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Control strategy for GARN underground feed DHW/radiant?

varadhammo Member Posts: 27
edited October 2014 in Thermostats and Controls
Hi all,

I am a DIY designing a system with a GARN wood boiler (~1800 gallon tank).The building requires both DHW and radiant floor zones. The heating load is around 50,000Btu/h, all radiant floor zones, single temperature. The underground pipe run to the building, reasonably well insulated, is about 90' each way of 1-1/4" PEX. My idea is to run the lowest possible temperature through the trench in order to minimize heat loss to the ground. To do this I would mix to the radiant supply temperature at the boiler shed. Then I would use zone valves at the boiler shed to bypass the mixing assembly only when there is DHW demand. Is this reasonable to attempt, or should I just keep it simple and run full temp underground to the building and do my mixing/zoning there?

I've attached a sketch of my idea, in this version using a 3-way mixing valve. Assuming it's a reasonable control strategy, would there be a way to do this using injection mixing, either 2-way valve or variable speed circulator? This would eliminate passing the full system flow through the 3-way valve and possibly allow a smaller circulator for the radiant system (zoned by valves with a taco 00-vdt circ in my current design).

Any guidance would be most welcome!


  • Zman
    Zman Member Posts: 7,467
    I think you a heading in the right direction. I like the simplicity of it.
    Have you figured out your flow rates?
    How long are the radiant loops?
    How many loops?
    I would want to calc that before proceeding.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Gordy
    Gordy Member Posts: 9,537
    Could be Unintended consequences.

    If the trench insulation is questionable I think it would be preferable to mix down in the dwelling. Your water supply temps could be a floating variable through out the heating season with 100' of supply line with poor insulation. What is the radiant detail? Sandwich, slab, gypcrete,staple up, with plates?

    I guess I would rather be able to dial in radiant mix temps in the house instead of running out to the boiler 100' away if supply temps were to vary wildly. Preferably I would pound the table for better quality insulation detail of for nothing else supply temp consistency.
  • hot_rod
    hot_rod Member Posts: 19,896
    spend the time and money to insulate and keep the piping in the trench dry, and loss to a minimum. Maybe spray foam around the dual pex encased tube.

    I would also suggest sending the higher temperature into the building.

    I'm not sure you will have 180f all the time anyways, you may need to drive the Garn pretty hard to assure a constant 180.

    You don't kneed 180F to generate DHW either. Choose an indirect with a lot of coil surface, or buy a dual coil tank and series the upper and lower HX to double the HX area.

    Another option if you have a good flow with those 1-1/4" lines you could generate DHW with a plate HX inside the building, design around a close approach, even with 140 supply you could create endless DHW with about 30- 40 plate 5X12 HX.

    What about summertime DHW? Are you prepared to fire the Garn year around? Maybe a gas or electric tankless downstream of the plate hx?

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream