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Viessmann Vitocrossal 300 CU3A for an old gravity system

ShahrdadShahrdad Member Posts: 14
edited January 9 in THE MAIN WALL
Hi everyone! I live in St. Louis and own a 2 1/2 story masonry house built in 1897 with an original gravity hot water system with cast iron radiators. The house is somewhere between 5,500 and 6,000 square feet in size. The original coal boiler was replaced with a Weil-McLain cast iron gas boiler in the mid 1970s. The boiler specs tag indicates an input of 245,000, an output of 196,000 BTUs, and a Water MBH of 170.4. There is Taco 110 circulator pump on the return side of the boiler. The label on the motor has fallen off, but the engraving indicates that it's a 1/12 HP pump. There seems to be a temp difference of between 10 and 15 degrees F between the supply and return pipes right at the boiler. The chimney is lined with a 7" stainless steel liner.

The system was installed by the previous homeowner himself with the help of his friends. He was a structural engineer at the architectural firm HOK, and he tells me that HOK's lead HVAC guy specked out the system and determined the required boiler size. Throughout most of the winter, the radiators are around 100 degrees, and even when the temperature is below zero, they are never so hot that they cannot be touched. All three floors of the house stay evenly warm and very comfortable.

I drop a few drops of oil in the three oil ports on the old Taco circulator every few weeks, and so far, both boiler and circulator have performed perfectly. Even during the polar vortex a few years ago, the house remained nice and toasty. Last year, a few pieces of insulation from inside the boiler dropped on top of the burners, but I cleaned it all up right away. There is a dusting of iron rust on the floor of the boiler and inside the burner tubes, which I clean up a couple of times a during heating season. The only repair I have had to do on the boiler was to replace the thermocouple some 20 years ago, but after close to 50 years of service, I think it's a matter of time before something major goes wrong with either the boiler or the circulator.

I have been doing some research on high efficiency boilers, and it seems that the Viessmann Vitocrossal 300 CU3A (199,000 BTU) could be a good fit for an application like mine. I like the concept of a high water volume boiler with large passages. And the heat modulation should obviate the need for several small boilers. Since my old 196,000 BTU boiler has kept the house perfectly warm all these years, I don't see a need for a higher capacity boiler. Viessmann says that this boiler can be dropped right into an old gravity system without the need for primary and secondary piping. And since my system operates at low temperatures, I think that this boiler should be able to achieve a very high efficiency. Also, I really dislike the look of steam coming out of the sides of these old homes when the boilers are exhausted through the side wall (it looks like a dryer running all the time), and if at all possible, I would like to use the existing chimney fitted with a smaller liner. I am planning to use this only for heating and nothing else.

Does anyone here own or has anyone installed the Vitocrossal 300 CU3A on an an old gravity system? If so, what do you think? What kind of circulator did you use? Have you had any issues with the system? And do you happen to have any photos of the installation? Also, would you continue with a high volume, low head pump such as the Taco 110 or B&G 100 series, or would you use something different to achieve a higher Delta T?

I would appreciate any advice before making this big investment. Thank you all in advance.


  • Paul PolletsPaul Pollets Member Posts: 3,359
    I'm involved with a current project exactly as you describe. We're using a CU3A to replace a WM, and added the 80g Vitocell indirect.
    All 18 radiators were re-valved with TRV's and a mixing valve and motor was installed to heat a basement zone. Additional wall panel rads were installed in the basement. We re-used the existing 4" gravity mains, but had to repipe parts of the returns.
    A Vitotrol 300 will be used to control the system. Almost finished and will post pics. I like this boiler, and have used it before.
  • ShahrdadShahrdad Member Posts: 14
    Paul, would you use a variable flow pump or something like the low head, high flow Taco 110 that's on the system now?
  • IronmanIronman Member Posts: 6,142
    That's an excellent boiler that will work great with your system.

    It was customary back in the day to use large 3 piece circulators like that, but you don't need anything like that energy-hog now that much more efficient wet rotor ECM circs are available. Remember, the system can actually flow from gravity, the circ is really only needed to pump the boiler.

    Also, you'll need to line the chimney with PPL or a similar approved plastic. The over-sized SS liner is not gonna work.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • IronmanIronman Member Posts: 6,142
    Also, make sure the system is thoroughly flushed and a good dirt separator added. The new boiler and circulator won't like any of the goodies that a 100 year old system can bring to them.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    ShahrdadAlan (California Radiant) Forbes
  • ShahrdadShahrdad Member Posts: 14
    Bob, that was one of the reasons I like this boiler: It has huge water passageways and a place on the bottom of the boiler for any debris to settle down in far away from the burner.

    Interestingly, the few times I have drained the system to strip and repaint the radiators, the water has been incredibly clean and clear. The previous owner said that the inside the pipes looked brand new when they installed this WM boiler in the 70s. And that was after almost 80 years in service.

    I'm also thinking a wet rotor circulator will be even quieter and more efficient. What would be the advantage of using an ECM circulator vs. a single speed on a one-zone system such as this?
  • IronmanIronman Member Posts: 6,142
    Basically, energy savings and an ECM can easily be set to deliver the correct flow.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • GWGW Member Posts: 3,903
    Excellent boiler. Only large mass high efficiency unit that I’m aware of (other than adding a buffer). 

    Got jammed a little on one job, couldn’t get HC1 (with vitotrol 300a) and DE1 to cooperate with each other. The HC was overshooting its set point, pump wouldn’t shut off. Put a regular stat on the main zone and now both zones go to DE1, 
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • Paul PolletsPaul Pollets Member Posts: 3,359
    I always use ECM pumps for the heating circuits, and fixed 3-speed wet rotor pumps for the indirect tank
    delta T

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