Connecting a new radiant heat zone for a garage slab to an existing baseboard boiler heating system

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Meng

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I have a Burnham 204HNSL-BEI2 boiler controlled by a single thermostat which currently serves about 1400 square feet of living space via copper pipes and varying sizes of baseboard fixtures in the various rooms.

I'm building a 24x26 garage addition with radiant heat in the slab. A contractor is installing the appropriate amount of radiant pex loops in the slab, and I'm planning on finishing the installation by connecting the radiant pipes in the slab to my existing boiler (the boiler is about 20 feet away from where the radiant pipe will connect). I have had a heat load analysis done on my home to confirm that my boiler has the capacity required to serve a new zone.

I'm interested in doing this work myself, and would like some help with the planning process. My understanding is that I will need to add a zone to my existing boiler along with a heat exchanger to separate the glycol system that heats the slab from the rest of the home baseboard fixtures, a mixing vale to regulate temperature of the glycol, a manifold to connect the delivery and return loops, and I'm wondering what else I need to think about for planning purposes.

What else should I think about as I'm doing this early planning work? Let me know if you need anything additional from me (further description, photos of my boiler, etc) in order to help point me in the right direction.
 

Dana

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A four-plate cast iron Burnham is enough boiler to heat TWO houses that size, plus a 650' garage with capacity to spare! It probably has something on the order of 80,000 BTU/hr of output, currently serving a load that's probably less than 1/3 that, even if you're in chilly International Falls or similar.

Did you calculate the heat load of the garage? That would be a critical first-step for specifying the rest.

Hopefully the contractor spelled out the loop sizing & layout for the radiant already, there's only so many unknowns you can have before the equations can't be solved.
 

Meng

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Dana, thanks for that information! You're correct that my Burnham boiler has 80,000 BTU/hr of rated output.That's great to hear my current project (624 sq ft garage) as well as future radiant heating needs may likely be able to be served by my existing boiler. Here are a couple of future projects I have in mind from a radiant heating perspective:
1) my current garage project is replacing a 20x24 tuckunder garage which I will eventually reclaim as living space once the new garage is active and functional, and am planning to re-pour the concrete slab in the old garage with radiant tube installed prior to building bedrooms in that existing garage space. I would like that radiant pipe to be on it's own zone.
2) My basement is unfinished and unheated at this time, and I would like to put an 8' baseboard heating element in the largest room (13' x 30') which is connected to the existing boiler zone which serves all baseboard in the rest of the house.

I used an online tool to calculate the heat loss for the new garage project (taking into account the R-44 blown-in insulation in the attic space in the trusses, four 2'x4' Andersen windows, one 36"x80" steel door, one 8'x18' R-18 garage door, and R-19 insulation batts + 1/2" drywall on the exterior walls). Here's what I came up with:
Heat Loss* (BTU/hour)
1947:Misc. Infiltration
5565:Under Floor
610:Ceiling
973:Exterior Wall: 1
422:Window: 1 (Wall 1)
946:Door: 1 (Wall 1)
245:Door: 2 (Wall 1)
10708:Total Loss

The garage project has not started yet. We break ground next week. I don't know the loop sizing and layout the contractor is planning, but I'll ask detailed questions now that I have a more clear picture of how this all works and what questions to ask. Is there anything in particular you recommend that I ask now that you know more about my project.

Thanks for your help!
 

Dana

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The R18 garage door isn't real- that's a center-panel R value, not it's actual performance. With the thermal bridging of the steel at the panel edges + leakage true performance is probably more like R8 (which isn't bad, really.) If they publish a whole door "U-factor", the actual effective R = 1/U.

See: http://www.greenbuildingadvisor.com/blogs/dept/musings/energy-efficient-garage-doors
http://www.greenbuildingadvisor.com/blogs/dept/musings/energy-efficient-garage-doors
R19 batts are really R18 when compressed in to a 2x6 studwall cavity too, and with the thermal bridging of the framing the "whole wall R" (including the half-inch wallboard, half-inch sheathing, and some siding) comes in at about R13, but that's usually factored into heat load tools that let you pick from a menu of wall types.

So with the true performance of the door the actual heat load is going to be more than the ~11K you came up with- the infiltration losses are probably higher too, but you're still probably not looking at more than 15K. A radiant heating contractor would be able to tell you from the tubing spacing what glycol-water mixture temps are required to deliver 15K or more of heat, and from there you can also specify a heat exchanger.

Go big on the boiler-water to gycol-water heat exchanger. Glycol reduces the heat transfer efficiency, and if it's only marginally oversized using it's water-to-water heat transfer performance the boiler-side temp requirements might be much higher than a napkin-math model would tell you. But given that the load is that low, even smallest of the line Bell & Gossett can work, provided you get sufficient flow on both sides, which is a function of both the pumps and amount of pumping "head" you get from the PEX as-configured.
 
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