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Boiler Sizing

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Sarah_3
Sarah_3 Member Posts: 9
Hi folks,



I will be replacing my old oil-fired boiler with a new atmospheric gas-fired boiler this fall. I did a heat loss study using free online software and came up with 68.000 BTU heat loss for the house. I've called two heating contractors so far and both want to do the heat loss study simply by multiplying my existing baseboard by some number. One came up with 75.000 BTUs, the other even more. I've explained that since the original baseboard was installed, we've had new replacement windows put in, but they don't want to do a more detailed heat loss. Both of them, and a couple other contractors I've talked to informally, say my numbers seem low, but I'm the only one actually measuring windows and doors and outside walls here.



I should say that I've worked with the first contractor before and have always been really happy with his work and his work ethic, so I'd like to use him if we can come to some agreement about the boiler size. He was going to put in a Utica 90,000 BTU boiler.



Money is a real consideration here--we are stretching to be able to do this, so condensing boilers and so forth are not an option this time around.



We have a 1925 single family in Southern Connecticut that's 1620 square feet with 3 zones, an insulated attic but uninsulated walls, double pane replacement windows, a full basement. The baseboard is all copper fin. HWT will remain separate.



What could happen if we put in a boiler that IS too small for the house's heat loss? do we have to take out sections of fin if we use a smaller boiler? A couple of our rooms underheat and a couple overheat, so it's not like the baseboard was that well measured to begin with. (Each zone is one loop).



Any comments or suggestions would be greatly appreciated.



thanks in advance for your help.

Comments

  • LarryC
    LarryC Member Posts: 331
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    Clarifying questions.

    Sarah,

    I am NOT a heating professional, but I hang around this site.

    Bunch of questions:

    1)  What did you pick for your outside design temperature?

    2)  What are the three zones? (Bedrooms, First floor, Basement)

    3)  How long do you plan on being there?

    4)  Why are you replacing the boiler?  Age, noise, efficiency, uneven heating?

    Comments: 

    A)  High tech boilers initially cost more but USUALLY cost less to feed and maintain.  Depending on the length of time you folks plan to stay in the house, it MIGHT be cheaper in the long run to go with a more efficient boiler.

    B)  Money invested in sealing the building envelope is essentially fuel you buy once and it keeps you warm for multiple winters.

    C)  Often heating systems need to be looked at in a big picture mode.  Replacing just the boiler may or may not fix the "problem" but may introduce other issues.  It is possible that just minor tweaks may address the heating system issues. 



    Good Luck

    Larry C
  • Sarah_3
    Sarah_3 Member Posts: 9
    edited September 2010
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    your questions

    Hi Larry,

    outside design temp is 7 degrees.

    We are replacing the boiler because it is ancient and inefficient, and the oil burner *stinks* when it's running--servicing it has not helped.

    zones are sun porch, first floor, second floor. the rooms that overheat and underheat are on the same continuous loop, so it's an issue of wrong length of baseboard, not a boiler problem per se. that's a separate issue, germane only in that the guy who put in the baseboard can't be trusted to have done such a perfect sizing job the first time around.

    While a condensing boiler may be more efficient in the long run, NOW is when we have to pay for it, and there's a limit. Besides, I'm not going to install a condensing boiler--which must be sized exactly right-- if I can't get a more accurate heatloss study than simply counting the last guy's feet of baseboard! : )

    cheers,

    Sarah
  • CC.Rob
    CC.Rob Member Posts: 130
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    stretch it another year

    Reducing household energy use starts with conservation, then efficiency. I suggest living with the oil-sucking monstrosity for another year and putting your money into  totally unglamorous but far more economic improvements to the building envelope (insulation and air sealing) to reduce the heating load. The best $200-400 you can spend is a blower door test with thermal IR imagery to evaluate the insulation and leakage of your building. Then do as much insulation and air sealing as you can. The idea is to reduce the heating load. 1600 sf and 68k BTU/hr heat loss is a really leaky, poorly insulated building. It needs help. Think of insulation and air sealing as fuel you buy only once. It pays back quickly and forever.



    Once you improve the building and reduce the heating load -- perhaps by as much as half -- then start thinking about new, much smaller boiler that will have lower first cost and lower long-term cost.







