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Community and Q&A

Swapping out a Conventional Boiler for a Modulating Condensing Boiler

1910duplex | Posted in General Questions on

Hi all,

Starting to plan for an install of a modulating condensing boiler in the offseason later this year. Currently, we have an older-than 25-year McLain Weil boiler with the pilot light turned up too high (if it’s normal, it blows out we learned after our first service call).

Our basement tends to be in the 50s in the winter, often mid to upper 50s (zone 4a; rare for the temperature to stay below freezing all day), and warms up by 2 degrees when the boiler is firing (which is not that much of the time to keep the house at 68). Will we need to add any heat source to the basement to keep pipes from freezing when we go to a modulating condensing boiler? Basement is unfinished, but does have washer, dryer and utility sink in addition to hot water heater & boiler.


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  1. charlie_sullivan | | #1

    What climate?

    You'd be better off adding insulation and sealing air leaks rather than adding a heat source.

    1. 1910duplex | | #4

      4a, as I said in my post. :)

      Understood that adding insulation would be better than adding a heat source. I guess I should be clearer -- do I have to do anything to the basement with this shift, or will it stay in a decent range anyway.

  2. joshdurston | | #2

    Sounds like it might be worth thinking about some basement air sealing and insulation.
    Oops Charlie got here first.

  3. joshdurston | | #3

    Often it's not just a slide in replacement, especially if you want to take advantage of the efficiencies a mod con offers. You really want to make sure flow and temperature's are minimized, and that your emitters (rads/baseboard) are taken into consideration.
    Far to many mod con swaps are left running at high temps and flows leaving a lot of efficiency on the table.
    And do a a heat loss calc, often modcon's get over sized because "they have a 10:1" turn down. But if the space only has a true 30,000btu/hr heat loss (and the boiler's rated output is 100,000btu/hr), then the usable turn down might only be 2 or 3:1. Ideally most of the heating season it should just purr along and modulate and you want a low minimum fire rate (ideally less than 10,000btu/hr).

    If you can try and follow the instructions here to get a heat loss measurement. It will help you be a more informed shopper when your contractors are pushing 2x or 3x oversized boilers.

  4. 1910duplex | | #5

    Thanks, Josh, I have been saving my natural gas bills, because I have read that article, along with this one  

    I also got advice on a previous Q about the switch:

    One trick to get lower RWT and better efficiency out of modcons with high temp rads is to run them at much higher delta T (provided the unit can handle it, most can). For example you can supply a rad 170F water but with a 110F return. The rad would put the same heat is if supplied by 150F and 130F return but much better efficiency out the modcon. A delta T circulator can do this adjustment automatically but you can also do it by tweaking the flow rates.

    1. mlavigne | | #6

      Your basement will be colder- but I have never heard of a boilers standby losses being the difference between pipes freezing and not- and I deal with boiler swaps in northern ME and AK where temps get below -20°F. Most people only add radiation to the basement if it is finished, in my experience.
      You can also use an outdoor reset curve to lower the supply temperature and gain efficiency, too. Your design day is likely in the ball park of 15-20°F, but over 1/2 of your total heating-degree-days likely occur when it is over 35-40° outside. That means your heating load for a majority of the season is likely only 15-20k BTU/hr (if your design load is 30-40k). You can provide much lower water temperatures during these more temperate days. One Brookhaven study showed that you could heat with condensing temperatures (<135°) for up to 92% of the year with traditional cast iron radiation (designed for 180° water) with outdoor reset properly utilized.
      Be careful with the low flow rates to increase deltaT- it can cause comfort issues, because the last radiator in the string might be a different room, and its only getting 120° water in your example. Also, some boilers have minimum flow requirements of 5+ gpm and require primary/secondary piping which recirculates water if the system flow is too low (raises your return water temp and lowers efficiency). Keep your flow rates up, or look for boilers that dont require primary/secondary to avoid this (there arent many mod-cons that can do this).

      Source- I'm the lead engineer at a ultra-high efficiency boiler manufacturer


      1. 1910duplex | | #7

        Thanks for the info, Michael! I already have comfort issues in our bedroom, which is the furthest radiator from the boiler (see, so your advice is well taken.

