ground-source heat pump with gas backup

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Folks,

I have a ground-source heat pump that operates during the day. At night, the pumps are shut off, and if the space needs heating to maintain the setback temperature, a gas furnace kicks off.

Said otherwise, only the furnace operates to maintain setback.

How would you model that?

_______________
Demba NDIAYE

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Hi Demba,

What is the building type and operating schedule? Why the gas furnace as
back up to cycle on only at night?

Carol

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Carol,

The building is a school. Operating hours are basically 7am - 4pm. The furnace operates as a backup only during the day, but at night is the only one operating to maintain heating setback.

I have two system types: one is a PVAV and one, a PVVT.

I can shut down the pumps through a schedule for night-cycling, but cannot see how to get only a furnace operating for night-cycling.

Demba.

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Demba,

Here are two options that come to mind:
1.) Add an electric HP-SUPP-SOURCE. Convert the electric use to gas
(by hand calculation, not within eQUEST) and use the furnace AFUE to
account for inefficiency.
2.) Add baseboard heating of type FURNACE. Use OA TEMP control,
create a RESET-RATIO baseboard schedule and adjust the OA temperatures
to minimize daytime operation of the baseboards. You will need to create
an hourly report to see when the baseboards (furnaces) are coming on.

Regards,

William Bishop, EIT, BEMP, LEED(r) AP

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Doesn't PVVT allow you to have supplemental gas heat for the backup? Using a
furnace? I think I just did that. You may be able to create a schedule for
it. I don't know about PVAV but you might be able to call it PVVT with out
too much trouble.
Try it and let us know. If it doesn't work do what Bill says to do.
Carol

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I have a building using a GSHP with electric supplemental heating
and I'm getting almost 30% unmet heating hours. Any ideas why
this could be occuring?

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It's very likely that your bore-field is undersized. Make sure that your configuration is correct and that the multiplier against the bore configuration results in the number of bores you have in the field. Depth, conductivity, etc. also play a key role, but this is usually the reason I come across.

Depending on the actual configuration of the field you may also want to split the simulated field in two and have two GSHX fields to "draw" from.

Matthew Higgins, ASHRAE-HBDP, LEED-AP

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Christian/Matthew:

Here is a procedure I dug up in the past to verify whether the GLHE is
doing its job or not:

Here's an excerpt from an archived message - full text is here
:

Actually, the better way to watch the performance of a GLHE is to let
eQUEST output the hourly report of the GLHE outlet temperature, which is
the entering fluid temperature to the heat pump if the thermal loss from
the loop and the pump temperature rise can be ignored. As an example, to
output the hourly loads on GLHE, outlet temperature of GLHE, and the
undisturbed ground temperature, you can save following text as a short
inp file and add it to your input file by using the "Import files"
feature of eQUEST in the detailed mode. For advanced users, you can
directly define the hourly report in the detailed mode of eQUEST.

$ ---------------------------------------------------------
$ Hourly Reporting
$ ---------------------------------------------------------

"GHX" = REPORT-BLOCK
VARIABLE-TYPE = "GLHX (VertWell-CM)"
VARIABLE-LIST = ( 1, 4, 5 )
..
"Hourly Report" = HOURLY-REPORT
LIBRARY-ENTRY "Hourly Report"
..
"Hourly Report 3" = HOURLY-REPORT
REPORT-SCHEDULE = "Hourly Report Schedule"
REPORT-BLOCK = ( "GHX" )

Original credits to Xiaobing Liu, whose name you may recognize as a
fellow whose work has been pivotal with regard to modeling ground source
heat pumps.

In any case, if you'll do this and check out the hourly behavior of the
ground loop temperature over the year, it will probably be obvious if
your 30% unmet heating hours is due to an undersized field.

Best of luck!

NICK CATON, E.I.T.

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