    A useful way to think about this is the efficiency pyramid:







    http://www.greenbuildingadvisor.com/blogs/dept/musings/energy-efficiency-pyramid







    Start at the bottom and work your way up. It's a useful and practical

    model. I do take some exception to the text of the article, however. In

    many places renewables are heavily subsidized which makes their

    economics a lot better and thus would move them down the pyramid. But as

    a general guide this is a great place to start. Note that heating

    systems come after a lot of other conservation and efficiency work.
  • Sarah_3
    Sarah_3 Member Posts: 9
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    thanks

    I"ll check out the article, thanks.



    Interestingly, we had a blower door test done and they said the house was fine as far as leaky doors and windows etc. (as a side note, there's an issue with getting blown in insulation that has to do with the possibility of old knob and tube wiring being inside the walls (gotta love these old houses) that has to be corrected before we can insulate). But yes, it makes sense to do that asap.



    so do I infer that if you think 68 K btus is a lot, you think a 90 K boiler is way too much, Given that the 90 K was assessed by a rule of thumb calculation and not a real heat loss study.



    cheers,



    Sarah
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
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    might

    want to squeak that design down to 0 and 72 indoor for some wiggle room. I'm using that for my area- Westport, CT. Your calcs are showing almost 42 BTU per sq ft. Wow. 

    If you can think of any reasonable envelope tightening improvements you will do in the future, account for that in your infiltration factor in the heat calc.

    A 1920's balloon frame is somewhere between "old farmhouse" and "average;poor vapor barrier" in construction..
  • Dave Yates (GrandPAH)
    Dave Yates (GrandPAH) Member Posts: 281
    edited September 2010
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    topsey-turvey

    Sarah,



    I'd strongly urge you to reconsider and revisit the modcon boiler thinking. They're a great investment that offers a better ROI than any atmospheric boiler on the market today. Here's another thought to contemplate:



    Suppose you do actually install a right-sized atmospheric boiler now and then, later as time and budgets allow, you decide to seal up the home's leaks (saying a blower door test is OK from a supposed pro is not only unacceptable, it's lousy business and smacks of unprofessional conduct - they should have provided you with the infil/exfiltration rate so that a more accurate heat loss/gain calculation could be performed), which often account for 40% of a home's heat loss/gain. Then you add insulation, windows & doors. Your new single-speed boiler is now grossly oversized and it's 82% efficiency rating has fallen off a cliff - most likely averaging less than 50%. Not what it seems you're striving to achieve.



    Take that same home and install a modcon boiler. As you avoid spending so much money on fuel, you can deposit the difference and start building cash reserves for other upgrades. As you make those upgrades, the modcon reacts intuitively to the lowering heat loss by slowing itself down. Unlike its atmospheric cousin, as a modcon slows down, its efficiency goes up! It will no longer operate at its peak capacity because that's no longer required.



    The upgrade to modcon won't break the bank normally (contact me for the average increased difference in costs as pricing issues aren't allowed here) and you'll be conserving both energy and dollars over the years to come.



    You can't save energy or money! You can, however, conserve energy and avoid spending excess money on fuel. If you bank the fuel-cost avoided, you can then save money.
  • CC.Rob
    CC.Rob Member Posts: 130
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    even a bit

    And to follow on Dave Yates' sage advice, if you can do even just a little air sealing and insulation, you get into the smaller modcon realm. The smaller you can go, the better. Take advantage of lower modulation rates for the shoulder seasons and above-design temperature days that comprise 95+% of the heating season. For example, if you can get down to a 10-50k or 15-60k BTU/hr modcon instead of a 19-80k BTU/hr model, that's great.



