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Thread: sizing a new gas boiler and indirect water heater

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    DIY Junior Member Rozie's Avatar
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    Default sizing a new gas boiler and indirect water heater

    I live in Boston, MA and am finally getting around to replacing a very old oil boiler (perhaps converted from coal and original to our 1926-built house). The house has about 1200 sf of heated space over two floors in a cube-like shape. One heating zone. Windows are original to the house, but storms were added. Someone blew insulation into the walls maybe twenty years ago. (I had a thorough energy audit last year and they could not recommend additional insulation.)

    The existing old system works just fine, but the oil tank is ancient and having oil heat in the house will be a liability when and if we decide to sell.

    I'd like to go with as efficient a system as possible for both bill savings and environmental reasons. The utilities here in MA offer deep discounts and rebates on Burnham Alpine boilers, which they show as available in "input" ratings of 80,000; 105,000; and 150,000. I've had three contractors in to get bids and they each have recommended the Alpine 105. One recommended the 150, but when I made some noises about over-sizing concerns and the product cycling on and off, he backed off to 105. The one who started with the 150 made his recommendation with having never left the basement; another walked around the house and counted the radiators; a third measured each radiator and counted the tubes. They are big, old cast-iron radiators from the 1920s. But they all ended up recommending the Alpine 105.

    Should I still be concerned that the 105 might be oversized? If I instead instruct them to put in the Alpine 80, is there a risk of undersizing?

    I'd hate to install a 95% AFUE boiler that actually performs like a much less efficient product.

    Should I be paying someone to do a manual J?

    I'm also planning on putting a SuperStor indirect water heater on as a second zone off the boiler. We've had a 40g conventional gas-fired water heater, which has been fine for my family of four. My instinct is to stick with a similarly-sized water heater, but should I consider going with a smaller Super Stor? Based on the Super stor spec sheet, the 20 gallon Super Stor has a first hour rating of 168 gallons of 115 degree F water, which sounds like plenty. Feels risky though to only have 20 gallons hot at a time. Next size up is 30 gallons with First hour rating of 212 gallons; and the 45 gallon tank (next size up after 30) delivers 292 gallons in the first hour, which sounds like an absurd amount of hot water for a 4 person household. However, some other posts have said the standby losses are so low on the Superstor that one shouldn't worry much about heating too much water.

    Any help with these questions would be very much appreciated!

  2. #2
    In the trades Dana's Avatar
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    Big old cast iron radiators are GREAT for gas-fired modulating condensing boilers- FAR better than fin-tube base board for two reasons: They put out predictable amounts of heat even at sub-100F water temps, and the thermal mass of the water keeps the boiler from short-cycling when running at low, low condensing temperatures.

    HELL yes you should be concerned about the -105 being oversized! While the number & sized of the radiators is an important factor in the system design, it's not how to size a boiler! It's highly unlikely that ANY 1200' house in MA that has glass in the windows would need a boiler the size of the ALP-105. Even an uninsulated house with single pane windows would usually come in under the output range of the ALP-80.

    For a sanity check, what's your exact ZIP code, and how much oil do you go through in a year? If you have a mid to late winter oil bill with a "K-factor" stamped on it, what's your K-factor?

    Edited to add:

    The burner on any boiler has more output than a standard 40 gallon gas fired tank, and operated as a "priority zone" you get the full output for hot water heating when needed. But you still have to size the tank for the biggest tub you want to fill. First-hour ratings are all about recovery rates, but what you really care about in filling bathtubs is the first 8 minutes, not the following 52 minutes. Most folks getting satisfactory hot water performance out of a 40 gallon standalone will do fine with a 30 gallon indirect, but you have to pay closer attention to go smaller than that.

    On the sizing issue- many crusty old-schooler HVAC guys will walk into a place and use a rule of thumb like 25BTU/foot for newer construction, 35BTU/foot if it's decrepit Victorian or older, and that will RELIABLY oversize it with quite a bit of margin. For a 1200' house even those overly-generous rules of thumb would still put you well under the output capacity of the ALP80.

    I live in a ~2400' circa 1923 bungalow in Worcester (about a 5F colder outside design temp than yours) and you could heat TWO of my houses with an ALP80. If you're inside the Boston city limits in a 1200' house even the ALP80 is going to be on the large size, but the thermal mass of the high-volume radiators will be sufficient to keep it from short cycling, and it'll still deliver good efficiency. Odds are pretty good that your heating needs at the 99% outside design temp are about 18-22,000BTU/hr, so even on "design day" it will be cycling on and off, but with a minimum input of 16,000BTU/hr the number of burn cycles you would get with the ALP80 over a season would be measurably lower than with the ALP105 (which has a 21,000BTU/hr minimum fire.)
    Last edited by Dana; 05-23-2013 at 11:26 AM.

