Water-to-water and water-to-air heat pump off same loop

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I would like to model a combination of forced air heat pump heating &
cooling with radiant slab heating. The ducted system would use a
water-to-air heat pump and the radiant slab would use a water-to-water
heat pump, both drawing off of a GLHP loop. Can I get there from here?

I thought of using a radiant slab system, and using the "Outdoor air
from system" to get conditioned air from a separate system. Is this an
option? Does the OA-from-system system have to be a 100% OA system?


William Bishop, EIT, LEED(r) AP

Bishop, Bill's picture
Joined: 2011-09-30
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Remember a cardinal rule of eQuest/DOE-2 - ONE SYSTEM-TYPE to ONE ZONE.
I suggest you split your space into two zones and give each the
appropriate system. Put the majority of the load into the Radiant System
(since I will assume you want that more passive system to operate
first), then put an AIR-WALL between the zones and let the ducted system
take the rest of the load.

This thinking is before my first morning cup of coffee, so perhaps it is
backwards. But you probably know which system should be able to handle
what load.

Good luck.

John R. Aulbach, PE, CEM

Aulbach, John's picture
Joined: 2011-09-30
Reputation: 0

Ok, so there are a few challenges in trying to model this system. I would
be cautious about splitting the zone and using an air wall, because this may
result in heating loads on one side and cooling loads on the other, when
really the space would balance itself, you just need to be careful about
where to make the delineation. I have split zones to model buildings that
have radiant heat under windows. In this case, I've created a perimiter
zone a few feet deep which is heated by a HW baseboards, and it seemed to
work ok.

Another option is to just put HW baseboards in, along with the water-to-air
heat pump. I believe the baseboards will pick up almost all the heating
load and am not sure if there is a way to specify that the air system pick
up some load as well. Here's the problem with this plan: eQuest's
loop-to-loop heat pumps don't work with WLHP or GLHP loops. They do work
with loops of type lake/well. You can schedule lake/well temperatures, so
you could run the building with a straight air-to-air heat pump system, and
track GLHP loop temperature. By averaging the monthly or weekly loop
temperature, you can create a temp schedule for a lake/well loop. Then
re-model as I've described and run both water-to-air and water-to-water heat
pumps off this loop.

Last option is to specify an air-to-air heat pump system only and adjust
your heating efficiency and performance curves to approximate a
water-to-water heat pump. This method overlooks a couple things:

1. no HW loop is simulated so the pressure drop through the system in the
zone is modeled as being the same regardless of whether you are in heating
or cooling mode, and pumping energy is not modeled as well as could be
2. fan power is specified once, you can't get rid of the fan power during
heating hours, although you could subtract this value on an hourly report

I would try the lake/well loop with baseboards. This seems to be the
closest approximation. Good luck and let me know how it goes--I'm curious.


No Username provide's picture
Joined: 2011-09-30
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