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Community and Q&A

Replacing boiler and perhaps radiators in small Massachusetts colonial

dtgriscom | Posted in Mechanicals on

I have a 2300 square foot colonial, built 1960 in Zone 5A, and I’m looking to replace the boiler with a direct-vent mod/con boiler, with indirect hot water. Goals would be increased efficiency (which isn’t just about saving money), reliability, low maintenance, and retiring the current chimney flue.

Current system is forced hot water through fin tube baseboard (with a couple of recently installed exceptions). Boiler is a gas Weil-McLean that’s probably over 30 years old (spec’d output is 95kBTU/hr), with a separate 40gal gas HWH. Insulation is not great: 2×4 walls with some fiberglass, and I’m about to reinsulate the attic with something like R47 cellulose or foam. New windows. House temperature is decently maintained, but the cold walls make it seem colder than it is. Two fireplaces, one of which we use from time to time. Two showers which are occasionally used at the same time, but the four of us don’t seem to take exorbitant showers. There is a 75gal whirlpool tub which we use once in a blue moon.

Here’s the radiator situation for the three zones. Note that the basement zone is a single run, but the first and second floor zones’ radiators are split into East and West loops.

– Basement: 20′ of baseboard. Wall-to-wall rug was installed that blocks the bottom vent, so not a lot of air gets through them, but the heat from the existing boiler keeps things warm.

– First floor: 37 1/2′ of baseboard, plus a recently-installed kickspace heater.

– Second floor: 45′ of baseboard, plus a small cast iron radiator in a remodeled bathroom (vents blocked because it put out too much heat)

I took last winter’s gas bills and heating degree days, whipped up a spreadsheet (based on a thread ; I think “Dana” is Dana Dorsett!) and consistently found that the boiler would use 30kBTU/hour to heat my house on a 0 degree F day. As a sanity check, when it was hella cold last winter I checked and found that the current boiler was running less than half of the time.

So, here are my questions:

– Should I consider replacing some of my baseboard with radiators?

– Is there any reason for me to get anything more than the minimum-sized mod/con boiler? Or, should I get a mod/con at all?

– Given the size of the basement baseboard, would putting the basement and first floor on the same zone avoid short-cycle risk?

– Do I need to bump the boiler size for the indirect HWH?


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  1. Dana1 | | #1

    If your fuel use against heating degree-day data calc indicates a 30,000 BTU/hr load @ your 99% outside design temp the smallest mod-cons in most lines are more than sufficient.

    Unless you have a monster-sized spa tub to fill, do NOT upsize the boiler for hot water when using n indirect fired tank. Size the TANK for the peak hot water load (which is usually the largest tub you need to fill.) If a 40 gallon standalone has been meeting your hot water needs, a 40 gallon indirect heated with a 40-60KBTU/hr mod-con (with the tank zone controlled as a priority zone) will deliver equal or better performance. If you plan to fill the 75 gallon spa on a regular basis you might want to go with a 50-70 gallon indirect.

    Low mass baseboard as heat emitters on the smallest zones and the minimum-fire output of the boiler determines how low the boiler temp can be without short-cycling. A typical 50 KBTU/hr mod-con has a min-fire output of about 13,000 BTU/hr. Dumping 13,000 BTU/hr into a 20' stick of fin-tube means the fin tube needs to deliver about (13,000 / 20' =) 650 BTU/hr per foot, which takes an average water temp of about 180F. Since the water returning to the boiler needs to be under 125F to get any condensing efficiency out of it, that obviously isn't going to work. You can fix that in a number of ways:

    1: Add enough fin-tube to the zone so that it's output balances with the boiler's output at 125F. Most fin-tube puts out only 250 BTU/hr or so per foot at that temp (see: ), so if your min-fire on the boiler is 13,000 BTU/hr, it takes (13,000/ 250 = ) ~ 52' of fin-tube to balance. You can cheat that a bit if you can program a wide differential on the boiler, but more radiation is always better.

    2: Combine zones (not recomended for basement zones), to make the loop balance at min-fire. The heat loss characteristics of basements are very different from above grade floors, and the temperature differences between the basement & first floor will vary quite a bit with outdoor temps if you try to combine them.

    3: Add enough thermal mass in the form of a massive hydraulic separator or buffer tank to the system to ensure minimum fire times greater than 3 minutes independtly of the water temperature.

    (I've explained the napkin-math version of those caculation dozens of times on that other forum.)

    It's worth running a I=B=R type heat loss spreadsheet to see just what your BTU / foot of baseboard is for each room. If the room-to-room temperature balances are reasonable, you could just guesstimate it, but there may be reasons to boost the radiation in some rooms/zone while you're hacking on it.

    A mod-con boiler is not a great DIY project, but with heat load numbers and a design sketch in your pocket you can weed out the real hacks from the professional hydronic installers.

    The smallest "sorta smart" mid efficiency boilers run about 55-60KBTU/hr for output, which is still 2x oversized for your load. But they still hit pretty close to their numbers even at 2x oversizing when equipped with heat-purge controls and internal low return temp protection. A direct vented version is preferable, especially in a house you plan to be tightening up. (The Burnham ESC3 wouldn't be insane, even if 2x oversized for the space heating load, if you opt to skip the mod-con approach.)

  2. user-2890856 | | #2

    I would inquire about any cold rooms that appeared this past winter . Series loop baseboard systems have always been a challenge to the average installer even if they have a good knowledge base . I ask about these possibly cold rooms to hopefully provide information for a real good recommendation . A heat loss calculation of the room by room type would be a good place to start as Dana has already mentioned .
    It sounds also as Dana has pointed out and you have demonstrated through actual BTU usage at near design conditions that low temp is a real possibility utilizing the existing baseboard . Beware of hacks though , they are abundant , especially in Middlesex County and immediate area as was witnessed by the amount of BAD installs we heard about on another site this past winter especially . What general area are you located in ? I may very well know a Quality contractor whom can help if that is your desire .

  3. dtgriscom | | #3

    Thanks, Dana, for the feedback. BTW, my design temp of 0F was too low; a more reasonable design temp of 5F cuts my load calc to 28kBTU/hour. But, what I'm hearing is that a boiler significantly larger than necessary only adds problems and complexity. I'd rather have a small, simple system than an oversized one with patches and fixes.

    In the basement, there's no way I can add any real amount of baseboard because the existing one runs the entire exterior wall. And remember that there's a rug blocking the baseboard input vent, so your BTU/ft*hr calculations are probably seriously high, meaning the real situation is even worse. The baseboards are semi-recessed, and the supply/return is in the slab, which makes things hard to modify without starting from scratch. I'd be willing to start from scratch, though; how about replacing the basement baseboard with some high-mass panel radiators?

    Hi, Richard. I'm in Melrose. The coldest room is the master bedroom; second floor, at the end of both of the zone's baseboard runs. I've blocked off a few sections of the other rooms' baseboards (the heck with the kids' comfort!), and the hall bath radiator, with some improvement.

    And everyone: should I insist on a formal Manual J heat loss calculation?

    I've talked about calculating heat loss with three installers, all who claim expertise in high-efficiency boilers. One said that I=B=R would be more appropriate for a retrofit. Another said he'll measure all my rooms, walls, windows and doors, and send the info to his supplier for a recommendation, which sounds like a conflict of interest. A third didn't respond (although I may not have asked clearly).

    I'd be willing to pay someone to come and do a heat loss if I could trust the results. I've also found a number of websites that, for a moderate fee, will take my data and do the calculations; the data collection seems complex, and it's not all that clear what's what, but I'm an engineer so with some assistance I'm sure I could do it.

    I'm reasonably confident in my past-use-based load calculation, but would a Manual J give me room-by-room information on heating requirements, helping me adjust and improve my radiation?

