Modeling in-unit DHW heater as hybrid hydronic space heating system in eQUEST

asked 2021-11-29 12:35:58 -0600

JoyLiao gravatar image

updated 2021-11-29 14:26:37 -0600


We are modeling in eQUEST (DOE engine 2.3) a multifamily residential building (300+ units) and the proposed model will be using a Hydronic Aquatherm AHU with hot water heating coil + RINNAI RU160i gas-fired tankless water heater (Energy Star, UEF 0.93, HW delivered temp: 140F) + Rooftop DX split system AC (SEER 14.5-15) to serve each apartment unit.

And I am just wondering if anyone has experience in modeling the same type of system as these are more and more common today (DHW-space heating hybrid system)? And does the below modeling descriptions sound correct to you:

  • We set up two DHW heating loops that represent residential space heating and DHW separately. (because we want results that can separate space heating and DHW end uses)
  • Both the system efficiency inputs are the same (HIR 1.075) as it's the same piece of equipment. (RINNAI RU160i gas-fired tankless water heater)
  • The capacities are modeled differently: DHW space heating loop 11.988 Mbtu/h (this is the sum of all AHU heating capacity); while the DHW loop has a 55.84 Mbtu/h capacity (this is the sum of all RINNAI RU160i water heating capacity, 160,000 Btu/h*349 units=55.84 Mbtu/h)
  • The schedules and loads for the two loops: DHW space heating loop has a 4.1 gpm (per equipment spec sheet) peak flow with a schedule of 1,907 (eQUEST default schedule); while the DHW loop has a 23.3 gpm peak flow with a schedule of 2,628 hrs (per Es MFNC simulation guideline)
  • The residential spaces associated with the system are linked to the DHW space heating loop on the air-side via 'DHW Loop'.
  • These residential space heating system heat source and zone heat source settings: heat source as 'None'; zone heat source as 'DHW Loop'.

A couple of concerns I have right now are:

  1. Is the DOE 2.3 engine really capable to model the DHW-space heating hybrid system? I checked the eQUEST DOE2.3 New Features Volume 6 and it did seem to mention that the DHW as a heating source and zone heat source is currently not implemented...
  2. I know that for space heating water loops you should specify Process loads instead of Process flows. But I cannot seem to find how to input process loads for DHW loops.

Will be very appreciative of any guidance. Thank you.

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