water boiler temperature settings and circulator

Discussion in 'HVAC Heating & Cooling' started by jsherk, Dec 9, 2009.

  1. jsherk

    jsherk New Member

    Messages:
    4
    Location:
    USA
    I have a weil-mclain cg-5 series 12 water boiler in my basement, and am trying to maximize the efficiency.

    The way it is currently wired, when there is a demand for heat from thermostat, the boiler kicks on and once the water temp reaches about 145 degrees, the circulator will turn on. When the circulator starts running, the temp slowly drops over about 30 seconds back down to about 120 degrees and then circulator will shut off. The water temp will rise back up to about 145 again, circulator will turn on, temp will drop to 120 and the cycle continues.

    The gizmo that adjusts these settings is some kind of Honeywell thing, and there are two settings I can adjust:
    #1 - The temp that circulator shuts off at (currently 120), and
    #2 - The difference (how much hotter) above the shut off temp before it turns back on (currently set to max of 25 degrees).

    I have read that boilers are more efficient at hotter temperatures. Should I adjust any of these settings to improve the efficiency?

    Thanks
  2. Dana

    Dana In the trades

    Messages:
    2,986
    Location:
    01609

    The opposite is true- you get higher combustion efficiency (and higher system efficiency) at LOWER temperatures, but the critical temp is the that of the return water, not the output water. See:

    http://www.csemag.com/photo/167/167345-boiler_efficiency_vs_temp_2.jpg

    But for cast-iron boilers you have to take precautions that it operates primarily ABOVE the temperature at which the flue gases condense. For natural gas boilers you're generally safe it the return water stays above 130F. You can sometimes go somewhat below that, but at a risk of condensation in the flue. If you have a stainless flue liner and something to catch & dispose of the condensate you can take it right to the limit, but don't let it condense on the boiler's heat exchangers or you'll shorten it's service life considerably. The condensate is slightly acidic- similar acidity to red wine, and can be dumped in normal drains.

    At 120F you'll be getting condensation on the plates, but if it spends most of the burn time above 125F it's not a disaster. Setting the high-limit at 145F seems on the low side though- since the return water from the radiators/baseboards is likely to be 20F colder and in the condensing zone. But if the boiler is plumbed with a "boiler bypass" pipe mixing some of the output in with the water returning from the radiators, or if it's plumbed with dual-circulators in a primary/secondary setup, the water entering the boiler might still be sufficiently hot to protect it:

    Boiler bypass:
    [​IMG]


    Primary/secondary:
    [​IMG]

    It sounds like somebody cranked it down to max out the efficiency already- hopefully they didn't go too far. But if you can carefully measure the temperature at the point where the water enters the boiler you can verify that.

    How long do the burns take to go from low-limit to high limit? If it's under 5 minutes you're probably losing much of what you've gained in lower temperatures to cycling losses, but if it's over 10 your good. If it's really short, raising the high limit to lengthen the burns may be beneficial- I wouldn't take the low limit any lower- it may be too low already. If the boiler is just plain too big for the heating load (which is often the case), you can replace the aquastat (aka "the gizmo") with a smart controller like the Intellicon HW which will raise/lower the high limit based on how fast the water temps are changing to maximize efficiency. See: http://www.intellidynellc.com/02_pgHW.htm There are other smart boiler controls from other vendors, but this one is an easier install for the DIYer, and it does a decent job of it.
  3. jsherk

    jsherk New Member

    Messages:
    4
    Location:
    USA
    The way this system operates is completely based on the main floor thermostat... when it gets cold enough and the thermostat says "I need heat", the boiler kicks on... water temp can be as low as 100 depending how long it has been sitting... it takes less than a minute for the water temp to rise to the 145 mark, and then the circulator kicks in. It then takes about a minute and you can watch the temp drop back down to 120 at which point the circulator will shut off and the temperature will start to rise again. The boiler continues to fire the whole time (does not shut off or cycle), and after several circulator cycles the water temp does not drop as much. When the thermostat on main floor says "It's hot enough", both the boiler and the circulator will shut off. The boiler will remain off until the thermostat calls for more heat again. There is no boiler bypass on this system.

    Thanks
  4. Dana

    Dana In the trades

    Messages:
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    Location:
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    OK I get it- these are the circulator aquastats, not the burner aquastats- I should have read more carefully. The radiation is so big that the slug of startup water cools the boiler into the condensing zone at which time the low-limit turns the circulator off to protect the boiler.

    In that case, raising the high limit will reduce the number of cycles it takes before it stays on and will not affect efficiency one way or the other, but it'll still cycle. Cast-iron boilers are susceptible to thermal stresses if the delta-T between incoming & output temps over ~30F, so perhaps it's best leave it alone.

    Installing a boiler bypass (preferably with a thermostatic mixing valve, but it could be hand-tweaked with a much cheaper ball valve as well) would mix boiler output with the return water to above 120F and the system would never need to cycle and it would limit the delta-T on the system. Depending on the combustion air mixture, the dew point of the exhaust can be as low as 122F, so 120F is right on that margin. Don't tweak that one any lower- if anything, raise it to 125-130F to more reliably protect the boiler.

    If the temp stays below 145F for the whole cycle once it stops cycling and still manages to heat the place (you said it didn't drop "as much", implying that the temp still drops, but not all the way to the low-limit), it means you're about as efficient as you're ever going to get with that cast iron boiler. But with a modulating condensing boiler you'd likely hit the mid-90s on average efficiency(!).
  5. jsherk

    jsherk New Member

    Messages:
    4
    Location:
    USA
    ok thanks alot... i think i understand!:)

    What about PSI... after bleeding all the air out and filling with water, I have the PSI at around 13 or 14 PSI.

    Thanks
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,270
    Location:
    New England
    That's a good normal water pressure. You are likely to have a low-pressure safety cutout that would shut it down if it gets much lower. that should be in the spec sheet. As long as it is above that, and not above the overpressure safety release, it should work. You don't want the water to boil (same idea as the pressure cap on your radiator of the car). More puts stresses on things, so you don't want it too high. 12-16 seems about average. Your boiler will say what the min/max is. Many boilers have an autofill valve, and that is set to 14# on mine, your results may differ. The hassle with an autofill is you may not notice you have a slow leak. The benefit is, you should not get a low pressure shut down since it is always sitting there maintaining the pressure. This also requires a backflow preventor to ensure the stagnent water in the boiler doesn't get into the potable water supply.
  7. jsherk

    jsherk New Member

    Messages:
    4
    Location:
    USA
    Ok thanks...
    I have to manual fill (no auto fill) so just wanted to make sure the pressure was ok.
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