GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Weil-McLain WM97+155 CT Boiler Recommendation

Jumboski | Posted in Mechanicals on

I chose the title to get Dana’s attention knowing it sounds over sized for any residential project.  But is it?  Here is my scenario.

I am a builder remodeling a home for my family.  We call it a remodel but for all practical purposes it is new construction.  We tore the existing house down to the bones and are now on our way back up.  Sound crazy?  Yes, it has been.

I plan to install my own heating system and dhw consisting of a boiler and hydronic joist heating, and slab heating in part of the basement and garage.  A local heating contractor recommended the following.

Boiler: Weil-McLain WM97+155 CT Boiler
Water maker: Amtrol
Between Joists: Upnor hepex suspended (no plates) with insulation below.
Basement: hepex 1’oc in slab with 2″ xps underneath(already installed)
Garage: hepex 1’oc in slab with 2″ xps underneath (already installed)

The heating contractor did do a room by room (more like a zone by zone)  heat load calculation for the home.  I live in northern Michigan and our design temp is -9 degrees F (zip code 49855).  

Basement 18,623 btu/h
First Floor 46,210 btu/h
Second Floor 30,529 btu/h
Garage 24,300 btu/h
Total: 119,662 btu/ h 
+ dhw ?

I have read all I can on this site and did my own paper and pencil calculations according to Martin’s articles and my results were similar.  So here are some details on construction of the house.

To be upfront, the house is “Ridiculously Over-glazed”   We have a “million dollar view (i feel like i just let a lot of you down, i don’t know what to say, sorry).

Basement 2280’^2 Mostly sub grade
First Floor 2280’^2 With 524’^2 of glazing
Second Floor 1720’^2 with 278’^2 of glazing
Garage 810’^2

Exterior Walls – 2×6 with damp spray cellulose, osb sheething, wrb, 2″ rockwoll comfortboard 80, 3/4″ rain screen, cladding

Ceiling- mostly r-60 cellulose

Rim joists – closed cell

Basement- 2″ exterior xps – not sure on inside yet.

So here are my questions.

1.  After reading as much as I can and realizing that most boilers are oversized by 2 or 3 times does this sound correct?  

2.  Any opinions on the WM 97+ boilers?  One of the main things I like about it is that it is common in my area.  Many of the local shops install them and parts are readily available.

3.  Plates or no plates According to the heatloss analysis i will need around 20 btu / sf on a design day (we do get more of them than I would like) .  On those days I could turn up the supply temp to around 160 degrees F and meet the demand but most of the time I could have 130 is supply temp to keep up and keep condensing.  ( I am getting these figures from Upnor’s design guide).  Though I do read a lot of posts online where no plates couldn’t keep up)

I know i probably left out some details so feel free to ask,

Thanks for taking the time to read this,

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Jon_R | | #1

    There are some low cost stamped plates available - without doing any calculations, I'd guess that it would pay (in more condensing operation) to install them.

    Usually it's something more like 100F needed to get good condensing efficiency.

  2. Jumboski | | #2


  3. Jon_R | | #3

    If accurately done, I'd go with Manual J numbers + margin. If you are going to oversize, use a buffer tank (or only high mass radiators).

  4. Expert Member
    Dana Dorsett | | #4

    The load calculation seems high by nearly 2x. Do you have the detailed inputs they used on the Manual-J?

    Can the radiation even emit the full output of the 155? (would need to be verified)

    The -9F design temp is suspect. What county are you located in?

    or are you near any of these cities:

    Jon R has it right regarding the value of plates, or at the VERY least convector fins. Even if it's possible to heat the place with 180F water on design day with the suspended tube, you'll get more condensing efficiency out of it with something better.

    The stainless heat exchangers of the WM97+ is a big improvement over earlier Weil McLain boilers with aluminum heat exchangers, but the 5:1 turn down is not. The minimum fire input of the -155 is 28,000 BTU/hr, with output more than half the design load of any single zone (which are probably higher than reality to boot.) When it's above freezing outside it can potentially short cycle. The -110 is probably a better fit.

    Are you really going to be heating the garage to 70F when it's -9F outside?

    [edited do add]
    In rare instances it might be important to upsize the boiler for domestic hot water service rather than simply giving priority to the water heater. Any big spa tubs to fill?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |