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Heat loss, what size boiler do I need?

MickeyHMickeyH Member Posts: 14
Hey everyone, I have another post running about contractor recommendations for an oil to gas conversion. I am just waiting on a couple more responses before I start making some calls.

I took some time today to install the SlantFin app on my iPad and went around the house measuring and entering everything into the heat loss calculator.

The result was just about 33,000. With this I started looking at boilers and most have a smallest size around 80,000/70,000 input/output. If my calculations was right those "small" boilers are about double of what I need.

What number relates to my heat loss number, the input or output? Do I need to factor in something for the indirect hot water heater? From what I read I will probably get a 40 gallon. Does that have a heat loss number I need to add to my 33,000 house number?

Sorry for all of the questions. Just trying to educate myself so I can have an intelligent conversation with the contractors.

Thank you.


  • SWEISWEI Member Posts: 7,356
    Building heat loss

    plus system losses should equal boiler output at design conditions.  The efficiency of a mod/con boiler drops a bit with higher return water temperatures, so there is some interaction with system design here.

    Assuming the heat loss is accurate, I'd probably look at a Triangle Tube PTS60 or a Lochinvar WHN055 for that heating load.

    Proper indirect sizing depends on your use pattern and the boiler size.  Be sure to look closely at manufacturer's performance data.  Most indirects show performance with ~190F boiler supply, but you can increase efficiency with a larger indirect (by supplying lower temperature water from the boiler.)
  • heatpro02920heatpro02920 Member Posts: 991
    edited February 2013
    Whats your budget?

    Obviously that is going to be a big factor...

    On the low end, I would recommend a Weil McLain CGa25 and a Bradford White 40 gallon direct fired water tank.. Or you could upgrade the water heater to a Rinnai RL75... If you wanted a little more economy have the installer add an outdoor reset control or thermal targeting {hydrostat makes a few} and delta t circulator {taco bumble bees}...

    On the higher end, I would say Swei is on the rite track, the TT 60 boiler with a 50 gallon smart tank, with some delta t circulators, and programmable thermostats will save you a few bucks, but cost you much more...

    I have a hard time wrapping my head around spending a lot more money on a High eff heating system with such a small load... IMO the savings are not going to be enough to ever warrant the upgrade... Now with a much larger home, yes it can make sense if done properly...

    If I were you I would do the math before pulling the trigger,

    Just for round numbers say a basic system costs you $4000 installed and the high eff. system costs you $10000 thats a difference of $6000, that at a minimum you want to make up in the life of the system which for round numbers say is 20 years {normally I would want to see the bulk of it within the warranty period but that will never happen with mod cons},, thats $300 a year which doesnt sound like a lot, BUT when you are heating your home for $1200 a year, your savings would have to be almost 30%, and thats probably not going to happen... Plus factor in Mod Cons in general tend to be more expensive to maintain than conventional chimney vented gas boilers as well as not have as great of a lifespan. One of my rental properties has 6 Weil McLain small gas boilers and they are much older than any mod con is going to last without drastic repairs costing more than it would cost to replace an entire conventional boiler...

    Just something to think about, there are a bunch of reasons to go mod con too, smaller carbon footprint, they look awesome, technology is cool, they can be setup to make your property more comfortable {priority DHW, different temps for different zones, ect}, fuel savings, quiet running, no chimney vent, zero clearance, Concentric venting with PVC, wall hung, takes up less space, plus more....

    So anyway do some research like you are doing, I get good results using delta t circulators and thermal targeting controls to squeeze a little extra economy out of conventional boilers....

    I hope no one takes offence to this, the bulk of my business is installing high efficiency systems, but I always explain this to my customers when they have smaller homes...
  • SWEISWEI Member Posts: 7,356
    No offense taken

    and we also upgrade conventional boilers with ECM circs and ODR controls.  I have found that the the increased first cost of a mod/con usually goes away by the time a buffer tank, additional circulator, controls, piping, and labor are added to the cost of a conventional boiler.

    If the oil boiler is in good shape, then a gas gun, buffer tank, and ODR may well be the best option.
  • MickeyHMickeyH Member Posts: 14
    Thanks, here is some more info

    SWEI and HeatPro, thank you for the replies. It's a lot to consider/think about.

