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Help sizing a new mod con

CabGuy Member Posts: 4
I've read through this forum quote a bit and learned a lot. I want to thank everyone that has contributed.

I have a 2750 Sq ft. home in the Northeast and we need a new heat system. I have performed the Slant/Fin heat loss calculation and have come up with 46,000 BTU needed at 0 outside temp and 70 inside temp. We have a 40 gallon indirect, radiant on the first floor, and baseboard on the second floor. We have been quoted a Vitodens 100-w at 118MBU. From reading here it seems that this unit is at least twice the size needed and only has a 5:1 turn down. It seems to me that a much smaller unit would be fine, something like the Vitoden 19k or Lochinvar KHN085. We have more baseboard than needed upstairs and it seems to me that we can run the system at a lower temperature most of the year and that it would condense.

So, should I go with the larger unit that the installer says is best or smaller?

Thanks in advance for helping.




  • heathead
    heathead Member Posts: 234
    I went with the viessmann that has min of 12,000 btu. You need to trust the heat loss. The only reason to add is if you need or have a high hot water domestic use, i.e. Large bath tub or lots of shower usage. I have the 80 gallon tank, and it takes a while to recover because it has max of 68,000 btu. But have yet to run out of water. Not even close. If so I would look a GFX drain heat recovery to increase supply temp for Idwh. I went veismann, but the lochivar was a close second. If you do the viessmann set it up with low loss header and let the computer in it do it's thing.
  • CabGuy
    CabGuy Member Posts: 4
    Thanks Heathead!
  • the_donut
    the_donut Member Posts: 374
    46 mbtu seems a bit low for that square footage unless your house is really tight.
    Rich_49rick in Alaska
  • Rich_49
    Rich_49 Member Posts: 2,766
    edited March 2018
    the_donut said:

    46 mbtu seems a bit low for that square footage unless your house is really tight.

    Why in the world would 16.7 BTU per square foot sound high . This is the median or average for a built to code house Donut . Nothing special about it
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
    CanuckerMark Eathertonrick in Alaska
  • CabGuy
    CabGuy Member Posts: 4
    Rich and donut thanks for your responses.

    Our house is 1850's but completely redon 20 years ago with new insulation, windows, storms etc. Do you think going with the larger Vitoden will be problematic?

    Is the slant fin heat loss accurate? I don't want to undersize.
  • lchmb
    lchmb Member Posts: 2,997
    I would be more inclined to the Lochinvar. Closer to the heat loss and leaves plenty for a call for the indirect for fast response. I will say the most important aspect is the install and setup. I would go with the unit your service provider is more comfortable with. They have to take care of it for you..;)
  • SeymourCates
    SeymourCates Member Posts: 162

    The issue with a mod-con is not the maximum input firing rate. Just about any one of them is more than sufficient. What you must pay attention to is the minimum firing rate. This will determine how far into the warmer weather you can go before the boiler starts to cycle.

    If your heatloss is 46K and the 100W has a minimum firing rate of 21K, you can easily see that the boiler must cycle at all ambient temperature above about 32F. This is undesirable and will limit the life of the boiler. If the installer says that it is "best", you really need another installer. Either he's clueless or dishonest.

    If you still desire to use Viessmann, utilize the 200W B2HB which has a minimum firing rate of 12K and you might get up to 45F before the cycling begins.

    Before you pull the trigger on Viessmann, consider the Lochinvar KHN or the economical HTP UFT-80, both of which can offer a minimum firing rate of approx. 8K. Now you have a machine that can run 24/7 for most of the heating season.
  • Zman
    Zman Member Posts: 7,606
    On the sizing , as indicated above, trust the heatloss calcs.
    I prefer the Lochinvar. To me they are simpler and more intuitive.

    What type of radiant do you have? It would be nice to run your whole house at the same temp. If you have a room by room heat loss and the length of baseboard for each space, it can be figured out pretty easily.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • CabGuy
    CabGuy Member Posts: 4
    Thank you lchmb, SeymourCates, and Zman .

    I spoke with the gentleman doing the install and he will be doing a heat loss.


    The radiant is for the first floor only and is screwed to aluminum plates under the sub floor. I have done the slant fin room by room heat loss and believe we can use 140 water 80% of the time.

  • SuperJ
    SuperJ Member Posts: 609
    The higher mass vitocrossal would be fine and offers the benefits of no min flow and variable primarily simplicity and efficiency.
  • 4Johnpipe
    4Johnpipe Member Posts: 484
    Sounds like the additional baseboard was intentional to match the temperature of your radiant zones and keep supply water temps down. Larger unit is not your friend...
    Considerate People, Considerate Service, Consider It Done!
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