        Does a Delta T circulator cause the same issues as trying to increase the delta by adjusting flow rates? I was planning to use outdoor reset, and I don't even know if a Delta T circulator is compatible with that strategy.

        Does stripping silver paint (or white paint) from cast iron radiators make any difference, or is it not really noticeable?

        Yes, winters here are pretty mild. It will fall to temperatures in the 20s overnight with some frequency, but most days' highs are at least 35, and often in the 40s.

        1. tommay | | #8

          You say you have cast iron radiators. I assume this a force hot water system. Is it piped as a monoflow system or is it piped in a loop or in series?
          If it is a monoflow system do you or have you purged each radiator at any time? Air in a radiator will build up pressure and the water will not flow correctly. If your current boiler still works and doesn't leak why change it, especially if it's just a circulation problem. Have you flushed the system at any time? How old is the circulator pump? What pressure does the boiler show? Flow rates aren't much of a concern if you are heating up a cast iron radiator since they take time to heat up, slower may be better but you need enough flow pressure to make a monoflow system work correctly. Check any flow checks to ensure they are working correctly and there is no flow obstruction by fully opening them and flushing, then close them when done.(The screw cap on top)
          If the radiators have valves going to them, you can also adjust the flow to each one by adjusting the valve.
          The pilot issue may be one of two things. If the system was converted from steam, it may still have a power pile thermocouple which requires a larger flame. If it keeps going out it may be going bad. If it is a regular millivolt thermocouple and the flame is to big it will burn out the thermocouple quicker so it may have to be changed. The pilot can be adjusted through the gas valve. There are usually ports with a screw or allen wrench cap, one of which will have the pilot adjustment.

          1. mlavigne | | #9

            I agree with everything Tom said (particularly, are you sure you want to replace your bulletproof beast of a boiler with a mod-con? many have a spotty longevity track record - unless the current unit is broken [and thus you have a large sunk replacement cost], high efficiency units often have a long payback- if you already need to replace the boiler, the extra cost for more efficiency is a no-brainer, payback wise).

            If you have comfort issues in your bedroom, you can make it a separate circuit too, by cutting the pipe and creating a zoned parallel pass like this:
            This way your bedroom will get the full boiler temperature.

            The delta-T pumps are overkill, you can just balance everything with a ball valve (throttle it to get the desired delta T). If your zoning, delta-P pumps (like many of the ECM/high-efficiency ones, or most of Taco brand pumps) are better because the output is fairly constant as different zones open.

          2. 1910duplex | | #10


            Yes, it is a hot water system; it was gravity circulation system when it was first designed in 1910. (Does that answer the monoflow system Q?)

            There are valves on radiators, but when the boiler guy came, he said they cannot be turned, since they have been in the same position for decades. He did bleed the radiators. Only a couple had any liquid come out. We still have a bang from time to time after the bleeding, for what it's worth.

            I would be interested in trying thermostatic valves to see if I could get the upstairs closer to the first floor temperature, but not sure if that could be done without stripping paint from radiators to restore them. (Also, not sure it's worth the trouble and expense for a two to four-degree difference). The radiators are warm upstairs, but I guess because of worse heat loss, they don't catch up to the downstairs temp before the boiler cuts off (except in the sunniest room, sometimes).

            The reason we were planning to replace the boiler is because the man who serviced it when we first moved in two years ago said we should! Do not know how old the circulation pump is. It is a grundfos type up 15-42-F made in Clovis Calif.

            Also, would like to use less natural gas if that is possible. Used 162 therms for 33 days in December/early January for 1300 sq ft of finished space (plus uninsulated basement and walkup attic insulated at roofline and airsealed with foam on eave walls). Only three exterior walls, because it's a side-by-side duplex. That was keeping the place at 68 degrees 14 hours a day and letting it fall to 63 overnight, then warm up gradually for two hours; used 159 therms for 28 days in Feb 9-March 8, keeping it at 70 degrees (foster care placement it's required), but still with a 63 setback for 10 hours.

            Boiler is cold right now and pressure is about 9? Water temperature is about 90.

            Now boiler is firing and working on getting thermostat from 67 to 68; pressure is 10, water temperature is 100...

            Now boiler stopped firing, looks like pressure got to 11, and temperature maybe to 125 to 130? Since it's 45 degrees outside, it doesn't run very long to go from 67 to 68. I think less than 10 minutes. Water temperature falls back to 100 quickly.

            I'm not sure what he adjusted the temperature to earlier this winter to make the boiler water temperature 10 degrees hotter, but our gas use is definitely higher than last year (but we also had average temperatures in January and February, while last winter, temperatures were much above average).

            The initial service call, I'm not totally sure what he did, besides "cleaning out" the boiler and adjusting the pilot light down (and then returning to turn it back up). It has never gone out again since it was returned to the too-high level.

            Adjusting the pilot light is definitely beyond our DYI capacity but we did buy the tool to bleed the radiators ourselves in the future. :)

            Michael, not sure if we can create a zoned parallel pass, because DC Contrarian said the gravity circulation design (2 inch iron pipes that run the length of the basement with a pipe to each radiator from it) makes that impractical. Is that true?

            He's the one that suggested either using the knobs (which are frozen in place) or installing thermostatic balancing valves.

            I wish I knew how to find a boiler expert that serves D.C. that could consult on this! ( only has Baltimore firms and a guy 45 minutes away in Virginia suburbs) Like I said, the one firm we've had out said we should replace the old boiler, but didn't really explain why. We don't really know how old it is, just that it was here before the previous owner, who bought the house in 1993. I can't see any CP-style serial number on it; there is a 9 digit number at the end of the sticker on top that gives directions for installation.

      2. Expert Member
        Akos | | #14

        Lot of times with modcon swaps, the system flow rates are not adjusted and the modcon is not configured properly. I have fixed over-pump the system like this where it left set at fixed 170F with such high flow rate that the delta T was barely measurable, turning a modcon into a non-condensing boiler.

        Something like an HTP UF80 or a Navien NHB-80 can run all the way down to 4 GPM when running high deltaT (40F). Combined with outdoor reset, this can get your system into condensing mode almost all of the time.

        You do have to take care and make sure the pump is set up properly, with modcons over-pumping the system is just lower efficiency and more electricity cost to run the pump.

        As others have pointed out, cleaning the system (plus a good separator) is a must. It is incredible the amount of sludge that can build up in a 100 year old cast iron system.

        When your system is drained that is the time to install TRVs or replace stuck valves. TRVs can be installed without repainting your rads. Not easy as 100 year old fittings will not come apart without a fight. Usually they are a good upgrade as now you have individual temperature control for your rooms. Just make sure you don't put one on the rad near the thermostat.

  5. mlavigne | | #11

    with that much water (2" pipes are huge, most residences can get away with 3/4"), there is no reason the boiler should be short cycling like that. If you dont get a cast iron boiler up over 135°, you have turned it into a condensing boiler and before long, you will have to replace it from rot.
    Do not try and squeeze efficiency out of a non-condensing boiler by running it below 150°.

    To me, it sounds like you have a flow issue- likely 30+ years of gunk in the system and your thermostatic valves on the radiators are stuck. The downside, is the cleaning (with a strong acid typically) can uncover all sorts of issues. The good-ish news, is that you need to clean the system anyways if you ever want to put in a high efficiency model- bad water quality will kill most mod-cons in a few years. It is now common practice to install an "air/dirt/magnetic" separator to protect modern system components (ECM pumps and mod-cons) whenever you do system maintenance- think of it like a filter.

  6. 1910duplex | | #12

    I didn't tell them what to set the water temperature at, I guess it was even lower before! But it has lasted at least 25 years so far...

    I will look into cleaning the water in the system!


