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Age Old Pump vs Zone Valve Question

Mudaero Member Posts: 46
Hey all. So I know the basic dynamics of this question but just wanted to get some input. I am rebuilding my hydronic system and it will contain 2 zones (about 30000 - 40000 btu total per zone [cast rads]). I've done this before using zone valves and a single circulator, but am wondering if I should move to 2 circulators this time simply because of the cost. It's actually cheaper initially (not talking life of system, I know pumps will consume more power) to purchase 2 pumps rather than 1 pump and 2 valves. Anybody have any thoughts on this? It seems to be a half-dozen in one hand, 6 in the other kind of issue. Just looking for any feedback. Thanks.


  • STEVEusaPA
    STEVEusaPA Member Posts: 5,786
    2 zones, 2 pumps. New pumps use very little electric. Just properly size them.
  • dopey27177
    dopey27177 Member Posts: 873
    Two pumps is fine. Key to this is to find the right horsepower pumps for each circuit.
    Because the head pressure may be less on each circuit the HP can conceivably be less than the one pump you now use.

    If you use two pumps install pumping away method which will assure that after the system is filled and operating air removal will be continuous.


  • Mudaero
    Mudaero Member Posts: 46
    Yeah, I was going to just go with the Grundfos Alpha to make things easy and will also have an air separator attached to the supply.
  • Mudaero
    Mudaero Member Posts: 46
    Definitely will be using the pumping away method. One last thought, is it necessary to use a flow control valve after the circulator? This will only be a radiant heating system and I can't imagine a little extra gravity flow into the zone that is not on would be that detrimental. If I am wrong, please let me know.
  • GroundUp
    GroundUp Member Posts: 1,440
    With rads, it will be overpumped no matter how small of a circ you go with. Even an Alpha will have trouble trying to decide where to run, not to mention an Alpha in a CI system isn't great idea anyway unless other means of magnetic filtering are integrated. Balancing valves will be a must on each zone to avoid overpumping. I would go with zone valves like a Taco Sentry in a system like this, personally, and no need for flow check.
  • Mudaero
    Mudaero Member Posts: 46
    I should mention that each zone will be connected to a manifold with 1/2" PEX Al PEX running to each rad (5 rads per manifold, 1" supply, longest single run between the 2 being no more than 80' and most at 40' - 50'). Yes, there will be a magnetic dirt separator attached to the system. I can definitely see how balancing valves would be needed. After looking at some specs, I was thinking that a Taco 007 (or similar) would be overly sufficient on each zone. But I can also see how even a single alpha with zone valves would work just as well. I appreciate the response, like I said, I'm sure both approaches would work, but I it's also nice to get input from those who do this more often than me. Thanks again to all who respond.
  • geno907
    geno907 Member Posts: 10
    0018e probably would be overkill but you’d fine tune the GPM in nice and tight.
  • The Steam Whisperer
    The Steam Whisperer Member Posts: 999
    I looked up the pressure drop running 1 gpm (10,000 btu/hr @ 20F drop) through 200 equivalant feet of 1/2 copper (probably close to 1/2 pex) and you end up with about 5.5ft/hd. The fittings on the ends will probably add a couple feet of head, so a total of 7 feet. That's enough heat probably for 400 sq ft of living space @ 25 btu/sq ft @ -5F outdoor. A well insulated and air tightened home using conventional design walls will probably be only about 15btu/sqft, so that line would cover about 666 sq ft. If using 30F water temp drop, the head for 10,000 btu/hr would be only about 3 feet for that 200 feet of tube. For cast rads I'd use 30 or maybe even 40F temperature drop ( max head of 2 feet or so) for my design, especially if they are way too big.

    So using a single pump you need about 7 GPM @ 7 feet at the largest (20F drop), 4.5 gpm @ 3feet (30F drop), or 3.5 gpm @ about 2 feet (40F drop).

    If you are running full outdoor reset the pumps will run nearly continuously throughout the heating season.. A single 007 would be using probably about 65 watts or so since it will be backed down its pump curve and max power draw. That would be about 1.5 kwh/day or about 47 kwh/month.

    Based on my own home's winter electrical usage (1500 sq.ft main floor and basement in Chicago) that 47KWH a month is about a 14 % increase in electrical usage versus no pump in use.

    I'd use a small variable speed ECM pump with zone valves, this would cut the power usage dramatically. IIRC my previous home had a heat load of around 55,000 btu/hr and the ECM pump was only drawing about 10 to 20 watts.

    I expect to be converting to steam in the next year so I will be eliminating the pump and will down only to a little 24v transformer that I can power from a car battery in power outages.
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  • HomerJSmith
    HomerJSmith Member Posts: 1,685
    edited August 2021
    Do a cost analysis on the two systems. How many thermostats? If two, then the costs change.

    Two Taco Sentry zone valves, one ECM pump, a DirtMag, and piping (flow controls, etc.).

    Two pumps, a Taco pump relay box, piping (see above), etc. A recommended DirtMag.

    Sizing circulators is more complicated, especially old system that were converted from steam to hot water, steel piping vs. copper piping. I would pay more attention to flow and how you are going to control it.

    There is a happy medium between flow and BTU delivery and you have to stay within that medium.

    BTU delivery is dependent upon Supply Water Temperature (SWT) and flow. Flow is determined by pipe size and material and pressure losses in the sys.

    It is better to have higher flow capacity than required that can be throttled down with added restrictions (higher pressure loss) with balancing valves. Inadequate flow can not be compensated for and will lead to under performance of BTU delivery.

    Just like Marriage, it's complicated.

  • Paul Pollets
    Paul Pollets Member Posts: 3,524
    I'm a proponent of design and installation that saves energy and runs more efficiently. This would be 2 zone valves and a smart pump. If your boiler has ODR, you wouldn't even need zone valves, unless thermostats were being used for each manifold. My shop with 1 manifold and 600SF of slab heating ran on 12 watts. (pump output) the boiler drew 4 amps when running.
  • The Steam Whisperer
    The Steam Whisperer Member Posts: 999
    One other thing to add is if you are running a conventional on/off cast iron boiler (draws about 0.2 amps when running), then your pump on time is probably about 1/5 the time or less than with a full outdoor reset boiler set up. The electrical usage of the pump is then far less, so a standard pump would probably make sense. With the cast iron radiators ( particularly with older high water content models) this provides a natural "outdoor reset" of the system. The boiler would run on very efficient long on and long off cycles, especially if you can find which zone needs to run the most and only let that zone actually turn on the boiler. If you kept the zoning at just the 2 zones this would probably be a nice comfortable system with both low electrical usage and low gas usage. Very economical to run, maintain and very long life ( assuming, oh course, proper installation)
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