, every day, for 365 days. Because the demand is the same and the incoming water doesn't really change much. It matters little to me how someone compensates for DHW use when taken off a heating boiler. You better have a thorough understanding of heat loss in water or you may one day, be faced with a situation that you can't explain.

    What would your answer to going into a house, being rented for big $$$$$$'s, and no hot water, none, with a 80 gallon electric water heater and the top element is drawing 17.5 amps? And there are no faucets running anywhere in the house? And you plumbed it new and have maintained it since it was built.


    Then, there's the heating where a house when it is 10 degrees outside and the boiler is shutting off when the room is 70 degrees, but a week later, it is 28 degrees outside and the house won't go over 62 degrees. The boiler is cycling on and off.

    I've dealt with those problems on more than one occasion. All had the same root cause.
  • Zman
    Zman Member Posts: 7,433


    I don't think that your 80 gallon electric producing 13,188btu/hr is sized correctly for  a high demand house.That's only .37 GPM of recovery.

    It sounds like the other system you describe should have been set up with priority with "timeout".

    My point is that if you have heating load that is 62,000 btu/hr on the coldest day and you have normal DHW needs, you really should not have an issue. My own indirect fires twice a day for a total of less than an hour. During the typical 20 minute dhw call there is no noticeable fluctuation in indoor temps.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • icesailor
    icesailor Member Posts: 7,265
    I'm not saying:

    I'm not saying anything other than to be aware of the DHW load as something that is constant, all the time when heating is variable. All that "Priority" gives you is a way to dump heating loads. on a temporary as needed basis.

    If you had a house with a 100,000 designed heat loss and 4 zones the same size, if the boiler couldn't quite keep up, and you turned off a zone, the input remained the same, the loss/load dropped by 25%. A Priority switch is just a load dumping control. When activated, it will dump 100% of the heating load.

    If you're at design temperature on a design day, can you afford 100% of your heating load for a  few hours while an indirect is running on Priority.

    Maybe my diagnosis of the problem I saw with over designed, over pumped and under piped and under radiated systems that didn't heat houses up when it was facing extreme conditions were all wrong. Whenever I disengaged the propriety switch, things improved.
  • HydronicRookie
    HydronicRookie Member Posts: 54

    Thanks to everyone for offering feedback - I was glad a proper discussion come out of it.

    I finally checked out the current boiler and its output is rated at 144,000 btu, which now explains more or less how the hydronic contractor arrived at his number.

    I'm going to stick by my heat estimate for now - and scale it up a tad to roughly match the calculated radiator output, and follow through with proper insulation and air sealing and insulation.