  7. tommay | | #13

    Yeah, they always want to sell you a new boiler telling you things can't be done.
    If you have a main loop around your basement with two pipes leading to each radiator then yes, it is a monoflow system, normal for previous gravity systems.
    First the valves should be able to be turned. Sometime you have to loosen the packing nut and use pliers to turn the stem, work the valve handle back and forth, then re-tighten the packing nut. Thermostatic valves won't help your situation nor would stripping paint.
    Bleeding the radiators is easy, either use a screwdriver or square nut wrench, which is probably what you bought, to open, or just loosen the bleeder itself since sometimes they get clogged. Just don't take them out all the way. If you had air in them then that's a big part of your problem and what is causing the banging. But in order to bleed them correctly, you need to bring the pressure up. Do this by lifting the lever on the automatic boiler feed, if there is one, or else use/open the main boiler water feed valve while keeping an eye on the pressure gage and bring it up to at least 20-25 lbs.. The pressure will drop as you bleed each radiator, so you have to bleed one or two then check the pressure again. Normal pressure should be at least 15 lb cold. The water/pressure feed should maintain this, if it is not, then it needs to be ignored, adjusted or replaced if not working correctly. Once all the radiators are bled, drop the pressure back to 15-20 lb by opening the drain or any of the vents.
    If he had to keep the pilot flame high, then either it is a power pilot thermocouple ($60)or the millivolt thermocouple is on it's way out ($10). A pp tc is around 3/8" in diameter and will have two wires that connect to the gas valve, a mv tc is 1/8" and will have a copper tube with a connector that screws into the gas valve. Pilot adjustment is just turning a screw under one of the caps on the gas valve, it's similar to a needle valve on a tire.
    Zoning could be done but a little bit of work disconnecting and re-configuring and not really worth it. Once you get all the radiators working correctly, you may find it works well which may be linked to the reason you used more gas, water not getting to the radiators and just circulating around your basement.
    If you take off the front panel of the boiler there should be a metal tag with model number if you feel the need to see how old the boiler is, but some of the older, simpler boiler are better than what you get today, which is why it has lasted this long. You just have to keep them clean by flushing, especially with CI radiators. This is where you will also find the boiler temperature adjustment, a rectangular box with a chrome wheel with temperature markings on it that can be turned and aligned to the pointer,or a box with hi/lo setting. Max/cut out temp should be set to at least +/-170 with cut in temp usually preset or set to +/-140. Smaller boilers will not have a min. cut in temp and will just start from a "cold" start, which may be why your temp got down to 100.

    1. 1910duplex | | #24

      By the way, Tom, I just looked at the dial as it's working to get up to 68, and since is the one time of day it runs for longer than 20 minutes, the pressure eventually climbed to 24 psi. Water temperature climbed to 140 before cutting off as thermostat has reached 68.

      Front bedroom at 66; attic at 59. About 30 degrees outside.

      1. tommay | | #26

        If you are getting pressure swings like that, there's definitely air in the system, most likely upstairs. The air is heating up, expanding and causing the pressure to rise. So just turn off the power to the boiler,let it cool down, bring the pressure up, open the air vents and bleed them. Do you have the model number, if so, post it and I'll see if I can find a manual or photo to see what type of temp. control it has. Like I said it should be a simple dial.

        1. Expert Member
          Akos | | #27

          Air in the system actually reduces pressure swings as it allows for extra space for the water to expand into when it heats up. The trapped air works the same way as the volume in an expansion tank.

          High pressure swings are the sign of either too small of an expansion tank, expansion tank that is waterlogged or not charged to the correct pressure. Bladder type tanks have a life of about 5-10 years, if it hasn't been changed in a while, it would definitely be causing pressure issues.

          1. tommay | | #30

            Not true, water is for the most part is in-compressible, if you heat up a water balloon it will not get bigger, maybe microscopically, heat up air in a balloon it will get bigger. If you believe air in a system is okay, then why purge a system at all. Water and air are separated in an expansion tank. Air in a radiator is not. Expansion tanks are more for surges that may happen so as to not damage other components in the system, that's why it is a safety item, just like relief valves.

  8. 1910duplex | | #15

    Dear Tom,

    Thanks for the instructions! Found the age of the boiler -- September 1984!

    I think the radiators are working more or less correctly -- they do all get equally hot when they run for a long while in the morning. Programmable thermostat lets temperature start falling from 68 at 10 p.m. down to 63 overnight/early morning. At 7 or 7:30 it starts working on getting temp up to 66 and then at 8:30 it works on getting temperature up to 68. (Incidentally, the pressure also does go up to 12 when it has been running for a good while like that)

    Also, when we went away for the weekend in February and set the thermostat at 58, both upstairs and downstairs maintained the same temperature (a band of 57 to 59 for 24 hours). It's just when we're trying to do the 63 to 68 (or 70) temperatures that the upstairs bedrooms don't keep up. I guess there is more heat loss upstairs. It is a balloon framed home, and the only insulation in it is between the first floor joists at the basement ceiling (badly installed) and at the roofline/third floor eave walls. (2 inches of closed cell foam at roofline, then furred out rafters with polyiso and R-23 rockwool batts; one inch of closed cell foam on eave walls and drywall & closed cell foam over top plates.)

