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OLD FAN COIL REVERSE RETURN SYSTEM

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HAVE A CUSTOMER WITH A OLD OIL FIRED BOILER ,REVERSE RETURN PIPING , INDIVIDUAL UNITS FOR EACH ROOM WITH A COIL AND A FAN BEHIND THE COIL PIPED IN 1/2" (LIKE A TOE KICK HEATER) , THE BUILDING HAS ONE THERMOSTAT THAT OPERATES THE BOILER TEMP, AND IN EACH ROOM THERE IS ANOTHER THERMOSTAT THAT RUNS THE FANS FOR EACH UNIT BY TEMPERATURE. THERE IS ALSO A OUTDOOR RESET SET UP ON THE BOILER TO MIX THE BOILER WATER DOWN PER THE TEMPS OUTSIDE. THE CUSTOMER WANTS TO UPGRADE THE BOILER , BUT NOT DISTURB THE ACTUAL SYSTEMS WORKINGS SENSE THIS WOULD REQUIRE REMOVING THE HEATERS . SIAD NO TO BASEBOARD , AND RADIANT IS OUT OF THE QUESTION. DOES ANYONE HAVE ANY IDEAS OF HOW I CAN INSTALL A HIGH EFF. BOILER TO THIS SYSTEM AND SAVE HIM SOME MONEY ON HIS HEATING COSTS ? I KNOW A NEW BOILER WILL DO A LOT . IS THERE ANYTHING ELSE I CAN DO?

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  • Brad White
    Brad White Member Posts: 2,398
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    First and Foremost

    I would calculate a heat loss on that building, taking into account the current condition of insulation, air sealing, storm windows, what have you.



    (Let's also assume for the moment that you can keep the heaters in place. Clean them, spiff them up maybe, but keep them in place.)



    Then I would find out or at least estimate the output of those heaters. Often they were designed for higher temperatures, say 180F or 200F. Not often a problem in years past. But now you have to find out their output curve at lower water temperatures and how that intersects with your reset curve.



    So you DO want to be sure what your new highest required temperature will be. It could be 160, 150, 140, when you match the potential output to the best heat loss estimate you can do.



    You can use outdoor reset as you are now, but take note of what the range is now and see if you can improve upon that.



    But with the lower temperatures if lower than now scheduled in milder weather, you may experience the following:



    -Too low a discharge temperature. Enough to heat the space but cool enough that the moving air is felt by occupants, especially if dry/low humidity and a few feet away from the heater.



    -Too low a water temperature if the FCUs are controlled by a low-limit aquastat internally. The fans may not start if the water temperature is too low and you have to jumper these out of the equation. Then see above.  The solution is to raise the minimum HWS temperature but lose some savings over the season.



     There should be enough hours at milder temperatures to show some savings and the modulation alone is a key factor in efficiency. But watch space comfort when you commission the system.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
  • icesailor
    icesailor Member Posts: 7,265
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    Rads:

    This should work.

    Do the fan coil rads with blowers have variable speed control switches? If they do, this sounds like "Commercial" system where thr output of the heater is controlled by the speed of the fan. Giving the units mulitple BTU outputs. The thermostats give room control.

    If you put outdoor reset in, be sure to change it to primary/secondary with a four way valve to keep the boiler above 140' or it will condense and make crud like you will never see before. I do a nursing home with a system like this and the outdoor reset causes me to remove over 30# of cemtoious slag every summer when I clan it.

    The owner will save lots of money. 
  • New Hampshire Heating
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    THANK YOU ALL

    THE REPLYS ARE VERY HELPFUL. THIS GIVES ME A FEW OPTIONS TO CHAT WITH THE CUSTOMER ABOUT . HE IS CONSIDERING SWITCHING THE WALL HEATERS OUT WITH A NEWER VERSION. MAY TAKE A LITTLE WOOD WORK , BUT IT IS POSSIBLE. THIS WOULD BE NEXT YEAR AFTER WE SWAP THE BOILER . WHAT DO YOU GUYS RECOMMEND FOR N OIL FIRED BOILER. I KNOW VIESSMANN WILL BE COMING OUT WITH A HIGH EFF. OIL BOILER IN A MONTH OR SO, BUT I DONT WANT TO BE THE GINNY PIG THAT TRYS IT OUT OVER HERE . THEY SAY THAT BUDERUS HAS A BOILER OUT THAT QUALIFYS FOR THE OBAMA BUCKS , DOES ANYONE KNOW ABOUT THIS BOILER YET. IT SOUNDED RATHER COMPLICATED WHEN I TALKED TO THE SALES REP ABOUT IT.



    ANY HELP WOULD BE GREAT , THANKS , JOHN
  • Brad White
    Brad White Member Posts: 2,398
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    When you said, high efficiency

    I was thinking gas-fired modulating condensing. Icesailor is correct, if oil or a high-temperature (above condensing) boiler, you need to decouple the system circuit from the boiler circuit to keep the boiler at 135 minimum on the return.



    That said, if you do change it out to one of the newer 3-pass eutectic CI boilers, you might squeeze that down a bit more.



    Now, if and when you DO change out your fan coil units, the key will be to get more coil rows. Right now you may have one (well, you ALWAYS have at least one!), but maybe two rows.



    For low temperature systems, I use 3-row and 4-row coils. For air handlers, sometimes, 6-rows and get very high delta-T's.



    They key to occupant comfort though, has to do with discharge temperature and humidity and then how the air is distributed. The higher the humidity, the lower a temperature one can tolerate on average. Not advocating that you humidify, but just to illustrate. 



    If the air blows directly on the occupants versus up the wall or across a ceiling, that hurts comfort.



    Most designers recommend a discharge temperature of 105-115F, which will mix down within a foot of discharge. So when you select your coils, look for a benchmark discharge temperature of about a 110F target.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
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