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Replacing boiler and hot water tank for 5 plex, old building

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Torontoheat
Torontoheat Member Posts: 11
edited February 2023 in THE MAIN WALL
Looking for advice on what boiler I should get to heat a;

- 4500 sq ft building
- 120 years old, with 5 showers and 5 laundry, 5 units.
- Cast iron radiators....30 of them.
- The existing boiler for heat only is a slant fin, natural draft, 175,000 btu input, approx 130,000 output. It runs for 14 hours on the coldest days.
- Domestic Hot Water is supplied by power vented 75 gal HWT.
- it gets -5 F here in Toronto.

I've installed 3 cast iron slant fin boilers before (including this one), but not very familiar with combis and I did it 15 years ago.

I'm located in Toronto, Canada.

Thanks for any input.

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  • Hot_water_fan
    Hot_water_fan Member Posts: 1,917
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    Welcome! It sounds like your existing boiler is a good bit oversized and you have high DHW demand. That’s not really a good application for a combi. You can use a modcon and either keep the separate DHW tank or used an indirect tank instead. That’ll get you the high demand draws it sounds like you’ll need while not being too oversized for central heating. 
    IronmanTorontoheat
  • Steamhead
    Steamhead Member Posts: 17,008
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    Is this a steam or hot-water system?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • hot_rod
    hot_rod Member Posts: 22,476
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    I’m impressed that 75 gallon tank is adequate? Toronto water runs a consistent 4C, being drawn from deep in the lake.

    An indirect tank would be another option, perhaps a bit more efficient when compared to a fired tank.

    Do you store the tank at 60C for legionella protection?

    That boiler could be pretty accurately sized, 28 btu/ sq ft in an old building is not unreasonable.
    On that design day condition a non stop run would indicate it is darn close to the correct size. 14 hours is a good long run time, also
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Torontoheat
    Torontoheat Member Posts: 11
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    Steamhead said:

    Is this a steam or hot-water system?

    Hot water system
  • Torontoheat
    Torontoheat Member Posts: 11
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    hot_rod said:

    I’m impressed that 75 gallon tank is adequate? Toronto water runs a consistent 4C, being drawn from deep in the lake.

    An indirect tank would be another option, perhaps a bit more efficient when compared to a fired tank.

    Do you store the tank at 60C for legionella protection?

    That boiler could be pretty accurately sized, 28 btu/ sq ft in an old building is not unreasonable.
    On that design day condition a non stop run would indicate it is darn close to the correct size. 14 hours is a good long run time, also

    These old Cast iron boilers (slant fin) are rated at 80%, but what do you think the actual rating is using old cast rads? Should I do a heat loss and size the boiler that way? Is over sizing a mod con a thing?? Thanks
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,167
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    the Cast Iron Slant Fin is 15 years old. Is that the one you are replacing? Seems like a short life for a cast iron boiler... Is it leaking? If not. why are you considering replacing it?

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • Torontoheat
    Torontoheat Member Posts: 11
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    hot_rod said:

    I’m impressed that 75 gallon tank is adequate? Toronto water runs a consistent 4C, being drawn from deep in the lake.

    An indirect tank would be another option, perhaps a bit more efficient when compared to a fired tank.

    Do you store the tank at 60C for legionella protection?

    That boiler could be pretty accurately sized, 28 btu/ sq ft in an old building is not unreasonable.
    On that design day condition a non stop run would indicate it is darn close to the correct size. 14 hours is a good long run time, also

    I have a few multiplexes and one HWT installer convinced me to use a smaller tank (50 gallon for 3 units (3 showers), and it works great. I use low flow shower heads 1.5 gpm I believe). I sized this 75 gal based on the 50 gallon install. It replaced 2 40 gallon twinned tanks. Fire rate was higher on new tank.
  • hot_rod
    hot_rod Member Posts: 22,476
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    If you need to run 180 to cover the loads, a mod con may not add much efficiency. 85% is reasonable on a new, properly tuned cast boiler

    You would first do a load calc, then see how your heat emitters match the load and at what temperature.

    You would need to get down 130 supply, 120 return or lower to get the benefit of the 90% on a condensing boiler.

