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second stage heat

Ken_40 Member Posts: 1,320
The heat load is the heat load - period.

If only 100,000 BTU's can be delivered via radiant on a design day, the supplimental, in your case a hydro-coil, will kick in and make up the difference.

If the concern is slow response time, same thing. If a day/night setback has the radiant side too slow to satisfy comfort need, the hydro-coil would suppliment as designed.

There is no need to change the calcs. They are "fixed." How you get there, is a design option you already have a handle on.

BTW, Hydro-coils and radiant are a great design!

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  • radmix
    radmix Member Posts: 194
    second stage heat

    I have a project that has a heat loss of 120,000 btus. the job will be all quick trak for first stage heat. She would like second stage heat delivered via hydro-coil. do you double the btu.
  • Brad White
    Brad White Member Posts: 2,398

    I would first calculate how much of a net radiant contribution you will have and what the difference is to your total heat loss. I would then size the hydro-air for the difference plus maybe 10%. I would not double anything- use moderation. This assumes that your radiant contribution may not cover your calculated heat loss of course. It well may!

    Key to controlling the hydro-coil is modulation, avoiding on-off cycling and keeping the airflow quantities low to avoid drafts. This is especially true if you are running lower outlet air temperatures.

    An ideal setup although more cost, is to have the AHU respond to an indexed discharge temperature based on return air temperature. The coil would be set up with a circulator loop and primary hot water injected into that to change the coil temperature- much smoother than "direct valve into coil".

    Bottom line is to avoid high swings in discharge temperature and to avoid cycling that unit where perceived intermittent noise and airflow might be objectionable.

    All told, you may find that your hydro air never comes on! I have designed several systems and seen others where the air system only runs to add humidity, despite calculating otherwise. Still, I slept better knowing that there was a reserve.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • ALH_4
    ALH_4 Member Posts: 1,790

    There is also the option of installing radiant ceilings or radiant in the lower porion of the walls in high heat load rooms. It certainly simplifies the controls and piping compared to combining hydro-air and radiant, and it keeps the system invisible. It also lets you maximize flue gas condensation to keep the boiler combustion efficiency high. I assume this project will include a condensing boiler?
  • Weezbo
    Weezbo Member Posts: 6,232
    i wouldn't ...

    instead ,Brads idea of keeping lower flow tempered air makes more sense. secondary heating and cooling mixed with outside air should save a few ducketts and help the indoor air quality a dash :) as a secondary additional heat some radiant panels would be another idea that might keep the entire "Theme" of low temp radiant supplemental heat....
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