  4. gusfhb | | #4

    You do not mention how much gas you are using now, or if the boiler struggles or keeps up well in very cold weather. Does the master get colder the colder it gets or does it always just lag the thermostat? It is likely you have more than enough baseboard since the house would have been built with single pane windows.

    It is likely you cannot buy a boiler small enough without going mod con,

    Since you cannot get one small enough an accurate heat loss is somewhat pointless. What would be interesting is figuring out why the master is colder. Probably it is on the corner and has more windows. Maybe it is leaky. Perhaps a blower door with some attention paid to that room might be useful.

    You 'should' be able balance the system by doing what you have done, Blocking some emitter in the kids rooms until the master evens out should not cause them to be cold. That can be done permanently by cutting fins off of the copper tube once you are sure you are correct.

    'Outdoor reset' is also useful with condensing boilers, as it lowers the setpoint temp in warmer weather, mine can run down to 100F water at times.

    The heating guys will want to count the feet of baseboard, cause they is boobs. Failing that they will want to replace exactly what you have.
    You are best off doing your research and getting quotes on exactly the boiler you want. has a lot of really smart guys and a lot of massachusetts guys that can help you out.

  5. user-2890856 | | #5

    Are the rooms closer to the beginning of the zone piping warm , too warm ? What type of circulators are there now ? Many BB systems everywhere you go were designed with best intentions but suffer from specific design flaws that the designers / installers did not even think could happen . Like presuming that the system would operate at a 20* Delta but sizing the pump incorrectly or going with the factory installed 007 . More often than not these systems that were designed for an AWT of 170* have lost so much energy through the loop that your master bedroom may be receiving 140* fluid . Not a good scenario . Proper circs that are sized based on the heat loss may a big help too , so I am saying just sizing and installing a new boiler may not produce the desired result . Supplier / distributor performing heat loss can be OK with right company , not many though . If you can be assured that say Emerson Swan was performing it you could be confident that it was good in most cases .
    Contact Dennis Foley plumbing and heating , he is honest , capable and will stand behind his work . He will also perform the heat loss as requested and he knows about thermal and fluid dynamics .

  6. user-2890856 | | #6

    Also , if you want an unbiased heat loss calc and recommendations based on what is existing you may contact me . Many of the internet based services that you mentioned have in the past performed poorly under the guise that if they are not there the work they perform cannot be guaranteed or they will just point a finger at the installer . I can certainly guarantee my work based on the accuracy of the information provided by you . Must have room square footage , window and door sizes and type , length and height of exposed wall , R value of the assembly ( U value determined ) Finish floor coverings including pad of carpeted , Desired IDT . You may also find me on and Taco's Flopro neighborhood . I consult and design for all of N America and all my work is done through sites such as this and through referrals .

  7. dtgriscom | | #7

    BTW, just had another plumber in. Great rep on Angie's List and the BBB, nice guy. Had a fifteen minute look around the house, and then said he'll sell me a Bosch 131, to make sure that I have enough hot water on the indirect. I pushed ("but my existing boiler ran less than half the time on the coldest days!"), and pushed again ("my 40gal 40kBTU/hr standalone HWH never ran out"), but he said it would be "unethical" for him to sell me a smaller boiler so I could save a couple of hundred dollars.

    Boy, is this harder than it should be...

  8. user-2890856 | | #8

    Well that is not your guy . Unethical is him installing anything , anywhere without the proper knowledge to do so . It is unfortunately harder than it should be . 732-581-3833 , Rich . What theu don't realize is that the couple hundred now saves 10xs that over the lifecycle , which might I add will be shorter with oversized equipment short cycling like a monster .

