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separate boiler/system pump required?

MarkusM Member Posts: 1
Hi All,

I am designing a new hydronic system to replace steam, using new radiators (runtal) and a knight ModCon boiler, approx 70000BTU. I want to have individual thermostats at the radiators (european style). Massachusetts.

It seems the standard is to have separate boiler and system pumps to not overheat the boiler. I am, however, considering to use a single pressure regulating variable speed pump. It seems to me that should work great to minimize return temp to get boiler to condensing temperatures. I would plan to couple this with a flow switch that only activates the boiler at the allowable minimum flow (no bypass valve since constant pressure).

Any reason not to do this? The only disadvantage that I could see is that if only a single radiator requests water the flow might be too low to start the boiler. I think I could live with that, since in a well balanced system another boiler should request water soon, and with a single radiator the system might short cycle anyways... .

Thanks !!

- Markus


  • SWEI
    SWEI Member Posts: 7,356
    edited April 2013
    if you size the radiators correctly

    for a 30F or 35F drop, a single pressure-regulated circulator with TRVs should work quite well.

    You could also use a 0-10V controlled circulator with the Lochinvar controls and run ∆T.
  • Paul48
    Paul48 Member Posts: 4,469

    Size for DT, or size to emit at lower temps?
  • SWEI
    SWEI Member Posts: 7,356
    same thing

    Wider ∆T = lower average temp (for a given supply temp.)
  • Paul48
    Paul48 Member Posts: 4,469
    Yes and No

    Without knowing how he is designing.....It's hard to say.Wouldn't he be better off with enough emitters to be able to satisfy the demand with 140* water, and let the DT fall where it may? That wide a DT would seem to suggest emitters in series, and if he's starting from scratch, wouldn't he be better off paralell? Just tossing out thoughts
  • SWEI
    SWEI Member Posts: 7,356
    30F is do-able on panels

    so I'd probably size @120F mean temp (supply at 135F and return at 105F.)
  • MarkusMM
    MarkusMM Member Posts: 1

    Hi All,

    thanks for the replies.

    @ SWEI: that is kind of what I had in mind. Either pressure regulated with careful balancing, or delta T. My worries were what happened if all or most TRV close - the flow would get lower then allowed, and the boiler should go in overtemp mode. I thought installing a flow switch would eliminate that problem.

    As I think more about it, maybe the deltaT via the knight together with a mechanical bypass valve would do - if the bypass valve opens the T goes up, and the pump slows down as low as the knight allows it to. I would still expect short cycling with no load, though. Do you have experience with such a scheme?

    @ Paul48: heaters in parallel, at fairly low flow rate to achieve high delta T. Runtal allow very low flow rates (in contrast to baseboard where flow becomes laminar).

    the supply temp will be set by outside temp, I expect 110F-160F depending on weather.

    PS: can not log in to old account, my new name is MarkusMM
  • Steamhead
    Steamhead Member Posts: 15,769
    Or you could

    fix whatever is wrong with the steam system and get close to the same results without spending all that time and effort on a complete replacement.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Paul48
    Paul48 Member Posts: 4,469

    If you have enough emitters,you can get away with much lower temps, even at outdoor design temperature. This will ensure that the boiler stays in condensing mode, even when the outdoor temps are moderate.
  • SWEI
    SWEI Member Posts: 7,356

    would be my first choice.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Just Use

    Radiator valves that have built in bypasses and it doesn't matter where the TRV is at any position. You want constant circulation. Size the panel rads for 140 Supply at design and a wide delta. Panel rads should be fed home run off a radiant manifold and 3/8" Pex should be more then enough pipe size. Let the boiler control the variable speed pump. Your heat loss is 70K so 70,000/(30x500) = 4.6 gpm is what you need to move around.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • unclejohn
    unclejohn Member Posts: 1,778
    Why go through all this

    It's a 70k boiler. Spend the extra money on a pump and P/S the piping and sleep like a baby.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Thru What?

    A direct piped system with one pump that the boiler control will actually vary the pumps speed to match the boiler btu/hr output. How is pri/sec better? He is doing panel rads this type of application is a mod/cons dream.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
This discussion has been closed.