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When to replace an old boiler (natural gas)

I have a 1984-vintage McLain-Weil boiler. It is 83,000 btu/hour for a duplex that has 1300 sq. ft of living space, plus unfinished basement and floored attic with no radiators but flash and batt at roofline and airsealing at eave walls, so its temperatures now track pretty close to 2nd floor.

Cast iron radiators. Balloon framed 1910 construction, no insulation in walls. Storm windows on all windows (which are original everywhere except enclosed porches outside original stucco walls).

Live in climate 4a (Washington, D.C.). With the fairly mild winters here, the boiler doesn't fire that much of the time, except from 7 or 7:30 a.m. to 9 a.m., when it gradually is recovering (in two stages) from a five-degree setback (63 to 68). Starts from cold start, and usually only runs long enough to get to about 135 degree water... haven't checked how hot the water gets when it runs for an hour.

Have a carbon monoxide detector in the basement, and no worries there. But the gas flame is set too high (when the boiler servicer turned it down, it went out within eight hours) and it burns yellow. I believe the burner tubes were vacuumed out in early 2019, when servicer first came out. Have not asked about replacing power pilot thermocouple or millivolt thermocouple, which someone at Green Building Advisor said could be to blame for the too-high flame. It does smell of gas right by the pilot light.

No water leaks, have never had to add water, no failures other than that one time the pilot light went out when it was turned down to normal levels.

An energy consultant claims the boiler operates at 78% efficiency (I don't know that I trust that, in that there was no real report, and he didn't even give me an air-change number, and while he gave me a CFM number after the blower door test, there was no information about where the leaks were).

There is a small comfort issue on the second floor, even after insulating & air sealing the attic (second floor bedrooms almost always are 2 degrees cooler than first floor, no matter what the temperature outside/thermostat setting, unless there's a lot of solar gain at the smallest bedroom which borders the south-facing enclosed porch), I'm not convinced that's a boiler issue per se. [Second floor exactly tracked first floor when we turned down thermostat to 58 degrees when we went out of town for a weekend in February; all weekend, it stayed between 57-59 in that north facing bedroom with two walls. Thermostat is in middle room of three, only has one exterior wall.]

Had same servicer come out to bleed radiators. Most didn't need bleeding, but there is still banging in pipes from time to time. Have never had radiators flushed in two years here.

So. Here's my question. Should we replace the old boiler with a modulating condensing boiler (it wouldn't be much more than a standard boiler, because the city would require a chimney liner if we went that route)? Should we keep the boiler as long as it's working more or less okay but ask about a thermocouple replacement? Would flushing the radiators make economic sense at this point, or wait until we're switching over to mod-con? The servicer was not willing to wrestle with frozen knobs to see about trying to balance radiators to see if that would help with the cooler rooms upstairs... but at the time of a flushing, I guess that could be done?

If a technician says we should replace the boiler, should we ask for reasons? Or just figure when you have a boiler this old, folks think that's the wise thing to do?

Thanks for your thoughts!


  • Jamie Hall
    Jamie Hall Member Posts: 23,443
    Couple of thoughts. Several, in fact. First place, I would assume from your description of the comments and work from the servicer that... you need a better one. The thermocouple you mention has nothing at all to do with the colour of the flame -- whether it is a power pilot or millivolt, which are two completely different critters (about the only things they have in common is a pilot light and they both burn gas...). The person at Green Building Advisor is clueless.

    If the flame is yellow, like a candle, at the tips either there is not enough air or the gas pressure is set too high. Both of these can be fixed quite easily by a competent technician with the correct measuring devices. The same technician can give you a real report on the combustion efficiency of the boiler -- using the same instruments.

    If the flame is more orange, that may be dust or sodium containing chemicals in the vicinity.

    In either case, step one is to get someone out there who actually does know what he or she is doing and have them thoroughly clean the fire side of the boiler, check the breeching and the chimney, and properly adjust the pilot and the burner using instruments. If nothing else, you'll probably save some fuel.

    Otherwise I'd keep the boiler as is. Hard to say how much longer it will live, but might be quite some time. When it does develop a leak, then that is the time to replace it. Perhaps not, however, with a mod/con -- that would depend on exactly what kind of radiators you have, as they work best at lower temperatures and you may not have enough radiation.