    Also check out federal and state tax credits for high-efficiency equipment, as well as utility rebates. www.dsireusa.org keeps a great listing. This can often drop the cost of a good modcon install down to what a simple fixed-fire boiler install would be.
  • Sarah_3
    Sarah_3 Member Posts: 9
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    so are you saying...

    that oversizing an atmospheric boiler will result in a massive drop in efficiency? makes sense, just want to be sure I'm understanding you. Also, the blower door test was just $25 after an energy co. rebate and didn't involve any infrared anything...maybe another one is in order...
  • Ironman
    Ironman Member Posts: 7,397
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    Something else to consider

    Chosing a mod/con will also give you boiler reset (the boiler adjusts water temp. lower as outside temp. rises). This can cause significant savings that are not reflected in the a.f.u.e. rating. In your area one of my reps. is claiming 40% savings with outdoor reset! The fact that your radiation may be oversized only makes this better since the reset control could lower the water temp. even more due to oversizing. The lower the water temp. returning to the mod/con., the higher its efficiency. It could be running at 98-99% with cooler return water. You can't do this with a standard cast iron boiler: it can't take the thermal shock and / or condensation that would be produced.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • World Plumber
    World Plumber Member Posts: 389
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    Load design

    Your contractors are measuring your baseboard because that is the load that will be connected to the boiler. If you have zones the boiler can usually be downsized by a percentage. Standard baseboard gives off 500 BTU per foot. So if the boiler isn't sized to the load you may not have heat at the end of the loop.

      You say you have hot and cool rooms on the same loop. Be sure the damper are open and there isn't anything blocking airflow through the bottom of the baseboard in the cool rooms. Such as carpeting that blocks the bottom of the baseboard. Close the dampers on the hot rooms. If they are still to hot you can wrap a few feet of the fin with aluminum foil.

         Check out the new condensing boilers and tax rebates. You may find yourself spending less in the end. You don't want to grossly oversize because the coldest day of the year is you efficient day.
  • CMadatMe
    CMadatMe Member Posts: 3,086
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    Heat Loss

    Sarah,



    Grew up in Stonington and worked for years in the Groton area. Design temp should be 0, have to take the wind off the sound into consideration. Your contractor is telling you that you need roughly 40 btu's sqft which is the old school methodology. The other by measuring the board, same methodology. Heat loss is proably in the high 40's to low 50's dependent on the insulation in the walls and attic. As CC Bob said. Invest in the insulation if need be and get the loss down. Blower door test is telling you where the nooke and cranny leaks are.



    While you may not have the budget for a mod/con I would strongly suggest a Burnham ES2. You can always plug in the outdoor reset card down the road and budget for it. It's a card. Won't require any piping change just a sensor that would need to be placed on the North Side and wired back with Thermostat wire. Able to take back lower water temps than other tradiational atmospheric vent cast iron gas boilers. Would have to line the chimmney though which you may have to anyway with any boiler you install. Did anyone inspect that chimmney?

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  • CMadatMe
    CMadatMe Member Posts: 3,086
    edited September 2010
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    What?

    Who cares what the baseboard load is!  The load could have the capability to put out 200,000 btu's but if my heat loss is 100,000 your telling me that I have to install a 200,00 btu boiler? I would size my boiler for the heat loss and take advantage of the extra board by running lower water temperatures resulting in system efficiency savings which means fuel savings.



    Baseboard also does not put at 500 btus ft standard. When the flowrate of a system is unknown ( you didn't do the heat loss) you use the baseboard output at 1gpm flowrate with 180 degree water which varies from 550 to 590 per foot dependent on manufacturer. That said, you are also doing your customer wrong.



    There is also another take on the methodolgy of rule of thumb. Estimates are free (that methodology) but exacts (my methodology) cost money. So if you would like a free estimate on replacing what you have no problem but If you would like an exact well that's $$$$ but I will refund it to you if I do the job. Exacts also give you 3 options to choose from estimates generally get you one.

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  • Chris,,,

    was just wondering how long it would take you to chime-in on this one!! ;-)
  • CMadatMe
    CMadatMe Member Posts: 3,086
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    Dave, Dave, Dave

    I need you for my organizations web site forum. You just light up everyday! :) Come over and register. Just stick .org at the end. Did it myself, use the web guy to bail me out when I screw up but think I found a new trade. Still not 100 percent complete though. Have to work on the cooling side over the winter and am adding daily.

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  • Unknown
    edited September 2010
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    AHHH Chris,

    I`m sure you`re a GREAT fellow, I was not "pulling your leg", I really DO believe in you!! ;-) Tough to get me away from "Dan the Man" though!!
  • CMadatMe
    CMadatMe Member Posts: 3,086
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    Don't Want to Steal

    Just borrow for some enlightenment.

    There was an error rendering this rich post.

  • WHAT A GUY!!!