  3. #3

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    I was in the same situation as you. I now have a 1200sqft cape which is fully insulated and renovated with an Alpine 150 boiler. Don't fall into the same trap as me and make sure you get the proper heat load analysis done. I think you'll find that the 60ishk model might work or might even be a little over sized. But last time I looked that was the smallest one you could get in the utility company deal.

    You can post the results of the manual J up here and we can go over them.

    Cast iron rads and one zone is an optimal situation.

    Also, I'd highly recommend getting an Active Voltage Regulator and use that to run the boiler. The electronics are very sensitive in these units. Power surges during and after Sandy blew the control module on mine and I had no hot water or heat for 4 weeks after the storm because the replacement units were back ordered. I now have this unit installed running my setup.

    http://www.amazon.com/dp/B0000512LA/...0_M3T1_SC_dp_2

  4. #4
    In the trades Dana's Avatar
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    mage182- do really think sizing this one calls for a Manual-J?

    The minimum fire output on the ALP80 isn't a whole lot bigger than most 50-60KBTU boilers from other vendors, so it's going to do about as well as any, despite being oversized for the actual load.

    And I've yet to see a house of any age or style in MA come in anywhere near 50BTU per foot of floor area (most come in well under half that), which would STILL be well within the output range of the ALP80, even at non-condensing temperatures. The risk of undersizing is zero.

    The contractors proposing either the -105 or -150 here appear to be real hacks, and may not be fully up to the task of installing & adjusting a mod-con to it's optimum performance. Call US Boiler (1-888-432-8887) to find out who your local Burnham distributor is, then get the distributor to recommend a few of competent installers working in your area. They will know who has been a real PITA calling the tech line for information that's clearly written in the installation manual, and who has been installing dozens of Alpines per quarter without any hand holding, call-backs, or complaints.

    But don't let anybody talk you into the bigger boiler- there's no upside, only downside to buying too much boiler.

    Once you're rid of the asbestos coated oil boiler, be sure to seal up the chimney opening- it's just a parasitic load sucking infiltration air into your house 24/365 for not good reason.

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    DIY Junior Member Rozie's Avatar
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    Thanks for the responses.

    Responding to Dana's first post: Looks like we used about 450 gallons of oil this year. I've not kept track year by year and the bill does not show a K factor. Zip code is 02131. Does that give you enuf info to do a "sanity check"?

    Also, great idea about calling the Burnham distributor for recommendations.

    Also appreciate your insights on sizing the indirect water heater -- makes good intuitive sense to size storage based on how much you hot water you want to be able to draw within a short period.
    Last edited by Rozie; 05-23-2013 at 05:41 PM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    How big is the tub? How many people will be showering at the same time (at what draw)? Will you be running the DW and WM at the same time as trying to bath? Could you add a drain water heat recovery unit (only helps with showering)? First hour can be important if you're using a sustained low/medium use, but not with a fill as fast as you can tub.
    Jim DeBruycker
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  7. #7

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    Dana - Perhaps you are correct. But my thought was that by the time the total cost is calculated, The 80 model with the discount should be around 1100 I think. Based on the boiler that is being taken out it's probably best to rip out everything and start from scratch. That should put the total cost for all new P/S piping, pumps, controller box, expansion tanks, flange kits, check valves, etc. to around 7000 as a rough estimate. at that point what is another 150(?) for a manual J?

    Rozie - If you're pretty much set on getting the Alpine, go to the USBoiler site and download the install manual for it. It's long, but I'll assume since you're here that you're capable of reading it and understanding what is going on and how the install should look once its done. I wish I had read it and understood it earlier, it would have saved me a lot of money and time.

    When you talk to USBoiler to find a contractor, make sure they deal specifically with the Alpine. The top 8 Burnham registered contractors in my area that are listed on their site had absolutely no experience with the Alpine line of boilers.

    Are you considering an indirect hot water tank? It will up the cost but I highly recommend it. I put a 50 gallon Burnham tank in during my install (Check the install manual, the way in which the tank has to be piped varies with the size of the tank). It took some calculations and fine tuning, but now even my oversized boiler only fires once per call, meaning when I take a shower the unit fires to make hot water once and never hits the max temp to shut down. You should have no problem running a 50 gallon tank with the 80 model. I currently have one bath but will soon be adding another.

    There is also a setting adjustment you should look into for your thermostat that will change when it calls for heat based on the heating unit and how many times per hour calls are made. This saved me even more money in gas once I made the change.

  8. #8
    In the trades Dana's Avatar
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    Quote Originally Posted by Rozie View Post
    Thanks for the responses.

    Responding to Dana's first post: Looks like we used about 450 gallons of oil this year. I've not kept track year by year and the bill does not show a K factor. Zip code is 02131. Does that give you enuf info to do a "sanity check"?

