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Thread: Vito den

  1. #1
    Join Date
    Feb 2010
    Posts
    7

    Vito down -Viessman less efficient than 50 year old boiler

    I have recently had two Viessman Vitodens 100 (120000 btu) gas boilers installed to supply radiant heat for our cast iron radiators, and which replaced our 40 year old 400,000 btu monster. So when I looked at my January gas bill I notice only 10% in fuel savings compared to January last year. Surprising especially since last January year was almost 5 degrees colder. Now the target temperature for the radiator water on the controller was was set to 170 F. Could this be too high?

    We have also had some problems with the pressure dropping bellow 12kpa on about 6 occasions causing the system to shut down. While the source of the problem looks like it comes from the HBX controller, this issue has not been confirmed. Another contributing factor of the system shut down was that the water intake flow value's threshold was below 10kpa. The valve has been rectified to above a threshold of 15kpa and the system has not shut down in almost two weeks. There was some leaking from the poor welds in the old piping but I would say the total water loss would be less than 1 litre( 1/4 gallon) day

    Is the pressure drops the cause of the inefficiency or is the low efficiency due to improper setting or other unknown problem on the HBX controller?
    Last edited by tortin; 02-10-2010 at 07:27 PM.

  2. #2
    Join Date
    Feb 2010
    Posts
    7
    any speculations on what the problem could be would also be appreciated

  3. #3
    Join Date
    Feb 2010
    Posts
    7
    any speculations on what the problem could be would also be appreciated as I am at a complete loss.

  4. #4
    Join Date
    Nov 2001
    Location
    Moose Jaw, Saskatchewan, Canada Occupation:Interprovincial Plumber, Commercial Gasfitter Interests:
    Posts
    2,412
    Your water leaks caused the shutdown.

    Your combustion efficiency is about the same as the old boiler when running at 170. But the new boilers modulate so you get extra savings.

    Hard to compare year to year without having the EXACT same conditions, wind, temps, sunlight, doors opening/closing etc.
    I love my job, but paydays Thursday

  5. #5
    Join Date
    Apr 2007
    Location
    Pa
    Posts
    758
    are you comparing dollars and cents or actual gas usage.?

  6. #6
    Join Date
    Feb 2010
    Posts
    7
    actual gas usage.

  7. #7
    Join Date
    Mar 2001
    Posts
    1,213
    Were you comfortable? Does the HBX have outdoor reset, & if so, are you using it? If not, why not? As rich said, 170f hardly allows condensing. Outdoor reset should give more savings. Maybe you don't need 170 if your rads are big enough. What controls the rads?

  8. #8
    Join Date
    Feb 2010
    Posts
    7
    Yes the HBX does have a sensor on the outside and the temperature target adjust according to the weather outside. Lately the target temperature has dropped to 115 while the outside temperature is around 32. The latest meter reading is about 48 cubic meters of gas a day down from the much colder last February of 54 cubic meter of gas a day.

  9. #9
    Join Date
    Mar 2003
    Posts
    7,323
    here is a guess. your old boilers operated at a preset temperature span, say off at 180 degrees, and on at 160 degrees. your new boiler now operates on a reset scale. the problem coud be associated with that scale. see, the only true way to set up an outdoor reset schedule is to do a heat loss calc for varioustemperatres, and creat a scale that tries to mimic the numbers, so that say at zero degrees your heat loss is 250000 btus, and your emitters, to provide that value need 174 degree water, then do the same for higher temps, and finally set up your reset scale to match the curve. unfortunately, most controlers cant and wont do this, so they are set up for a range. many installers dont realize the importance of actually doing follow up to adjust the schedule. for example, if they saw your old boiler set to 180, they probably used that number as design at say ten degrees, and then use say 100 for 65 degrees. the computer then does the math for every other degree on the scale. this is not 100% accurate, especially with old cast iron rads.many homes with cast iron rads are now oversized due to better windows, insulation, siding etc, than what was available when the home was built. your heat loss effectively is lowered. with large cast iron rads, you probably dont need 180 degree water any longer. the only way to know is to live it and adjust. for jobs like that, i start out at 160 max. dod your boilers cycle a lot? have your pipes been insulated? has asbestos been removed? there are soo many things that can cause poor performance. it will take a real pro to determine if fixing is necessary or not.

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