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determining boiler size

homeowner2 Member Posts: 8
I am replacing my gas hot water gravity fed boiler. 9 out of 9 plumbers did not conduct heat calculations to determine boiler size. When I ask them about conducting the heat loss calculation they tell me it's not necessary based on their experience and knowledge. Some have counted and measured the radiators. I am confused, is it absolutely necessary to perform the heat loss calculation to determine boiler size?  What does this calculation entail?  Why would so many plumbers be opposed to conducting the heat loss calculation?


  • Greg Maxwell
    Greg Maxwell Member Posts: 212

    My experience with gravity systems is to fire the connected load. Now with that said, I have been in this business going on 30 years, and Im still open to learning.  You should also do a heat loss calc. Compare that against the current load calc. You dont want to be too much on the small side however, because you also have to remember the amount of water in that type of system. Take a look at both the heat loss calc, and the load calc, and neet somewhere in the middle. If you go too small, and dont install any sort of bypass, you may create condensation because of the potential for cooler water returning to that boiler. If you can, also look at modulated water temp with outdoor reset. That will give you greater comfort, with lower fuel bills.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    Funny thing about contractors...

    The LAST thing a contractor wants to hear, is a complaint from the consumer that they are not warm, when it is cold outside. So, they do what they have always done, which is to oversize the boilers. They use rules of thumb (30 to 50 btu/s/ sq ft/hr) or "This is what we did on the last job like this one and they never complain", or the stand across the street with cut outs and determine how many sections are needed to heat the home. None of them are a valid means of determining load, but they never get the cold call, and when the appliance breaks down due to short cycling, it is simply charged off the the nature of the beast...

    The consequences of over sizing the boiler is short cycling which leads to inefficiency and short life expectancy. In reality, with a bang bang (On/Off) boiler, it WILL be over sized, even if properly sized, for 98% of the time. Even a perfectly sized mod con boiler will short cycle during the shoulder season, and an improperly over sized mod con boiler will also short cycle.

    The problem you are working against is that if you were to hire someone to do a design and heat loss, the contractor is not going to want to install it because bottom line, HE is responsible for the end results. It may require some sort of limited liability release in order to get them to play along.

    It has been my personal experience, that even on jobs that I know for a FACT that the boiler was properly sized to the theoretical load, at design condition, the appliance is doing a 50% duty cycle, which means it is 2 times larger than necessary. But no contractor in their right mind will look at a heat loss calc and say "I'm going to cut the boiler size in half..." With that said, installing a good mod con boiler will alleviate, but not completely eliminate a lot of the problems with short cycling due to over sizing.

    The one factor that will KILL a decent heat loss, is the air infiltration factor. My suggestion would be to hire a BPI certified energy auditor or equal to perform a heat loss and blower door test on your home. The emitters (radiators) are most probably oversized, and that is to your advantage when considering a mod con appliance. It equates to a lower fluidic operating temperature which equates to appliance efficiency and savings.

    Energy conservation BEFORE appliance replacement will give you the biggest bang for the bucks, and will also result in a smaller appliance being set. I would also recommend that you consider the placement of non electric thermostatic operators on all of the radiators to control flow and heat output. Why heat a space that doesn't need it...

    Unfortunately, common sense is not so common any more...

    The soap box is now free...

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Greg Maxwell
    Greg Maxwell Member Posts: 212
    Heat Loss

    I know that different areas have different types of supply houses, but, there are very capable people working in that arena. They can go in, and do a heat loss/load calculation, using experience, as well as a verified heat loss program, (we use Wright Suite) and come up with a very good recommendation for pretty much any job. The contractors should have a relationship with someone they trust at their supplier. We dont go in and measure air change, because by looking at the job, whether or not they are insulated, how much, old or replacement windows, etc., we can be very accurate in our calculations.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    My boiler is oversized, too.

    "It has been my personal experience, that even on jobs that I know for a

    FACT that the boiler was properly sized to the theoretical load, at

    design condition, the appliance is doing a 50% duty cycle, which means

    it is 2 times larger than necessary."

    My contractor sized my boiler by pacing around two sides of my house and decided to offer a 105,000 BTU/hour mod-con boiler. What I had was an oil burning boiler with a 1/2 gallon/hour nozzle, which figured out to about 70,000 BTU/hr. It always gave enough heat. And that thing always short-cycled. I calculated the heat loss of my house and got from 30,000 BTU/hr to 35,000 BTU/hr heat loss when it was 0F outside. Design day around here is 14F. So I insisted on the 80,000 BTU/hr boiler that modulated down to 16,000 BTU/hr which is not low enough at the bottom end.

    Trouble is that this is the smallest unit in the mod-con line from that manufacturer. There are now a few that are a little smaller..

    But even if the contractor does a correct heat loss and has the nerve to install a boiler that meets that, perhaps with a small margin of safety, it is likely to be oversized since you cannot get a small enough boiler for an 1100 square foot house. I wish they made a 40,000 BTU/hr mod-con that would modulate down to 5,000 BTU/hr or even a little lower. But it is too late for me now.
  • Greg Maxwell
    Greg Maxwell Member Posts: 212
    Boiler size

    We have to size a boiler for the capability of heating the house when it is at design conditions. Typically, in our part of the state that is -7 degrees. That is an actual load, not theorectical. It is however more than the house needs for a great majority of the time. This is when outdoor reset comes in. Modulate that boiler temp down to meet the design conditions of any given day.
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