    Also, great idea about calling the Burnham distributor for recommendations.

    Also appreciate your insights on sizing the indirect water heater -- makes good intuitive sense to size storage based on how much you hot water you want to be able to draw within a short period.
    02131 Roslindale has had fewer than 5500 heating degree days (base 65F outdoor temp) over the past 12 months, but let's round up, eh? The 99% outside design temp there is about +10F. There are 138,000 BTUs in a gallon of oil, and assuming 85% efficiency (it's really probably more like 50-70% if it's the original 1920s boiler, even with a retrofit 86% flame-retention oil burner), that's about 117,000 BTUs/gallon delivered as heat to the house.

    450 gallons in 5500 heating degree-days is (450/5500=) 0.082 gallons per HDD, or (0.082 x 117,000= ) 9594 BTU per heating degree day.

    With 24 hours in a day, that's (9594 / 24 =) 400 BTU per degree-hour.

    With a 99% design temp of +10F, that's (65 - 10 =) 55F heating degrees. So at the 99% outside design condition the heat load is no more than (400 x 55 =) 22,000 BTU/hr.

    Given the efficiency is probably lower as well as the true heating degree-days, the heat load is actually lower than 22K.

    At full fire at the edge of condensing temperature the DOE output of the ALP-80 is about 72,000 BTU/hr, so no matter HOW little radiation you had (which determines how high the peak temp has to be), you would have more than 3x the amount of output required to heat the house, and you'd be good down to lower than -100F for an outdoor temp (seen those temps in your neighborhood lately? :-) ) from a boiler-output point of view (but probably not from a radiation sized point of view.) If you went with the ALP105 you'd be almost 4x oversized with enough boiler to heat the place even below -150F. Do you really need that extra burner output, for ANYTHING? (methinks not!)

    The min-fire high-condensing output of the -80 is going to be about 15,000 BTU/hr and if you set up the boiler to run under outdoor reset control and adjust the flow rates correctly that's about where it will be firing ALL of the time, delivering higher efficiency than it's AFUE rating. Only when the indirect hot water heater is calling for heat would it ramp up to the ~70,000 BTU/hr range (which is over twice the burner output of your standalone gas HW heater- so it will recover in half the time.) Go with the 30 gallon indirect, and set the temp on the indirect to the lowest temp that still fills the tub without having to wait for recovery. People with smaller tanks may need to store water at 180F to get the tub-filling capacity, but I suspect you'll be fine at 130-140F, which will have MUCH lower standby loss, and would result in higher combustion efficiency on the boiler while serving hot water calls.

    While I'm a big fan of drainwater heat recovery, it may not be cost effective on fuel savings at current natural gas prices unless your family all takes long daily showers, not tubbys. But with 70K of burner behind it a 50% efficiency drainwater heat exchanger would make continuous/endless showers possible, with NO recovery time required between serial showers. SFAIK no MA utility is currently giving rebates on them, but if you buy them through EFI (a MA company), you can get them at wholesale pricing for the extra trouble of opening an account over the phone with a credit card. (I installed a 4" x 48" PowerPipe on my combi-system for the enhanced capacity reasons, and it works like a champ. It's efficiency is ~53% according to Natural Resources Canada third party test data.) No matter how you heat hot water, they save energy on showers. But when used with a high efficiency boiler burning historical low-priced natural gas, any concept of "payback" has to be about performance, not financial, unless you're running the shower an hour or more per day.

  9. #9
    In the trades Dana's Avatar
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    To correct the record, using the past 12 months weather database on DegreeDays.net from the KMAQUINC2 weather station in Quincy on DegreeDays.net it's more like 5700 HDD, North Canton KMACANTO2 logged about 6000 HDD, not the 5500HDD estimated. (The dataset from KMAROSLI2 Roslindale only had 6 months of data, and couldn't even be estimated.)


    So, revise that max-possible heat load number up to (22,000 x 6000/5500= ) 24,000 BTU/hr, not that it changes anything of substance in the discussion. I'm sure there are errors in exact filling dates and oil use to skew it a bit too, but unless you were burning over 1500 gallons a year you wouldn't be running short of boiler with the ALP 80.

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    In the Trades Tom Sawyer's Avatar
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    In New England we use an outdoor design temp of -10
    We frequently have temperatures at -10 and lower for a couple weeks a year.
    Most folks are not real happy not being able to get the house over 55
    K factors are often skewed because many folks use supplemental heat like pellet stoves, wood stoves and electric space heaters to keep the cost of fuel down so that needs to be taken into account when using the k factor
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Outside design temperature has little to do with the extreme mean--it is why they call it extreme--and everything to do with percentage of time. There is a standard for sizing all space heating appliances. It is used nearly exclusively by real professionals in virtually all of N.America and beyond. It is called Manual 'J'. If you are trying to size a new boiler for residential heating you first must find the proper heat load on a design day--see ASHRAE--and choose a posted design day in the 98 percentile or less. These numbers are come to by using empirical meteorological data gleaned over a meaningful number of years.

