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MarketManager » Domestic Hot Water
Q: How do I model a HP water heater that extracts heat from the chilled water return loop to use to heat service hot water?


The easiest way to do this would seem to be the following:

  1. Set up the HP Water heater(s). The current "HP water heater" type in MarketManager assumes air-to-water HP, not appropriate to your application. Fortunately, return chilled water varies over a relatively narrow range of temperatures, as does the DHW tank inlet temp -- so it should be easy to determine a quasi-constant COP (or efficiency) to plug into a custom WATER HEATING item. Let's assume this COP to be 2.5 for now -- i.e. a "Recovery efficiency" of 250% in the WATER HEATING form.

  2. Set up a pseudo-equipment to model the reduction in chilled water coil load from running the hot water heater HP. To do so, Create a piece of equipment and label it, say, "WAHP load." Zero the equipment energy input (e.g. KW), and enter a *negative* total heat gain (all sensible) in the Detail screen equal to <minus sign> DHW input rating * (COP - 1) on the grounds that this is the peak amount of heat subtracted from the chilled water loop.

  3. Now copy to this equipment item all of the DHW schedules. In the example above, if your DHW input energy is 80,000 Btu/hr (temporarily switch input fuel to "Solid Fuel" to have the program do the conversion for you) and COP=2.5, then you would set the equipment total heat gain to -120,000 Btu/hr. If all goes well you should have created a negative internal heat gain which at all times removes exactly as much heat from the zone to which it is attached as the DHW item. Again, make sure that this pseudo equipment has 0 input capacity, so as not to "cost" you any real energy.

Note that we have thus approximated the chilled water coil load reduction with a zone cooling load reduction. Not quite the same thing, but acceptable *if*

  1. the zone cooling load is greater than the negative heat gain at all times;
  2. this is the only zone served by this chiller.

If either (1) or (2) are not true, I suggest you follow essentially the same strategy, but with proportionately smaller pseudo-equipment in all zones.

Finally, with regards to heat recovery:

In this particular application heat recovery would not do what you want. When heat is recovered from, say, a HEATING item (the provider) to a CHILLER item (the receiver), the program assumes that the heat goes to offset the CHILLER input energy. I.e. it assumes implicitly an absorption-type chiller.

In sum, you are modelling the water heating in (1) and the heat recovery off the CHW loop in (2) as reduced chiller load.

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