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Under seafloor Heating?

Jumbomqt Member Posts: 1
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 and my results were similar. So here are some details on construction of the house.

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 installs with 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,


  • Zman
    Zman Member Posts: 7,468
    edited April 2019
    The WM boiler is the same firetube design you see from many manufactures. I have not worked on one personally, but if it is well supported in your area, it should serve you well.

    The plate vs no plate is a little more complicated that you might think. If you try to run the slabs at 160 degree, you will overheat the space. In addition, if you have a cold slab come on line during a call for heat upstairs, you will not get 160 degree water because the slab will suck all the energy.

    I think you either need to go with plates and run it all at the same temp or put in controls integrated mixing valve on the slab zones.

    I would highly recommend the plates. I have never seen a plateless system that can put out 20 btu/sf.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,630
    Your numbers do sound high. Here in Maine (-40F design) a new well build home like what you are describing is typically 15btu/ft2.

    I have installed a few of Weil-mclain's ECO series boilers, same boiler as the 97+ but without the fancy controls.

    For reference I heat my own home 3200 ft2 R30 walls R60 ceiling, slab on grade tubing 12"OC, with an ECO70 and its throttled to 50,000 btu.

    I'm willing to bet the 110 model would work well for your project. I would also recommend plates, a dry (staple up) system will require very hot water and waste boiler efficiency, as well as underperform on cold days.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!