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variable spped circulators

of low mass and zone valves, you are likely to need a boiler-dedicated circulator regardless of what else you do.

That you are using positive closure zone valves, Delta-P is the way I would go. As the valves modulate closed, you will generate back-pressure to which the circulator will respond.

The alternative is Delta-T as you note, great for single-zone systems, injection and boiler protection. However you might need a bypass or a control arrangement that would take on Delta-P control when that exceeded the Delta-T based requirements.

If say you have TRV's on most but not all radiators, (let's face it, some you just cannot find a good spot), then there is your bypass. I would then use Constant Delta-T.

As for high mass cast iron mod con (Ray boiler I assume), let me be clear, the mass in question is water volume not the mass of the boiler itself, when it comes to this kind of control.

Low mass if in terms means "low water volume" creates an issue of flashing into steam if the flow is too low. The use of a buffer tank is all about water volume.

I do not know what the water volume of the Ray is (the IRON volume is quite high, no question!). For all I know it may be so low a water volume that you would need a buffer tank anyway. I really do not know specifically regarding the Ray.


  • Dan H
    Dan H Member Posts: 15
    variable spped pumps

    I see a lots of information on variable speed circulators. many of which aren't even available. However which is better a delta P or a delta T circulator.

    I'm designing some hydronic baseboard heating sytems using a low mass mod/con boiler and I would like to use as few circ's. as possible along with zone valves.

    In adition, what about using a high mass cast iron mod/con to eleiminate short cycling instead of using a buffer tank?
  • MechTech_2
    MechTech_2 Member Posts: 84

    We've installed a number of Ray boilers, and have noticed very little short cycling. The combination of the mass of the cast iron, 4.75 gallon fluid volume, and the 5 to 1 turndown ratio certainly seems to help. The pressure drop across the boiler is so low, that we've actually had a primary pump fail, and the secondary pumps have still drawn enough fluid through the boiler to keep it running.
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