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circ pump selection, ECMs and Delta-T's

I've only ever worked on super simple floor radiant systems with <$100 dumb circ pumps and have rarely had any issues. I guess i've gone a little over my head with this sytem that i helped install at a friend / client's house. The grundfos UPS15 pump i installed has been freezing up on trapped air bubbles, so i'd like to upgrade it to an ECM pump, but because i'm using 2-wire zone valves (specifically the blue-fin manifold type) and this system has both in-floor radiant and panel radiators, I'd also like to use a pump that manages temperature and pressure more intelligently. I kind of don't know where to begin with selecting a better pump.

My question with delta-T pumps: I understand that a delta-T pump should modulate down when the temperature delta drops below the set point, to manage over-circulating when, say, only one zone is open and the return water temp climbs. That seems pretty straight forward, but what about the first 10 seconds after a cold start, there should be no temp difference between the upstream and downstream sides of the pump . Are delta-T pumps intelligent enough to handle that?

Regarding delta-p features. Because the 2-wire zone valves take a minute to open up, i don't want the pump struggling for the minute that the valve is still closed. I was thinking i should install a differential pressure bypass valve to handle this, but the Taco 007 claims to "eliminate the need for a pressure bypass valve", even though it doesn't specifically list "delta-P" as a feature. Is this really true?

I'm trying to decide between the Taco 007, the Taco Viridian, and the Taco 0015e3, but will take other suggestions.


  • hot_rod
    hot_rod Member Posts: 19,854
    A delta P circ will work fine with even slow acting ZVs. They were designed to play well with TRVs which also take some time to open. The pump will ramp slowly as the valve opens slowly.

    A pressure bypass valve in not a good match with a delta P circulator 

    im not sure about you comment that the circ is freezing up on trapped air bubbles??
    ECM circs will not moved air either😉

    Find and eliminate the cause of trapped air. All hydronic system should and can be 100% free of all air, large, small, entrained air
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • EternalNoob
    EternalNoob Member Posts: 42
    Thanks hot rod. Your willingness to tackle amateur questions is always appreciated.

    I understand that air elimination on it's own is a priority. When I replace the pump, which will involve some re-sweating because of the flange geometry, i plan to add a discal air eliminator just upstream of the circ pump, where currently I only have a hi vent. I have been tinkering with the system a bit, moving some things around, and probably introduced air without properly purging everything. Nevertheless, because the system has seemed to be more vulnerable to air elimination than what i'm used to, i think it only wise to upgrade to an ECM pump for a minor additional cost. That's kind of a secondary issue though...

    What was actually confused about is this delt-p vs delta-T pump selection and finding a pump which ideally does both things. Supplyhouse is my go-to for parts specing, and their spec sheets leave a little something to be desired. Seems like taco's website provides better details.

    I decided to go ahead and buy the Taco Viridian Delta-T pump because it seems to have all the features I may or may not need. Possibly a more expensive pump than necessary but a minor cost upgrade for some peace of mind.

    The question still remaining for the hydronics community which would be helpful to me for posterity, perhaps for the next project, is, what kind of pump is best for a manifold with 2-wire hot-wax zone valves [the issue being that pump will be pushing against a closed system in the minutes before the valve(s) open?] Is an ECM pump like the 007e or the 0015e good enough or is it necessary to use a pump which modulates to provide constant pressure?

  • hot_rod
    hot_rod Member Posts: 19,854
    Depending on the thermal actuator, and the ambient temperature around it, the valve starts to crack open, allow flow within a minute. The low current draw version actuators 250mA, may take 3 minutes to be 100% open.
    I think when ∆P circulators are at rest, maybe the 7 W indicated, they are still spinning. But not enough flow if any to cause harm. The pump will increase flow as the valve(s) open wider.

    I've seen no evidence that ∆T circs on multi zoned low mass emitters add any value, and if ODR is used?? A solution looking for a problem.
    There are excellent applications for ∆T circulators, yours is not one of them.

    Yes the circulator needs the pressure function(s), for zone valve application not a plain ECM.

    Here is a few graphics showing where the different pressure modes would be applied.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Dave H_2
    Dave H_2 Member Posts: 527
    To answer your question with the VT2218 Delta T circ, on startup, yes the temperature readings may be identical and in theory should tell the circ to not speed up.

    The sequence of operation of the circ is when it is first powered up, it starts on the lowest speed and will slowly work it's way up to the maximum speed within about 3 minutes. However, about halfway through, it then starts to look at the temperature sensors and decides if the Delta T is met and therefore will either continue ramping up or slowing down.

    Your other question regarding the 007e as a Delta P circ, it is. It just doesn't have all the settings and changes like the other do. It's performance curve is similar to the 007 however, it will speed up and down when actuators or zone valves open and close on a 10' head constant pressure curve.

    Now if you are having air issues in the system, upgrading to an ECM circ will not cure that ailment. Granted the above mentioned circs do have an air purge function, it is only for the circ itself and not for eliminating air from the system.
    Treat that issue with air elimination and proper fill pressure.

    Your next question with slow acting 2-wire actuators and worried about "dead heading" with either of these circs is not a concern. These are very low hp (1/25). Don't use a pressure differential bypass valve. They are designed for fixed speed AC circs.

    Dave H.
    Dave H
  • EternalNoob
    EternalNoob Member Posts: 42
    edited January 2022
    Once again, Thank you guys.   So, sounds like next time the 007e would be sufficient for manifold valve zoning.

    In my defense, my, perhaps mislead logic behind the DeltaT functionality was that I wanted something that will bring down the pump speed for the radiators and back up for the floor radiant or if all zones were calling.    I have three zones on three tstats with different heating characteristics.   1, a small converted-to-studio garage slab, 2 half the 950 s.f.  house with staple-up radiant floor, and 3, the other half with two small (2'x4') panel radiators.   Each zone is plumbed as a home-run.  The panel radiators were sort of a band-aide decision after starting into the staple-up and realizing that half the house had a 3-inch thick wood sub-floor.  Haven't worked with radiators before so i'm kind of backwards designing and trying to make this work.  Maybe this wasn't the best decision, but I opted not to do TRVs or balancing valves, because I want the thermostat dedicated to the radiators to command heat and not give the user another knob to tinker with.   Without TRVs, when circing to the radiator zone alone, the return temp from the radiators gets pretty hot.  I suppose i could choke down the flow with TRVS until the return temperature drops, but it would use less energy over-all if for the pump modulates down when the return temperature climbs, and an extra $100 for delta-T pump functionality compared to two $50 TRV's, cost-wise it's a wash. 

    Having said all that it sounds like a delta-P pump would more or less achieve the same goal but with pressure modulation rather than temperature modulation, and I could save 90 bucks on the delta-P viridian over the delta-T viridian.