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Installing a Condensing Modulating Boiler with an Indirect Hot Water Tank; still efficient w/ old plumbing and cast iron radiators?

user-788447 | Posted in Mechanicals on

Is installing a condensing modulating boiler w/ indirect hot water in an old home with cast iron radiators in Minneapolis still efficient if the house has old 2”+ dia. cast iron piping and cast iron radiators? I’ve read that if the water returning to the boiler isn’t under a certain temperature the heat exchanger won’t function efficiently. Also need to add an additional zone for a previously unconditioned space where we would like to use a wall hung radiant panel. My understanding is that old boilers w/ cast iron radiators typically operate with water temps at 180 degrees F, the wall hung radiant panel wants to run at 150 degrees F, and the hot water will be heated to about 120 degrees F. Is it a problem for a single modulating unit to supply different zones with different temperature demands?

This is for a frugal household – 2-3 people, 1 shower, 3 sinks, clothes washer, no dishwasher.

Any recommendations of manufacturer’s based on field installation experience would be appreciated.



  1. Riversong | | #1

    You'd really need to have a competent HVAC contractor inspect your house, but I suspect the answer will be NO. And no heating plant is going to be efficient until you first improve the efficiency of your thermal envelope. Then you can significantly downsize your heating load and heating plant with a small mod-con boiler.

    You can, however, run each zone at a different temperature with mixing valves, but that requires setting the boiler output to the highest required temperature, which will make it less efficient.

    I've been very impressed with Triangle Tube Prestige boilers and Smart Tanks and their new Excellence combo unit (both heat and hot water - no separate tank required).

  2. J Chesnut | | #2

    Thank you for your response Robert and including models you have had success with.
    Agreed on the thermal envelope upgrade. Your comment on how the use of mixing valves requires the output to run at the highest temperature is helpful.

  3. Envirocon | | #3

    Last year I installed a Raypak xp120 in a 7 bedroom 4 bath house that was built in 1907. It had 2.5 inch pipes that fed original radiators. The unit has worked admirably and cut the heating costs in half. The key is outdoor temp reset and a boiler controller(fancy thermostat) like the Tekmar 256. The Triangle Tube Prestige Excellence 110 would serve your needs quite well when coupled with a boiler controller and an installer that knows how to get proper thermal separation between the primary loop and the secondary loops that will need zone circulators to run this system right. Since the Raypak uses an aluminum hx, I would stay away from it and others like it. They require precise pH control and the longevity is unknown past about 15 years.

  4. Graham Mink | | #4

    Doesn't using a dishwasher actually save hot water and energy? I can't remember the numbers but I think it was pretty significant.

  5. Riversong | | #5

    Doesn't using a dishwasher actually save hot water and energy?

    Compared to what? Running the tap continuously while washing and rinsing dishes?

    Using a machine to perform a simple and meditative task saves nothing and wastes an opportunity for responsible action. It's very easy to wash dishes without hot water and with less than a gallon per day. How quickly we forget that most of what we consider necessities are wasteful and unnecessary extravagance which the earth cannot support.

  6. GBA Editor
    Martin Holladay | | #6

    The most important study comparing the energy use and water use of a dishwasher compared to hand washing was conducted at the University of Bonn in Germany. That study concluded that using a dishwasher saves energy and water compared to the way most people wash dishes by hand.

    If you are very frugal, however (like Robert) you may be able to beat the dishwasher. Maybe. But it's hard.

    More here:

  7. Riversong | | #7

    The study that Martin refers to was sponsored by the dishwasher manufacturers, and meta-studies have proven that the funding source has a strong influence on study outcome (duh?).

    What such studies ignore is the embodied energy (and water) in the manufacturing and shipping of large household appliances, and the fact that most dishwasher detergent has environmentally destructive phosphates and must be abrasive to facilitate scrubbing.

    While the latest Energy Star qualified dishwashers must use no more than 5.8 gallons of water per cycle (until last year, there was no water limit), that's efficient only if full loads are washed, and its comparatively water-efficient only when compared to the most wasteful hand-washing methods, such as continuously running hot water.

    It's quite simple to use two wash basins, one for washing and one for rinsing, that contain together no more than the nearly six gallons required of the best dishwashers. Relying on an automatic washer does nothing to encourage the thoughtful and conservative practices that we need to foster in order to create a sustainable society.

    Wash the dishes relaxingly
    as though each bowl
    is an object of contemplation.
    Follow your breath
    to prevent your mind from straying.
    Do not hurry to get the job over with.
    Consider washing dishes
    as the most important thing in life.
    Washing the dishes is meditation.
    If you cannot wash the dishes in mindfulness,
    neither can you meditate while sitting in silence.

    Thich Nhat Hanh

  8. user-788447 | | #8

    Thanks Bill Bradbury for sharing your experienc in the field.
    I ended up purchasing a Buderus GB-142 condensing gas boiler. It does have an outdoor reset but it also has an aluminum hx. I was wondering what the differences where between the stainless steel and aluminum hx so your comment on that was informative.
    Another concern I found with coupling state-of-the-art boilers with older plumbing systems is dirt particles getting into the boiler; so the manufacturer advises to add a particle filter in the return circuit.

  9. badgerboilerMN | | #9

    There is hardly anything more efficient than an condensing boiler connected to old cast iron radiators.
    First, the radiators have mass, which presents a load to any boiler. This increases "system" efficiency.
    Second, the old cast iron radiators were designed for relatively low "design" temperatures. We find most our replacement boiler operating in the low 120's Fahrenheit. This means the return temperatures are below body temperature and the combustion efficiency is in the 98%+ range!
    Third, all condensing boilers now have weather sensitive controls built-in. This means that the boiler will be modulating flame (output) and keeping the radiators warm, day and night, in cold weather while burning about half the fuel of a conventional low-efficiency cast iron boiler.
    Finally, on improving the thermal envelope. Always a good idea, but not always practical. We find that installing a condensing boiler in older homes saves fuel and makes folks more comfortable at once.

    Where natural gas is available there is only one informed choice for driving old cast iron radiator heating systems.

    Aluminum vs. Stainless steel heat exchanger for your condensing boiler. It is the installer, not the boiler that really matters.

  10. pjwst6 | | #10

    I agree that high efficiency boilers are a great way to go with an old cast iron system.

    A consideration is that the more you address the building shell through air sealing and insulation, the the more efficiently the boiler will operate. Your cast iron radiation was sized for a leaky, poorly insulated building. If you improve the shell, the radiation will be over-sized for a traditional boiler. But, this is perfect for a condensing system, where the water temperature can be run lower, allowing for the highly efficient condensing operation.

    One word of caution. Many heating installers will install a high efficiency boiler, but skip installing the outdoor reset. This means that the system will operate at a high constant set point, and much of the boiler efficiency will be lost - all to save spending 45 minutes installing this temperature sensor and doing the setup. I recently was in a new Energy Star gut rehab for a non-profit, and all five high efficiency boilers were installed without the manufacturer included outdoor reset sensor. When we pointed this out, the contractor refused to install the OR, saying this wasn't in the contract. So, make sure your contractor does a proper install.

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