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replacement of lennox complete heat boiler with ? and sizing

Hi, I have currently 150,000 BTU aging lennox complete heat boiler supplying DHW. Forced air fan coil for shoulder seasons heat and heat exchanger loop which has to supply 1500 sq ft in concrete @ 110 F and 1900 sq ft of staple- up pex for the floor above at 150 F (my heat exchanger only allows 140 F across at best) I have R 20 bsmt and main floor walls and R80 ceiling and looking at my gas bills I use .73 Gj per day or about 30,000 BTU per hour on average for November-February (our average temperatures appear to be in the -10 C or about 0-4 F ) Given lennox has cashed in on the complete heat I want to replace it with a modulating, condensing boiler. I need a minimum of three temperatures if I am going to use an Indirect Hot water tank ( could run the DHW and air handler at the same temp and then need 110+ 150 F options for the other two radiant heating zones. Weill McLean appears to have issues with its aluminum block, and am not sure if they have fixed that problem or not. I keep hearing I should go with what the local chaps are using. The IBC modulating boiler made in Vancouver BC sounds like it has exotic controls and stainless steel heat exchanger.


1) can one get away with oversizing the boiler as long as it is a modulating boiler?

2) Weill McLean/Lochinvar/IBC +/_ tekmar controllers?

Any help would be appreciated as I would like to replace it before I have the -25C week when it crashes and parts are unavailable. Thanks for the help, Rogerbc


  • Ironman
    Ironman Member Posts: 7,376
    Boiler Selection and Sizing

    You need to do (or have done) an accurate heat loss calculation. This is the only proper way to size a boiler. If the contractors who are bidding can't do this, then you need to look elsewhere. You don't want to over-size even though the boiler modulates. If the boiler is sized to match your heat loss, then you only need its full capacity at design (coldest) temp. At any other temp, it's over-sized. That's the purpose of modulation - to mach the load as closely as possible at any temp. Why try to put more in than your gonna need?

    The most important thing is not choosing a particular brand; it's choosing a particular contractor. An average boiler will perform like a champ if installed and maintained properly. And the best boiler made will be a constant source of grief if an incompetent idiot installs it. Unfortunately, many fall into the latter category. Do your home work on choosing a contractor and then listen to his recommendations. If you have questions about what he recommends, post them here for more input.


    Tekmar and others have controls that will allow multiple zone temps.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    issues with its aluminum block

    Since I have a W-M Ultra 3 with an aluminum block, I would be curious what documented issues there are with it. Did they, after investigation, have to do with corrosion or cracking of the block? Were the problems because the block was aluminum, or because the boiler was improperly installed? Or that the water quality was not in accordance with the installation instructions? Or that the installer did not put the required Sentinal X-100 water treatment into the system? Or is the aluminum really an unacceptable material with which to make the block? If so, I wonder why W-M persist in using it; I cannot imagine they would wish to risk their reputation on a material with which they have no confidence. I suppose there was a Series 1 Ultra boiler that was a lot like the Series 2 and Series 3. So there must be quite a few of these things out there. If they were unsatisfactory, perhaps W-M would not recall them, but surely they would do something in the newer models to correct for problems in the older ones.

    My (former) contractor said that the X-100 was not necessary, but the first thing my new contractor did was to add it.

    I read of people installing these units without primary-secondary (or equivalently, without low-loss headers) even though W-M specifically requires this. Contractors who refuse to read or follow the I&M manuals seem pretty common. I imagine any improperly installed boiler can give unsatisfactory results. Mine has been in operation since mid-May 2009, supplying the indirect hot water heater all this time, as well as heating the house during the 2009-2010 winter, and it is starting the 2010-1011 season fairly seriously now (24F outside at the moment putting 100F water into the radiant slab downstairs and 127F water into the (oversized) baseboard upstairs as I type this.
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