    This journal, Idronics 25, a free download takes you through the steps, and has a cast iron derate example.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Torontoheat
    Torontoheat Member Posts: 11
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    hot_rod said:

    If you need to run 180 to cover the loads, a mod con may not add much efficiency. 85% is reasonable on a new, properly tuned cast boiler

    You would first do a load calc, then see how your heat emitters match the load and at what temperature.

    You would need to get down 130 supply, 120 return or lower to get the benefit of the 90% on a condensing boiler.

    This journal, Idronics 25, a free download takes you through the steps, and has a cast iron derate example.

    Wow...thanks! I did the slant fin heat loss and it came to 81,000 btu using 0 F and 70 indoor...is that correct? I would have to now see what rads I have and calculate to see what temp the emmiters will run at. That makes sense. A low return temp is what I'm hoping for to get the efficiency out of a mod con, correct?

    I read that the old convection type hotwater systems were often oversized and can work well with mod con due to the return water temps being low. Does the efficiency decrease on super cold days when supply and return temps are high?

    Would a new cast slant fin mid efficiency boiler be much better than my 30 year old slant fin? One issue looks like it is quite over-sized.



  • Torontoheat
    Torontoheat Member Posts: 11
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    the Cast Iron Slant Fin is 15 years old. Is that the one you are replacing? Seems like a short life for a cast iron boiler... Is it leaking? If not. why are you considering replacing it?

    We have the same name....... Two Ed's are better than one.....lol

    My slant fin is about 30 years old...works fine, doesn't leak, but seems inefficient.
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,917
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    Wow...thanks! I did the slant fin heat loss and it came to 81,000 btu using 0 F and 70 indoor...is that correct? I would have to now see what rads I have and calculate to see what temp the emmiters will run at. That makes sense. A low return temp is what I'm hoping for to get the efficiency out of a mod con, correct?
    We don’t know your heat loss, but if your 130kbtu boiler only runs 14/24 hours, it’s in the ballpark. 

    If you have sufficient cast iron radiators, you can condense! 
  • Torontoheat
    Torontoheat Member Posts: 11
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    hot_rod said:

    If you need to run 180 to cover the loads, a mod con may not add much efficiency. 85% is reasonable on a new, properly tuned cast boiler

    You would first do a load calc, then see how your heat emitters match the load and at what temperature.

    You would need to get down 130 supply, 120 return or lower to get the benefit of the 90% on a condensing boiler.

    This journal, Idronics 25, a free download takes you through the steps, and has a cast iron derate example.

    I read a bunch of the material in idronics.....super helpful...thankyou. I calculate that the heat losses in most of the rooms can be heated by the old radiators with 130 F water temp. This is at design temps which is worst case scenario or coldest days. Am I correct to say that most of the heating will take place with water temps below 130 and so I would take advantage of the condensing portion of the mod con?

    Hot_water_fan
  • hot_rod
    hot_rod Member Posts: 22,476
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    Some other helpful data is the weather occurrence. That would show you how many days you are at, below, or above design condition. Obviously the more you are above, the lower the required SWT and the more potential to operate in condensing mode. So balance all three factors, load, emitter size, and days at what %

    I doubt you will find a cost offset to replace that working boiler with a mod con with the small % of gain?
    But that is why we crunch numbers.

    A mod con also brings the modulation to the party. The system efficiency of a fixed output boiler can easily drop into the 70% range if it short cycles constantly at low load or multi zoned systems. Called the run time factor.

    Yes there are formulas for the different efficiencies in a system based on fuel use, run time, etc, etc.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Torontoheat
  • Torontoheat
    Torontoheat Member Posts: 11
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    I've been reviewing this heating system.

    Issues that I am having;
    -top floor (3rd floor) on a very cold day was 70 F and the main floor was 65F. and then on warmer days around 32F, it flips, and the top floor is cooler and lower floors are warmer??
    -I did a heat loss and came in at 80K BTUH. The boiler is 175K Input and 120K output....over-sized?
    - I've attached some data on running times for the boiler. Does it look like its cycling? or maybe just kicking on and bringing up the temp quickly and shutting off.

