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Community and Q&A

Choosing an Energy-Efficient Boiler

GreenHomeowner | Posted in General Questions on

We are looking to replace our home’s 1960s “Ack-o-Matic” boiler.  We were hoping to replace it with a 90% efficient model, but have been steered away from doing so by 3 HVAC contractors due to reliability of these models and annual maintenance.

The 3 quotes we received were different brands of ~83% efficient boilers, and different materials – cast iron and copper.

Consumer Reports did a review of boiler brands earlier this year, but had just 2 measures – reliability and owner satisfaction, and interestingly, there was not much correspondence between those 2 measures.

We would greatly appreciate links to any sources that would help us select a boiler, or any personal recommendations or thoughts.

Thank you.

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  1. Patrick_OSullivan | | #1

    Do you plan on doing any significant energy improvements in the coming years? Boilers are typically oversized to begin with, and if you do any meaningful energy improvements later on, they become even more oversized.

    Example: my uninsulated house had a 120K Btu/h steam boiler. After an addition and adding insulation, air sealing, etc., my new modeled load is under 28K Btu/h.

  2. joshdurston | | #2

    The biggest variable for reliability is often the quality of the install and overall system design.
    If you go cast iron, make sure you have a robust low RWT protection strategy. (this is often missed). Typically a thermostatic valve that allows the primary pump to bypass the house and recirc flow until the return temperature is high enough. But there are other strategies with varying levels of effectiveness. Having a properly sized boiler sometimes makes the low RWT problem more evident since it may hit thermal equilibrium at a lower temperature. But oversizing a boiler isn't really a valid low RWT protection strategy and comes with it's own problems.

    There is some validity to the argument that the savings from a modcon (90%+ condensing), are eaten up with increased maintenance, but I'd still go for one though mostly due to the sub 10k min fire rate.

    I would recommend cross posting on there are a lot of experienced boiler guys.

  3. GreenHomeowner | | #3

    Thank you. Each of the quotes received was for a 150k BTU boiler. They measured the baseboards in the house, but I'm not sure what else they took into account in the sizing. I've been reading some of the posts on this forum about appropriate sizing, and trying to digest all the information as a newbie.

    We are upgrading the attic insulation and airsealing. No insulation in the main floor walls of our ranch-style home. I would love to add it, but I think it would be a fairly extensive project since the house is brick with new siding installed by the former owners, so we'd have to drill holes from the inside. Insulation was added to the walk-out basement walls when the previous owners finished it.

  4. joshdurston | | #4

    Measuring the emitter size (baseboards or rads) is really only the method for sizing steam boilers. It's good to know with hot water, but the heat loss calc is king.
    Steam systems need to be matched to the emitters to ensure the whole system sees steam at approximately the same time (undersized will end up serving the rads closer to the boiler only), but no so much as to build pressure quickly.

    He probably measured the baseboard, multiplied the length by 600btu/ft and come up with a number high enough to justify a 150k btu boiler. Baseboard length was often determined as much by room geometry as it was by heat load. 600btu/ft is the approximate output at a 180f average temp. With your upgrades you should be running much lower temps, ideally as low as 120f in the shoulder season.

    Hot water systems should be sized according to the heat loss calc. If you have extra baseboard length then you can run the system at a lower temp.
    You can use the slantfin heat loss calc, but keep in mind it will probably give you numbers 25% high, so don't add too much extra to your boiler size on top of your heat loss.

  5. Expert Member
    Akos | | #5

    With the cost of cheap gas, the payback on a modcon install is probably very long. Since it sounds like you do have a lot of baseboards you can definitely make one work and it would get you above 90% efficiency.

    It is a shame that installers charge so much for a modcon install as they are similar cost and the only thing you need to do extra is run the vent pipes through the side wall of the house.

    While replacing the unit, you should try to get it sized closer to your house loads though. Sizing on baseboard length is not it.

    I would run through this and see what your exact loads are:

    My own home has 95000BTU of rads but needs 36000BTU for heat.

  6. GreenHomeowner | | #6

    Thank you for all your advice! Much appreciated.

  7. gusfhb | | #7

    REaserach boilers and have a real heatloss done
    I had the same experience and ended up having to spec a boiler and find an installer.

    High tech boilers are no more problematic than low tech ones.
    I have had condensing gas furnaces at work for 17 years and have replaced one igniter and one pressure sensor in that time. My buderus condensing oil boiler has had one overtemp sensor fail, which was troublesome to diagnose, but in 10 years that is the only failure.

    Continue searching for installers.

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