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condensing gas boiler with constant circulation

bob eck
bob eck Member Posts: 929
when installing condensing 95% AFUE gas boilers with primary secondary piping using circulators does it make the system more efficient if the secondary circulators are on constant circulation?


  • Unknown
    edited November 2010
    From an efficiency standpoint,

    I would have to say no.

    From a comfortability standpoint, I would say yes.

    The house temperature will be more even(depending on what zone, and the radiation required within that zone), also solar-gain depends-on various window exposures wherever the stat may be.

  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    secondary circulators are on constant circulation?

    As a home owner, not a professional, I would say it depends on what you mean by efficiency, and even there I would have to equivocate. I imagine it could be arranged to have the system use less gas if you could adjust the reset curve just right. But your electric bill would go up with the circulators running continuously. In my system, these are Taco 007-IFC, which, while not power hogs, run about 75 watts, I would guess. If I wanted to run this continuously, and I were starting over, I would use a delta-P circulator and use zone valves; I would use an ECM type circulator. I believe an ECM delta-P circulator costs a lot more than a regular circulator, but it should pay for itself after a while. I believe you should measure "efficiency" by including both the gas bill and the electric bill when running a heating system.

    In this, I see I assumed you meant that you got the circulators to run all the time by having the reset curve either too low, so you never satisfied the thermostat (which you would not do), or just right, so you just failed to satisfy the thermostat, but satisfied the people in the building. Of course you could wire the circulators to be on all the time, and control everything with the thermostats, but that would probably lose a lot of the condensing a mod|con could deliver, wasting, I would guess, about 10% of the nominal efficiency. I do not think you could match the reset curve to the needs of the house accurately enough unless there is no air leakage at all which could change the heat loss even if the outdoor temperature remains the same, and you had magic windows that allowed the same amount of heat in and out irrespective of the insolation on the house. These are unlikely.

    One advantage of constant circulation, especially if you can get it by having exactly correct reset curve, is that the expansion noises you get with the circulators going on and off should disappear. My downstairs zone is copper tube in concrete slab, so no noise. BUt my upstairs zone is fin-tube baseboard; two 14 foot pieces, but they are 32 feet apart, so there are two 32 foot pieces of 1/2 inch copper tube in the floor that expand and contract and "click." Today after being off for almost 12 hours, it started running 125F water up there and it was pretty noisy for about 15 minutes. If I remember last year correctly, when it gets colder outside, the circulator up there runs for longer periods of time so there are fewer incidents of this. Last year we did not have enough really cold days to get the reset curve up there the way I wanted it. I did get the downstairs pretty well, and on cold days the circulator for that would run 18 hours at a stretch.
  • CMadatMe
    CMadatMe Member Posts: 3,086

    I wouldn't pri/sec would use a low loss. I would also be using an Alpha or similar system pump as well as some type of indoor feedback to the boiler. I wouldn't be doing constant circulation with mutiple zones though. I would be using panel rads with thermostatics.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Paul Pollets
    Paul Pollets Member Posts: 3,515
    The Paper

    Here's the paper on constant circulation...

  • CMadatMe
    CMadatMe Member Posts: 3,086

    Paul pulls out the Godfather of Low Temp Hydronics in the US. I have all of Joe's writings from 1989 to present in Chronological order by year. Where did you find this article? Absolutely beautiful...
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Paul Pollets
    Paul Pollets Member Posts: 3,515

    the article is from Richard Trethewey's website:

  • Henry
    Henry Member Posts: 995

    All our commercial and residential instal are done P/S. And they are all continuous circulation as it balances out better heat transfer. In the residential market, we use the Raypack residential with the built in bypass and pump. In the light commercial we prefer the Knight and Synch. At times we can use mid efficiency boilers due to cost considerations and code. When we have to, we use the RBI as they have an intergrated modulating indoor/outdoor control.

  • Constant vs Continual

    Consider this - the difference between constant vs continual circulation.

    Constant assumes there is no flow change in the application (primary or secondary zone).  If the load of the zone(s) is consistent or the output of the boiler is constant than it is a constant flow app.  But I think it is safe to suggest all heating apps see varying outputs as loads change.  Varying outputs translates to varying # of water (a function of BTU's) or varying flows.

    Is a primary loop constant circulation?  Yes, if the boiler output is constant (or there is only one boiler piped to the primary loop.  Or, what if we want to modulate the return water temp to assist a condensing boiler to condense by lowering the primary loop flow velocity (allowing more BTU's to get off the train).

    I am a huge supporter of continual circulation, all zones flowing just enough to provide enough output to provide comfort (this also means extremely low power requirements to provide these low flows).  No banging valves, no air traps, no sediment build-up, and so on.

    Food for though boys...
This discussion has been closed.