Buderus GB142 adjusting for heating room via air handler

Discussion in 'HVAC Heating & Cooling' started by rayguy, Oct 23, 2011.

  1. rayguy

    rayguy New Member

    Oct 23, 2011

    I just had a Buderus GB142 mod con boiler installed and I'm beginning to see how the system works.

    My question: Is there a way to adjust the boiler response curve for each zone? Is there a way to individualize the boiler supply temp to each zone if only one zone is calling for heat?

    I have a 3500 sq foot home from the 1920's in southern CT. Four zones: 1) Main house radiators, single thermostat 2) Master bedroom radiators single thermostat 3) basement baseboard radiator single thermostat and 4) Family room, forced air via air handler single thermostat.

    Boiler settings are factory and an AM10 outdoor reset module is installed

    Just turned on the family room zone and find the response to heat call is poor even with outside temp 45 degrees. Boiler is supply temp is 100 degrees.

    The family room loses more heat than other parts of the house.

    I did not find an answer on review of the Boiler and AM10 manuals.

    Thanks for any help.

  2. BadgerBoilerMN

    BadgerBoilerMN Hydronic Heating Designer

    Mar 4, 2011
    hydronic heating designer/contractor
    All condensing boiler replacements should start with a proper computer generated Manual 'J' heat load. Once a high efficiency boiler is specified to meet the total load radiation can be taken into account. Design water or "operating" temperature is determined by sizing the radiation for a particular room. The more radiation you have, the lower the operating temperature can be. Naturally this is all determined before the first pipe is turned.

    The Buderus AM10 has to be set for the climate and the radiation employed. Controlling the water temperature for individual zones - especially when they are of the same type of radiation - is rarely needed.

    Your installer should know how to do this, if he doesn't I would find a new service company for you annual service. High efficiency condensing boilers like the Buderus GB142 series deserve professional care.
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    There are ways to do this, but it can get complicated. Moving air generally needs to be warmer than the hydronic radiators to be comfortable because of the 'wind chill' factor (one reason why I don't particularly like heat pumps - on a cold day, their outlet temp is too cool for me). They do make mixing devices that can do this, but it generally needs to be designed in from the beginning. You'd need something to temper the water for the radiators while tapping the hotter water for the air handler's heat exchanger. I know there's some built-in logic to handle an indirect at high temperature. If you don't have an indirect (you probably should!), you might be able to tap that circuit for the air handler, but the logic means that is a priority zone, and the rest of the house wouldn't be getting heat while that one is calling for it. Otherwise, you'd need to add in a mixing circuit to provide multiple outlet temperatures, customized for your needs.

    the system can be set for a minimum temperature, which might resolve the situation, but it wouldn't be as efficient.

    You need to contact your installer/designer and discuss your options.
  5. Dana

    Dana In the trades

    Jan 14, 2009
    From a comfort point of view it's best to keep the lower-limit of the boiler feed to a hydro-air coil at 120-125F or so, to keep the exit air at the registers at human body temp or higher. That will still keep the return water to the boiler well into the condensing zone. It may or may not keep up at outdoor design temp with 120F water though- it just depends. With 100F water it's neither comfortable nor efficient, since the heat-transfer across the coil is much reduced, and the power used by the air handler can equal or sometimes even exceed the amount of heat being delivered to the air through the coil.

    That's not much of a restriction for the low-mass baseboard zone either, since below 120F the heat output becomes less predictable (the dust-kittens can factor into convection rates considerably with 100F water), and it could be prone to short-cycling.

    Crank the curve up, and see what it takes. Efficiency is usually still pretty good in an air handler with 140F water out, and the heat transfer rates through the coile will be ~3-4x as much at 140F than at 100F. Odds are that you won't have to go higher than that except perhaps at design condition, maybe not even then. The 100F output temp is only appropriate for high-mass radiation or radiant floor applications. As long as the return water entering the boiler is under 120F you'll be getting a very significant condensing boost. In southern CT the heating outdoor design temps are in the +10F<->+15 range, so even at 45F outdoors you're at well over 1/3 the design condition load, so I suspect the curve is currently set up for 140F out @ 0F or so. Cranking the curve up 10-20F isn't going to impact combustion efficiency much, and would improve both comfort, and the chance that the air handler keeps up. You can mess around with the curves empirically a bit to see what delivers 120F @ 45F outdoor temps, or just set the min out of the boiler to 120F and see if it all keeps up during colder weather before tweaking the high end of the curve.

    At 120F water, measure the air-temps at the register- for wind-chill comfort it's best to keep exit-air above 100F (unless you have a continuously variable speed ECM drive air handler, which isn't likely, since you probably would have mentioned that.) If it's delivering 105F air @ 120F incoming water temp you can back it off another 5F and buy yourself a percent or two in condensing efficiency, but depending on the air handler and your utility rates that won't necessarily mean a cost savings, and bumping the low limit up to 125F for better comfort is probably a wash too. Setting the low limit to 135F would probably lower the net efficiency measurably though.
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