Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Circulators in Boiler/Indirect Htr/Radiant System

Hi all,

I have gone through Mr Holohans books and the forum on this site and am just about to start on upgrading my heating system. Because I had to re-level all the floors in my house I decided to install radiant heat. I am all set to start to pipe up the boiler "room" but I just became a little unsure of the circulators for the system.


Single level home, 2300 sq ft, heat load 25k, ~2300' radiant.

Prestige Solo 110

Smart Indirect 40 Water Heater

Uponor EP Heating Manifold.

I was going to use a NRF-36 to feed the radiant loop and the Prestige internal UPS15-58F for the boiler/water heater loop. I just noticed that the Prestige internal circulator is on the radiant output of the boiler. So now I am a little confused on what to do. Should I use a smaller NRF and put it in series with the internal circulator for the radiant loops and then get a separate circulator (NRF-25) for the boiler/indirect water heater loop?



  • EricAune
    EricAune Member Posts: 432
    Page 18

    Not knowing the head loss for your radiant manifold I would suggest following the diagram on Figure 6, page 18 of the manual although integrating buffer tank would likely help decrease the short cycling your boiler is almost certainly going to experience.

    You listed the heat load at 25K.  The lower modulation input rating for the 110 is 30K.  Adding a buffer tank should help if there are multiple zones on the manifold pictured or any day above design conditions.

    You should have plenty of hot water though and that is always nice! 
    "If you don't like change, your going to like irrelevance even less"
  • bob eck
    bob eck Member Posts: 930
    prestige PS110 & Smart 40 and radiant

    you can use the ups1558 circulator already in the boiler and pipe to the supply on the indirect water heater. at the back of this boiler where you would pipe it to the indirect come off there and pipe it to the radiant manifold supply. circulator I don't know what size? both returns get piped into the boiler return. if there is only one radiant heating zone. if more than one heating zone pipe it into a caleffi hydrolink

    then out to your heating zones.

  • CMadatMe
    CMadatMe Member Posts: 3,086
    edited March 2011
    Buffer Tank

    Eric gives a very valid point. You have a boiler that modulates way more then you need. Use a buffer tank and I would be looking at using a Grundfoss Alpha or simliar pump for the radiant. My indirect would get piped in on the boiler side of the buffer tank. Looking at the pic hope you left enough over head room to service the boiler.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Wrencher
    Wrencher Posts: 7
    Buffer Tank

    Ah yes good point I see what you are saying. The 6 gal through the indirect heater just isn't enough. I will look into the buffer tanks. Concerning the top of the boiler yes I have about 3 ft.
  • Wrencher
    Wrencher Posts: 7
    Grundfoss Alpha

    If I use this circulator is it still a good idea to use a differential pressure regulator?
  • Steve Whitbeck
    Steve Whitbeck Member Posts: 669

    Stick with the Grundfos UPS-15-58 pumps. I don't like the NRF pumps.
  • Gordan
    Gordan Member Posts: 891
    Nope, not needed.

    The circulator will ramp up or down to maintain a constant differential pressure. Ideally, you'll have three circuits: the indirect circuit, the boiler circuit, and the space heating circuit. If you use a buffer tank, as suggested, then the buffer tank can be piped to hydronically separate the boiler and space heating circuits. In essence, a very large low loss header.

    Using the indirect as a buffer is sub-optimal not (only) because its boiler-side volume is too small, but because you would not be able to use lower temperatures for space heating than what you use for the indirect. This would negatively impact thermal efficiency.
  • Gordan
    Gordan Member Posts: 891

    When I was considering the PS110, that's how I was going to pipe it up. Then you just rewire the internal circulator to the DHW circulator terminals. But I was going to do this because I always intended to pump through the boiler with the Alpha - no pri/sec. I don't see much of a point to doing that if you're just going to hang an external fixed speed circulator to do the same thing that the internal one would have done.

    I changed my mind alltogether and got the PS60 instead. Yes, this limits my DHW output but it should be manageable given my usage, and the boiler is much less oversized for my space heating load. Pumping through the boiler with an Alpha; 15-58 on low speed is on the DHW side (I'm using a Turbomax reverse indirect which has a very low boiler-side pressure drop.) Zoning is via TRVs, so there will always be adequate flow through the boiler for minimal modulation. The call for heat is controlled by a simple thermostat from my biggest (non-TRVd) zone.
  • Wrencher
    Wrencher Posts: 7
    Loops in Boiler/Indirect Htr/Radiant System

    Thanks everyone for your replies. I think I get it and then I read something else and then start questioning myself. The internal circulator is what is confusing me.

    It appears I need to take Fig 8 page 20 and Fig 13 Page 23 and combine them. The internal circulator would go out to a boiler closely spaced tees loop. I would also add a buffer tank to this loop.

    The DHW heater loop with its own circulator would run off the other outlet on the boiler (which is between the internal boiler and the input side of the internal circulator) and go back to the common return on the boiler.

    The radiant loop would go off of the Boiler loop.

    If this is correct (fingers crossed) where should the expansion tank and air separator be? Since the circulator is internal should it be near the return of the boiler like in Fig12 page 22?
  • CMadatMe
    CMadatMe Member Posts: 3,086
    edited March 2011
    Pick One

    Pick an application in the attached. If you need info on buffer tanks then I would suggest you visit www.taco-hvac.com and join the Flo Pro Team. John has a nice webinar in the Neighboorhood on buffer tanks.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Wrencher
    Wrencher Posts: 7
    Recommended Gauges

    Thanks Chris that was super helpful. I am an EE through schooling/work but I find this heating/hydro stuff really interesting.

    One last question, I was wondering where it is recommended to have gauges (temp, pressure) in the system.

    - Temp ones on the radiant supply and return

    - Pressure one on the expansion tank

    Is that it?

    Thanks again. Coffee is going and I am starting to assemble!
This discussion has been closed.