    -Would an outdoor reset help?
    -Would the modulation of a mod/con help?

    -Coldest days, the boiler ran 14 hours total in 24 hrs.

    Thanks again, for any help




  • Hot_water_fan
    Hot_water_fan Member Posts: 1,917
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    I did a heat loss and came in at 80K BTUH. The boiler is 175K Input and 120K output....over-sized?
    - I've attached some data on running times for the boiler. Does it look like its cycling? or maybe just kicking on and bringing up the temp quickly and shutting off. 

    -Would an outdoor reset help? 
    -Would the modulation of a mod/con help?

    -Coldest days, the boiler ran 14 hours total in 24 hrs.
    I like those run time visualizations, what program does that? 

    You’re oversized but not egregiously so. If you went with a mod con, you could roughly get max inputs of 150, 120, or 100kbtu. With 10:1 turndowns, the lowest modulation would be 10kbtu, the highest 15kbtu. Not really a big difference between those. Both modulation and ODR would help - between those and an indirect you could have very low run times. 
    Torontoheat
  • Torontoheat
    Torontoheat Member Posts: 11
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    I did a heat loss and came in at 80K BTUH. The boiler is 175K Input and 120K output....over-sized?
    - I've attached some data on running times for the boiler. Does it look like its cycling? or maybe just kicking on and bringing up the temp quickly and shutting off. 

    -Would an outdoor reset help? 
    -Would the modulation of a mod/con help?

    -Coldest days, the boiler ran 14 hours total in 24 hrs.
    I like those run time visualizations, what program does that? 

    You’re oversized but not egregiously so. If you went with a mod con, you could roughly get max inputs of 150, 120, or 100kbtu. With 10:1 turndowns, the lowest modulation would be 10kbtu, the highest 15kbtu. Not really a big difference between those. Both modulation and ODR would help - between those and an indirect you could have very low run times. 
    The NEST thermostat gives this info.

    With mod/cons I've read that even though the output is variable, you don't want to oversize the boiler. Do you know if they have higher efficiencies at highest output?

    I'm thinking that if the mod/con input temps are too high for max efficiency, I'd have to size the boiler at 80% efficiency....just in case the boiler doesnt go into condensing mode (especially on very cold days with high water temps)....does this sound reasonable?

  • john_james
    john_james Member Posts: 39
    edited February 2023
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    I like those run time visualizations, what program does that?

    That's the nest app. Android version.

    Edit: he already answered while I was reading. How do I delete a comment?
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,917
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    With mod/cons I've read that even though the output is variable, you don't want to oversize the boiler. Do you know if they have higher efficiencies at highest output?


    The opposite - they are slightly more efficient at lower outputs, given the same water temp.

    I'm thinking that if the mod/con input temps are too high for max efficiency, I'd have to size the boiler at 80% efficiency....just in case the boiler doesnt go into condensing mode (especially on very cold days with high water temps)....does this sound reasonable?


    That's reasonable. Maybe 85%, but that's just splitting hairs.

    Lots of condensing available to you over the entire year!





    Torontoheat
  • Torontoheat
    Torontoheat Member Posts: 11
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    I'm trying to get the right guy to install a boiler. First call to a boiler installer.

    He is suggesting NTI 150K BTU with 80 gal indirect for DHW.

    He thought a 100K BTU was enough for the heating, but jump to the next boiler size to ensure enough hot water.

    Thoughts?....his price is $15000 canadian pesos

    What should I be asking him? low loss header?? anything else?

    Thanks Again


  • Paul Pollets
    Paul Pollets Member Posts: 3,658
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    I'd be using a Viessmann CU3A and either an 80g or 120g indirect. Viessmann has good support in Toronto. No low loss header required for the CU3A's
    Torontoheat
  • Torontoheat
    Torontoheat Member Posts: 11
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    I'd be using a Viessmann CU3A and either an 80g or 120g indirect. Viessmann has good support in Toronto. No low loss header required for the CU3A's

    What do you like about the Viesmann?? relative to NTI or others? Thanks. Is it like the Toyota of reliability or the porsche of high tech?