Reheat on GSHP
Reheat on GSHP
(OP)
I have an existing dormitory building with rooms that open onto balconies in South Carolina, USA (near coast & humid environment). During a recent renovation, contractor was given a design to use through-wall PTAC room units with electric reheat. It was later decided have the contractor replace the PTAC units with GSHP's. Question is should the GSHP units have reheat for humidity control?
In a normal application where the supply air is ducted through out having some reheat to reduce the RH in the SA stream makes sense to prevent condensation inside ductwork. (in the past I've allowed fan heat to provide this reheat.) However, when the AHU is located inside the space such as with a hotel room PTAC, mini-split or GSHP, is this reheat really necessary since the air stream enters the occupied space almost immediately. The electric heat coil on an air cooled PTAC is reasonable to provide general heating.
In a normal application where the supply air is ducted through out having some reheat to reduce the RH in the SA stream makes sense to prevent condensation inside ductwork. (in the past I've allowed fan heat to provide this reheat.) However, when the AHU is located inside the space such as with a hotel room PTAC, mini-split or GSHP, is this reheat really necessary since the air stream enters the occupied space almost immediately. The electric heat coil on an air cooled PTAC is reasonable to provide general heating.





RE: Reheat on GSHP
Anytime I could add a hot gas reheat coil to a heat pump, I would. Depends on budget but it's usually not much more expensive and uses very little extra energy. I wouldn't go cheap in a humid environment like that.
RE: Reheat on GSHP
RE: Reheat on GSHP
RE: Reheat on GSHP
you would need to do psychrometric calculation, any decision without it is blind-folded one if you want to be aware of what are you doing.
the other story is how much data you are provided with for "simple" units planned for residential use. such units normally serve for partial, uncontrolled humidification and the question is which data you can obtain for the unit - supply air temperature for instance.
if you have limited data available, the best what you could do is what tys mentioned - performing detailed calculation of cooling loads and taking care not to oversize units.
RE: Reheat on GSHP
simultaneous cooling and heating (or de-humidification and humidification) is not allowed by ASHRAE 90.1 - one would say that reheat is out of the question, unless it is used as primary heat for winter heating only - reheat should not be enabled in summer.
As tys90 mentioned, OVERSIZING is generally the problem. Consider downsizing the AC unit, it will work like a horse all day long and keep dehumidifying continiously - Use Manual J calculations instead of running Trace for such system.
RE: Reheat on GSHP
RE: Reheat on GSHP
Why do we put reheat in an AHU?
I have my own thoughts on it, but I want to hear from others in the community to confirm or rethink the way I design and review systems in the future.
RE: Reheat on GSHP
RE: Reheat on GSHP
if you have some link for small heat pump that uses hot gas for space reheat, i would be interested to see it.
RE: Reheat on GSHP
Best to you,
Goober Dave
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RE: Reheat on GSHP
http://www.daikinmcquay.com/McQuay/ProductInformat...
http://www.waterfurnace.com/products.aspx?pl=60&am...
http://trane.com/COMMERCIAL/DNA/View.aspx?i=2354
RE: Reheat on GSHP
it's revelation to me to see so small units quipped with dedicated hot gas reheat, together with ec fans. I will try to find time to study how minimum outdoor air can be achieved.
RE: Reheat on GSHP
RE: Reheat on GSHP
I'd recommend looking at AAON literature, as they write much gooder English than me.
http://aaon.com/literature.aspx?id=1
RE: Reheat on GSHP
RE: Reheat on GSHP
McQuay has heat pump design manuals on their website that will show this configuration.