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Community and Q&A

Radiant or Forced Air Minneapolis

Ryan Picha | Posted in Mechanicals on

Ok guys what do I do here is my situation haven’t started building yet but this is what I’m dealing with…

-80x52x20 slab on grade well insulated barn type building going with a BSC type building approach
-52×35 two story home inside open floor plan with bedrooms upstairs, remaining 45×52 for shop/garage space heated to around 35-40 degrees in the winter
-my wife doesn’t want ductless minis doesn’t like the look
-there is natural gas available on the property was thinking of doing a boiler for both dhw and radiant heating
-two stage/speed regular natural gas furnace
-Polaris/Phoenix type electric hot water heater for both space heating/dhw

Either way I’m going to need a a/c unit with ducting as I can’t see how else to do it without using minisplits and those are out of the question

i pay 11 cents/kwh and 69 cents/m cubed for gas any help would be appreciated thx

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  1. Expert Member
    Dana Dorsett | | #1

    A cubic meter of gas is about 10.6 kwh of source fuel energy. Burned in a 95% efficiency burner you get about 10kwh of heat for 69 cents.

    The same 10kwh of heat delivered by a cold-climate ductless heat pump would average COP of about 3 in a Minneapolis climate, using only 3-4kwh, which would run you 33-44 cents. It would be signficantly cheaper.

    A radiant slab would be more comfortable than a 2 stage condensing hot air furnace, but the boiler or combi-heater would be signficantly more expensive up front.

    Whether it makes more sense to heat the slab with a condensing boiler vs. a hot water heater depends a bit on your actual heat load, which must be calculated up front.

    Cold climate multi-splits that can use interior cassettes other than everybody's least favorite wall-coil units are now available from Mitsubishi (the "M-series Multi-zon": ). There is a modest efficiency penalty for going multi-split + cassettes rather than the wall-blob-only single head mini-splits, but it would still be cheaper to operate than condensing gas.

    But without the room by room and total heat load figures there's no way to say what really makes sense here.

    Heating the garage to 35F-40F with hydronic systems would require anti-freeze in the loop which would otherwise have a high risk of freeze-up in the event of a power outage, which takes a toll on heat transfer & pumping efficiency. Heating it with ductless would be an issue, since the minimum setpoint in heating mode is typically 60F. If it shares a common wall with the conditioned space, insulating only the exterior walls of the garage and leaving the partition wall uninsulated would keep the garage above freezing, if you have a tight & insulated garage door.

  2. Ryan Picha | | #2

    Those mitsu multi zone minis look pretty sweet I think I can sell my wife on the horizontal mounted ducted units. Mitsu also has a "P" style commercial heat pumps that go down to 0F maybe that would work in the garage? I know the floor plan and where windows/doors will be guess my next step is to do a heat calc. Are there any websites that make that process easier? Thx again!

  3. Expert Member
    Dana Dorsett | | #3
  4. Morgan Audetat | | #4

    We design and install radiant floor heating systems in Minneapolis every day. With all due respect to my physics hero. The answers are not that difficult.

    Natural gas is the fuel of choice here in Minneapolis since it is only by comparison to single-head minisplits (my own M series boasts COP of 4) that condensing boilers look bad.

    In every other comparison a condensing boiler with an indirect-fired water heater wins.

    Comfort is unmatched. Domestic hot water production, ditto.

    I just tried to use a mini-split on a recent SIP design and found the loads, both from the Manual 'J' design software and personal experience could not be handled (due to unbalanced solar gain from windows) with the limiting factors presented by minis.

    We rarely use anti-freeze in an attached garage or in any of our hydronic designs, save the snow melting driveways, where we may include the garage and use anti-freeze in all to save installation time and money.

    We start all HVAC projects with a room-by-room Manual 'J' heating and cooling load analysis. Since the cooling degree days in Minnesota are a 10 that of heating, we like to focus on the heating. So i t follows that most of our cooling system are 13 SEER high velocity and most of our heating systems both new and retrofit are a combination of radiant floors, walls and ceilings .

  5. Jacob Weel | | #5

    Morgan: Makes sense that in a climate with lots of heating and a little cooling, you'd design the heating system for efficiency and the cooling system for cost. That sort of thing seems to get lost a lot in prescriptive-type subsidy and rating programs.

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