    Just to provide some more clarity about my house. It is an expanded cape, 2 floors and about 2200 Sq Ft. Every room has baseboard, I measured that too but was not included in the heat loss calc, the SlantFin App did not ask for it, total for the house is 118 ft of baseboard.

    Does this amount of baseboard affect/increase my heat loss number?

    After doing more searching on the wall this morning, I have seen that some say to take the house sq ft and multiply it by 40 to get a rough estimate on the heat loss. With that I should be at 88,000 not the 33,000 that the SlantFin app gave me. What should my real number be?

    Thank you.
  • heatpro02920heatpro02920 Member Posts: 991
    edited February 2013
    A properly performed heat loss...

    A real heatloss is much more accurate than the 40 rule... Is your house well insulated? what year was it built? A lot of windows and glass doors? 2x4 framing or 2x6, also your location makes a difference as well as your desired temp...

    2200 sq ft in this part of the country in a middle of the road house not super tight but not an uninsulated 2x3 built home from 1880... I would say around 55K btu...

    As far as 118 f of board, is that all element, sometimes contractors use black with just pipe in it no fins....

    118 X 600 BTU per foot is a little over 70Kbtu, but that depends on a lot of things, temperature, manufacturer, ect...
  • SWEISWEI Member Posts: 7,356
    feet of baseboard

    70k of baseboard with a 55k loss would allow you to run lower water temps, which would be a good thing, particularly with a mod/con boiler.
  • MickeyHMickeyH Member Posts: 14
    Heat loss

    Thanks guys, I am learning but still a little confused.

    The house was built in 1952 but not sure when the dormers were done to expand the upstairs. The windows are fairly new double pained, not sure about the insulation in the walls but the exterior of the house has 1/2 inch foam under the siding. The attic has new r30. This is the info I used for the heat calc along with 68 for the indoor temp and 30 for the outdoor temp.

    For the base board I took a look and it is mostly all fins the last couple of inches of each run is pipe so lets loose about 5 feet for that, 113 total.

    So with a 33,000 heat loss and 67,800 for the baseboard does that mean I need to get a boiler rated for 100,800 output? Or do I just need to fulfill the 67,800 btu needed by the baseboard?

    Just trying to understand how the heat loss number and the base board btu numbers should be used to select the right size boiler. Thanks you for all the help.
  • heatpro02920heatpro02920 Member Posts: 991
    wheres waldo?

    30* outdoor where are you located?
  • MickeyHMickeyH Member Posts: 14
    Long Island NY

    Long Island NY
  • SWEISWEI Member Posts: 7,356
    sizing boilers and radiation

    The capacity of your baseboard is probably stated for180F water at a relatively high flow rate.  If you look at design data (something like you can see what temperature and flow will produce the required heat for design conditions.  By lowering the temperature of the returning water (a combination of lowering both supply temp flow is best) you will increase you comfort level (since the boiler will run longer cycles) and efficiency (on a mod/con.)

    Design temp seems a bit optimistic -- someone with direct knowledge will no doubt chime in on that.
  • heatpro02920heatpro02920 Member Posts: 991
    edited February 2013
    30 is high

    Im getting negative 5 for your area.... I think your average may be in the 30's but I use the coldest day in the coldest month...

    Now as far as the baseboard, what that tells you is, you have the ability to deliver almost 70K btu to your living space.... Which is good, because if your heat loss comes in well under that, that will allow you to run a lower high limit on your boiler... this saves energy, the ideal spot it 160 because thats hot enough to run an indirect and most homes are oversized at least that much with the old rule of thumb guys doing the installs... Plus mod cons like 160, I just always try to shoot for 160 high limit unless its radiant of course...

    So Im thinking your heat loss is going to be a little different after you change your design temp to -5, I also ussually set my desired temp to 70...
  • MickeyHMickeyH Member Posts: 14
    edited February 2013
    Revised heat loss

    OK, were on a roll here.  Thanks for all the input.

    I changed the inside temp to 70 and outside to -5 and I get 64,792 for the heat loss.  So that is now getting close to the 67,800 of the base board.  Is that good or bad?
  • heatpro02920heatpro02920 Member Posts: 991
    thats fine

    Them heat loss cals are a little over anyway...

    So now you just have to figure out what you want to spend on a system...

    Triangle Tube excellence 110 will make hot water and enough btu's to heat the building. or a solo 110 with an indirect...