    There is no chrome wheel inside. Just a box that has a dial that says pilot, on, off, and below that, a place where it says low and high, though it's not clear how one switches between the two settings.

    It definitely goes from a cold start -- current water temperature is below 100.

    Perhaps of interest, the BTU/hour is 83,000.

    I know old boilers can go a long, long time -- toured a house for sale with an oil-using boiler that dated back to the 1930s!
    But I also wonder what it was about my boiler that had the servicing company say I needed a replacement. Weil McLain's 'tell tale signs your boiler needs to be replaced' says consider after 30 years (check), increased frequency of failures (no), leaks (no), discoloration of flame -- yes, ours is yellow, and there is a smell of gas right by the boiler. We have a carbon monoxide detector in the basement, and it has never gone off, so it's not a dangerous level of improper burn, I guess, but it's not right. Perhaps this is related to the thermocouple part.


    1. tommay | | #16

      Mara, sounds like your all set and know what needs to be done and/or at least what's going on. The yellow flames are probably from dirty burner tubes. They can be removed easily (except for the one with the pilot tube attached) and washed or just blow out any and all debris with a vacuum. Gas smell is probably from incomplete or bad combustion from the dirty tubes.
      Just realize that CI radiators are not an instant heat, they take time to heat up and radiate so there is going to be some fluctuations and temperature swings from set backs compared to""just set it and forget it".

      "But I also wonder what it was about my boiler that had the servicing company say I needed a replacement.".....MONEY!

  9. 1910duplex | | #17

    Ah, I'm guessing cleaning out the burner tubes is one of the things the servicing company did in early 2019. There was definitely vacuuming involved. Is that something that should be done annually?

    I do understand that radiators are not instant heat. It's just weird that the upstairs is a little cooler than the downstairs during heating season. But what can you do, it's an old house with quirks!

    1. tommay | | #18

      Yes if they are dirty. You can usually see debris , rust and dust on them or below them that tells you it's time to clean them, along with the yellow flames you see. Sometimes just blowing air over them, blowing debris around, can clog them more, so best to remove them and clean/wash/rinse and dry them as they can fill up with debris. Watch some videos on how to remove them, easy enough.
      The radiators upstairs, being higher. cause a head pressure that fights the circulation pressure and the water will take the least path of resistance, the downstairs radiators, which is why you should bring the system pressure up from what it is now to where it should be at, 15-20 lbs, by adding in some water, and make sure they are bled, air will rise to and collect at the top of the system.

    2. Mark_Nagel | | #19

      In order to achieve the proper end result/expectation you're going to have to have to have a full accounting of your current system, you need to get a baseline. And, I'd get an energy assessment of your building/structure.

      I'm in no way an expert in this (or in anything), but while doing some research on HVAC for a proposed new build of mine I ran across the site and have been actively reading there. In one forum there was an extremely long thread (over 500 posts?) with someone having a not-so-different issue/question as yours. The professionals posting in response were having a heck of a time trying to figure out (diagnosing stuff like this via Internet can be an impossible task) why things weren't working: I'll puff my chest out and say that not too far into the thread I started to have a hunch, and it turned out to be correct! It turned out that the person didn't have a FULL energy calculation done on their building. They had had one done, but it didn't take into account a basement and an attached garage! The basement wasn't disclosed until there was a comment by the owner that they had problems with pipes freezing in their basement and that they needed to run their wood stove to help with that situation. The new system was struggling to meet demand because of those "hidden" energy sinks. This is the kind of thing that gives new equipment bad raps. Old stuff was WAY oversized; energy was mostly a lot cheaper so just ramp it up! AND, old boilers pretty much have to run hard/hot; trying to conserve energy with them is a sure way of decreasing overall system performance until it's decided that it all needs to be replaced (sooner than it otherwise would have been needed).

      As noted above, get those radiator valves working. Everything running in series means that those closest to the boiler are going to have the hottest water and each subsequent radiator in series will have lesser hot water. I'm pretty sure there's a recommended way to orchestrate these [valves] to help.

      Tom May mentioned increasing pressures. I believe for a two-story building 15 psi is the norm.

      The suitability of the pump is a question I have. That pump is good for a 15' head. Piping distance, radiators and all might be straining it.

      I'm NOT a professional. If you want proper results you're best off finding and paying for a good one.

      ALWAYS look first to KEEP the energy inside your house. Find out how you can improve heat loss. THEN look to your boiler situation.

  10. 1910duplex | | #20

    Thanks, Mark! I did add insulation in the attic (it had none), see (if you read that story, you'll see I also tried to get an energy assessment, but they did kind of a half-ass job, giving me a CFM number but no ACH, showing me thermal camera images that were unsurprising, telling me my boiler was at 78% efficiency, did not give a written report at all.)

    And I did upgrade storm windows, and one basement window, and did some minor air sealing in the basement. (Basement tends to be 55-59 degrees in winter). I was just reading from someone else here today that his upstairs is colder than his downstairs even after doing the same kind of attic retrofit ... also an old stucco house with no insulation in the walls. His delta was narrowed after the attic insulation. Ours might be too, it's a little hard to know, as we only started measuring after we finished the job.

    I have reached out to a company recommended by the Wall as a boiler expert. If they are able to get those valves to move again, or if they turn up the PSI or add water & it makes a difference next winter, I'll update!

    If nothing else, I found out that my boiler was made in 1984, which is good to know! Before all I knew is that it predated the owner before me, who bought the place in 1992 or 1993.

    1. Mark_Nagel | | #21

      Sounds like you're on track. Might be good to document things at the heatingwall as well: folks there can help optimize things for you.

      The last posting (at this time) to the article you referenced is excellent:

      I'll note that it's probably best to NOT set back temps at night as that requires the boiler to work extra hard to try to bring temps up to a reasonable level in a reasonable amount of time (which kind of sounds like it's not happening- boiler is having to do a lot of catch-up).

      Keep in mind that heat loss is also via conduction. The basement is likely sucking heating down and out of the house. Not said is if the basement is conditioned space or not. If it's conditioned then it's likely sucking heat out and down into the ground (floor and walls): a lot of work to, at least, insulate the exterior of the walls: folks here can probably better state how to approach. If it's not conditioned, and that's OK (you're fine with leaving it that way), then look to insulate real well between there and the upper floor.

  11. 1910duplex | | #22

    Basement is unfinished, and will remain so.
    We don't mind how long it takes boiler to bring temps from 63 to 66 and then, after a pause, from 66 to 68. It tends to be most comfortable in that final hour/first hour past recovery since that's the only time the radiators are actually noticeably warm for any length of time! Just now, for instance, the boiler ran for 20 minutes to get the temperature from 67 to 68 in the room I'm in. [It is 49 degrees outside now.] The water in the tank was 130 immediately after it cut off. So the radiator doesn't feel that warm.

    1. joshdurston | | #23

      With a modcon, OAT reset and constant circulation, your rads will be more consistently just warm enough instead of intermittently hot. This can make things a lot more comfortable and even.

      On a cast iron boiler you can potentially install a valve (taco I-series 3/4way) to perform both an OAT reset and boiler protection to achieve some of the same benefits.

      If the older boiler is sized properly and in decent shape is use it as long as possible.

      1. 1910duplex | | #29

        Of course, it is not sized properly. It is 83,000 BTU and I only need 50,000, according to Dana Dorsett's therm-usage calculation.

    2. Mark_Nagel | | #25

      What I was wanting to know was whether your basement is part of your conditioned space or not. Is there insulation between it and your upper floor?

  12. 1910duplex | | #28

    There is insulation between basement and first floor, but not well installed. (Fiberglass batts in floor joists)

    1. Mark_Nagel | | #31

      That means that the basement is a separate conditioned space. This is probably best as the walls and floor aren't likely well insulated (if at all). I'd look to insulate between these two areas as well as you can. Don't take this lightly. The basement is sucking heat from your upper floors.

      Others can chime in as to how best to do this. (Figure that there needs to be a focus on creating thermal breaks.)

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