    Great to know I`m not taken SERIOUSLY,,, only "enlightenment",,, in the old days didn`t we call them clowns?? LOL ;-)
  • Sarah_3
    Sarah_3 Member Posts: 9
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    Chris,

    I'm happy to pay for a heat loss study. The trick seems to be getting someone to do one. Can you run lower temperatures with a traditional atmospheric boiler?
  • Sarah_3
    Sarah_3 Member Posts: 9
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    ES2

    Yeah, I was looking at that boiler. Why can it take lower water temps than other cast iron boilers? Anyone have a strong feeling about the ES2? I'm really not looking for a debate on the virtues of modcon versus atmospheric, just experience with this boiler.



    what do you think about the issue of potential boiler undersizing (which would go away once we get the walls insulated)? Do I have to take out baseboard fins in order to keep the upstairs zone from getting cold?



    What I mean is, even if there's low heat loss post-insulation, if I have too much baseboard will some of it not heat up? This is a 3 zone house, and we don't usually have all 3 zones calling at the same time.



    The design temp for new haven is listed at 7 degrees, that's how I got the 7.



    thanks!
  • Sarah,,,

    you can,,, then it gets "tricky",,, and even more expensive,, Chris will help you ALOT!
  • CMadatMe
    CMadatMe Member Posts: 3,086
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    Limited

    You can but are limited to 140 degree return water temp in order to protect the iron from shocking. Since this is an existing sytem heat loss needs to be done. Need to also measure existing radiation in each room and by zone. This way you can then calculate what water temp you need at design to overcome the loss. On average most older homes are 35% over radiated which can allow for a lower water temp. Standard industry formula for savings based on water temp. For every 3 degrees I can run my system below 180 degree water I can save 1% of fuel. If you could start at 160 then that equates to 6% savings. Need to also have that chimmney inspected.

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  • mark schofield
    mark schofield Member Posts: 153
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    slant fin hydronic explorer

    to double check your calculations, this neat program is still available



    http://www.pvsullivan.com/Downloads.html
  • CMadatMe
    CMadatMe Member Posts: 3,086
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    Too Much Board

    Is a good thing. Allows you to run lower water temps. Baseboard has outputs at difference temps. Remember, the baseboard and boiler are designed to be able to overcome the heatloss at design conditions. You do not need the full load when it's, 10, 15, 20,25,30,35,40,50 degrees outside. That's why the mod/con is really the best bang. The burner will modulate and give you the required btu's and water temperature for that given outdoor temp providing its installed, designed and set up correctly. Here is an example that might help you understand.



    Let's say I have a bedroom and the heat loss is 3,500 btu's. The existing baseboard is14 feet. The bedroom loss combined with the other rooms on the zone total 9,500 btus. So, we get out the baseboard chart that every mfg supplies. We use the 1gpm rating for the board with the 180 degree water temperature which in the case of SunTemp is 560 btu's a foot. 560x14 = 7,840 btu output.But I only need 3,500 btus to heat the room and the chart only takes me down to 150 and at that rating I still have 5,180 btu's coming out of the board.



    I happen to get a hold of a chart that Mestek (makers of the board) have. It has correction factors for the output of baseboard using 100 degree water all the way to 240 degree water. At 130 degree water baseboard will put out 257 btu's a foot. 257x14 = 3,598. So at design meaning 0 degrees I only need 130 degree water to heat this room. That is 50 degrees less than 180. 50 divided by 3 = 16.6% fuel savings over utilizing 180 degree water. How many days is it zero in New Haven?



    I may have to tweak it here and there for real world conditions, but I think you may now understand the benifit of using low water temperature.

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  • tim smith
    tim smith Member Posts: 2,774
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    RE: sizing etc etc

    Sarah, have you taken the tax credit yet for windows or insulation. If not then the modcon comes down in price and makes it even more attractive. My best recommendation for you is a Triangle Tube prestige solo boiler. Great piece of equipment, been very reliable for us. The $$ pencil out good against cast iron with tax credit. Even if you need to go to the 110 size, it modulates down to appx 30,000 btus. Built in computer. Low on maintenance that we have found over the last appx 4 yrs. You will be happy as long as you find a good installer in your neck of the woods. Good luck, Tim
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