    A good buderus gas boiler with an indirect will do a good job too...

    I use TT smart indirects, they have yet to let me down...
  • MickeyHMickeyH Member Posts: 14

    Great, i just looked up those TT boilers and will look up the Budurus tomorrow.

    So as long as I select a boiler with an output that can cover 67,800 I should be good, right?  Sorry to keep asking questions but I am not catching on to the sizing...

  • heatpro02920heatpro02920 Member Posts: 991
    wont find exact btu output

    I would look for something between 60-75K
  • MickeyHMickeyH Member Posts: 14

    Thanks for that. I do have one more question, kind of revisiting one I had earlier.

    Do I need to increase the BTU number to account for the indirect HW heater? I was looking at the Triangle Tube Smart 40 which lists that it needs 112,000 BTU per/hr.

    Again, I really appreciate your help and guidance.
  • SWEISWEI Member Posts: 7,356
    SMART 40

    can transfer 112k, but will work with less.  If you try to feed more than that into it, you will not get it out.  Remember that typical gas-fired tank water heaters produce less than 30k -- even a PTS60 with a SMART 30 will significantly outperform them.
  • heatpro02920heatpro02920 Member Posts: 991
    I don't add btu for DHW

    I run priority zone controls {unless mod/con, they do it automatically for you}, so you will never call for heat and hot water at once, your space heating wont circulate until the DHW is satisfied...

    But anyway as practice I don't upsize for DHW...
  • MickeyHMickeyH Member Posts: 14
    Thank you

    Thanks you, again, for all of the help and putting up with my questions.

    I learned a lot from this and am now confident that I will be able to ask the right questions of the contractors I will call this week for proposals on my conversion.

    HeatPro and SWEI, do you guys work on LI?
  • heatpro02920heatpro02920 Member Posts: 991
    There are plenty of goo contractors out there..

    Sorry I do not work on LI, look at the find a contractor link you should find someone..
  • SWEISWEI Member Posts: 7,356
    Long Island

    Is 3,300+ miles from here, but we have several pros in the area.
  • Paul48Paul48 Member Posts: 4,258
    Design Temp

    For some reason they don't list L.I. cities with N.Y. for temperature. You are just across the Sound from me, and mine is 9*. I would think you are in that range.
  • MickeyHMickeyH Member Posts: 14

    Thanks Paul, I will try that out too.
  • MickeyHMickeyH Member Posts: 14
    Not many...

    I used the find a contractor page and only 1 is listed in my area. Can you recommend any others?
  • SWEISWEI Member Posts: 7,356
    I'm guessing

    that you might be able to get by with a PTS60 there once you re-run the heat loss.  If you come up just a few BTUs short, consider some envelope upgrades.
  • R ManninoR Mannino Member Posts: 422
    We Have

    A TT PS60 in a ranch home in Dix Hills. It was running about 140 degree water on a twenty something degree day last month (it has convectors on monoflow for radiation).
  • MickeyHMickeyH Member Posts: 14

    Thanks R M, I will discuss that option as well. I will be making some calls tomorrow.
  • JStarJStar Member Posts: 2,668
    Heat loss

    "Design condition days" only account for less than 5% of your system's design. Every other day of the year it will be oversized, and by a good amount. Find a local average, and start there. Don't be scared of low ratings. They work!
    - Joe Starosielec
    [email protected]
  • SWEISWEI Member Posts: 7,356

    Design days typically represent 2-1/2% of the use case.  If your building has significant thermal mass you can probably move to a looser design day spec (there are several) or just be willing to tolerate 66F instead of 70F or whatever.  With a well-controlled modulating system (whatever the heat source) it will not be an issue.
  • bigmamabigmama Member Posts: 1
    I downloaded the mr combo app and did a calculated heat loss factoring a bunch of info on my home and got a 54,423 heat loss how now do I calcul what size boiler do I need and I have a boiler already installed at 151 btu that does my heat and hot water. The boiler is well over 20 years it's a Weil McLain oil
  • RobGRobG Member Posts: 1,850
    What type of boiler are planning to install? Cast iron, mod-con? What type of venting do you plan to utilize? You should start a new thread and include pictures of your existing system and tell us what your wants and needs are.
  • ZmanZman Member Posts: 3,517
    It sounds like you need a boiler with an output rating around 55,000 btu/hr.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
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