  9. user-2890856 | | #9

    Bosch Greenstar 131 Combi . Bad choice as the DHW performance would be 3.25 GPM but the low end modulation is 36K , probably more than you require at design much less the other 90% of the heating season . For this unit not to short cycle you would need to install a buffer tan of at least 29 gallons to combat the short cycling . Now the 100 is no better at a low end modulation of 34.6K and providing a dismal 2.65 GPM I believe . You need something in the 50K range with a 30 gallon indirect storing at 150 and mixing down to 120 for distribution to the fixtures . The boiler should have a low end modulation around 12,000 for space heating . If the BTU usage you have stated is correct you'll require something along these lines . Remember you don't want to store water at below 140* , 50% of municipal water supplies in this country contain Legionellosis bacteria . We need to kill that and not allow it to multiply in our storage vessels , 140 kills 90+% , I like 150* because it also allows a longer draw between calls from the tank holding out the boiler just that much longer while not compromising your comfortable shower . Storing water up to 160* is not an issue with a quality indirect .

  10. Dana1 | | #10

    What Richard McGrath said- combi boilers are rarely the "right" solution, and definitely not the right solution here.

    What you need is something that modulates as LOW as possible (lower==better, especially with low-mass radiation on micro-zones like your basement) , to limit the short-cycling potential, but have a max output north of 30,000 BTU/hr to be able to provide comparable domestic hot water performance to your 40 gallon free standing tank. (A 40K non-condensing burner on a non-condensing hot water heater delivers about 32K to the water.) IIRC Lochinvar has some mod-cons that modulate down to ~9K in, which may be appropriate here.

    But the LAST thing you should install here is a low-mass boiler with a min-fire of 36K, which is GUARANTEED to short cycle on your 20' fin-tube zone. You could make it work by designing in some buffering thermal mass, but that's more of a band-aid to a poorly chosen boiler like the Bosch combi than a "right" solution. There are better options out there.

    Just because they can correctly spell "plumbing" and "heating" doesn't make them competent hydronic designers. You can be a great plumber and still have huge gaps about what it takes to actually DESIGN a heating system. If they don't even offer to do a Manual-J heat load calculation before recommending a solution, they're probably not the right contractor. (And even those who DO offer may not be competent to run the calculations.) I eventually gave up and began designing my own hydronic hacks, given the wealth of ignorance out there, even if I still hire contractors to do the installation. (The designer for one of those contractors said straight out that my solution wouldn't work and bet his boss $100 that it wouldn't cut it. Six heating seasons later I beg to differ- he had to pay. :-) )

  11. user-2890856 | | #11

    Cadet CDN040 , HTP EFT55 , Dunkirk DKVLT-050 . There you go , if in fact your calcs are correct any one of these will be well suited paired with a quality indirect , I would go with the SuperStor40 or the Dunkirk indirect . If it matters Dunkirk is made west of you in NY by ECR Intl and HTP is made right in East Freetown Mass . Find someone capable of installing these and you'll have it beat . First though have that heat loss done to verify adequacy . Dana , what say you ?

  12. dtgriscom | | #12

    Thanks for all the help. Even with a small boiler, my basement loop (with 20' of obstructed fin tube baseboard) is likely to be a problem. Would it help if I ripped out the baseboard and put in a couple of high-mass radiators?

  13. Expert Member
    Dana Dorsett | | #13

    "Would it help if I ripped out the baseboard and put in a couple of high-mass radiators?"

    Short answer: Yes.

    Longer answer: Don't guess- DO THE MATH !

    It's not rocket-science- even the napkin-math is fairly predictive of the actual performance- you'll know when it at least has a shot of making it without short-cycling, and when it doesn't stand a chance in hell!

    Calculate the total thermal mass in the system on that loop, including the water (and iron, if a cast iron boiler), and look up the maximum differential that the boiler can be set up for. With the thermal mass, the temperature swing, and the output of the boiler (min-fire output, if mod-con) you can estimate the minimum burn times when serving only that zone.

    Say your 20' zone of fin-tube is 3/4" copper, as is the plumbing, and there is 100' of total length in that loop, and your boiler is a 3-plate cast iron beastie with 2 gallons of water and a dry weight of 170lbs, that can be set up with 20F swings between high/low temp, and a D.O.E. output of 60,000 BTU/hr.

    The water volume of 3/4" pipe is about 0.027 gallons per foot (, so at 100' of plumbing you're looking at about 2.7 gallons. Add in the 2 gallons in the boiler you're looking at 4.7 gallons. At 8.34 lbs per gallon that's 39 lbs of water in the loop.

