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Flow Settings for Home Run Piped Emiiters with TRV

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tropostudio
tropostudio Member Posts: 43

Quick summary of my hydronic heating system, commisioned in 2018:

59 KBTU/h heat loading at coldest design day. 80 KBTU/H mod con, fire-tube boiler. Calculations are for 30F Delta T and 140F supply/110F return for -10F design day.

A single Grundfos Alpha Circulator (no P/S pump). 15 home run emitter loops - 5 per floor, distributed via Uponor SS manifolds. 12 loops use 3/8" PEX, 3 loops loops use 1/2" PEX.

I've calculated a 3.9 gpm system flow; 0.70 ft. head loss across combined parallel loops w/ emitters; and 2.06 ft. head loss across largest single loop with emitters. Every panel has an Oventrop TRV.

Here is how I have been setting loop flows: First, I set each TRV at '5', which should have them 'wide open' while I set flows. I run the Alpha at it's lowest constant pressure mode setting, which ostenibly maintains 6 ft. head. This is well above the calculated drop for my total system, but is as low as the Alpha will go I then adjust each loop's flow valve to match my calcs. Calculated loop flows range from 0.1 (a bathroom) to 0.5 gpm (living room).

I still have a nagging feeling I missed something in how I balanced it. The flow and head loss calcs include an equivalent length allownace for a 'valve' at each loop, but how do you actually account for the fact that each loop has a flow valve and a TRV - with the pressure drop across the TRV varying over time? Should I be following a different procedure

FWIW, my TRV's are set to '3' (68F) and I leave them alone. The house temps stays at a consistent 68F all Winter. The house is quite tight and well-insulated. The system generally just cruises along, with very little ramping up and down (except after the indirect DHW steals a call for heat).

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 27,775

    Don't overthink it. If your TRVs are controlling the space temperatures the way you want them… what's not to like?

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 28,260

    the TRVs and the Alpha should do the balance for you.

    TRVs are proportional valves, not open / close like zone valves.

    If the Alpha is getting the job done at its lowest setting, what’s not to like?

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • tropostudio
    tropostudio Member Posts: 43

    Guilty as charged. I have a tee shirt that says 'Hold on a minute while I overthink this!' I design and build science museum exhibits. Sorta can't help it.

    I was thinking in terms of having run through all the calcs, if I could match the design flow with the Alpha (really can't - it has fixed options), the TRVs shouldn't even be necessary. And if the boiler control and outdoor reset curve are set up right, it should work across the range of outddor temps. Does that sound correct?

    Conversely, if I'd skipped the flow valves, piped everything with 1/2" PEX, and used the TRV's, would it be running exactly as it is now?

    This is more of an academic exercise than practical. TRVs definitely keep everything consistent.

  • hot_rod
    hot_rod Member Posts: 28,260

    it is hard to beat a TRV for individual room or radiator control,they do the thinking for you. With the Alpha set correctly it ramps up and down as valves regulate. It’s about the perfect control logic.

    Take the time to dial in the ODR and you will get close to constant circulation, which is a great target to hit. Boiler and pump modulates to the changing load, rarely shutting off.

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