    If you have cash to spend on stuff, your first priority should be locating and doing something about draughts. Pick a nice cold windy day and wander all over the house, looking and feeling for them. Make sure the storm windows are in good shape. If the main windows are original, just see about making sure they operate well and are well maintained -- the combination of old windows and storms is just as good as all but the very most expensive replacement variety.

    You might look into having the walls insulated. This depends a lot on how they are built as to whether it is a good idea or a horrible one. If they are plaster over lathe on the inside, plus good wood sheathing and outer finish on the outside, insulation may not be worth the effort.

    You ask if you should ask for reasons if the tech. says you should replace the boiler. Indeed you should. He won't give you the correct answer (which is something along the lines of "I don't know how to work on this old girl and I need the paycheck for the work") but it never hurts...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • mattmia2
    mattmia2 Member Posts: 9,747
    I would add that getting someone that knows what they are doing to get the radiator valves cleaned up and re-gasketed or replaced so you can balance things would also be a good investment. May need to look at strategically adding some TRVs in areas that heat first if you can't balance it with the valves alone or if the mixture of the load varies a lot with different weather conditions(solar gain, wind, even cooking).

    A mod con is a possibility when the time comes, if the radiation is large enough for it to make sense, running lower temps for longer cycles also could help even things out, but it doesn't sound like your current boiler needs anything beyond someone that knows what they are doing to work on it.
  • unclejohn
    unclejohn Member Posts: 1,833
    A mod con is usually twice the price of of cast iron boiler so unless your gas bills are really high which I doubt you are better of with a cast iron replacement. Do you know if the the pump or gas valve have been replaced. That boiler may have a mercury type pilot sensor that plugs into the gas valve. I would seriously think towards replacement in the near future especially if any original parts are still on the boiler.
  • 1910semi_attached
    1910semi_attached Member Posts: 1
    Thanks for the feedback! The consensus both here and on Green Building Advisor seems to be that the boiler guy we had who said we have to get a new boiler just wanted the business. :)

    Tom from Green Building Advisor didn't think the flame color had to do with the thermocouple, rather he said maybe the reason the pilot light couldn't be turned down was because of a bad part. Here's what he said:

    "If he had to keep the pilot flame high, then either it is a power pilot thermocouple ($60)or the millivolt thermocouple is on its 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."

    He thought the flame color was because of dirty burner tubes. The technician did clean the fire side of the boiler two years ago... but don't know if he checked the breaching. Told me that the city would require a liner in the chimney if I replaced the boiler with another traditional boiler, but I don't know if that means he looked inside the chimney.

    I feel confident we have enough radiator capacity for a mod-con: the boiler fires very little of the time except for 90 minutes in the morning when it's recovering five degrees of setback, and when it comes on for a short period to move the thermostat from 67 degrees back to 68, the water temperature only gets to 135 before it cuts off again.

    We upgraded all the storm windows but five of them, and had windows made operational that had broken ropes, and cracked window panes replaced. (Only two of the storm windows are old now; the other three were pretty new already).

    We spent about $4,000 airsealing and putting insulation in the attic -- it had none before -- but we have plaster and lathe walls and have been advised because of the way the stucco is applied, insulating the walls is not practical. Used 159 therms from Feb. 9 to March 8, and 162 December 10 to Jan. 11... a lot more than those months last year, but we are home during the day because of covid, and we aren't doing as severe a night-time setback now that we have attic insulation; plus it was colder this winter than last.

    We will probably spend the money we would have spent on the new boiler on some minisplits for cooling upstairs! We hate window units so we mostly just sweat in the summer.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,578
    The majority opinion here is that temperature setbacks for short periods of time, (under 24 hours), will not save money, and will only cause uneven heat recovery, and discomfort. It is best to set a slightly lower temperature, and leave it constant.Keeping the boiler clean, and serviced is most important.—NBC
  • Steamhead
    Steamhead Member Posts: 16,901
    Get in touch with @Dan Foley at Foley Mechanical to service your boiler- go here:

    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • 1910semi_attached
    1910semi_attached Member Posts: 1
    Thanks, I will contact him. At first I thought Lorton was too far, but I saw some DC folks recommending him just before you posted that.