    A proper heat load program will be based on Manual 'J' and have the design day information built-in.

    As for your three contractors; first, you can't size any appliance from the basement--monkey see, monkey do is not an accepted procedure--. Second; measuring radiation can be useful but it is marginally so when cast iron radiators are the sole source of radiation. The only time cast iron radiators should be used to size the output of a boiler is when installing a steam heating system.

    When sizing a new condensing boiler for retrofit to old existing cast iron radiators, the only information you must have is the heat load of the structure. The most practical way to obtain a proper and accurate heat load is by measuring the exterior of the house, calling out insulation, window size and quality and plugging this information into an ACCA Manual 'J' software program. If you local heating "professionals" can't manage this (common) hire a professional HVAC designer, preferably with some hydronic heating experience to perform a heat load and specify the size of the boiler.

    Unfortunately sizing the boiler properly is but one of many hurtles the "average" boiler contractor must overcome, the most challenging seems to be reading the installation manual. Ask to see a filled-out start-up sheet before paying the final bill.

    Beyond this, most of the old steam and hot water systems featuring cast iron radiation were operated on a single zone (one thermostat), furthermore, if you have a hydronic system, not steam, the piping is likely over-sized, if this is case primary/secondary near-piping is of no value and in fact unnecessarily wastes both the installation cost and perpetual operating cost of a redundant pump. Fortunately the smart and considerate folks a Burnham boiler recognize that smart contractors will not waste their time or your money by adding the extra pump and will not void the warranty if everything is properly sized and installed as so many others will. I give them great credit for this and use Burnham Alpine boilers in my local installations and our national hydronic design practice.

    If your heating contractors can't show you a sample of a proper Manual 'J' heat load, get a new contractor. If your contractor wants to charge you for the estimate or offers to do the heat load for a fee, pay him. You'll be glad you did.

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    In the trades Dana's Avatar
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    The 99% Manual-J design temp listed for Boston, where this house is located is +12F.

    Ever seen a house with (as reported ) both wall insulation (any value) and storm windows that came anywhere NEAR the output of the ALP-80 at +12F in a full-on Manual-J calc?

    Unless Rozie is interested in changing the radiators or micro-zoning there's simply no point to running a Manual-J. Boiler sizing from the Burnham Alpine series is a-priori a done deal- the smallest boiler wins.

    But getting the rest of the system design right is more than a mere plumbing exercise. Hopefully the distributor recommended contractors will have a better handle on it than those making the first round of proposals.

    News for Tom: Boston 02131 is still "In New England...", and any practitioner designing to -10F in any Boston ZIP code arguably needs a "license-adjustment", since that would significantly oversize the system. For most of coastal Maine -10F would be a bit bogus too, if not to the same extent as Boston. Even in central MA (also in New England) the number of hours over the last century colder than -10F are in single digits. Last time it was that cold in my town was in the mid-90s, though it's been colder than -5F several times since. The 99% design temp for Worcester is +5F, but even in the past 5 years I had a hydronic heating professional tell me with a straight face that the design temp for Worcester was -5F. (He didn't get the contract.)

    The point about supplemental/auxilliary heating sources is well taken- all heating source fuels need to be accounted for in a fuel-use linear approximation of the heat load the 99% design temp. Far fewer people in Boston are using pellet/wood burners for supplemental heat, and electricity in Boston is 2x as expensive as in Maine, making it somewhat cost-prohibitive to heat even partially with resistance electricity, so I don't generally ask, but perhaps I should. Usually people will offer that up, but not always.

    Rare is the heating system in Boston (or anywhere in MA) that would be so undersized as to only be able to make 55F on design day. The only time I've seen anything remotely that bad-off was a small barely insulated house with single-panes (formerly a summer vacation cabin) heated solely by a ~20KBTU/hr propane fired wall furnace, but the home owner was complaining about only being able to make 60-65F (well above 55F.).

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    "if you have a hydronic system, not steam, the piping is likely over-sized, if this is case primary/secondary near-piping is of no value and in fact unnecessarily wastes both the installation cost and perpetual operating cost of a redundant pump"


    No p/s needed on oversized piping ? Why is that?

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    In the trades Dana's Avatar
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    Short answer:

    The pumping head of oversized piping is very low, low enough that it doesn't need a separate pump. The min-flow requirements of the boiler will always be met even if it's pumping directly through the low-head radiation & fat distribution plumbing.

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    So, p/s needed only on what? Undersized piping? Why anybody would undersize piping? And even on undersized piping if you get bigger pump would it help to eliminate need for P/S piping?

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