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Warming return water temps with Mod. Fan Powered Economizer (MFPE)

Hey there,



First post on The Wall....exciting stuff.



I watched a webinar yesterday on the benefits of employing a Modulating Fan Powered Economizer in a commercial/industrial/multi-family application. The MFPE will harvest btus that were headed up the chimney for use in the building and will moderate draft. Both positives.



One of the touted benefits proclaimed in the webinar: "an MFPE can warm return water, allowing for more aggressive resets to be used."



I am a residential guy new to larger building systems, and I don't understand the application here. Lets say we are in a multi-family building with a hydronic system. I'm in NYC, so lets suppose thats where this is too.



Aren't cooler return water temps and cooler stack temps advantageous to combustion efficiency? Why do we want the return water to be warmed? How will this allow for more aggressive resets?



Thanks!!!!



-Art

Comments

  • Tim McElwain
    Tim McElwain Member Posts: 4,608
    Most economizers simply

    reroute flue gases across a coil connected to return piping. The theory is that the heat that would have gone up the flue will be reclaimed into the coil with the return water coming back to the boiler. On conventional boilers both commercial and residential this makes sense to some degree that if we can reclaim some of the heat back into the boiler it will be some savings of fuel. The down side many times is on the combustion side. The added restriction into the flue gas flow reduces draft and causes poor combustion conditions (carbon monoxide) and many times the reclaimer gets sooted. Another possible advantage to one of these is if we have low return water temperature which could cause thermal shock to the boiler the reclaimer would heat up the return water to bring it above 140 degrees.



    I am not sure what the advantage of the modulating fan on the unit is unless they are going to determine its speed relative to flue gas restrictions.



    Yes lower return water temps and lower stack temps are part of non-conventional Modulating/Condensing boiler technology. Those do not however apply to conventional boiler systems.



    I am not a fan of these devices as I have seen many different attempts at this over the years and most of the time it causes combustion problems.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Too new to rate???

    I've never heard of this concept, but that doesn't mean that it hasn't been around for a long time.



    To your questions, "more aggressive reset" to me indicates that they intend to violate the normal recommended minimum return water temperature, which is around 140 degrees F for natural gas. That would be beneficial to energy savings, provided that the heat emitters can handle the lower fluid temperatures. There ARE limitations to what can be done in that arena.



    Thou shalt not expose a conventional boiler to too low a temperature of operation for fear of condensate production in thine combustion processes, and the resultant negative effects of same.



    In addition to avoiding condensate in the combustion chamber, one must ALSO maintain enough residual heat in the flue gas products to avoid the production of condensate in the flue piping, while maintain adequate and proper vertical movement of flue gasses to eliminate them, unless the flue piping is conducive to seeing the acidic condensation.



    Controlling draft is extremely important to efficient combustion processes, but has typically been addressed by the use of barometric dampers. Too much draft over fire, and you are sending paid for BTU's up into the atmosphere. Too low a draft, and you are not completely burning the paid for BTU's and generating excess carbon monoxide. Just right draft, and you are avoiding flue pipe condensation, and wasted energy.



    What concerns me about the use of fans is their parasitic cost of operation, and the complications that adding another component to a already critically sensitive process.



    Too many moving parts, and too many things to go wrong. When things go wrong in the field, and service is requested from unqualified people, safeties get bypassed to get things back up and running again, and then all hell starts breaking loose.



    Most boiler manufacturers will not approve any of these devices being applied to their appliances, so I would proceed with caution.



    Conceptually, it sounds great. In reality, if it is not properly operated, and maintained, it may shorten the life expectancy of the equipment, AND the building occupants...



    Proceed with caution. And welcome to The Wall.



    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • ArtVandelayLEEDAP
    ArtVandelayLEEDAP Member Posts: 9
    Another reset question

    Mark:



    "To your questions, "more aggressive reset" to me indicates that they intend to violate the normal recommended minimum return water temperature, which is around 140 degrees F for natural gas. That would be beneficial to energy savings, provided that the heat emitters can handle the lower fluid temperatures. There ARE limitations to what can be done in that arena."



    So does this sound to you like the suggestion is to increase delta T between supply and return temps, or operate the system at a cooler supply temp to begin with? Surely there is a limit to how low you can go with the supply temps.



    Agreed about the potential headaches involved with mechanical parts (although the webinar also demonstrated redundant safety features, but those could potentially be bypassed). The product claims to produce "perfect draft" in all scenarios.



    http://hpac.com/bse/modulating-fan-powered-economizers-0611/

    --> brief re-cap of said webinar



    thanks.



    -Art
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    There are only a couple of ways of increasing temperature differentials...

    Decrease flow, increase load or increase emitter surface areas.



    Decreasing flow will result in poor system performance on the last half of the emitter circuits. This can be addressed with the use of a 4 way reversing valve, but again, has its limitations and adds complications and is only compatible with non directional critical flow control valves.



    Increasing the load is not conducive to energy conservation :-)



    Increasing the emitter surface area is doable (radiant floors, walls, ceilings etc) but is generally considered cost prohibitive in most situations.



    The article correctly states that although "design" differentials are typically 20 degrees F, it is rare to actually see them unless you are always at "design" outdoor conditions. I think I have seen it once, maybe twice...



    The article is somewhat confusing in its use of fin tube and radiators. Not sure if they are talking about the heat recovery or the heat emitters...



    Cast iron emitters do typically have a deeper delta tee because you have a LOT of mass that needs to be accelerated before the heat hits the emitting surfaces. Cast iron radiators, in my experience, also have larger surface emitters, experience low flow conditions without creating too much problem if properly designed and installed, and work well with low temperatures.



    They are making some claims of some fairly healthy $ savings, but I find it hard to believe. In order to fully diagnose the potential, ALL loads must be taken into consideration.



    Fixed industrial loads are easy to ascertain. Multi family settings are much more variable, and difficult to nail down. And then there is the question of existing conditions... What condition was the combustion process in BEFORE the application of the equipment?



    It is my experience, that most MFD boilers receive little to no preventative measures, and are more likely to be in poor shape on the fire side, as well as the water side of the system. Preventative maintenance is predictable and results in quantifiable savings. Crisis management is expensive and generally results in budget busting repair bills.



    Quite honestly, for me and my money, I'd prefer to replace the existing boilers with high efficiency mod con boilers, where I KNOW that at a minimum I will see a 30% reduction in energy consumption, possibly as high as 50%, and the waste heat recovery is built right into the unit.



    And in the course of redesigning the system, it has also been my experience that the sizing of the appliance (should) result in a serious reduction in the size of the appliance. Pay attention to the DHW function, which is typically the reason that the boiler is oversized in the first place. Consider the use of reverse indirect DHW production that will keep the heat source in the condensing mode, even when in DHW production routine.



    Times, they are a changing, and I try and keep an open mind, but my previous experience with this waste heat recovery scheme has not been positive. It increases the need for regular maintenance.



    HTH



    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
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