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.

Boiler Ratings

Peter_26 Member Posts: 129
Hello all,

I was checking boiler ratings from different manufacturers and was thinking if all boilers are spec'd at 180º SWT? If so, let say a homeowner has traditional fin-tube baseboard and wants to replace them with panel radiators for some fuel savings and planning on some possible future equipment changes. I know the panel radiators have a higher btu output at lower SWT than the fin-tube, so is a future boiler replacement still chosen using the heat loss of the home? I know that is the standard, but if I'm using a CI boiler for now and I'm contemplating a replacement of the the same construction versus a HE boiler, then is there a reason to not go with the heat loss when choosing a replacement since the panel radiators produce more heat output at lower SWT? I am thinking that the decision would be mute if the boiler used is an HE one since they modulate and in turn lower water temp will be used. Can the heat loss be met with a lower rated boiler since the SWT will be less than the rated 180° SWT needed? My thoughts are that it wouldn't be a problem, but I just want to be sure I am thinking this through in a rational manner and there is something I am missing.

Thanks for the help


  • psb75
    psb75 Member Posts: 834
    The building heat-loss is a constant. It is always worthwhile to make it better--first, before upgrading boiler equipment and radiators. More efficient equipment is good in two respects: 1. that the boiler can adjust (modulate) to the changing demands of the load (heat loss of the bldg.) and the outdoor temperature. 2. Better radiators can be more efficient by producing useable/detectable heat with lower supply water temperatures which will allow the modulating boiler to 'condense' and extract more btus out of the fuel consumed. The boiler should still be sized according to the "design degree day" or the coldest day of the year--which seem to be getting warmer.
    EBEBRATT-Ed Member Posts: 15,527
    The heat loss is the heat loss that does not change. The radiation installed and the amount of radiation determine the supply water temp and btu output of the radiation. With a mod con this is not an issue you can install a large amount of radiation and force the supply water temp down into the condensing range.

    If you don't have enough radiation the water temperature and radiation output has to come up to match the heat loss.

    The lower the SWT you can run the more fuel saved but the installed cost rises with the added radiation.

    Circulator capacity has some affect on this and a cast iron boiler brings the low return temp protection into the picture.

    Calculate heat loss
    lay out radiation
    determine the supply water temp & gpm

    Once you have those #s you can determine the boiler size.

    Boiler output needs to be determined from water gpm and supply and return water temp which will give you the BTU output the boiler needs.

    gpm circulated x 8.4lb/gallon x60min =LB of water circulated /hour x td of water =btu needed

    example: 1 gpm x 8.4 x60=504 504x20td=10,080btu/hour

  • Hot_water_fan
    Hot_water_fan Member Posts: 1,850
    Size any boiler to the heat loss. Supply temp doesn’t matter - except when right on the end of condensing. For example, a condensing boiler supplying 120F water will be more efficient than one supplying 150F, so the 120F scenario could work with a 80kbtu boiler that outputs 76k Btu. But if the heat loss is 76kbtu and you won’t be condensing, you’ll have to size up to a 100kbtu boiler (typical boiler sizes are 80k, 100,120k, 150k, etc). 

    In either situation, you size output of boiler to heat loss. Stick to that and you’ll be fine. 
  • hot_rod
    hot_rod Member Posts: 22,144
    The extra efficiency comes from operating in condensing mode. Another option is to spread the operating delta T to get back colder water water. Its the return temperature that drives condensing 

    Panel radiators are often designed around a 30- 35 delta. No more than 3 in series so the last one has adequate supply temperature
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Sylvain
    Sylvain Member Posts: 125
    edited February 24
    Whether your boiler is efficient or not it must deliver enough heat for the design day.
    Altough the needed rating doesn't change, the fuel consumption may change.

    Whatever the type of radiators, the btu output will be proportional to [flow X deta(T)].
    Lower delta(T) means more gpm.