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MarketManager » Modeling HVAC Systems
Q: I have a boiler serving both reheat and perimeter radiators, but reheat goes off at night while the perimeter radiators stay on to keep the building from freezing. What do I do?


This assumes a VAV or other reheat system with appropriate AHU. If the same boiler serves both the radiators and the AHU hot water coils, there is really no reason to model the radiators at all. Just ignore them. In MarketManager, a BTU is a BTU, whether it is delivered by radiant heating or through an AHU. MarketManager does not consider human comfort, but whether the zone temperature setpont has been satisfied. MarketManager will apply heating energy to the zone until either the load is met, or the heating equipment has reached maximum capacity, making no distinction as to how the energy is applied to the space.

So in your case:

  1. do not model a zone heating unit for each zone

  2. use a AHU system with a central boiler supplying reheat.

To have the system come on just to keep the building from freezing in the night:

  1. set the HVAC operation to "Day only"

  2. check the HVAC standby box for heating (This means the fan will cycle on at night to meet zone temperature requirements only, not ventilation requirements.)

  3. and set the thermostat schedule to a lower acceptable level for nighttime operation

  4. schedule the boiler on 24 hrs. (HVAC standby is for fans, not boilers. For equipment, on means available.)

Even though the AHU is off as the zone temp gradually floats downward, if it ever reaches the lower setpoint, heating is expected to come back on line.

In your case there is a slight problem in that MarketManager will also assume that the AHU comes back on, though only just enough to meet the small load. Even if the minimum stop is high, MarketManager assumes that the AHU cycles just enough to meet this small load. Once again, MarketManager does not really distinguish where the heat comes from -- radiators or reheat coils.

If the "erroneous" fan consumption is a big problem, consider setting up 2 supply fans - a "day fan" w/ regular kw, and a "night fan" w/ 0 kW.

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