    A dry weight of 170lbs means the boiler has something like 130-140lbs of iron in the plates (the jacket, burners & insulation weigh something). At a specific heat of 0.11 BTU/degree-lb (see: ), that's the water-equivalent mass of 0.11 x 130 lbs (thinking conservatively) , or 14.3lbs, bringing the total thermal mass of boiler + plumbing to about 50-55lbs of water-equivalent. (Ignoring the much smaller thermal mass of the copper itself, etc.)

    To change 50bls of water 20 degrees is only 20F x 50lbs = 1000 BTU. How long is that going to take?

    With 20' of baseboard, assuming a water temp averaging 180-190F you're looking at at the baseboard delivering about 12,000 BTU/hr, but the boiler is dumping 60,000 BTU/hr into the system, leaving a 48,000 BTU/hr excess. At 48,000 BTU/hr of excess, it only takes 100o BTU/48,000 BTU/hr = 1/48 th of an hour, or about 1.25 minutes, or about 75 seconds to deliver that 100 BTU- it'll short cycle like CRAZY (as is surely does when your 95K output boiler is serving only that zone.) In practice it probably will overshoot the high-limit (unless you have a ridiculously oversized pump too.)

    If you swapped it out with a big 500lb column radiator with a volume of 15 gallons (125lbs) of water that can emit 30,000 BTU/hr @ 180F, you'll come up with different numbers. You have about 150 lbs of water and 50lbs of water equivalent iron for a total of 200lbs water-equivalent, and only 30K of excess BTU input. To swing that mass 20F takes 200lb x 20F= 4000 BTUs which gets delivered in 4000/30,000 = 0.13 hours or 8 minutes. An 8 minute burn time is just fine for a cast iron boiler, and if that's your smallest zone it'll do great, even at 140F water temps.

    It the 20' of fin-tube/100' total plumbing situation, instead you installed as small modulating condensing boiler with a mid-mod output of about 15K, the thermal mass of the boiler & contents will be much smaller than the cast iron solution. The loop volume is still 2.7 gallons, the boiler maybe one, for 3.7 gallons/ 30lbs of water, and no metal-mass of consequence. With the boiler output running 180F in non-condensing mode the output of the boiler is only 14K, the fin-tube is emitting 12K, for only 2000 BTU/hr of excess. To swing 30lbs of water 20F takes 30lbs x 20F= 600 BTU. So with 2000BTU/hr of excess heat entering the loop it'll take 600/2000= 0.3 hours or about 18 minutes to swing 20F. At 130F (the beginning of low-90s efficiency condensing) the fin tube is only emitting about 5-6K, and in condensing mode the boiler is delivering 14.5K for about 9000 BTU/hr of excess. To deliver the same 20F swing then takes 600/9000= 0.066 hours, or about 4 minutes, which is just fine for a mod-con boiler. But at 110F high-90s combustion efficiency (or with only a 10F temperature swing) you start getting more than ankle deep into short-cycling territory. The lower the water temp, the more condensing efficiency you get out of it, but the greater the BTU/hr excess going into the loop.

    But replacing that 20' fin-tube with even some flat-panel radiators of equivalent output that have some internal water volume/ mass will definitely help. Something like a Biasi B-36.48 ECO has same output, but also adds more than 38lbs of water-equivalent thermal mass roughly doubling the thermal mass of that loop (assuming 80-85' of 3/4" plumbing instead of 80' of plumbing + 20' of 3/4" fin tube.)

    To play Jr. Hydronic Designer you have to look up some specs, dig into the operating & installation manuals for the boilers, etc, but in the end you won't be shooting yourself in the foot with an overprices underperforming system doomed to an early grave from short-cycling. And unless you hire a true hydronic professional your napkin-math sketch of a design & spec will outperform the HVAC-hack-designer solutions, so at least have that design in the back of your pocket for reference for vetting the designs & recommendations of others.

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