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Question regarding boiler sizing and IDW

John_in_Buffalo Member Posts: 3
I am replacing my old (1960’s era) Peerless Boiler with a WM Ultra 3.


I did a heat loss calculation (using Slanted Fin software) on my modest house (1600 ft2) and came up 57MBH (outside temp 0F).


We have only one zone in the house (but will be adding a second for the hot water)


My own research suggested the smaller WM boiler (80 MBH).


My installer suggested that I go to the next higher size boiler (105 MBH) due to the indirect HW tank.


Not ssure if it matters but it is a 1 bath home with a 5 gpm showerhead and 4 people living here.


I don’t want to second guess my installer, but I was under the impression (based on what I’ve seen discussed here) that an IDW set up as a priority zone doesn’t get included in the heat loss calc.  I could be wrong on that as much of what you folks discuss is over my head.


My question: Are there any drawbacks (other than the small additional cost for the larger boiler) to use the larger size (105 MBH) boiler? 


Am I correct in assuming that, since it is a modulating boiler, if I oversized it a bit the “modulating” aspect would help me minimize some of the lost efficiencies (due to an oversized boiler).


Any advice/comments would be much appreciated.


  • Robert O'Brien
    Robert O'Brien Member Posts: 3,385

    There is a lot of fudge in most heat loss programs,the true number is probably much less than the calculation,go with the right size boiler. Determine your DHW needs and go with the size tank you need,size the tank to boiler not boiler to tank.
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  • John_in_Buffalo
    John_in_Buffalo Member Posts: 3
    Size the tank to the boiler?

    Thank you for your response.

    The comment "size the tank to the boiler" raises another question.

    On the WM Site w/specs for tanks, they say (for the 35 gallon tank at 180 degrees boiler water):  "Boiler out = 150 MBH"

    This is confusing to me.  Are they suggesting that I need a minimum 150 MBH output boiler to use with this (their smallest) tank?

    Which would mean their 80 MBH and the 105 MBH Ultra3's wouldnt work with their smallest IDW tank.  I must be missing something. 
  • SWEI
    SWEI Member Posts: 7,356
    size the tank

    those numbers are the maximum boiler output the tank can transfer.  Lower boiler outputs will work, but produce lower first hour numbers for DHW.

    The Ultra has an aluminum heat exchanger, which would not be my first choice for any boiler.  I'd probably choose a Triangle Tube PTS60 to meet that heat loss, and pair it with a SMART (30, 40, 50, or 60 depending on DHW needs.)
  • unclejohn
    unclejohn Member Posts: 1,749
    I would go with

    The triangle Tube 110. That will heat the ID quickly and and keep you nice and cozy.
  • SWEI
    SWEI Member Posts: 7,356
    TT110 is a fine boiler

    But its minimum firing rate is over half of his maximum load.  The 60 would be the right match for the heating load, but might need a larger indirect to satisfy the DHW demand.  
  • heatpro02920
    heatpro02920 Member Posts: 991
    New boiler fun...

    I would go solo 60 {ultras are, ehh...} and either a rinnai condensing tankless or a larger phase 3, set it up for DHW priority {your trimax control will handle all of that}.

    A couple notes on your system, since you have one single zone you wouldn't notice the oversized woes others with multiple small zones would, still not a great idea to do but.....

    For DHW, why a 5gpm shower head, that seems excessive to me, I would think about swapping that out with a high sierra head and save your self some money... I have them in all 4 of the showers in my house and they are amazing, kind of loud but they feel like a water waster head, but they are some of the tightest in the industry...

    So imo I think a tt60 would serve you very well, you will save on the yearly service and you will save on the unit being almost perfectly sized with such a low turn down... If you go with an indirect I would check the specs but go with something like a smart 60 with the solo 60, I did this in a 2 bath town home with 4 people living there and they have no issues, I prefer a seperate rinnai unit for dhw but that is just me, cost is about the same...
  • SWEI
    SWEI Member Posts: 7,356
    shower head swap

    excellent point!  I'll just add that the Kohler Forte 1.75 GPM is a favorite here.  Spray pattern is closer to that of a conventional showerhead than other low-flow models we have tried.
  • heatpro02920
    heatpro02920 Member Posts: 991
    edited May 2013
    I didn't know the forte was low flow

    I have installed a bunch of forte shower valves, is that the one that comes in the kit? I thought they were 2.5gpm like the rest of them....

    You have to try the high sierra, it definitely flows very little water I got 1.35 gpm in my master bath when I tested mine, but it feels like a lot of water.

    I have a Rinnai unit {of course} so no water mixing just put the shower on full hot and jump in, I noticed the high sierra shower heads almost cut my hot water gas usage in half!!!! At first I didn't believe it but after I did the math a few summer months it added up, so now I have them in all of my properties and I offer them to my customers...

    They have an orifice instead of 50 small holes, so they never get clogged up and they spray the same pretty much no matter how much or little pressure you have.... Good idea, not the prettiest thing you have ever seen and not the quietest but nothing saving 40% of your DHW bill can't make you feel better about....

    Check them out...
  • John_in_Buffalo
    John_in_Buffalo Member Posts: 3
    Phase 3?

    Thanks to you and everyone for the responses.

    Not to display my ignorance but, What is a Phase 3 IDW?
  • heatpro02920
    heatpro02920 Member Posts: 991
    Sorry they are called a bunch of things

    TT indirects, Phase 3's, smart tanks, tank in tank, double tank, ect... they are imo the way to go when it comes to indirect water heaters... And with a Traingle tube solo 60 you can't go wrong, just have your contractor size the tank accordingly, they make them from 30-120 gallons... I don't add BTU's for DHW, think of it this way, using priority the DHW and central heating will never run at the same times, so your tt 60 will make around 50K btus, thats 50k for the DHW tank and when that is satisfied you get your 50k btus for your heat loop... The house isnt going to drop more than a degree by the time the tank recovers, 1 because they recover quickly and 2 because you are running that hot water in the house some where so the heat is most likely transferring to the house in one way or the other {in the winter when we take showers in our master bathroom it raises the master beds heat about a degree with the door closed}, and 3 50K BTU of hot water is a good amount a direct fired gas water heater has less than that and would be fine for your needs...

    I would seriously consider getting a smart 50 or 60 and a TT60 boiler, then change that shower head for gods sake 5gpm is a waste, you will save money and water just by doing that alone get something 1.75 or under... this will give you much longer showers, which I dont think you will have a problem with the tt60 and a 50, 4-15 minute showers back to back with 5 minutes of recover in between each, that will keep up no problem with one bathroom...
  • unclejohn
    unclejohn Member Posts: 1,749
    Pretty sure

    The 110 turns down to 30K his stated load was 57k. I personally think the TT60's are to small for a quick recovery on your indirect.
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