Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Additional BTU margin for the boiler

Abe Member Posts: 3

TIm - thank you for hosting the website. As a home owner and someone looking to replace the boiler system, I learned few things that will help me ask the right questions to hire the potential contractor for the job. Good job.

QQ - The house heat loss is 72k BTU. I have option of installing Viessmann WB2B-26 and 35 with output of 85k and 114k DOE BTU (74k and 99k IBR) respectively. Which boiler is appropriate for the house? In 2-3 years I will upgrade windows and that will reduce heatloss to about 54k. The house is in MA.


  • Tim McElwain
    Tim McElwain Member Posts: 4,593
    First of all what do you have

    for heat transfer, baseboard, radiators etc?

    Keep in mind the Viessmann units only have a maximum temperature of around 167 to 176 degrees depending on which model.
  • Abe
    Abe Member Posts: 3
    Additional BTU margin for the boiler

    I have mix of cast iron baseboards in two zones and copper/fin in the other two zones.
  • Zman
    Zman Member Posts: 7,472
    I would..

    Do you have a domestic  hot water indirect? If so what are your water needs?

    Since you have already done a heat loss, compare the size of your copper baseboards to the heat loss in those rooms. you can go to the manufactures website and get an output table. I think you should look at how those heaters will perform at the lower  temps that mod/con boiler love.

    Generally you do not need to add a BTU margin.Domestic water may be an exception.

    I would also look at the triangle tube and lochinvar boilers. especially if you need the higher output temp.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Abe
    Abe Member Posts: 3
    Additional BTU margin for the boiler

    We are looking to install 60g tank that will support our worse case usage. All rooms have more linear feet of baseboard than the heat loss for that room. As a homeowner (not hvac prof) pardon my ignorance - are you suggesting that if the radiating heat from the baseboard is more than the the heat loss for that room I should be able to run it at lower water temp? I guess I am not following your comment on looking up heat loss per room at manufacturer' website - baseboard manufacturer or the boiler?
  • JStar
    JStar Member Posts: 2,752

    Correct. If your heat loss is less than the available radiation, you can heat the space with lower temperature water. That's what I do in my house. My water maxes out at 165F. When I converted my boiler from oil to gas, I went as far as calculating the BTUH loss of all of the copper pipes in my basement, and added that to my firing rate. I heat my 1,000 square foot ranch with about 30,000 BTUH.
  • Tim McElwain
    Tim McElwain Member Posts: 4,593
    Abe in order to get

    the maximum efficiency from your Mod/Con boiler the return water temperature has to be below 130 degrees (F). This can be accomplished if your radiation exceeds the heat loss in the rooms. By lowering the send out temperature to say 160 or so with a 30 degree delta T (temperature difference) you can begin to approach the condensing range of your system. As the system gets up and running you can continue to adjust the send out temperature and see what level gives you the comfort level you are looking for (room temps).

    With out door reset you add another feature which is the ability to control input based on outside temperature. As the outdoor temp goes up you want to use less fuel and still support your comfort level. ODR does that for you automatically based on the setting of the heating curve and measurement of radiation.

    The domestic hot water will be a priority setting so when hot water is needed it will operate over the heating demand until domestic requirement is satisfied or a set time depending on set up.
  • Henry
    Henry Member Posts: 996
    Design temperature

    Well done Tim! Design temperature is in our area less than 200 hours of 1050 hours of operation for maximum capacity of heating. So, near Boston this is even lower! ODR is a must and will save you a lot of energy. I like to ad 50% of the hot water load to the heating load. With a modulating boiler in mod/con this works out well. This keeps the comfort level in the coldest months. We have had numerous instances whereby competitors ignored the hot water load with a degeneration of the the capacity to heat the building! If you have rain forest shower heads and non restricted flow faucets, it can cause the boiler to stay in a DHW cycle for too long.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Multiple Temps

    I'd take full advantage of being able to run multiple heating circuits with thenVitodens 200. Add the Viessmann Mix package. Have the boiler rest for the fintube and the mixing vlv reset for the cast zones..
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • icesailor
    icesailor Member Posts: 7,265
    DHW Load:

    Whenever I bring that issue up, I get a blank stare like I am speaking in another language.

    DHW load, 365 days a year.
This discussion has been closed.