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Help: Designing Retrofit for Boiler

Constantin Member Posts: 3,796
Pat, you're doing all the right things by insulating your home first and thinking about a proper boiler choice later. If the auditors came up with a 100kBTU heat loss, you're either in a large home, a very cold location, or you have a lot of insulation/heat loss improvement potential. Did the energy auditors make any recommendations regarding the envelope of your home?

If you are going to stick with gas-fired appliances and have lots of standing radiators, you should go for a modulating, condensing gas boiler like (in alphabetical order): the Buderus GB142, the HTP Munchkin, the Weil McLain Ultra, the NTI Trinity, or the Viessmann Vitodens.

Your contractor should size any of these boilers to just meet your maximum expected heat loss. The Ultra 105, the GB142-30, and the Vitodens WB2 8-32, all produce about 105kBTU at maximum output. However, don't discount the Trinity 150 or the Munchkin 140 because both of them can modulate down quite a bit also.

The modulation feature then allows these boilers to then reduce their output down to around as low as 25kBTU/hr whenever you don't need full-bore output. A condensing boiler is also an ideal heat source for an indirect hot water tank for hot potable water use.

Mike T. in Swampeast MO saved himself 44% on the gas bills by switching from a 80% AFUE cast iron boiler to a Vitodens that was feeding radiators fitted with thermostatic radiator valves (TRVs). Thus, it is conceivable that you could improve your comfort level and reduce your gas bills at the same time.

Lastly, click on <a href="http://www.heatinghelp.com/getListed.cfm">"Find a Professional"</a> to see if a installer is listed in your area. If so, ask them some of the many excellent questions that Paul Pollets and others have contributed to <a href="http://forums.invision.net/Thread.cfm?CFApp=2&&Message_ID=149481&_#Message149361">this thread</a> to see if they get it right.


  • Pat Justis
    Pat Justis Member Posts: 3
    Need help with design/practical load and boiler size

    Hi. I'm planning a retrofit of my existing boiler and piping. Keeping existing radiators. I performed heat load calcs and found that I need about 102,000 BTUH. A local energy auditor found that I needed 99,000 BTUH. I did not consider internal heating inputs such as people, appliances, or electronics. I've owned the house (approx 2100 sq ft including basement I am finishing) for 3 years and we have never had any problems with getting enough heat. We typically keep it pretty low (65F) and on the occasions when we cranked it up for guests we didn't have any problem. I feel that we've never had to max out the boilder (150,000 BTUH American standard, likely about 20+ years old). I have just recently added insulation to the attic and done air sealing. I do want to upgrade to a high efficiency boiler and better controls, maybe with outdoor reset. I am in the process of finishing my basement so I'll be adding some heat to that. The load calcs above include the basement load. Here's my dilemna: I don't want to oversize the boiler and wonder if my current boiler is oversized since I get some cycling that reduces our comfort somewhat. Right now it is a two-pipe direct return system with 8 radiators. I intend to put a formerly removed radiator on Floor 1 and add radiators for the basement. I'll also likely do three zones, basement, floor 1, and floor 2 and pipe using primary secondary loops. I was hoping to get away with a small boiler that would run more continuously rather than the cycling we currently get.
    Could I go with, say, an 80,000 BTU Munchkin rather than going up to a 140,000 BTU Munchkin? Perhaps this is a dumb question, but I definitely would rather be cool a few days of the winter and have the boiler sized right for the majority of the winter rather than having 40,000 extra capacity I don't need most of the winter. Comments?
    Thank you for your time. This web site is awesome.
  • jalcoplumb_2
    jalcoplumb_2 Member Posts: 172
    Match your.....

    heat-loss as close as possible. I like to match or slightly exceed. The modulation of the boiler can and will compensate for the discrepancy.

    I have done quite a few radiator retrofit jobs. I have left the radiators and removed the boiler and all the piping. I then ran 1/2" PEX feed and returns to each radiator off a radiant manifold.

    I like to use Munchkin boilers with the Vision 1 system. I like to reset the system and run constant circulation.

    I do a heat-loss calculation and determine the system load. I also do room by room calculation in order to figure out the required heat for each room.

    Then I size the existing radiators to determine the heat output.

    Then I backwards engineer the system by changing the outside design temperature on the heat loss program to determine the amount of heat for each room at that outside temp. With this information I can reference the Radiator chart to find out what the water temperature will need to be to provide the proper amount of BTU's for that given outside temperature. Then I can set my reset curve based on these numbers.

    Depending on the radiator sizes some rooms might over heat. Then you can use the radiant manifold to adjust the flow to balance the system out. Using a manifold with a flow indicator helps.

    Once this is all done you almost don't need an indoor thermostat.

    The comfort and savings are outstanding.
  • Mark Eatherton1
    Mark Eatherton1 Member Posts: 2,542
    My house...

    is set up almost exactly as you described. My loss calcs came in at 80K. When I first started converting the home for GFA to hydronics, it had a 150K GFA furnace that short cycled like a banchee, even at design conditions. I replaced it with a dedicated water heater with outdoor reset and it did a fine job of keeping us comfy, even at design condition. I then upgraded to a Munchkin T-50 wall hung boiler (50K max input)and reduced my energy consumption by 30% after compensating for weather differences.

    I know of two other people that replaced 82 AFUE cast iron boilers with outdoor resets with modulating sealed combustion boilers and they too saw their gas bills drop by 30%. Modulation is a wonderful thing. When it gets real cold outside, my boiler never shuts off. It just idles down to meet the actual load.

    Speaking of actual load, none of the loss calcs we perform take into consideration the mass effect, nor solar and internal gains and it has a substantial influence on the "real time" energy consumption.

    Beginning in June, I'm starting a 3 part series of articles in Contractor Magazine that goes through the process of auditing existing equipment to determine "real time" energy requirements. While this probably won't do you a lot of good between now and the time you plan on replacing your physical plant, I think it's safe to say you'll be OK slightly undersizing the plant.

    However, if you want to also do your DHW, it pays to size it to THAT load and allow it to modulate to the space heating loads.

    As for your long term goals, look into thermostatic radiator valves (TRV's). THey too will make a world of difference in the overall comfort and energy consumtion of your home. Google Danfoss or Oventrop or even Honeywell. They all make them and they are compatible with the older style of cast iron radiators. The work is best left to a pro with proper tooling and experience IMPO.

    Good luck, and remember to report back with your fuel savings next spring.


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  • jeff_51
    jeff_51 Member Posts: 545
    ya know, for a home owner

    Constantine is usually right on the money, and he is here. As you zone, you will actually reduce your btuh load except on the very coldest degree day when all the zones will be fully open. The modulating boilers will give you a wide range of btuh capacity in the same unit. Ya, 100,000btu does sound a bit high for your house. I live in mpls with a comperable house, 1 1/2 story cape with full basement, 1 1/2" of insulation in the walls, original windows with combination storms and have a 75,000 btu furnace only for the ac load. Otherwise 50 would have prolly done for me. We size for 5 below zero here. House was built in '54. Just on more idea, get rid of the water heater and go with a hot water maker off the boiler
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